DESIGN OF VENT GAS COLLECTION AND DESTRUCTION SYSTEMS

Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries

Web Site: www.GBHEnterprises.com

GBH Enterprises, Ltd.

Process Safety Guide: GBHE-PSG-019

DESIGN OF VENT GAS COLLECTION AND DESTRUCTION SYSTEMS

Process Information Disclaimer

Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the information for its own particular purpose. GBHE gives no warranty as to the fitness of this information for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.



DESIGN OF VENT GAS COLLECTION AND DESTRUCTION SYSTEMS CONTENTS 1 INTRODUCTION 1.1 Purpose 1.2 Scope of this Guide 1.3 Use of the Guide 2 ENVIRONMENTAL ISSUES 2.1 Principal Concerns 2.2 Mechanisms for Ozone Formation 2.3 Photochemical Ozone Creation Potential 2.4 Health and Environmental Effects 2.5 Air Quality Standards for Ground Level Concentrations of Ozone, Targets for Reduction of VOC Discharges and Statutory Discharge Limits 3 VENTS REDUCTION PHILOSOPHY 3.1 Reduction at Source 3.2 End-of-pipe Treatment 4 METHODOLOGY FOR COLLECTION & ASSESSMENT OF PROCESS FLOW DATA 4.1 General 4.2 Identification of Vent Sources 4.3 Characterization of Vents 4.4 Quantification of Process Vent Flows 4.5 Component Flammability Data Collection 4.6 Identification of Operating Scenarios 4.7 Quantification of Flammability Characteristics for Combined Vents 4.8 Identification, Quantification and Assessment of Possibility of Air Ingress Routes 4.9 Tabulation of Data 4.10 Hazard Study and Risk Assessment



4.11 Note on Aqueous / Organic Wastes 4.12 Complexity of Systems 4.13 Summary 5 SAFE DESIGN OF VENT COLLECTION HEADER SYSTEMS 5.1 General 5.2 Process Design of Vent Headers 5.3 Liquid in Vent Headers 5.4 Materials of Construction 5.5 Static Electricity Hazard 5.6 Diversion Systems 5.7 Snuffing Systems 6 SAFE DESIGN OF THERMAL OXIDISERS 6.1 Introduction 6.2 Design Basis 6.3 Types of High Temperature Thermal Oxidizer 6.4 Refractories 6.5 Flue Gas Treatment 6.6 Control and Safety Systems 6.7 Project Program 6.8 Commissioning 6.9 Operational and Maintenance Management APPENDICES A GLOSSARY B FLAMMABILITY C EXAMPLE PROFORMA D REFERENCES DOCUMENTS REFERRED TO IN THIS PROCESS GUIDE



TABLE 1 PHOTOCHEMICAL OZONE CREATION POTENTIAL REFERENCED TO ETHYLENE AS UNITY FIGURES 1 SCHEMATIC OF TYPICAL VENT COLLECTION AND THERMAL OXIDIZER SYSTEM 2 TYPICAL KNOCK-OUT POT WITH LUTED DRAIN 3 SCHEMATIC OF DIVERSION SYSTEM 4 CONVENTIONAL VERTICAL THERMAL OXIDIZER 5 CONVENTIONAL OXIDIZER WITH INTEGRAL WATER SPARGER 6 THERMAL OXIDIZER WITH STAGED AIR INJECTION 7 DOWN-FIRED UNIT WITH WATER BATH QUENCH 8 FLAMELESS THERMAL OXIDATION UNIT 9 THERMAL OXIDIZER WITH REGENERATIVE HEAT RECOVERY 10 TYPICAL PROJECT PROGRAM 11 TYPICAL FLAMMABILITY DIAGRAM 12 EFFECT OF DILUTION WITH AIR 13 EFFECT OF DILUTION WITH AIR ON 100 Rm³ OF FLAMMABLE GAS



1 INTRODUCTION 1.1 PURPOSE The purpose of this guide is to provide guidance on the safe design of vent gas collection and destruction systems including, in particular, thermal oxidizers and their associated equipment for destroying volatile organic compounds (VOCs). It is based on experience gained from operating units and capital projects and on the application of sound engineering practice and good safety principles. The standards which are applied to any particular project or plant will differ based on the geographic location and local legal requirements as well as site and business preferences. Any relevant company, local, national or international codes or standards should therefore be applied to the design of the system. Most operating problems that are experienced with thermal oxidizers derive from process deviations upstream of the unit. Therefore, in any project or installation it is essential to consider the vent collection headers and the destruction unit as a complete system and not as an assembly of separate entities. 1.2 Scope of this Guide

This guide does not replace, or provide a substitute for, national or international standards but should be considered in conjunction with them. When consulting this document it should be remembered that it is intended as a guide and not a set of hard and fast rules. Good engineering judgment should be applied to the design at all times in order to produce a safe and efficient collection and destruction system. This guide is applicable to the safe design of:

o Vent collection headers whether connected to destruction units, flare stacks or vent stacks;

o Ancillary equipment including knock-out pots, fans, pumps etc.;

o Thermal oxidizer units;

o Process and vent gas burner control systems.



It also covers:

o Flammability and explosion hazards in vent headers;

o Environmental aspects of vents treatment and destruction systems;

o Heat recovery systems;

o Flue gas scrubbing;

o Specification and purchase of destruction units. This guide does not deal with:

o Detailed mechanical or engineering design of the thermal oxidation unit itself, except where applicable to safety issues;

o Choice of materials of construction for oxidizer refractory linings;

o Choice of specific type of oxidation unit, except for general

considerations around environmental and safety performance. Guidance on different types of VOC abatement technology can be found in Process Safety Guide: GBHE-PSG-017 PRACTICAL GUIDE ON THE SELECTION OF PROCESS TECHNOLOGY FOR THE TREATMENT OF AQUEOUS ORGANIC EFFLUENT STREAMS . Guidance on the detailed design and operation of flare stacks can be found in Process Safety Guide: GBHE-PSG-008 PRESSURE RELIEF



1.3 Use of the Guide This guide is split into six main Sections: 1 Introduction. 2 Environmental Issues. 3 Vents Reduction Philosophy. 4 Methodology for Collection & Assessment of Process Flow Data. 5 Safe Design of Vent Collection Header Systems. 6 Safe Design of Thermal Oxidizers. Section 2: discusses environmental issues, mechanisms for ozone depletion and air quality standards. Section 3: provides guidance on reduction at source in compliance with the principles of inherent SHE. Section 4: outlines a methodology for collecting and assessing the data required to design a vent header system. This is based on previous experience on a number of previous projects in GBHE. Section 5: contains guidance on the design of vent header systems. This is equally applicable to all header systems whether venting to atmosphere, flare stack or thermal oxidation unit. Section 6: deals with the design of thermal oxidizers. These are the most common form of destruction system used for VOCs. Specific guidance on the design of flare stacks can be found in GBHE-PSG-008 PRESSURE RELIEF



2 ENVIRONMENTAL ISSUES 2.1 Principal Concerns Some VOCs are toxic and some are implicated in damage to the stratospheric ozone layer. However, the principal concerns with most VOCs are: (a) Their involvement, together with oxides of nitrogen and in the presence of sunlight, in the production of photochemical oxidants in the lower atmosphere (see Section 2.2). (b) Odors which may be offensive at concentrations well below the Occupational Exposure Limit (OEL). VOCs can be classified according to their Photochemical Ozone Creation Potential (POCP) referenced to a standard of unity for ethylene (see Section 2.3). Ozone is the photochemical oxidant that has been studied most widely but there are others including peroxyacetyl nitrate (PAN) and hydrogen peroxide. Ozone can pose a health risk and cause environmental damage (see Section 2.4). Some VOCs also present an odor nuisance, even at very low concentrations. For example, ethyl acrylate has an odor threshold of about 0.02 ppb. This can create major difficulties for design and operation as the emission to atmosphere of only a few mg/sec can cause odor problems. It is therefore vital that odorous materials are contained within process equipment. Where this cannot be achieved, then destruction or capture techniques should be very efficient and stacks discharging directly to atmosphere should usually be very tall. 2.2 Mechanisms for Ozone Formation The atmospheric chemistry of ozone formation is very complex and involves a multitude of interacting chemical reactions [Refs. 2 & 3]. The principal reactions are shown below which illustrate the involvement of VOCs in a simplified form.



Nitrogen dioxide absorbs natural radiation and breaks down into nitric oxide and oxygen radicals:

The oxygen radicals combine with oxygen to form ozone:

However, ozone oxidizes nitric oxide to nitrogen dioxide:

Hence there is a natural balance of ozone concentrations at ground level involving oxides of nitrogen. However, peroxy radicals (RO2) produced by the attack of hydroxyl radicals (OH) on VOCs act as a sink for nitric oxide and thereby disturb the above equilibrium towards higher concentrations of ozone:

It is believed that hydroxyl radicals are formed in the atmosphere by photochemical dissociation of ozone and subsequent reaction with water. It should be noted that the above reactions require the simultaneous presence of precursors in the appropriate meteorological conditions. Furthermore, not only are some of these reactions slow, but ozone, once formed, can persist for several days and so may be transported long distances. Therefore, elevated ozone concentrations often appear over widespread areas up to several hundred kilometers from the sources of the precursors. 2.3 Photochemical Ozone Creation Potential As stated above, VOCs and other substances can be classified according to their POCP referenced to a standard of unity for ethylene [Ref. 5] as shown in Table 1.



2.4 Health and Environmental Effects A high concentration of ozone can affect human soft tissues such as the eyes and nose. It may also affect respiratory functions including changes to the airways and an increase in the sensitivity to some inhaled allergens such as pollen. Although there is no evidence that it can cause asthma, it has been claimed that it might trigger allergic reactions and it is widely reported to be involved in the significant rise in reported cases of asthma. It is recognized that ozone at commonly found concentrations can damage a wide variety of crops and other vegetation including grapevine, beans, beet, spinach, clover, peanut, cotton and turnip. It has been reported that soybean yield is reduced by up to 15% by concentrations of ozone at about 50 ppb. Ozone and other photochemical oxidants cause material damage to rubber, plastics, painted surfaces, dyed fabrics and synthetic elastomers which is estimated to cost billions of US dollars annually.



It is well known that smog in warm still air, such as regularly experienced in the Los Angeles area, can be caused to some degree by photochemical oxidants. It is worthy of note that ozone is the only atmospheric pollutant that is commonly present in concentrations that can be significant fractions of the occupational exposure limit (OEL). Further information on the health and environmental effects of ozone can be found in Refs. 4 and 5. 2.5 Air Quality Standards for Ground Level Concentrations of Ozone, Targets for Reduction of VOC Discharges and Statutory Discharge Limits The World Health Organization guideline for ground level ozone concentrations on an 8-hour average basis is 50-60 ppb. The National Ambient Air Quality Standard for ozone in the USA is 120 ppb hourly average, not to be exceeded on more than one day per year. The UK Expert Panel on Air Quality Standards has proposed an Air Quality Standard of 50 ppb as a running 8-hour average [Ref. 4]. The 8-hour time weighted average (TWA) occupational exposure limit (OEL) for ozone is 100 ppb; the 3-minute TWA limit is 300 ppb. A 1991 Protocol to the 1979 United Nations Economic Commission for Europe (UNECE) Convention on Long Range Transboundary Air Pollution, calls for voluntary reductions in VOC emissions across Europe and North America by at least 30% by 1999 relative to 1988 levels. There is increasing pressure from both legislative authorities and public opinion to completely eliminate all vents containing VOCs. In general, discharge limits for VOCs are set at national level and are usually in the form of emission concentration limits. Some of these are defined by statute as in TA Luft [Ref. 6] in Germany whereas others appear as strict guidance limits as in IPR Guidance Notes [Ref. 7] in the UK. Although the principles of POCP are becoming generally accepted, it is likely to be some time before they are adopted formally by the statutory control authorities.



3 VENTS REDUCTION PHILOSOPHY 3.1 Reduction at Source It is most important that, wherever possible, vents should be eliminated at source according to the principles of inherent SHE. This is not only environmentally responsible, but also good business practice as vented material is wasted material and, furthermore, end-of-pipe treatment is invariably expensive. If vents cannot be eliminated at source, they should be reduced as far as possible or mitigated. Large volumes of vented material will require proportionately larger and more expensive collection and treatment systems and have higher operating and maintenance costs. Vents minimization can therefore have a large positive benefit on the overall project cost. Technical options for control at source include the:

o Increased vessel design pressure may eliminate the need for pressure relief systems at minimal extra cost for the stronger vessel. Consideration should also be given to the possibility of uprating the design pressure of existing vessels, tanks and pipe work. Stock tanks should be fitted with PV valves instead of open vents;

o Instrumented, high integrity protective systems may be fitted

utilizing reaction quench technology or dump tanks. It should be noted that in North America and some countries subscribing to ASME codes, containment or instrumented protective systems may not be allowed;

o If water-based solvents or solvents with lower volatility can be used,

VOC discharges can generally be reduced significantly;

o Subject to considerations of safety, cross-contamination and plant layout, a number of stock tanks can sometimes be connected to a common venting system to reduce the overall volumetric flow rate. This is particularly effective when transfers are made between the tanks in question;

o Similarly, the vent on a road tanker or other transportable container

that is being loaded or unloaded to a stock tank should, wherever possible, be connected (i.e. back-balanced) to the stock tank vent system;



o Where it is necessary to exclude oxygen or moisture by using

nitrogen, this should be achieved by means of a pressure-controlled nitrogen supply and a pressure-controlled vent rather than a continuous nitrogen sweep in order to minimize the volume of gas vented. Furthermore, the nitrogen inlet and the vent outlet should be located close to each other in order to minimize the concentration of VOCs in the vent. Disturbance of the vapor space should be minimized by connecting the nitrogen flow via a large nozzle thus reducing the gas velocity;

o It is claimed that floating-roofs can reduce evaporative losses from

stock tanks by up to 90% compared to conventional fixed roof tanks. Multiple and secondary seals also reduce evaporative losses;

o The liquid inlets to stock tanks should, wherever possible, be below

the liquid level in order to minimize the disturbance of the vapor space. This reduces evaporative losses;

o Hydraulic and pumped liquid transfers, rather than pneumatic

transfers, can significantly reduce VOC losses as vapor and mist in the vent at the end of the transfer;

o The charging of material through an open lid or charge port into a

vessel containing VOCs usually results in VOC losses to atmosphere;

o If the vessel is at or above atmospheric pressure, the losses occur

locally. If the vessel is under some vacuum, there will be an ingress of air which could result in a VOC discharge to atmosphere remote from the charge point. Furthermore, air sucked in could result in fuel-rich mixtures becoming flammable in the vessel or in downstream vent collection pipe work;



o Ιf the material to be charged is a liquid or can be dissolved in a

liquid, a closed charging system should be used. Where this is not possible, a charge hopper should be considered with a narrow entry point and a rotary, ball or slide valve into the vessel;

o As a general rule, the flow rate of inerts that come into contact with

VOCs should be minimized. Unnecessary purging and draughting should be avoided. Attention should be paid to poorly designed or faulty pneumocators, valves on nitrogen blowing or blanketing systems that are passing or left open, etc. Correct location of nitrogen blanketing on the vent line to the thermal oxidizer can reduce vapor losses, but in some cases it may be necessary to sweep the vapor space (e.g. if corrosive gases are evolved from the liquid);

o High quality maintenance can reduce fugitive losses from poorly

seated relief valves, pin holed bursting discs, flanged connections, control valve stems, pump glands, etc.. Fitting bursting discs to relief valve inlets may eliminate fugitive emissions but their effect on the relief stream capacity should be checked;

o Alternative process equipment may reduce fugitive losses e.g.

glandless or canned pumps, soft seat relief valves, bellows sealed valve stems and improved gasket materials.



3.2 End-of-pipe Treatment Some possible types of end of pipe treatment are:

Condensation;

Adsorption;

Absorption;

Thermal oxidation;

Catalytic oxidation;

Biological filtration;

Membrane separation. End-of-pipe solutions should always be regarded as a last option in view of their capital and operating cost. Destruction systems can also have inherent problems of statutory authorization and social pressures which invariably take a significant amount of time, effort and money to overcome. The additional cost to the business of these factors should not be underestimated. The overall energy and environmental impact balance should be considered carefully before selecting the appropriate, if any, vents destruction system. The impacts of such things as additional support fuel usage, discharges to atmosphere of thermal oxidizer flue gas, discharges to water of scrubbing liquor blowdown or waste solids disposal of spent adsorbent should be addressed opposite the environmental improvement of treating the vent gas in question. This exercise is required by statute under Best Practicable Environmental Option (BPEO) assessments in the UK and under Best Available Control Technology (BACT) assessments in the USA. The above principal end-of-pipe treatment options are described in more detail in GBHE-PEG-015 which also provides guidance on the selection of the appropriate option together with names and addresses of suppliers. It may be advantageous to use a combination of techniques such as refrigerated condensation, adsorption or membrane separation in order to concentrate or reduce the amount of VOCs prior to destruction by thermal oxidation. This will result in a smaller and thus cheaper destruction unit.



The safety and environmental aspects of thermal oxidation are discussed further in Section 6 of this guide. 4 METHODOLOGY FOR COLLECTION & ASSESSMENT OF PROCESS FLOW DATA 4.1 General Vent gas collection and destruction systems are complex plants in their own right. Hence, in order to ensure a safe design, a methodical approach to the design basis and basis of safety is essential. This Section provides a framework methodology which can be adapted to specific project requirements. Considerable effort is required to collect the information on flows, compositions, component data, flammability data and scenarios which is needed to produce the basis of safety for the system and the Hazard Study. The use of a spreadsheet will assist in this process. This process is especially difficult for batch plants where flows are intermittent and highly variable. For existing plants and processes it is essential to obtain the full co-operation of the plant personnel in the information gathering process since they will have experience of many of the possible deviations from normal operation which can occur. It should be noted that some possible occurrences may never have been experienced in the life of the plant due to their extremely low potential frequency. The range of possible scenarios should be established by consultation with the plant operations team and by examination of the Hazard Study records for the project. If necessary, further Hazard Studies may be required to establish a range of worst cases. Full transmittal of this information from the plant to the project (or between members of the project team for new plants) is essential. For new plants, all possible operating scenarios should be identified at the design stage, again using information from the Hazard Study process. Other useful techniques for hazard assessment and reduction are fault tree analysis, process hazard review, failure mode and effect analysis and consequence analysis [Ref. 17].



The use of a standard proforma may be helpful in allowing clear and concise collation of the data. It is essential to ensure that the person responsible for completing the proforma is aware of the importance of the data being supplied. One-to-one discussions are invaluable to avoid confusion. The proforma should be comprehensive in the information requested. An example proforma is shown in Appendix D. If the information supplied on the proforma is incomplete or incorrect, it will have serious consequences for the design of the system, possibly even making it unsafe. If errors are discovered in the information on vents flow and compositions the rework required will almost certainly be costly in terms of both man hours and new equipment. There are examples where VCDS have been grossly undersized or there have been fluctuations of the composition into the hazardous region due to a failure to identify the maximum short term flows. If possible, the vent collection system should be installed at least a year before final design of the destruction system in order to provide time for comprehensive monitoring of the flows and compositions in the header system under operational conditions. This has benefits to the project in that the data collected during this period enables a more efficient destruction unit to be designed with consequent savings in design and operational costs. Regulatory authorities, however, generally require the collection and destruction systems to be installed simultaneously. The proposed methodology for safe design consists of the following steps:

Identification of vent sources;

Characterization of vents;

Quantification of process vent flows;

Component flammability data collection;

Identification of operating scenarios;

Quantification of flammability characteristics for combined vents;

Identification and quantification of possibility of air ingress;

Tabulation of data;

Hazard Study assessment.



The processes involved at each step are described further in Sections 4.2 to 4.9 (inclusive): 4.2 Identification of Vent Sources It is essential that all vent sources should be identified before starting the design of the header system. These may include:

Tank breathing vents;

Relief and breather valves;

Tanker loading points;

Reactor vents;

Vacuum pump exhausts;

Lute pots and siphon breakers. It is important that all sources are identified, as the number and location will have an impact on the size and complexity of the collection system. It may be possible to identify a number of vents which could be eliminated, recycled economically or minimized by other means at this stage. Any existing vent or flare header systems should also be identified (e.g. common purging of tank farms), and a strategy for dealing with these included. During this part of the project, the plant engineering line diagrams (ELDs) should be updated for existing plants and vent sources for new plants clearly marked. This information should also be carried over onto site plot plans and general arrangement drawings and will aid both estimation of project costs and mechanical design of the header system. 4.3 Characterization of Vents The results from this part of the design process will have major implications on the number, type and size of headers, the conditions in the system and ancillary equipment needed. Vents may be characterized in several different ways. Typical characterization groupings are:



Fuel-rich, fuel-lean or flammable;

Continuous or intermittent;

Condensable or non-condensable;

Corrosive;

Toxic;

Wet/dry;

Mixed or variable properties. This activity will indicate which of the vent headers is the most appropriate to use for each vent stream and any treatment which is needed to make the header safe if it would otherwise operate within the flammable region. During the characterization process, the effect of any interactions between vent compositions should be evaluated to ensure that the flows and compositions in the system do not operate in the flammable region and that there are no undesirable chemical reactions between the different materials. This is particularly important where there may be polymeric material which can clog the system. Any base load of inerts, support fuel or dilution air should be included. An interaction matrix should be used to ensure that all possible combinations are identified and assessed. Interactions should also be examined between the VOCs and the materials of construction of the header system. An example of this is shown in Ref. 17. Certain conditions such as fire relief and other types of emergency vent may be exempt from treatment on the basis that they are likely to occur extremely infrequently and have such large flow rates that they would need the construction of a much larger destruction unit. Such matters should be assessed during the quantification of process vent flows and, if appropriate, discussed with the local regulatory authorities. Vents often have varying compositions depending on the particular operating scenario at the time; hence the "mixed or variable properties" heading. These may need special consideration if they can transit from fuel-rich to fuel-lean or vice versa. Similarly, consideration may be required if the composition in the vent can change drastically or if a material with extreme combustion properties such as hydrogen or a material with an unusual flammability diagram such as ethylene oxide can



be present. The effect of such changes will have an impact on the design of the headers in dictating the type of ancillary equipment or systems needed (e.g. flame arresters or inert gas provision). This is of particular importance in batch process where several reaction steps or unit operations may be carried out in a single vessel. 4.4 Quantification of Process Vent Flows Vents collection and destruction systems can only be designed safely with full knowledge of the range of flows and compositions which may be encountered not only during normal operation but also in abnormal conditions (e.g. relief valve operation, process deviations etc.). For most processes, whether batch or continuous, both the vent flows and compositions are likely to be highly variable. Typically, the following operations should be considered:

• Flowsheet (normal operation); • Batch operating cycle; • Tank breathing as a result of thermal expansion and contraction, pumping etc.; • Process deviations; • Relief situations; • Maintenance purging of some or all plant items; • Start-up, shut-down and stand-by modes; • Other abnormal operations.

Where possible, monitoring of flows and compositions should be carried out over an extended period of time where applied to existing plants to ensure that all normal situations are covered. Where this is not possible, soundly based estimates should be made. It is unlikely that worst case conditions will be seen during the monitoring period since the frequency of combined events occurring may be very low. A judgment should therefore be made as to the worst credible case, taking into account equipment failures, process deviations, operator error, etc. Some of this information



may be carried over from pressure relief documentation, especially relief philosophy and bursting disc/relief valve data sheets and. If possible, the vent collection system should be installed prior to the destruction system in order for performance monitoring to be carried out. This will yield valuable design information for the destruction unit. In-depth plant knowledge will be needed to fully identify all the possible deviations and resulting vent compositions and flow rates. Once again, any base load of inerts, fuel or dilution air should be included. As stated above, a proforma may be useful for the transferral of information from plant and operations personnel to the project team, although this is no substitute for face-to-face discussions with plant personnel and should not be used in isolation from other information sources. The data can be classified into a number of flow rate/composition scenarios such as:

Zero;

Normal / flowsheet;

Minimum flowsheet;

Maximum flowsheet;

Maintenance condition;

Maximum plus over-design allowance. It may be impracticable to install a vent gas collection and destruction system that can cope with the simultaneous occurrence of the "worst case" flows from all vent sources. The likely frequency and duration of deviations from flowsheet should, therefore, be estimated in order to determine which combination of vent flows will be accommodated and which will be dealt with by other means. Common cause events should be identified as these often lead to comparatively large vent flows e.g. power failure. When calculating the flows due to relief valve operation, the relief stream capacity should be used rather than the required relief rate. A spreadsheet may be helpful to correlate the data in order to identify those scenarios which would cause operational difficulties or process hazard.



The quantification of vent flows may be particularly difficult for batch processes which, by their nature, have intermittent flows and compositions. In this case it is sensible to consider the maximum possible flows from the process and the full range of flows from zero to the maximum. For batch processes, consideration of the possibility of process deviation and cross contamination is especially important. The reduction of emissions from batch processes is discussed in Ref. 1. It may be advisable to carry out a Hazard Study on the upstream process plant at this stage to consider the feasible deviations which could occur resulting in different emissions to the vent collection system. When applying the Hazard Study guide words, consideration should be given to the special cases which may be generated (e.g. more fuel, more air, less fuel etc.). Typical deviations which should be considered for all process plant, but especially for batch processes, are:

Charging wrong reactants (other materials stored in area or wrong materials delivered);

High or low process temperatures;

High or low pressures;

Overfilling of tanks, reactors or distillation columns;

Purging, venting or pressure letdown;

Agitator failure;

Heating failure;

Cooling water failure;

Instrument air failure;

Power failure.

Overfilling can be a major problem as it may result in liquid entering the vent gas collection header system. This should be avoided as it can cause a number of hazards as described in Section 5.3. Frothing of reactor or tank contents may also result in liquid entering the header system.



Suitable precautions should be taken to prevent this situation occurring, including the provision of liquid interceptors (knock-out pots) or liquid level alarms if appropriate. Temperature, pressure, bubble and dew points for each component and composition may be needed if there is a possibility of flashing liquid entering the header and also to evaluate any possibility of volume shrinkage of the gas on cooling or condensation after entering the header. (Shrinkage may cause air to be drawn into the header giving rise to a flammable mixture). This will also give an indication of whether lagging or heat tracing of lines is needed and whether there are any potential solidification or icing problems. Incomplete quantification of data is likely to result in incorrect specification of equipment including the vent collection pipe work, safety equipment such as flame arresters, KO pots and downstream plant such as a thermal oxidizer. It is therefore vital that the quantification process is carried out in full. This can only be achieved by appropriate allocation of resources and time in the overall project program (see Section 6.7). Particular regard should be paid to the presence of more hazardous components such as hydrogen, acetylene, ethylene oxide etc.. Chemical interactions should also be quantified at this stage using the interaction matrix developed in Section 4.3. Undesirable reactions may occur when mixing vent streams causing, for example, polymerization, condensation or exothermic reaction. Such situations should be avoided.

4.5 Component Flammability Data Collection

Flammability data, particularly LFL, UFL and MOC, is required for each of the components in the vent system in order to construct the flammability diagrams for the different compositions and scenarios which may occur (see Section 5.2). If possible, experimentally determined flammability diagrams should be used. If flammability diagrams are not available then they may be constructed for each of the worst case compositions for each of the vents. For further explanation of flammability diagrams see Appendix C. In some systems there is synergy between the more reactive and less reactive components of the gas mixture, hence relatively small amounts of, for example, hydrogen have a disproportionate effect on the flammability characteristics. If there are multiple components or significant quantities of reactive gases present then experimental determination of flammability characteristics should be considered.



These diagrams can also be used to assess the possible consequences of air ingress into fuel-rich systems. The flammability characteristics for mixtures should be estimated using Le Chatelier's rule (see Appendix C). A spreadsheet may be useful for this. Critical flammability estimates should be backed up with experimental data. 4.6 Identification of Operating Scenarios The range of operating scenarios which are appropriate to the individual process sources should be identified. The scenarios to be considered may include: • Start-up from cold; • Re-start after trip; • Shut-down; • Stand-by; • Normal operation; • Low rate operation; • VOC/fuel excursion; • Oxidant excursion; • Inert excursion; • Commissioning standby equipment or after maintenance; • Depressurizing or venting down; • Vacuuming down; • Purging. Any other possible scenarios should be identified as part of the individual project.



There may be differences in the way that the system performs or in the conditions of each vent under the different operating scenarios which lead to differences in the flow rate or composition of the vent. The range of operating scenarios identified depends on the operation of the plant, for instance the conditions for a cold start up may differ from those which occur after a plant trip. The identification process is intended to detail the full envelope of operating conditions which can be generated by the plant. The list produced should include those scenarios which would be generated by failures of trips or controls. Obviously a full working knowledge of the plant and its associated control systems and safety trips is required to identify all the possible scenarios. 4.7 Quantification of Flammability Characteristics for Combined Vents A brief description of flammability diagrams and associated terminology is given in Appendix C. The flammability characteristics for the possible combinations of vent sources under each of the possible operating scenarios should be calculated. The compositions calculated can be placed into one of the following categories: • Fuel-lean; • Fuel-rich (oxidant lean); • Inerted; • Flammable. Fuel-lean vents are those which have fuel concentrations below the LFL and which are therefore safe under all air ingress conditions. Fuel-rich vents have compositions above the UFL which could, in theory, enter the flammable region in the case of air ingress whether they are oxidant lean or inerted. Flammable vents are those operating inside the flammable region. As stated previously in Section 4.6, excursions should be considered which could change the composition of the vent. Flammable vent compositions should be avoided if at all possible or treated to take them out of the flammable region (e.g. by inerting). If they cannot be avoided, a full risk assessment of the likelihood and consequences of incidents should be carried out.



Design of the system to cope with overpressure due to deflagration or detonation may be necessary in exceptional circumstances (see Section 5.2.6). It should be noted that air is not the only source of oxidant. In particular chlorine and oxides of nitrogen may act as oxidizing agents. These may originate in the upstream process. 4.8 Identification, Quantification and Assessment of Possibility of Air Ingress Routes Obviously, if the vent collection header is operating under positive pressure at all times then air cannot be sucked into the system from the atmosphere. Hence, it may be possible to eliminate all or the majority of possible air ingress routes in the header system by operating at positive pressure. However, this may not be possible for all vents or for upstream process equipment operating under vacuum. Therefore it is essential that all possible upstream air ingress routes are considered as well as those relating directly to the header system. There may also be a number of cases where failures mean that a nominally positive pressure system may become negative pressure. There may be a number of possible routes for air ingress into fuel-rich vent headers. For each source, all possible openings or paths for air leakage into the system should be identified and the potential ingress rates estimated. It is important to include all flanged joints, instrument connections and also possible failures of the header pipe work. Some typical situations and operations which may lead to possible air ingress routes are: • Maintenance operations involving removal of equipment such as isolation, control and relief valves, instruments, blank ends, flanges, slip plates, etc; • Failure of seal liquid supply to, or failure to top up, lute pots or leakage of seal liquid. This may lead to the seal running dry thereby opening up a route direct from atmosphere; • Accidental damage to pipe work (e.g. vehicle damage to exposed lengths of header adjacent to roadways);



• Corrosion of pipe work and fittings including cracks and weld defects; • Manual operations involving breaking/making connections such as road/rail tanker purges to vent collection headers, opening of inspection and charging ports, sampling operations, physical changeover of batch unit operations, etc.; • Process equipment operating under vacuum; • Failure to adequately purge equipment prior to start-up, causing air to be displaced into the header; • Failure to completely purge headers and laterals from process vents through to vents treatment unit; • Inadequate isolation e.g. leaving sample points open, failure to blank off, passing valves (including thermal oxidizer bypass valves); • Oxygen generation by process. It may be possible to eliminate a number of air ingress routes by minimizing the number of flanges, equipment connections etc.. Similarly, purge and sample points may be equipped with “dead man’s handle” type valves to prevent them being left open inadvertently. Other operations may also be modified to reduce the possibility of air ingress. Where stacks are involved, many nominally positive pressure systems may in fact be under a slight vacuum due to the chimney effect. This is particularly apparent where the vent gas is above ambient temperature or the molecular weight is significantly less than that of air. This effect is more pronounced at low flow rates and can result in air ingress causing a flammable mixture. The stack may be fitted with a liquid seal at the base to prevent the header system operating under negative pressure. Even systems that are nominally under positive pressure may in fact be under negative pressure due to the chimney effect where no seal pot is installed.



Once again, this identification process requires a thorough knowledge of the plant operation, layout and maintenance procedures if all possible hazards are to be identified. Some knowledge of the proposed operating pressure in the header system will also be needed to calculate the air ingress rates from the various routes. This may mean taking some early design decisions in order to get an early estimate of possible consequential hazards. The frequency and consequence of each possible air ingress combination should be estimated. Air leakage into the system will alter the composition in the header, possibly taking it into the flammable region. Additionally, the flows from other sources and the pressure profiles in the header may be affected by the leakage. The full possible range of operating scenarios for other vent sources discharging into the same header should be considered, including the effects of the air leakage. Similarly, the interactions caused by air leakage should be identified. An interaction matrix should be used for this evaluation process. 4.9 Tabulation of Data A "control chart" should be created that lists the activities and events (normal and abnormal) which would result in deviations from flowsheet conditions whether resulting from process variations or by air ingress. By inspection, those scenarios which would not result in a flammable mixture occurring in the vent header system should be eliminated. It is extremely important to identify the likelihood of any transient incursions into or through the flammable region, as well as new steady state flammable conditions.

The remaining scenarios are therefore the ones which would result in a potentially hazardous situation. The tabulation of data is an important aid to understanding the complexities of the numerous operating scenarios and simplifies the identification of potentially hazardous situations.



4.10 Hazard Study and Risk Assessment An assessment of the potential for flammable mixtures and the likely consequences should be carried out in the Hazard Studies. Whether or not required in order to satisfy the Hazard Study criteria, the opportunity should be taken to consider the possibility of making some of the hazardous vents inherently safe or more safe or of reducing the duration or frequency of potentially unsafe situations. It is most important that a trained, accredited practitioner carries out, or at least coordinates, the Hazard Studies and Risk. 4.11 Note on Aqueous / Organic Wastes One way of dealing with aqueous effluent contaminated with organic waste is to air strip it in a packed column. The air can then be used as combustion air in a thermal oxidizer. There are, however, a number of potentially hazardous situations that could arise including the following:

• Enclosing organic contaminated water in a tank may lead to a flammable mixture arising in the vapor space of the tank. The Henry's law coefficients of the contaminants should be examined to check for the possibility of a flammable mixture above the liquid;

• If a large quantity of organic liquid gets into the aqueous waste stream

there is a risk of free phase organics getting into the stripping column. If this occurs, then the air stream coming from the stripper may again be flammable;

• The column may also be prone to clogging due to dissolved or suspended

solids. Reaction of scrubbing liquor with atmospheric gases or constituents of the vent gas may also cause clogging (e.g. alkalis with CO2);



• During start-up, shut-down or process interruption the vapor space in the stripper column head may become flammable. On restart the flammable mixture may be carried forward into the header system with consequent risk of explosion hazard;

• Low air flow, however caused, can cause a flammable mixture to develop

in the air stripper (e.g. fan failure, damper failure, partial blockage). 4.12 Complexity of Systems There may be considerable pressure both from environmental and business sectors to increase the number of vents being treated by a single thermal oxidizer. As the number of vents increases, the number of cases to be considered may increase exponentially. This is reflected in the increased amount of work needed during the design methodology described previously. Cost and business pressures often dictate that a single large thermal oxidizer is installed rather than a number of smaller dedicated units. The provision of two or more smaller VCDS may make the system inherently safer due to the reduced complexity and lower number of possible failure modes. When all factors including maintenance, availability and the cost of down time are taken into account, the economics of a number of smaller, independent systems may in fact be better than for a single large system. 4.13 Summary The methodology described above is intended as guidance which can be adapted to the particular requirements of any project. It does, however, contain the basic steps which should be considered for the assessment of potential incidents in the formal Hazard Study of a vent header system. It is important that this work results in an auditable design trail. It is essential that, wherever possible, vents should be eliminated at source according to the principles of inherent SHE. This is not only environmentally responsible but also good business practice as vented material is wasted production which cannot be recovered and, furthermore, end-of-pipe treatment is invariably expensive to design, build and operate. If vents cannot be eliminated at source, they should be reduced as far as possible or mitigated.



5 SAFE DESIGN OF VENT COLLECTION HEADER SYSTEMS 5.1 General A typical vent collection and destruction system (VCDS), including a thermal oxidizer, is shown schematically in Figure 1. FIGURE 1 SCHEMATIC OF TYPICAL VENT COLLECTION AND THERMAL OXIDIZER SYSTEM

All pipelines carrying potentially flammable liquids or gases have some risks attached. These risks may stem from external factors such as corrosion or impact damage or internal factors such as process composition changes or the failure of a fan or pump. The risks can be minimized by good engineering design as described in the following guidelines. The quality of the design, maintenance and operation of the vent header system is critical to the safety of the thermal oxidation unit, since many safety problems with VCDS originate in the vent headers or upstream process plants.



5.2 Process Design of Vent Headers 5.2.1 Basis Of Safety The principal requirement of the basis of safety for vent headers should be control of the vent gas composition such that the header does not operate in the flammable region during normal operation or abnormal situations. This can be done properly only after systematically identifying, characterizing and quantifying the vent streams using a rigorous methodology such as that described in Section 4. This approach is based on inherent safety and is important since there may be possible sources of ignition in the vent header system itself (fans, pumps etc.) and there is, of course, a permanent source of ignition in the thermal oxidation unit. Even in the absence of obvious ignition sources in the header system there is still a possibility of static electricity discharge, especially in non-conductive or mixed conductive and non-conductive pipe work systems. The probability of an ignition occurring may be low but cannot be assumed to be zero. Operation in the flammable region could therefore result in an ignition in the header leading to deflagration or detonation. It can be difficult to design vent headers to have a sufficiently low frequency of deflagration or detonation, particularly if the consequences of such an event would be the rupture of a long vent header. Where a header passes through a number of different plant areas, the domino effects from the rupture of a header are potentially serious. Notwithstanding the precautions taken to prevent vent headers operating in the flammable region, process deviations, equipment failures or other unforeseen circumstances may arise which result in the formation of a flammable mixture within the system. Typical of such events are leakage of air into the system from maintenance activities, process deviations on start-up or shut-down or failure of instruments. These failures or deviations, however unlikely, will have a finite potential frequency. Since there is also some finite probability of ignition sources being present, it is prudent to consider installing a second form of protection to further reduce the possibility of a flame front propagating into the header pipe work or other pieces of equipment with consequent hazards. Secondary protective systems are not designed to provide continuous protection against the permanent or extended presence of a flammable mixture in the header but do provide protection for a limited period enabling the system to be shut down safely or the flammable condition to be removed.



Typical secondary protection systems are flame arresters or flame suppression systems and shutdown or diversion systems actuated by oxygen analyzers. Flame arresters are designed to prevent the propagation of a flame along a header from an ignition source. To be effective they should be placed as close as possible to the source and should be considered for fitting in the vent headers both upstream and downstream of any potential ignition sources in the header itself and also immediately upstream of the thermal oxidation unit. Positions of flame arresters in a typical vent header system are shown in Figure 1. All flame arresters, including those on diversion stacks or vents, should be fitted with high temperature trips or alarms to warn of an ignition occurring in the system. Some additional protection from flashback from the thermal oxidizer may be provided by the velocity of the gas through the burner nozzle if it is greater than the turbulent burning velocity. However, it is extremely difficult to estimate turbulent burning velocities. There is also some doubt as to the possibility of flame creep back along the walls of the burner nozzle and back into the header. Thus this approach cannot be used as the basis of safety against flashback.

It may be tolerable to operate very short sections of the header system in the flammable region, if this condition is unavoidable, depending on the hazard consequences and the probability of an ignition occurring. In this situation, the length of line operating in the flammable region and the probability of an ignition occurring should be minimized. An example of this is the section of line at the exit from a scrubbing system or reactor where dilution air or inert gas can only be injected after leaving the scrubber or reactor thus creating a small flammable region prior to dilution. In some cases it may be possible to design the plant to withstand deflagration or detonation where this condition is known to exist. The consequences of a detonation occurring in a line should be considered very carefully with particular emphasis paid to possible injury or plant damage from missiles. Domino effects by missile impact into other pieces of plant and equipment, e.g. tanks holding toxic or flammable materials, should also be taken into account.



Requirements of the basis of safety in addition to the avoidance of flammable mixtures, include protection against internal and external corrosion, mechanical impact damage, liquor logging and back pressures that could adversely affect upstream plant. It may be necessary to segregate vent streams in parallel headers in order to manage these issues or even to install a number of independent, smaller VCDS.

5.2.2 Process Design Basis for Vent Collection Header Systems Vent header systems should be designed to avoid the possibility of flammable mixtures as described above in Section 5.2.1. For the initial design, three main types of header should be considered: fuel-rich, fuel-lean and inerted. Several branches may connect into each header at the process plant end. It is, therefore, important to ensure that a flammable mixture cannot result from the mixing of a fuel-rich vent from one branch mixed with a fuel lean vent from another branch. The design basis should also take into account the quantity of material vented and the design pressure of the upstream process equipment. For example, low pressure storage tanks may not have a high enough design pressure to provide the necessary driving force for the required flow of material down the vent header and hence a suction fan may be needed. This may, however, introduce new hazards from pulling air into the system or sucking the tank in. Complications may also be introduced into the design by the presence of high and low pressure vents and high and low temperatures, especially if venting into the same header. For high pressure vents the possibility of back flow and over pressurization or contamination of low pressure sources (e.g. stock tanks) should be considered. High temperature may cause damage to headers or take the mixture above its auto-ignition temperature. High temperature and high pressure may also affect the UFL and LFL. For further information on the change of flammability limits with temperature and pressure see list of Best Contacts in Appendix B. Separate header systems or additional processing equipment may be required to avoid these issues. Low temperature (e.g. from vaporization of liquefied gases) may cause condensation in the line and liquid logging or even freezing. In cold climates it is often necessary to lag and heat trace headers to prevent condensation or icing.



If VOCs do condense in the header during cold periods and then vaporize on warming, the destruction unit can be overloaded. There are recorded instances where this has occurred. In carbon steel headers embrittlement and damage may result from sub-zero temperatures. (see Section 5.3.3).

5.2.2.1 Fuel-Rich Headers Wherever practicable, fuel-rich headers should be operated under positive pressure rather than under suction since a leak of gas to the atmosphere will usually be less hazardous than an ingress of air which could possibly result in the mixture becoming flammable. The exception to this is where the material in the line is not only flammable but also toxic or highly damaging to the environment; in which case the consequences of a release should be considered carefully against the consequences of air ingress. The pressure of a sub-atmospheric header will probably have to be raised above atmospheric at some stage upstream of a thermal oxidizer. Therefore, it is generally better to provide the boost in pressure as far upstream as possible in order to minimize the length of header subject to possible air ingress. A major consequence of a leak of non-toxic gas from a fuel-rich header operating under positive pressure is likely to be a torching fire which may impinge on other adjacent equipment. The possibility of consequential ignition or damage to other equipment in this event needs to be considered. With vent systems it is unlikely that sufficient gas will be released to cause a significant fireball or flash fire; however, if the release occurs in a confined space there is a risk of a confined explosion. Significant overpressure is only generated when the flammable cloud has a degree of confinement. Most vent headers run in unconfined areas so the risk of a confined explosion is generally small. For assistance with explosion and consequence modeling see Best Contacts in Appendix B.

5.2.2.2 Fuel-Lean Headers Fuel-lean headers can be operated above or below atmospheric pressure without increasing the risk of generating a flammable mixture through air ingress. If the vent gas is toxic or particularly damaging to the environment, then consideration should be given to operating at sub-atmospheric pressure.



5.2.2.3 Inerted Headers Inspection of the operating position on the flammability diagram is necessary to determine the effect of air ingress into an inerted header. Where it is possible for air ingress to result in a flammable mixture, the header should, wherever practicable, operate above atmospheric pressure. 5.2.3 Modifying Composition of Vent Headers Wherever practicable, flammable mixtures should not be sent to the vent collection system. If the vents arising from a plant or process are in, or very close to, the flammable region then they should be made safe prior to, or immediately after, entering the vent header system. Similarly, if it is possible for a flammable mixture to be generated within a vent header by, say, the mixing of fuel-rich and a fuel-lean vent streams or condensation of VOCs in a fuel-rich stream, the possible consequences should be evaluated and, where appropriate, corrective action taken. This can be done in a number of ways (see 5.2.3.1 to 5.2.3.4): 5.2.3.1 Enriching The vent gas may be enriched by adding fuel gas to take the composition above the UFL. Some of the thermal oxidizer support fuel can be added in this way. The amount required to make the vent "safe" should be calculated based on the variability of the composition and flow and possible air ingress rates. For reactive gases, such as those containing significant quantities of acetylene, ethylene oxide, hydrogen etc., it is difficult to specify an upper "safe" limit because of the size of the flammable region. Hence, the flammability characteristics of the gas mixture should be taken into account when specifying the appropriate amount of enrichment. 5.2.3.2 Diluting The vent gas can be diluted with air to below the LFL. From NFPA 69 a value considered "safe" for this would be LFL/4 without composition monitoring or up to 60% of the LFL with monitoring, but other values may be appropriate on consideration of the factors described above. Operation above LFL/4 with or without monitoring, should be considered very carefully. This value is chosen because of the variability of process flows and the difficulty of estimating compositions accurately for upset conditions.



Reasons for the choice of factor should be described in the basis of safety. The amount of confidence in the accuracy of flow and composition data should be considered when making the decision. It should be noted that, whereas application of the NFPA standards is mandatory in the US and Canada and may also be mandatory in some other countries and is strongly recommended for use throughout the Americas, it may not be accepted in others. It is necessary to check with local regulatory authorities before making a final decision. 5.2.3.3 Inerting Inert gas can be injected into the vent in order to reduce the concentration of oxygen in the header to below the minimum oxygen concentration (MOC) to sustain combustion. NFPA 69 suggests a limit of 60% of the MOC with monitoring or 40% of the MOC if the MOC is below 5%. If not continuously monitored, the oxygen concentration should be checked on a regular basis (see NFPA 69). Again, the variability of the vent flow and composition should be considered along with the measurement accuracy. There may be circumstances where it is appropriate to use a larger safety factor such as 25% of the MOC depending on the variability of vent flows, process deviations and confidence in the data. The reasons for the choice of dilution factor should be detailed in the basis of safety. As above, it should be noted that application of the NFPA guides may be mandatory in some countries. 5.2.3.4 Combination of Vent Headers Combining vent streams should be considered very carefully. Although mixing vent streams to ensure operation outside the flammable region is possible, the various combinations of flow and composition should be quantified in detail as deviations in one or other of the streams may result in the header becoming flammable [Ref. 17]. This method of ensuring operation outside the flammable region is not generally recommended unless there is a high degree of certainty about vent flows, compositions and equipment reliability.



The interactions of the components in each of the vent streams, along with the possible range of compositions, should also be examined to ensure that there are no undesirable reactions or other consequences of the combination. Chemical reactions may occur in the header causing, for example, corrosion, condensation or polymerization of material in the line. Using the combination of vent headers as the basis of safety will result in a significant amount of additional work in order to provide sufficient justification and hazard quantification. An interaction matrix should be used to check for undesirable interactions between the streams being mixed. Complex vent collection systems connecting several plants or units to a common destruction unit may cause the propagation of an incident from one plant to the others. It may, therefore, be preferable to have several smaller systems instead of one large system. A consequence analysis should be performed to consider the options. The benefits of scale for a single, large vent header and destruction system may not be significant when considered against the additional burden of design engineering needed to ensure the safety of the system. 5.2.4 Summary The decision on which of the above methods to apply depends on the starting composition of the vent. In order to decide the best route for altering the vent composition, the flammability diagram for the vent composition should be considered. Flammability diagrams are described further in Appendix C. 5.2.5 Flame Arresters 5.2.5.1 General Flame arresters are designed to prevent the propagation of a flame front. They are classified as a form of secondary protection and are effective for a limited period before burn through or overheating occurs. Each arrester is designed for a specific duty based on the composition, flow rate and operating conditions in the line. The presence of a flame arrester can provide time for the plant to be shut down or the fault condition to be rectified before an incident occurs.



The design of flame arresters is a complex subject. Only a short synopsis of the main features is provided here. For further advice on design and specification of flame arresters see the list of Best Contacts in Appendix B. 5.2.5.2 Types The most common type of arrester in use is the conventional crimped metal type (such as supplied by IMI Amal in the UK and Enardo in the USA) but other types are available including flat or perforated plate and liquid seal. Arresters are designed for a specific range of duties. An arrester designed to cope with potential detonation will be designed to a more stringent standard than one designed for deflagrations. There are a number of standards applicable to testing of flame arresters including ISO, BS, Canadian, Underwriters Laboratories [Ref. 14] and US Coast Guard [Ref. 15]. The USCG tests are reckoned to be the most stringent. It should be noted that suitable approval will be needed for flame arresters before installation in the USA (Factory Mutual or Underwriters Laboratories) and some other countries. Again, local authorities should be consulted. Plate type arresters are less common in use than crimped metal types and are limited to the less reactive gases and therefore are not suitable for mixtures containing hydrogen or acetylene. This type is made by several manufacturers, particularly in the USA, including Protectoseal and Westech. Liquid seal arresters are less common but are useful when dealing with gases containing particulates or mists. There are no known published design methods for this type; however, empirical design procedures have been used in GBHE. Under conditions of high gas flow the seal may break down and a gas path exist through the arrester. This type of arrester should not be specified without reference to GBHE. Pebble bed arresters are another example of a type which was used extensively in the past but is little used today. Again there are no known design criteria for this type of unit.



5.2.5.3 Specification Most arrester manufacturers have their own specification sheets which should be filled in as far as possible for the enquiry. Information is required on gas composition, flow rate, materials of construction and acceptable pressure drop. The key design parameter is the minimum experimental safe gap (MESG). This may have to be determined experimentally for gas mixtures although it is known for many single component gases. The location of the arrester in relation to the ignition source is also important as it affects the flame velocity and whether there is likely to be a deflagration or detonation. Based on this specification and the manufacturer’s knowledge of the performance of their own designs, a flame arrester will be proposed. Much of the design knowledge for the arresters is based on performance of actual units in operation and is not available in the public domain. Flame arresters are designed to stop deflagrations or detonations. The latter are significantly larger, stronger and more expensive than the former. The two types are not interchangeable. Deflagration arresters are intended to stop relatively low velocity flames whereas detonation arresters are designed for supersonic flame fronts and shock waves. 5.2.5.4 Pressure Drop There is always some pressure drop across the arrester which varies with the type and duty. Crimped metal and plate type arresters are designed with larger cross sectional areas than the pipe in which they are installed, partly to minimize the pressure drop and partly to reduce the flame speed. Where more reactive gases are present and the gas flow channels in the arrester smaller, the pressure drop will be higher. It should be noted that flow through the arrester is likely to be laminar due to the large diameter. For liquid seal arresters the pressure drop is dictated by the head of liquid needed to make the seal. Typically the liquid level is in the region 300-400 mm. Some liquid may be lost via the overflow due to level swell and liquid can be lost by vaporization to the vent gas. An adequate source of makeup liquid is therefore required. For some low pressure vent collection systems, the pressure drop may be critical and should be discussed with the arrester manufacturer.



Pressure drop can be increased by deposits forming on the arrester element, some of which may not be easily visible on inspection, or by liquid logging if the arrester is installed in the horizontal plane. Dust and polymeric materials are a particular problem in crimped metal or plate type arresters. Liquids which may polymerize on the element may also cause difficulty. When installed on the vent of "atmospheric" or low pressure tank vents, a separate emergency relief should also be fitted. High and low pressure alarms should also be provided to warn of the operation of emergency vents. NFPA 30 gives guidance on the design and use of flame arresters on storage tanks. 5.2.5.5 Instrumentation Instrumentation of flame arresters is an important part of the safety for the header system. In the event of an ignition occurring, a high temperature can be detected in the flame arrester. Since flame arresters are designed only to give protection for a limited time, it is essential that corrective action be taken to eliminate the source of flammable gas and extinguish the flame before burn through of the arrester occurs. For this reason, all flame arresters should be fitted with a thermocouple and high temperature alarm in order to detect the presence of a flame. The thermocouple may be installed on one or both sides of the arrester element. This enables the fault to be detected and rectified or the plant to be shut down safely. Where the arrester is critical to the safety of the system (i.e. the majority of cases) more than one independent temperature probe should be fitted. Thin wall thermocouple pockets should be used in this application for a rapid response time. Large arresters may need more than one thermocouple to guard against localized burning. For systems where pressure drop is critical, or those where clogging of the arrester may be expected, the pressure drop across the arrester should also be monitored. Typical systems which may cause fouling of the arrester are those containing solid particles and those containing monomers which are prone to polymerizing on the element of crimped metal types in particular.



Instrumentation for liquid seal arresters should include not only high temperature detection, but also liquid level detection and high and low level alarms. A low level in the arrester may indicate failure of the makeup liquid supply or excessive carry-over of liquid in the vent gas due to high flow. High level in the arrester may mean liquid flowing into the arrester from the vent or blockage of the overflow line. Level alarms should be of appropriate types to deal with the temperature, pressure and composition of any liquid which may enter the arrester. Float or capacitance probe instruments are prone to fouling or differences in composition which can give false readings. Ultrasonic or other non- intrusive types are therefore preferred for this application. In the event of a deflagration or detonation occurring in the arrester the instrumentation should be inspected for damage and replaced or re- calibrated as necessary. 5.2.5.6 Installation and Maintenance Installation depends on the type of arrester. Crimped metal arresters should be installed in a vertical plane so that the element is self draining. Offset designs are available which can be installed in the horizontal plane but are not recommended for use as they are more prone to blocking and corrosion. Arresters may be fitted with liquid drains if installed in the horizontal plane. If not luted then arrangements should be made for regular draining. Drains may be prone to blocking or freezing and precautions need to be taken to prevent blockage occurring (e.g. heat tracing). Luted drains may be installed and may further be protected by a "dead man’s handle" spring loaded valve on the drain line to prevent air ingress. Maintenance of crimped metal arresters is limited to inspection and cleaning of the elements. This should be done carefully, as the element is constructed from relatively thin metal strip and is prone to mechanical damage. If the element is damaged then it may result in enlargement of one or more of the passages through the element which may allow transmission of a flame. Endoscope ports may be fitted to the arrester body to check for blockages. These are particularly recommended if the arrester is to be used on a duty where fouling is expected.



Cleaning of the element should be done by non-contact means (e.g. solvent bath or low pressure steam). Regular cleaning should be considered carefully as it increases the chance of mechanical damage to the arrester element thus compromising its integrity. For systems where clogging is expected or experienced, it may be possible to fit two arresters in parallel, one on line and the other off line being cleaned or on standby. Isolation and purging systems will be needed and for larger sizes special lifting gear may also be necessary. Due to the physical size of flame arresters relative to the pipe and the necessity of designing for high pressure, lifting or support gear should be considered for pipe diameters above about 100 mm. Metal plate arresters are more robust but may still be prone to mechanical damage. Liquid seal arresters may require regular cleaning and flushing. The design of overflows and drains is important to minimize the possibility of blockage due to solid deposition. 5.2.6 Design Pressure of Header Systems If the probability of deflagration or detonation is unacceptably high after all reasonable precautionary measures have been taken, then it may be necessary to consider designing the line for containment. For hydrocarbons, deflagrations typically generate 8 times the initial pressure and it may be acceptable to design for containment. Detonations can generate up to 20 times the initial pressure in a straight line with impulses of up 40 times the initial pressure at bends and junctions and localized pressure spikes of up to 100 times initial pressure. Designing for these pressures will significantly increase the cost and complexity of both the header and supports which will have to be designed to cope with the pressure wave impulse. The temperatures and pressures generated by deflagrations and detonations can be predicted with a degree of accuracy, although whether a particular mixture will run up to detonation is not accurately predictable. Similarly, there are as yet no reliable design methods for predicting the effect of shocks on pipes, pipe supports and support structures. The amount of effort needed to ensure the safety and integrity of the design should not be underestimated. For calculation of deflagration and detonation pressures see list of Best Contacts in Appendix B.



The cost of designing the line for containment should be balanced against the cost of making the system safe by preventing the occurrence of a flammable mixture. As well as the line, the cost of designing vessels to cope with very high pressures may be prohibitive. Fans, in particular, are normally designed only for low pressures and, although they may be designed for containment of moderate pressures, damage to the casing should be expected in the event of an incident. In the limit, secondary containment may be necessary for equipment with low design pressures (e.g. locating in blast bays or explosion proof enclosures). Although the thermal oxidizer itself may also have a very low design pressure, it should not suffer significant damage from a deflagration or detonation in the vent headers since: · There should be no significant quantity of unburned fuel in the oxidizer, hence the explosion should be snuffed out; · The pressure surge passing through a flame arrester should be dissipated in the large volume of the combustion chamber. It should be noted that it is not generally possible to design the oxidizer combustion chamber for full pressure relief since the low design pressure of the unit would require a very large relief area. Additionally, the high operating temperature would make the construction and sealing of vent apertures on the combustion chamber impracticable. It is possible to design an instrumented protective system (IPS) to provide a similar level of integrity as a pressure relief device. 5.3 Liquid in Vent Headers Liquid (non-flashing as well as flashing) should be avoided in vent headers if at all possible. The philosophy should be to eliminate or minimize the carry-over of liquids into the header, especially those which may solidify or cause fouling through, say, polymerization. The possibility of condensation should also be minimized.



5.3.1 Sources Liquid may enter the vent header system in a number of ways, for example:

Liquid carry-over from over-filled upstream plant;

Liquid from relief valves (especially thermal reliefs);

Condensation can occur due to contact of warm gas with cold pipe work or by the mixing of hot and cold vent streams;

Storage tanks vents are normally saturated with vapor, hence diurnal

temperature changes may cause condensation;

Liquid can be collected in a knock-out pot and then re-enter the header if the pot becomes over-filled;

Water from, say, maintenance activities (although this should not

remain in a properly designed header after re-commissioning). 5.3.2 Potential Consequences 5.3.2.1 Liquid Logging Liquid collecting at low points in headers can cause partial or total obstruction. This may cause excessively high pressure drops or back pressure problems bearing in mind that many vent headers operate with only a few mill bars of pressure differential. If a vent header intended for gas is filled, even partially, with liquid then damage can be caused by the weight of liquid either by bending the pipe itself or by damage to the pipe supports. At best this can lead to low spots being formed, thus worsening the liquid logging, and at worst in a shear failure of the header.



5.3.2.2 Thermal Oxidizer Upsets Liquid, even as a spray, fed in a line to a thermal oxidizer designed to burn gas is likely to cause major upsets including the possibility of flame-out or even explosion. If the liquid is flammable, then it may cause a flame out followed by ignition of the flammable vapor from the hot refractory which could result in an explosion. 5.3.2.3 Flammable Hazard Due to Evaporation of Liquid The evaporation of a very small amount of liquid VOCs in a fuel-lean vent header can push the composition into the flammable region extremely quickly. 5.3.3 Design 5.3.3.1 Elimination of Liquid Carry-over Carry-over of liquid into vent collection headers should be eliminated at source wherever practicable. Possible methods include condensation of vapor, mist elimination, inclusion of knock-out pots and careful management of liquid levels in vessels connected to vent headers. Dilution of vaporized liquid is not generally favored as it is likely to cost more in downstream treatment than it would save in avoiding condensation by other methods. For small quantities of liquid carry-over, trace heating of the header may be sufficient. For guidance on design of pipe work systems see Best Contacts in Appendix B. 5.3.3.2 Sloping of Lines All headers should be constructed with an appropriate slope for drainage of any condensation forming in the system (water vapor or high boiling point organics), regardless of whether any liquid is expected in the system. A suitable gradient is likely to be of the order of about 1:100 depending on the type of pipe work and distance between supports. For small bore plastic lines which are prone to sagging, a steeper slope may be appropriate whilst for large bore metal lines a more gentle slope (not less than 1:250) may be used.



Gas superficial velocities should be low enough and line sizes should be large enough to prevent entrainment of liquid as this could cause excessive pressure drops, surging and liquid hammer as well as significant hazards in a thermal oxidizer. If possible, the slope should be in the downstream direction (i.e. with the gas flow) otherwise a steeper gradient may be needed to ensure liquid flow. Low points should be fitted with drain points. 5.3.3.3 Pipe Supports Appropriate pipe supports should be specified for the weight of the line filled with liquid if there is any possibility of significant quantities of liquid entering the system. If hydraulic testing of the line is required, then the design should take this into account with the provision of appropriate maintenance drain points and procedures. For design of pipe supports, the appropriate best contact should be consulted from the list in Appendix B. 5.3.3.4 Design Temperature Consideration should be given to the possibility of liquefied gases flash vaporizing down to very low temperatures. This can cause localized thermal stresses at low points and in KO pots. In particular, welded joints can be prone to thermal stress cracking. There are numerous examples of incidents occurring on vent header systems due to low temperatures caused in this way. The design temperature of the system should be low enough to cope with the flash vaporization temperature of any liquid that may be discharged into the header or the lowest credible ambient temperature, whichever is the lower. In certain parts of the world, winter temperatures can fall below the boiling points of ammonia, propane and chlorine. Steel becomes embrittled at low temperature, hence a suitable grade should be specified. On systems where steam cleaning may be employed, the upper design temperature should be appropriate. High temperature can cause damage to lines due to thermal expansion or melting of plastics.



5.3.3.5 Knock-Out Pots In long headers it may be necessary to install a number of KO pots or condensate drain points to remove liquid from the line at intervals. The complexity of equipment needed and relative cost makes the inclusion of KO pots generally unattractive. Ideally, KO pots should only be used to provide security against an abnormal occurrence of liquid carry-over into the header or of condensation in the header. Whereas KO pots can be used safely on fuel-rich or inerted systems, their inclusion in fuel-lean headers can cause problems if liquid vaporizes at such a rate that it sends the composition into the flammable region. KO pots should not therefore be used in fuel-lean headers unless the liquid being removed has a low vapor pressure or is inert (e.g. water). KO pots may be designed to either remove liquid flowing down the line or to disentrain liquid droplets from the vapor stream. A typical configuration is shown in Figure 2. If at all possible, the use of KO pots should be avoided as they are an additional cost and can also be difficult to operate and maintain. The design basis for a liquid collection KO pot depends on the draining arrangements for the system. If the pot is equipped with a continuous drain (luted or continuously operating drain pump) then the pot will have to hold sufficient liquid such that the drain system can cope with the maximum expected rate of ingress over any particular time period. The lute needs careful design to be able to cope with the maximum pressure and any possible fluctuations.



FIGURE 2 TYPICAL KNOCK-OUT POT WITH LUTED DRAIN

If the pot is equipped with a manual drain, then it should have sufficient capacity to hold the maximum amount of liquid which could collect between drain periods under normal and abnormal conditions including emergency relief. It is essential that a proper management system is in place to ensure that manual drains are properly maintained and operated. Calculations of the maximum inflow, drain rates and any allowances for vaporization should be detailed in the design basis for the KO pot and associated drain equipment and should form part of the basis of safety for the header. Lagging and heat tracing systems should be designed to the minimum anticipated ambient temperatures for the area. If designed for the 1% or 2½% lowest probability of recorded temperature then there is a finite likelihood that the system will freeze at some point during the life of the plant. The consequences of freezing are far more costly than the marginal extra cost of the increased specification. All KO pots should be equipped with suitable high liquid level alarm systems in order to prevent flooding of the vent header due to unforeseen circumstances or process deviations.



Alarms should give sufficient time for process operators to respond before flooding of the header occurs. Temperature monitors may also be necessary to check that heat tracing is working and to guard against freezing. Some liquid VOCs collected in a KO pot may be removed by vaporization. Heat can be supplied from a heating coil, trace heating or jacket and should be sufficient to vaporize any liquid collected during normal operation. The vapor flow from the KO pot should not overload the destruction system. During times of abnormal liquid flow, the level in the KO pot could rise. A level gauge and high level alarms are therefore necessary. There should also be a separate, independent high/high level alarm. Monitoring of the heat source may also be considered necessary as it is critical to the safety and operation of the system. Reliance on the integrity of a steam trap for the safe operation of the header system is not generally acceptable. Vaporization of VOCs from a KO pot may cause other problems with the composition of gases in the header. Selection of an appropriate level sensor is important as liquid density may vary. Pressure cell and float types may give a false reading under these circumstances. Capacitance probes or ultrasonic type sensors should be considered. For selection of appropriate instrumentation see list of Best Contacts in Appendix B. A drain system should be provided to remove liquid collecting in the pot which does not vaporize or which is being collected to be recycled. The pot may be some distance from the plant and the liquid collected may need to be pumped a considerable distance to be recycled to the process or treated prior to disposal. The equipment associated with this operation may be costly, and is likely to include a pump with power supply and associated pipe work. The operation of the pump should be monitored as it may be critical to the safe operation of the system. Drain systems which rely on pumps which are normally idle are prone to failure due to non-starting. Regular maintenance and checking is required to ensure that these systems remain operable. Automatic drain systems which operate by lutes can be reliable if lagged and heat traced, although solids deposition may still be a problem (rust, scale etc.) and the drain pipe work should be designed in such a way that it can be unblocked easily if problems do occur. However, lutes are prone to failure due to pressure surges and may not re-seal under these



conditions; they should not therefore be used on systems which are prone to extreme pressure fluctuations. Luted drains are not suitable for volatile flashing liquids as the liquid seal may fail on vaporization. If liquid in KO pots and drain lines can freeze, especially during cold weather, heat tracing and lagging should be fitted. The heat input and lagging thickness specified should be appropriate for the worst weather conditions likely to be experienced in the geographic area. In the UK a minimum temperature of at least -20°C should be used, but in parts of Canada or the USA a much lower design temperature may be necessary. The above problems with KO pots are likely to be more severe if the installation is located in a remote area of the plant or in an area not frequently visited.

5.4 Materials of Construction Materials of construction should be suitable for the range of process materials and operating conditions expected in the system. The possibility of getting water vapor into the system should also be considered, as it may cause corrosion in combination with other substances such as HCl (even in trace quantities). In this case, the use of corrosion resistant materials should be considered. Typical of these are Hastelloy, lined carbon steel or plastic/GRP. The suitability of materials depends on several factors including: • Temperature range; • Corrosion resistance; • Operating pressure range; • Electrical conductivity; • Whether design for explosion containment is required; • Cost.



5.5 Static Electricity Hazard Static electricity can provide a source of ignition. The flow of any gas or liquid through a pipe work system may be sufficient to build up a charge large enough to cause a spark although, in general, liquid droplets and mists are more likely to cause problems. It should be noted that solid particles can also create a charge when carried in a gas stream. The amount of energy required to ignite a gas mixture is known as the Minimum Ignition Energy (MIE). Some gases, such as hydrogen or acetylene, have very low MIEs. Even in systems of conducting pipe work, sufficient electrostatic charge may be generated to ignite flammable mixtures containing gases with very low MIEs. Non-conductive pipe work should be avoided particularly for gas mixtures which have a low MIE. It is possible to make plastic pipe work conductive by carbon filling. Pipelines and equipment should be checked for conductivity across flanges and joints prior to commissioning the plant and on a regular basis thereafter (and especially after maintenance). Any joints where the resistance is above 1 Ohm should be fitted with electrical bonding strips across the flanges. Electrostatic build up can be hazardous, particularly across changes in material specification (e.g. changes in pipe specification from metallic to non-metallic and across non-conductive gasket material). All fittings, valves and instruments should be assessed for static electric generation potential. Valves and fittings in the header should be designed and specified to an appropriate antistatic standard. Problems may occur in systems containing particulates or materials which can polymerize if a non-conductive coating is formed on the inside of conductive equipment. Static electricity is a complex topic.



5.6 Diversion Systems

5.6.1 General Diversion systems are used to redirect vent gases away from the normal feed inlet to a thermal oxidizer prior to oxidizer start up or in the event of an abnormal situation such as:

• Thermal oxidizer trip; • High oxygen in a fuel-rich vent header; • High fuel in a fuel-lean vent header; • High pressure in vent header; • High temperature in flame arrester; • High liquid level in final KO pot; • Failure of inert gas system.

The diversion system enables corrective action, such as re-starting the thermal oxidizer or shutting down upstream production units, to be taken in a controlled manner and is important for the safety of both upstream production plants and the vents treatment unit itself. The diversion system should also be used when starting up plants to avoid surges or unsteady operation which could cause problems with the oxidizer. Diversions can be to existing local discharge stacks or to a new bypass stack or stacks. Flashback protection (e.g. flame arresters) may be needed in the diversion system as for the thermal oxidizer.

5.6.2 Discharge The diversion system can either redirect the vent gases for direct discharge to atmosphere at a safe location (usually at high level) or else to a flare stack.



A flare stack may be necessary if the vent gas is toxic, strongly odorous or if the size of flammable plume could be hazardous. If discharge is to a flare stack, then similar precautions will need to be taken in the event of a flammable mixture being present in the system as for when the gas is being sent to a thermal oxidizer (i.e. flashback prevention using flame arresters). A flare has a lower destruction efficiency than a thermal oxidizer and it may cause visual or audible nuisance. There may also be a thermal radiation hazard and additional support fuel costs for the pilot flame system. In the event of an ignition occurring on a plain vent stack (e.g. due to lightning) it is normal to provide a means of snuffing the flame out. This can be done using a high flow of steam or another inert gas (typically nitrogen or carbon dioxide). If discharge is via a plain vent stack, then it should be located such that the plume will not cause a hazard such as fire or vapor cloud explosion (VCE). If the concentration of VOCs in the vent gas is above the LFL, then there is a chance that the plume could become flammable on dilution with air. In particular, the location of other ignition sources and the ground level concentration of the vent gas are important. Modeling of the gas dispersion of the plume can be done to determine ground level concentration of vent gases, distance to LFL, radiation from an ignition, explosion overpressure etc..

5.6.3 Period of Operation Statutory authorities will generally place a restriction on the amount of time for which a thermal oxidizer is allowed to be off line during any period. This is a topic for negotiation based on the projected availability of the thermal oxidizer and associated equipment and the environmental impact of the emissions. The amount of down time allowed for the thermal oxidizer will determine the level of installed spare equipment, maintenance effort required and also the level of spares needed in stores. The effect of unscheduled down time on the upstream plants should not be underestimated in terms of disruption and consequential losses.



5.6.4 Design

When designing a diversion system, the reliability of the equipment and the speed of action are important. Any equipment which relies on starting up an electric or other type of motor should be avoided as there is a risk that after a long period of idleness the motor will not start. This is of particular importance in collection systems which rely on suction from a fan or fans located near the treatment unit. Equally important are the phasing and sequencing of operations such as the opening of diversion valves and closing of isolation valves. Where equipment such as valves or electric motors are critical to the operation of the diversion system, sufficient instrumentation should be installed to allow for monitoring of valve positions, operation of motors etc..

5.6.5 Emissions from Diversion System If the vents are diverted to a stack or stacks without flaring, consideration should be given to the expected ground level concentrations of toxic, odorous or flammable materials. The possibility of ignition of the plume by external sources (e.g. lightning), should also be considered. If the plume from a vent stack is ignited, then the thermal radiation effects need to be considered as for a flare stack. The effects may include personnel hazard, heat damage to surrounding equipment and ignition hazard for other vents. Grouping together of untreated vents may cause the concentration of controlled materials to go above the allowable concentration or flow rate for a single vent. This is a topic for discussion with the local statutory authorities. If the diversion stack is not a flare stack, then the flammability of the gas mixture should be considered. A schematic of a diversion system is shown in Figure 3.



FIGURE 3 SCHEMATIC OF DIVERSION SYSTEM

5.7 Snuffing Systems Snuffing systems are designed to extinguish flames in vent header systems or stacks. They work by injecting a large quantity of inert gas (or occasionally powder) into the header. This inert gas takes the composition in the system below the LFL thus extinguishing the flame. Nitrogen, carbon dioxide or steam are commonly used for snuffing. The inert gas injected forms a "slug" of non-combustible material. The snuffing system may only have a limited inventory of gas. Once this inventory is exhausted, the composition in the header may become flammable again. The snuffing system provides time to correct the problem or shut down the system safely.



6 SAFE DESIGN OF THERMAL OXIDISERS 6.1 Introduction Information on some of the different types of thermal oxidizers, catalytic oxidizers, heat and mass balances, recuperative and regenerative heat recovery, suppliers, etc. Although there is some overlap between the two guides, this guide concentrates on issues concerning safety and environmental control and those issues not covered by. 6.2 Design Basis 6.2.1 Capacity The flowsheet duty is determined from the collection and assessment of process flow data for the vents to be treated (see Section 4). It should be noted that this duty should include any base load of inerts or fuel gas to ensure operation of the vent header(s) outside the flammable region. In many cases, the maximum duty that is specified initially is determined by the need to treat a high flow rate of vent gas for short periods of time from, for example, peak batch operation flows or relief streams. These specific needs should be examined very carefully because:

The size and hence capital cost of the thermal oxidizer and its downstream flue gas handling plant would have to be increased to cope with these peak flow rates;

The maximum turn-down on a thermal oxidizer is typically about 5:1

and often as little as 3:1, hence it would be necessary to use large amounts of excess combustion air and consume associated large amounts of support fuel outside the peak flow rate periods which would be a large proportion of the time;

There will be a limit on the maximum rate of change of flow rate or

calorific value of the incoming waste gas stream with which the control system on the thermal oxidizer can cope.



Therefore, wherever possible, peak flow rates of waste gas to be treated should be attenuated and relief streams should not, as a general rule, be fed to thermal oxidizer systems. Flare stacks may be used to cope with high flow rates due to relief conditions but their destruction efficiency is lower. 6.2.2 Destruction Efficiencies and Emission Limits 6.2.2.1 Destruction Efficiencies The key design parameters for high efficiencies of destruction of organic materials fed to thermal oxidizers are Temperature, Time, Turbulence (often known as the 3 Ts) and oxygen concentration. A minimum temperature of 850°C to 900°C is required to destroy most organics. However, if halogenated materials are present, the statutory authorities are likely to require a minimum temperature of 1100°C to 1200°C in order to avoid the formation of halogenated dioxins and furans (see below).

It is generally accepted that the destruction of organics is so fast at thermal oxidizer temperatures that reaction kinetics are not limiting. However, minimum residence times of about 2 seconds after the last injection point of combustion air are often required by statutory authorities to ensure full and adequate mixing which is of paramount importance [Ref. 8]. The geometry of the combustion chamber and the orientation of the main burner nozzle and air inlet ports are very important in order to ensure high radial turbulence without any unmixed or cold spots. Residence time, temperature and destruction efficiency will be specified on the permit to operate in the US and Canada and a number of other countries. A minimum oxygen concentration at the exit from the oxidizer of about 3% v/v, without correction for water vapor, is generally required to ensure a very high level of oxidation of organics. Monitoring of the excess oxygen in the flue gas is mandatory in many countries to ensure complete combustion. Carbon monoxide (CO) is a good surrogate indicator of other products of incomplete combustion (PICs) and, as such, it is customary to specify a maximum limit of about 100 mg/m³ CO in the flue gas. This concentration is often referenced to certain standard conditions, such as 1 atmos pressure, 0°C, 11% v/v oxygen and dry gas in Europe. Also, the period of time over which the concentration is averaged, such as 1 hour, should be specified.



The performance of thermal oxidizers is usually specified in terms of maximum allowable pollutant concentrations, as in Europe, or in terms of the destruction and removal efficiency (DRE), as in the USA. As with CO, maximum allowable pollutant concentrations are usually referenced to certain standard conditions. DRE is defined as:

where POHC = mass flow rate of principal organic hazardous constituent. 6.2.2.2 Acid gases Acid gases, such as hydrogen chloride (HCl) and oxides of sulfur (SOx), may have to be removed from the thermal oxidizer flue gas by scrubbing (see Section 6.5.1). Treatment of scrubber blowdown liquor will inevitably be required. The composition of any support fuel used should be taken into account when considering pollutant concentrations in the thermal oxidizer flue gas. For example, a fuel oil containing sulfur could produce more SOx than is produced from the sulfur content of the waste gases being treated. 6.2.2.3 Particulates Thermal oxidizer flue gas will contain particulates that may require abatement if the incoming waste gases contain suspended inorganic dust or if water containing dissolved or suspended solids has been used for temperature control. Various methods including liquid scrubbers or

dust filters may be used to remove particulates.

6.2.2.4 NOx In some countries, the statutory authorities set a maximum permissible limit on emissions to atmosphere of nitrogen dioxide (NO2) and nitric oxide (NO) (collectively known as NOx). Typically, the ratio of NO: NO2 in a thermal oxidizer flue gas stack is about 10:1 with the NO slowly oxidizing to NO2 in the atmosphere.



However, the emission limit is likely to be total NOx expressed as NO2. Even if a NOx limit is not specified, consideration of NOx control may be necessary to avoid a visible brown plume especially if burning organics containing bound nitrogen or if the combustion temperature is about 1200°C or higher. NO is not visible but NO2 is visible. As a general rule of thumb, the onset of NO2 visibility is characterized by the following Beer- Lambert law:

Where D = stack diameter in m. Organically bound nitrogen tends to form NOx (fuel NOx) in oxidizing atmospheres at any temperature likely to be found in a thermal oxidizer combustion chamber. The formation of fuel NOx can be controlled by staged air combustion (see Section 6.3.3). Elemental nitrogen in the incoming waste gases or in the combustion air tends to form NOx (thermal NOx) in oxidizing atmospheres at temperatures of about 1200°C or higher. The combustion chamber wall does not normally reach this temperature but higher temperatures can be generated in the flame. Low-NOx burners are readily available and are usually employed to control the formation of thermal NOx. These usually employ a short, highly turbulent flame with staged air injection into the flame.

6.2.2.5 Dioxins

Polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) (collectively known as dioxins), which can be formed in incineration processes if chlorine and hydrocarbons are present, have attracted considerable attention. However, their formation in the combustion chamber can be reduced to levels generally acceptable to statutory authorities provided: • There is sufficient turbulence within the combustion chamber; • There is sufficient time for good mixing (generally specified at 2 seconds);



• There is no bypassing or dead spots; • The temperature is high enough (generally specified at a minimum of 1100°C). It is known that dioxins can be formed as the flue gases cool downstream of the combustion chamber. Although the exact chemical reactions are not understood (de novo reactions), it is believed that the critical temperature range in which these reactions take place is about 200°C to 450°C and that certain metals, including copper and iron, act as catalysts. Dioxin formation has been observed more in old municipal solid waste (MSW) and clinical waste incinerators where the combustion temperatures were not well controlled and/or where particulates settled in the tubes of waste heat boilers. Although current regulatory dioxin emission concentration limits can be as low as 0.1 ng/m³, this should not be a significant problem with a modern thermal oxidizer burning gases and producing essentially no particulates in the flue gas.

6.2.3 Availability In order to achieve high availability it is most important to properly specify and install the refractory linings and then to care for them during operation (see Section 6.4). As a general rule, it should be possible to achieve an on-line availability of about 90% to 95% (i.e. typically about 20 to 35 days downtime per year) provided planned maintenance can be scheduled to coincide with downtime on the plant(s) producing the vent gases to be treated. If the thermal oxidizer services several plants that do not have planned concurrent downtime, the on-line availability of the thermal oxidizer could be less than 90%. It is likely that a system to divert the vent gases to a safe place will be required during thermal oxidizer downtime (see Section 5.6). 6.2.4 Heat and Mass Balance Combustion air and support fuel requirements should be determined by means of standard heat and mass balance calculations.



6.2.5 Turn-Down Thermal oxidizer turn-down is typically of the order of about 5:1 but can be as low as 3:1. The minimum turn-down can be governed by the main burner characteristics or by the need to ensure appropriate flow patterns for adequate turbulence without bypass routes in the combustion chamber. It should be noted that it may be necessary to use excess combustion air and support fuel at low flow rates of incoming vent gas in order to maintain minimum turndown. 6.3 Types of High Temperature Thermal Oxidizer 6.3.1 Conventional Oxidizing Combustion Chamber A typical simple conventional unit with a vertical combustion chamber is shown schematically in Figure 4. In this example, the support fuel burner assembly is housed in a sub-chamber attached to the side of the main combustion chamber. This arrangement protects the main burner assembly from the radiant heat in the main combustion chamber and also makes maintenance of the burner assembly and the refractory lining in the sub-chamber easier. The temperature in the main combustion chamber controls the feed rate of support fuel. The combustion air feed rate is ratio-controlled to the support fuel feed rate with the ratio control set point adjusted by the concentration of oxygen in the flue gas.



FIGURE 4 CONVENTIONAL VERTICAL THERMAL OXIDIZER

6.3.2 Conventional Oxidizing Combustion Chamber with Integral Water Sparger Lentjes supplies a thermal oxidizer with an integral water bath through which the feed of waste gas bubbles into the combustion chamber. This configuration minimizes the risk of flash back into the vent header especially if the mixture in the vent header can approach the LFL or the UFL during, say, upset conditions. Care should be taken with this design to ensure that operation at the maximum gas rate does not result in a continuous free gas path back through the liquid. This type of unit is shown in Figure 5.



FIGURE 5 CONVENTIONAL OXIDISER WITH INTEGRAL WATER SPARGER

6.3.3 Staged-Air Combustion

The formation of fuel-NOx (see Section 6.2.2.4) from organically-bound nitrogen (e.g. amines) can be minimized by the use of staged-air combustion as shown diagrammatically in Figure 6. The reduction chamber operates sub-stoichiometrically on air thereby converting the organically-bound nitrogen to elemental nitrogen. The gases leaving the reduction chamber typically contain about 10% combustibles such as hydrogen and carbon monoxide. In order to achieve a high destruction efficiency of organics within the reduction chamber, it is necessary to compensate for the oxygen deficiency by operating at a high temperature, typically 1100°C to 1300°C.



The temperature of the gases leaving the reduction chamber is reduced in the quench chamber to achieve a maximum of about 1000°C in the oxidation chamber so as to minimize the formation of thermal NOx (see Section 6.2.2.4). Quenching can be achieved by means of cooled recycle gas as shown in Figure 6, or by using excess secondary combustion air fed to the oxidation chamber or by introducing quench water or steam. Cooled recycle gas has the environmental benefit of producing the least volume of final flue gas and can be used to raise steam but it is the most capital intensive option. Cooling water requires an atomizing fluid which could be air but steam should produce lower NOx. Also, special care is required to avoid the possibility of water droplets impinging on hot refractory surfaces. Both water and steam cooling could give rise to a visible steam plume from the stack. Air is the lowest cost option but may not reach the very low NOx levels achievable with the other options. It should be noted that NFPA and Factory Mutual have special safety requirements for the use of recycle gas. Vendors should comply with these regulations for installations in North America. FIGURE 6 THERMAL OXIDIZER WITH STAGED AIR INJECTION

The oxidation chamber operates typically at a minimum of 3% v/v oxygen in the final flue gas.



6.3.4 Down-Fired Combustion Chamber Figure 7 shows a vertical combustion chamber down-firing directly into a water bath. This type of unit is suitable for burning gases containing fluorinated hydrocarbons which produce highly corrosive hydrogen fluoride. It is essential that expert advice is sought regarding materials of construction especially for the down comer insert into the water bath. The down comer should be cooled with quench water. Additionally, it may be necessary to irrigate the inside surface of the down comer to prevent localized overheating. FIGURE 7 DOWN-FIRED UNIT WITH WATER BATH QUENCH



6.3.5 Flameless Thermal Oxidation Thermatrix markets a flameless thermal oxidation unit shown diagrammatically in Figure 8. The unit comprises a slightly conical vertical upflow reactor packed with ceramic spheres. The bottom layers of spheres are smaller in diameter than those in the main body of the reactor and serve to distribute the gases evenly across the cross-section of the unit. Waste gases, combustion air and support fuel, if required, enter a plenum chamber at the base of the reactor and then flow upwards through the gas distribution zone and then into the hot reaction zone where oxidation takes place. Thermatrix suggest that the gas mixture in the plenum chamber should not exceed 50% LEL but a more conservative approach would be 25% of the LEL. The temperature profile can be established at start-up by feeding hot combustion gases from a standard burner downwards through the bed. Flameless thermal oxidation offers the following advantages: • High destruction efficiencies resulting from homogeneous mixing without bypass paths of the reactant gases; • Low thermal NOx due to the absence of high flame temperatures; • High thermal inertia to cope with varying feed flows and compositions; • The positioning of the reaction zone is, to some extent, self regulating in the slightly conical reactor in that the rate of advance of the reaction zone down the bed equals the upward velocity of the gas stream. Therefore, an increase in calorific value of the influent gases without an increase in volumetric flow tends to move the reaction zone down the bed, but this is countered by an increase in upward velocity of the gas in the narrower diameter of the bed; • The lower part of the bed may act, to some extent, as a flame arrester.



FIGURE 8 FLAMELESS THERMAL OXIDATION UNIT

However, the following factors also need to be considered: • The influent gases mix in the feed line, plenum chamber and lower part of the reactor. The likelihood, and possible consequences of a flammable mixture developing, should be considered. However, it should be noted that this issue also applies to catalytic oxidizers; • It is not known whether the ceramic spheres provide any active sites for surface catalysis. Therefore, caution should be exercised for gas mixtures not yet proven with this technology. 6.3.6 Regenerative Heat Recovery in Multiple Beds Figure 9 shows a multiple bed thermal oxidizer with regenerative heat recovery. This type of unit, which is more fully described in PSHEG 15, operates batch-wise by heating up a matrix by direct contact with hot flue gas and then uses the hot matrix to pre-heat the incoming waste gas stream.



FIGURE 9 THERMAL OXIDIZER WITH REGENERATIVE HEAT RECOVERY

6.4 Refractories There are usually several layers of refractory lining. The inner layer(s) should be resistant to any chemical attack as well as withstanding the high radiant and convective temperatures in the combustion chamber. The principal duty of the outer layer(s) is to provide a thermal insulating barrier to protect the outer metal shell. Refractory linings can be cast in situ or built from pre-formed bricks. The latter necessarily require suitable jointing or grouting material. It is vitally important to consult a Materials Engineer with regard to the specification of the refractory materials and the means by which they are fastened to the shell After installation, refractory linings have to be dried out and cured before commissioning. This is normally done using a small flame, ramping the temperature up slowly. This process may take up to three weeks to complete for a new lining. The refractory can be damaged by thermal or mechanical shock. Slugs of liquid entering the oxidizer can cause localized thermal shock damage. The refractory can also be damaged by erosion due to flame impingement.



6.5 Flue Gas Treatment 6.5.1 General It may be necessary to treat the flue gas before it is discharged to atmosphere by, for example, scrubbing to remove acid gases such as HCl or SOx. As stated earlier, it is unlikely that treatment would be required to remove particulates unless poor quality water is used for quenching or temperature control or if the thermal oxidizer is also used to burn particulate forming materials (e.g. TiCl4) or if the gases themselves contain non-combustible particulates. 6.5.2 Flue Gas Cooling

It will usually be necessary to cool the flue gas leaving the combustion chamber upstream of any flue gas treatment unit by one or a combination of the following methods. Recuperative heat recovery (see Section 6.5.3.1);

Adiabatic water quench. Great care should be paid to the choice of

materials of construction and to the mechanical design, especially in the area of first contact between the hot gas and the quenching water. For advice on materials, see list of Best Contacts in Appendix B. It should be noted that it may be possible to combine the duties of an adiabatic quench and an aqueous scrubber into a single unit;

Partial water quench. This can be achieved using water sprays into the

hot gas stream. It is likely that a quench vessel would be required rather than in-line spray quenching in order to avoid impingement of water droplets onto hot refractory surfaces. Although high pressure spray nozzles are available that do not require an atomizing medium, they have

• very limited turn-down and it is more likely that steam or air would be

required for atomization;

• Diluent air cooling. This is more likely to be used to achieve a modest temperature reduction upstream of a gas-gas recuperator rather than a massive temperature reduction upstream of a scrubber.



Typically, about 0.3 kg of air at 6 bara atomizing about 1 kg of water would give a turn-down in the region of 3:1. The amount of atomizing air can be reduced to 0.1 kg per kg of water if the water pressure is increased to 6-7 bara whilst maintaining the atomizing air pressure about 2 bar above that of the water. Turn-down ratios greater than 3:1 can be achieved by isolating a number of nozzles, but great care is needed to prevent the isolated nozzles from overheating or blocking. Steam atomization typically requires about 0.5 kg of steam per kg of water. Pressure atomizing nozzles using no steam or air can give turn-down ratios up to 4:1 but very high water pressure is required and some means of cooling or withdrawing isolated nozzles is normally needed to avoid damage. 6.5.3 Heat Recovery 6.5.3.1 Recuperative Heat Recovery Heat can be recovered from the hot flue gas using a recuperative heat exchanger to generate steam, pre-heat boiler feed water in an economizer, or to pre-heat combustion air or the incoming waste gas stream. An economic analysis should be carried out to determine whether it is viable to use some of the waste heat against the additional costs of the extra equipment and maintenance needed. Attention should be paid to the choice of materials of construction, especially the upstream tube sheet and tubes which should withstand not only the high temperature of the incoming gas stream but also the stresses resulting from differential thermal expansion between cold shut-down conditions and hot operating conditions. If halogenated organics are present in the waste gas stream, it may be necessary to limit the recuperator minimum wall temperature in order to minimize de novo dioxin formation (see Section 6.2.2). The acid dew point should also be taken into consideration in order to prevent corrosion.



If nitrated organics are present in the incoming waste gas stream, a proportion of the cooled flue gas can be recirculated back to the combustion chambers for fuel-NOx control (see Section 6.3.3). Where sulfur oxides are present, temperatures should again be high enough to prevent the formation of sulfuric acid mist and associated corrosion. 6.5.3.2 Regenerative Heat Recovery A ceramic or brick matrix, pre-heated by direct contact with the hot flue gas, can be used to pre-heat the incoming waste gas stream or combustion air as illustrated in Section 6.3.6. 6.5.4 Plume Suppression The UK Environment Agency may require steps to be taken to reduce the size or frequency of a visible atmospheric dew point condensation plume [Ref. 9]. Dew point condensation plume visibility increases with:

Increased temperature of flue gas that is saturated with water vapour;

Increased relative humidity and decreased temperature of the ambient air.

As a general rule-of-thumb, dew point condensation plume visibility in Europe should not be major issue if the flue gas relative humidity is significantly below 100% or if its temperature is below about 40°C. Dew point condensation plume visibility can be reduced by cooling to condense out some of the water content, by heating the flue gas or by diluting it with air. The most common approach is to dilute with hot air. As a general rule-of-thumb, diluting hot flue gas that is saturated with water vapor at about 70°C with an equal amount of hot air to achieve a mixed gas temperature of about 110°C will avoid plume visibility issues in Europe.



6.6 Control and Safety Systems 6.6.1 Control System (Burner Management System) One of the principal challenges for the design and operation of vent gas collection and thermal systems is that of control. Most of the problems that are experienced with these systems result from the variable flow rate and composition of the gaseous feed streams. It is generally not possible to measure these variables for use in a feed forward control system. Consequently, most control systems rely on feedback of the combustion chamber temperature and flue gas composition as shown in the simplified diagram in Figure 4. Sometimes it is appropriate, as in the case of staged air combustion (see Section 6.3.3), to use temperature difference across a combustion chamber as one of the measured control parameters. It should be noted that in some parts of the USA, analysis of the excess oxygen in the flue gas is mandatory. A burner management design guide has been produced [Ref. 10] which gives advice on design and specification of thermal oxidizer control systems. Design of control systems in North America is governed by NFPA 86. In Canada, fuel gas trains should conform to the Canadian Gas Association Code and oil burner trains to NFPA 86. Also in Canada, only approved fittings may be used, therefore the use of local engineering contractors is strongly recommended. Advice on control systems should be sought from the corporate insurer who is the "Authority with Jurisdiction" referred to in the NFPA codes. As feedback control is typical, it is important to study the process and control dynamics of the system and to ensure that, for example, rates of change of flow rate or composition of the incoming waste gases can be accommodated safely. It may be appropriate, in some cases, to install a relatively simple control system and use, for example, larger than normal amounts of excess support fuel and combustion air. Dynamic simulation may be used to model the operation of the control system. For advice on this topic see list of Best Contacts in Appendix B. Burner management systems may be complex, with large numbers of inputs and outputs. All control systems should be thoroughly tested at the factory as it may be difficult to carry out simulations of control conditions on site.



6.6.2 Burner Design, Burner Management System and Flame Detection A typical burner assembly comprises a pilot burner, a main burner and a burner management system (BMS). These should be designed to ensure that it is not possible for a large volume of unburned flammable gas to develop in the combustion chamber at any time. This is achieved by the following provisions:

Flame detectors, also known as "fire eyes" or "magic eyes", should monitor the presence of flames on the pilot and main burners. Flame detectors normally rely on the optical detection of specific wavelengths of light. Two detectors, set at different wavelengths, can be used to provide redundancy. The detectors should be set to "fail safe" with shut down being initiated only if both units indicate failure;

• Flame detectors are generally reliable once set up and operating correctly;

however, they do require cleaning periodically. Soot and other dirt can collect on the quartz glass lens sometimes giving a false "flame out" reading. Careful consideration should be given to the location of the flame detectors since exposure to the full flame temperature can result in damage and rapid failure of the unit. Flame detectors therefore should be located out of the flame in the side of the chamber, preferably adjacent to the burner nozzle. It is advisable to make provision to withdraw the detector head for maintenance without having to shut the burner down;

Self checking detectors should be used if possible. These rely on

detecting the difference between light and dark due to the rotation of an internal shutter. If, for any reason, the regular flickering is not detected, then an alarm can be generated. This could be due to either a flame failure or a detector failure;

Typically two detectors per burner are used, infra red for pilot flames

and UV for the main flame. The detectors may be air purged to keep the detector window clear;

The burner management system should cut off the fuel to the pilot

burner if the pilot burner flame is not established after a pre-set time, so as not to produce an unacceptably large volume of flammable gas mixture in the combustion chamber;



The burner management system should cut off the fuel to the main burner if the main burner flame is not established after a pre-set time, so as not to produce an unacceptably large volume of flammable gas mixture in the combustion chamber (as above);

The burner management system should include a timed delay after

automatic shut-down before it is possible to admit fuel to the pilot burner, fuel to the main burner or vent gases to the combustion chamber in order to avoid the possibility of hot refractory igniting a flammable mixture in the combustion chamber. This is intended to allow time for complete purging of the chamber but does allow for hot restarting;

The burner management system should ensure that the combustion

chamber is purged with air or inert gas equal to at least five times the volume of the combustion chamber before attempting to re-light the pilot burner. Air dampers should be opened fully to ensure the best purge rate;

Appropriate isolation standards should be used for fuel supplies. In

particular, for fuel gases at high pressure or those containing significant quantities of hydrogen, the provision of double block and bleed isolations should be considered. Even with liquid fuels, tight shut off of the fuel supply is critical to the safety of the unit;

A hard wired or non-reprogrammable logic system should be included

to take care of emergency trips and shut-down systems;

A detailed Hazard Study of the logic on both the normal control system and the non-reprogrammable system is essential. This should be done by a person conversant with programmable electronic systems and their hazard study.

NFPA 85C gives requirements for installation of flame detection devices, burner management systems, trip systems, purging and fuel systems as applicable to multiple burner boiler - furnaces. NFPA 86 gives guidance on burner management systems. BMS in the US and Canada should be approved by Factory Mutual or Underwriters Laboratories. Whilst it is possible to gain approval for a custom programmed plc, it does not usually make economic sense to do this.



One of the approved commercial devices such as those produced by Fireye or Honeywell should be used. Protective devices such as flame detectors, pilot light and start-up and shut-down sequence are all governed by NFPA codes and should be strictly adhered to in the USA. 6.6.3 Start-up & Shut-down The hazards associated with start-up can be minimized by the use of a proper procedure and the presence of suitable trips and alarms. The vents system should not be started up direct to the destruction unit. Vent flows should be established to the "cold stack" or bypass vent route prior to bringing the destruction system on line. Vent deviations at start-up can be a major cause of problems with destruction systems, due to variations in flow and the possibility of getting liquid in the header; therefore it is better to establish flows to the header prior to bringing the thermal oxidizer on line. A simplified sequence, which may be controlled wholly or in part by the burner management system, would be as follows:

Establish vent flows to diversion system or cold stack;

Purge burner chamber with at least five volumes of air or inert gas to ensure that any VOCs or other fuel gas is swept out;

Check that fuel gas is available to pilot/igniter;

Check combustion air flow;

Establish the presence of a spark or equivalent to the igniter;

Initiate the pilot burner gas flow;

Check for presence of the pilot flame;

Establish a flow of support fuel to the main burner at low rate;

Confirm the presence of the main flame;

Turn off the pilot flame;

Ramp up the main burner gas flow;



Bring on the vent gas flow at low rate to oxidizer;

Ramp up the vent gas flows to normal operating conditions and shut off flows to diversion system.

The failure to establish any step successfully should abort the start-up and initiate a purge of the burner chamber. It should be noted that after the fuel to the thermal oxidizer has been shut off, the refractory material may remain hot for a long period. If the VOC vent flow or support fuel is re- introduced during this period, then an explosion may result due to delayed ignition from the hot refractory. A hot restart should not, therefore, be attempted if the temperature of the refractory is above the AIT unless the burner management system is specifically designed to cope with hot restarts. The burner management system should have provision for hot restart unless there are exceptional circumstances which would make this event hazardous. Factory Mutual 6-11 [Ref. 13] states that there should be a mandatory pre- ventilation period to purge the combustion chamber, intake and exhaust systems of any fumes or fuel which may have accumulated during shutdown periods. It should be noted that in the US and Canada the start- up and shut-down sequences should be handled by the BMS as described above. This is also strongly recommended for installations in other countries. The purge should be proven by suitable interlocks or monitoring instruments. At least three volume changes of air are required by Factory Mutual 6-11 and the period should be timer controlled to prevent actuation of fuel valves or ignition devices during the purge. Dampers should be set to the open position during the purge and, if necessary, provided with interlocks or indicators to prove them open. The pilot flame is established using small flows of natural gas (or fuel gas) and air. The flow of gas to the pilot flame is small enough that it would take a significant time to fill the combustion chamber with sufficient quantity of unburned gas to cause damage to the chamber shell in the event of a delayed ignition. Factory Mutual 6-11 [Ref. 13] states that interlocks should be provided on fuel supply pressure (high and low as required) and also atomizing air or steam pressure where used. NFPA 85C also contains recommendations on purging and fuel shut off. The pilot flame is usually turned off after the main flame has been established. This minimizes erosion of the spark ignition electrodes and wear and tear on the pilot burner itself. Operation of the pilot and ignition system can normally be checked with the thermal oxidizer on-line.



Controlled shut-down of the thermal oxidizer should also be done in sequence, typically: Shut off waste flows to oxidizer and send to diversion system;

Shut off fuel to main, auxiliary and pilot burners;

Purge with air or inert gas (at least five combustion chamber volumes).

On shut-down, the burner chamber should be purged thoroughly to remove any unburned gas. Temperature measurement should not be taken from the exit gas but from the refractory lining itself. Measuring the exit gas temperature is likely to give a false (unsafe) reading considerably below the lining temperature. An automatic or manual quench flow (low pressure air or steam) may be incorporated to cool the refractory lining. 6.6.4 Trips and Emergency Shut-down The number of trips and shut-down systems should be kept to a minimum, commensurate with acceptable safety, as over-complicating the unit could result in the system being difficult to start up and operate. Typically, trips will be needed for the following:

Support fuel high / low pressure;

Combustion air low pressure/flow;

Instrument air low pressure; Combustion chamber high temperature;

Electrical power failure to ignition spark generator (during start-up);

Flame failure;

High temperature on flame arresters;

Low oxygen concentration or high carbon monoxide concentration in

the flue gas;

High oxygen in fuel-rich or inerted header or low oxygen in fuel-lean header.



It is likely that the statutory authority would require the flow of waste gases into the combustion chamber to be shut off automatically in the event of the last event on the above list occurring. Subject to the complexity of the system, other trips may also be required. There may be some degree of redundancy in the control system (e.g. use of a supervisory module to back up critical trips and sequence steps). 6.6.5 Explosion Relief It is unlikely that it would be practicable to fit suitably sized explosion relief panels to the oxidizer unit for the following reasons: It would be difficult to make a good seal in the refractory lining around the

explosion relief panel due to high temperature;

The large volume of the combustion chamber would generally require comparatively large explosion relief panels which are likely to be impractical to fit due to space limitations;

The large size of the explosion relief panels and the high temperatures in the

combustion chamber are likely to present problems of warping and gas leakage.

The basis of safety is, therefore, to prevent any significant explosion over-pressure from occurring by ensuring that there is insufficient unburned fuel gas in the system to generate a hazardous pressure in the event of an ignition. A small "pop" on ignition, involving only a small volume of flammable gas, is considered normal for many combustion units. 6.7 Project Program 6.7.1 Outline Program The length of time taken to execute a project of this nature should not be underestimated. An outline project program for the design and construction of a typical new vent collection and destruction system is shown below in Figure 10.



It can be seen that a total of about 4 years may be needed with about half of this required for pre-sanction work including, in particular, vent identification, characterization and quantification. Delays to the project may be caused by external factors including matters such as statutory approvals. The possibility of additional clarification being needed in order to obtain planning permission etc. should not be discounted. VCDS projects may be completed in less time than this if some level of parallel engineering is done. This may entail taking a number of financial and design risks in order to procure long delivery items. Obviously, the level of parallel engineering which can be tolerated also depends on the complexity of the project. There have been examples where the project time scale has been reduced to under two years by parallel engineering on new plants. FIGURE 10 TYPICAL PROJECT PROGRAM



6.7.2 Personnel As with all project program, it is important to appoint the appropriate personnel, including the more senior members of the commissioning team, early in the project program. Late appointments, temporary appointments or the use of part time personnel will most likely lead to difficulties including extended commissioning program and the possibility of an unsafe design. It is particularly important that members of the commissioning team are fully acquainted with the design and have a complete understanding of the operation of the system. Members of the plant operating team should also be involved in the design. 6.7.3 Preliminary and Detailed Enquiries Normally, a preliminary enquiry is sent to about 6 to 12 suppliers which usually enables about 3 to be selected for the detailed enquiry. The changes necessary to a vendor’s proposal in order to meet GBHE standards should be considered. Some suppliers may not have experience of units which are required to operate 24 hours per day for long periods, or units which are left for long periods without operator supervision. Preparation of detailed enquiries should involve all the relevant personnel, including a Materials Engineer and a Furnaces & Boilers Engineer. As part of the bid analysis on the detailed proposals, visits should be made to selected operating units in order to confirm the suppliers' claims. Valuable information on the design, construction and operation of the unit can be gained in this way. 6.8 Commissioning As with all commissioning program, it is essential that all systems are thoroughly checked out in a proper manner. Items should be tested against a checklist to ensure that there are no omissions. An outline checklist is as follows:

• Check that all mechanical equipment including process and service pipe work has been installed according to final approved drawings and specifications; • Ensure that all slip plates and temporary blank flanges or line blinds are removed;



• Where appropriate de-scale/condition the inside surfaces of metallic vent collection pipe work; • Ensure that all drain lines and overflows from lute pots etc. are free flowing (i.e. not blocked); • Check liquid top up supply to lute pots is flowing and that the correct fluid level is present; • Check lute pot maintenance schedule and that frost prevention systems are operable; • Blow out process, utility and instrument air lines to ensure no obstructions in the system which might block flame traps, instruments, burner nozzles etc.; • Physically check flame arresters (crimped metal and other in-line types) for integrity, physical damage or blockage; • Pressure/leak test all equipment where required or practicable; • Dead check all cable/wiring loops; • Live check all cable/wiring loops; • Check operation of fans, pumps etc.;

• Stroke check all valves including, where appropriate, that they move to their intended failure positions; • Zero and span check measuring systems; • Field test gas analyzers including response times; • Check emergency shut-down and voting systems (e.g. on gas analysis equipment and flame failure devices); • Full functional loop tests on all control and emergency shut-down systems;



• Check start-up sequence control system including safety interlocks (e.g. delay timers on repeat start ups); • Check all emergency shut-down initiation systems (e.g. high temperature and flame failure devices); • Check change over to emergency back up supplies (e.g. electrical power); • Start-up and check all equipment and process units down-stream of combustion chamber(s); • Test isolation of support fuel, confirm by gas sampling and analysis; • Purge vent headers and combustion chamber with air for fuel-lean systems and inert gas for fuel-rich systems with at least five volume changes; • Check ignition and operation of pilot flame visually via inspection port; • Dry out refractory lining according to supplier’s specifications; • Test stability of main flame throughout operating regime; • Ensure all interlocks and trip systems function properly by deliberately simulating shut-down situations in a controlled fashion (e.g. fan trips, low support fuel pressure, flame failure etc.); • Full load and control test on support fuel before admitting any process streams; • Combustion efficiency compliance tests.



6.9 Operational and Maintenance Management 6.9.1 Fundamental Principles It is most important that vent collection and thermal oxidizer systems are treated with the same high respect as afforded to production units. They should not be treated as service units of secondary importance, otherwise there could be serious safety, environmental and loss of production risks. The operational and maintenance management of these systems should be integrated into management arrangements for the associated production units. If a vent collection and thermal oxidizer system treats waste gases from a number of production units under the control of different groups of personnel, special arrangements should be made to interface the management systems of the production units and of the thermal oxidizer. 6.9.2 Communications If it is not possible for the thermal oxidizer to be operated by the same team of personnel that operates the associated production unit, then it is essential that good communications are established between the two teams. Each team should ensure that the other team has an awareness of the status of their plant, especially with regard to unusual or unexpected situations. 6.9.3 Maintenance Wherever possible, maintenance on a vent collection and thermal oxidizer system should be planned to coincide with downtime on associated production plants and vice versa. Major repairs to thermal oxidizer refractory lining can take up to about three weeks taking into account drying and curing time. Steps should be taken, therefore, to safeguard the integrity of the refractories and to carry out inspection and preventative maintenance accordingly. The critical steps are:

• Proper specification; • Careful installation; • Drying out slowly; • Minimizing temperature cycling by minimizing the frequency of shut-down and by avoiding rapid temperature changes; • Avoiding flame impingement;



• Avoiding impingement of quench water that may be used; • Regular inspection through viewing ports; • Regular inspection of outer shell for hot spots; • Expeditious minor repairs.

Where a thermal oxidizer is connected to a number of upstream plants via a complex header system, robust management procedures should be in place for maintenance activities. Full understanding of the scope of maintenance work and the effects on both upstream plants and the VCDS is essential to the safety of the operation. NFPA 85C requires that a formal maintenance training program be in place and that procedures should be established to cover routine and special techniques. 6.9.4 Management of Change Changes to equipment specifications or operating conditions should be managed in the same way as a normal operating plant in order to minimize the risk of hazards. All changes should be documented and a hazard analysis or hazard study carried out as appropriate.



APPENDIX A GLOSSARY AIT Autoignition temperature. The temperature at which a flammable gas mixture will ignite without the presence of a flame or spark. BACT Best available control technology. BATNEEC Best available techniques not entailing excessive cost. BMS Burner management system. BPEO Best practicable environmental option. CHC Chlorinated hydrocarbon. Deflagration A sub-sonic explosion where the flame front is preceded by the pressure wave. A deflagration will produce a pressure of up to 8 times the initial pressure. Detonation A supersonic explosion where the flame front and pressure wave are coincident. A detonation will produce a pressure of up to 20-40 times the initial pressure. Detonation arrester Similar to a flame arrester but designed to higher standards of pressure and generally with larger arrester elements. Designed to prevent the propagation of a detonation. DRE Destruction and removal efficiency. Flame arrester A device fitted in or at the end of a pipeline which is designed to prevent the propagation of a flame. See also "detonation arrester". FM Factory Mutual Insurance. US industrial insurance company. Fuel-rich Above the upper flammable (explosive) limit for the mixture. Fuel-lean Below the lower flammable (explosive) limit for the mixture. GRP Glass reinforced plastic, also known as FRP.



LFL (or LEL) Lower flammable limit or lower explosive limit. The lowest percentage (v/v) of fuel which will just sustain combustion. Specified at a particular temperature and pressure. Inerted A gas mixture in which the amount of inerts is sufficient to keep the composition out of the flammable region. KO pot A knock-out pot designed to remove liquid or solid particles from gas streams. MIE Minimum ignition energy. The minimum amount of energy required to ignite a flammable gas mixture. Generally spark energy. MOC Minimum oxidant concentration. The percentage (v/v) of oxidant below which the mixture will not burn. MSW Municipal solid waste. NFPA National Fire Protection Association (of America). Oxidant Generally oxygen but other possible oxidants include chlorine and oxides of nitrogen. PCDDs Polychlorinated dibenzodioxins. PCDFs Polychlorinated dibenzofurans. PICs Products of incomplete combustion. POCP Photochemical ozone creation potential. Generally referenced on a scale relative to ethylene. POHC Mass flow rate of principal organic hazardous constituent. Quenching diameter The diameter of orifice through which a flame will not propagate. Measured at set pressure and temperature, this parameter is used in the specification of flame arresters. QRA Quantified risk assessment.



UFL (or UEL) Upper flammable limit or upper explosive limit. The maximum percentage (v/v) of fuel which will just sustain combustion. Specified at a particular temperature and pressure. UL Underwriters Laboratories. US testing and insurance company. UNECE United Nations Economic Commission for Europe USCG United States Coast Guard. A regulatory body which has issued a specification for the design and testing of flame arresters. VCDS Vent collection and destruction system. The collective headers, destruction unit and discharge stack system. VOC Volatile organic compound. WHO World Health Organization.



APPENDIX B FLAMMABILITY B.1 GENERAL For combustion to occur, three things are necessary: fuel, oxidant and an ignition source. Many substances, particularly hydrocarbons, burn readily within certain composition limits. The limits, known as the LFL and UFL have been determined under standard conditions for a variety of substances. The flammable limits vary with temperature, pressure and composition and can be plotted on a flammability diagram. A typical diagram is shown in Figure 11. The flammable region is shown hatched. It should be noted that the diagram is triangular and, as such, the sum of the compositions at any point always adds up to 100%. Many diagrams originating in the USA are shown on rectangular plots with fuel and oxidant as the axes, the percentage of inert being 100 minus the sum of the other two. Figure 11 shows the major features of a typical flammability diagram. The "air line" shows the composition change on diluting 100% fuel with air. Where the air line crosses the bottom axis, the composition is 21% oxygen, 79% nitrogen. Some flammability diagrams show only the region to the right of the air line where the only oxidant available in the system is air. This section is then expanded to fill the full diagram. This is often done where only the composition of the gas when mixed with air is important. FIGURE 11 TYPICAL FLAMMABILITY DIAGRAM



The line to the right of the air line and just touching the nose of the flammable region shows the minimum oxidant concentration for combustion (MOC). The composition on the oxidant axis is known as the MOC on a fuel-free basis. This is the concentration of oxidant, below which no combustion can occur. The stoichiometry line shows the range of compositions which would result in complete combustion of the fuel and oxidant. Compositions just above the stoichiometric line produce the largest temperatures and pressures on ignition.

The LFL is essentially constant for the majority of hydrocarbons and hence is shown on the diagram as a horizontal line. The UFL is different in air and pure oxygen, hence it is important to know whether the UFL is in air or oxygen. Although for many hydrocarbons this line is essentially straight, there is, however, a significant number of substances which exhibit non-typical behavior. Where there is any doubt as to the characteristics of the diagram, experimental determination of the flammability limits is strongly recommended. Oxidants other than air/oxygen can also be expressed on this type of diagram (e.g. chlorine, NOx etc.). The flammability diagram should be annotated with the temperature and pressure at which the data were measured. Increasing the temperature or pressure tends to increase the size of the flammable region although it generally has a small effect for pressures greater than atmospheric. The size of the flammable region is also affected by the ignition source, e.g. whether hot wire, spark or fuse head. The more powerful the ignition source, the larger the flammable region. Most flammability diagrams are measured using a powerful spark or other ignition source and hence are applicable to vent gas collection systems where the typical ignition energy available is a few milliJoules. For detailed interpretation of flammability diagrams, ignition energies, flammability data etc. The lower flammability limit for a mixture of gases may be calculated using Le Chatelier’s law:



Where: L = Lower flammable limit of mixture Cx = Percentage of component in mixture (v/v) Lx = Lower flammable limit of each component Flammability limits for many substances can be found in Reference 12. Le Chatelier’s law can also be used to calculate the UFL of a mixture. Although this formula is generally reliable for calculating LFLs, it is less reliable when calculating UFLs due to the more complex reactions which take place in a fuel-rich mixture. B.2 CHANGES OF COMPOSITION The triangular diagram can be used to ascertain the effect of changes in composition on the flammability of a particular mixture. If the operating point is plotted on the diagram and gas of another composition is added, the composition moves in a straight line from the original operating point towards the composition of the gas being added. Figure 12 shows the example of a particular mixture being diluted with air. The initial composition on this diagram is: Fuel 43% Oxidant 32% Inert 25% As the initial composition is diluted with air, the operating point moves in a straight line towards the composition of air. At approximately 17% fuel, the operating point enters the flammable region. Figure 13 shows the effect of various amounts of diluent air on an initial volume of 100 Rm³ of gas of the above composition. From Figure 13 it can be seen that the composition enters the flammable region when 150 Rm³ of air has been injected.



FIGURE 12 EFFECT OF DILUTION WITH AIR

As the composition moves further away from the original operating point, an increased amount of diluent is needed to change the composition by the same percentage. This can be seen again in Figure 13 by considering the amount of air needed to dilute the now flammable mixture to a point below the flammable limit. The LFL on the diagram is 3% and by calculation a total of 1,350 Rm³ of air is needed to reach this limit. A total of 2,300 Rm³ of diluent air is needed to reach a level of 60% of the LFL and 5,700 Rm³ to reach 25% of the LFL (0.75% fuel). Thus it can be seen that the strategy for treating vents should be considered carefully in order to provide a safe and economic solution.



FIGURE 13 EFFECT OF DILUTION WITH AIR ON 100 Rm³ OF FLAMMABLE GAS



APPENDIX C EXAMPLE PROFORMA Operating plant name and unit number

Vent title and description







APPENDIX D REFERENCES



DOCUMENTS REFERRED TO IN THIS PROCESS SHE GUIDE This Process Engineering Guide makes reference to the following documents: NATIONAL FIRE PROTECTION ASSOCIATION NFPA 30 Flammable and Combustible Liquids Code (referred to in 5.2.5.4) NFPA 69 Explosion Prevention Systems (referred to in 5.2.3.2 and 5.2.3.3) NFPA 85C Prevention of Furnace Explosions & Implosions in Multiple Burner Boiler Furnaces (referred to in 6.6.2, 6.6.3 and 6.9.3) NFPA 86 Ovens and Furnaces (referred to in 6.6.1 and 6.6.2) PROCESS SAFETY / SHE GUIDES GBHE-PEG-008 Pressure Relief (referred to in 1.2 and 1.3) GBHE-PEG-015 Practical Guide on the reduction of Discharges to Atmosphere of Volatile Organic Compounds (VOCs) (referred to in 1.2, 3.1, 3.2, 6.1 and 6.3.6)

