Designs to Prevent Fires and Explosions

Potential Damage Limit

A 2-fold strategy is used to limit the potential damage from fires and explosions:

Prevent the initiation of the fire or explosion (fire prevention)

Minimize the damage after a fire or explosion has occurred (fire protection)

Design Methods

Fire and explosion preventive measures InertingStatic ElectricityControlling static electricityVentilation Fire and explosion hazard control measures Explosion proof equipment and instrumentsSprinkler systemsMiscellaneous design features

Sources of Inerting Gases

Possible sources of purge or inerting gases include commercially available gases supplied from high pressure cryogenic tanks or standard cylinders, or on-site air separation plants.

Cross-connections between the source of inerting gas and any other system should not be allowed.

The purging and inerting gas should be introduced and exhausted so that effective mixing is ensured and the desired reduction in oxidant concentration is maintained throughout the system being protected.

The gases from purged or inerted vessel must be disposed of in a safe manner.

Purging Methods

Several methods may be used to form and maintain a non-combustible atmosphere in an enclosure or vessel. These include batch modes for one-time use, such as purging equipment during a shutdown, and continuous inerting to assure safe conditions during normal operations.

Purging methods to reduce the oxygen concentration are:

vacuum purging, pressure purging, combined vacuum-pressure purging, sweep-through purging and siphon purging.

Vacuum purge cycles

Disadvantages of Vacuum Purging

-Relatively slow process of developing a vacuum

-Capacity of vacuum systems decreases significantly as the absolute vacuum is decreased

Pressure Purge Cycles

Pressure purging versus vacuum purging

Practical advantage of pressure purging versus vacuum purging is the potential for cycle time reductions.

The pressurization process is much more rapid compared to the relatively slow process of developing a vacuum.

Also, the capacity of vacuum systems decreases significantly as the absolute vacuum is decreased.

Pressure purging, however uses more inert gas.

Therefore the best purging process is selected based on cost and performance.

Vacuum –pressure purging with initial pressurization

Combined Pressure-Vacuum purging If the initial oxygen mole fraction is 0.21 , the oxygen mole fraction at this point is the same as the initial mole fraction.

Furthermore, the remaining cycles are identical to the vacuum purge operation. The oxygen mole fraction at the end of this initial pressurization is given by:

Where, P0 is the starting pressure.

H

oo P

Py 21.0

Advantages and Disadvantages of the Various Pressure and Vacuum Inerting Procedures

Pressure purging is faster because the pressure differentials are greater, however it uses more inert gas than vacuum purging.

Vacuum purging uses less inert gas because the oxygen concentration is reduced primarily by vacuum.

When combining vacuum and pressure purging, less nitrogen is used compared to pressure purging, especially if the initial cycle is a vacuum cycle.

Sweep-Through purging The sweep-through purging process adds purge gas into a vessel at one opening and withdraws the mixed gas from the vessel to the atmosphere (or scrubber) from another opening. This process is generally used when the vessel or equipment is not rated for pressure or vacuum. The volumetric quantity of inert gas required to reduce the oxidant concentration from C1 to C2 is Qvt and is calculated using:

Siphon purging The sweep-through purging requires large quantities of nitrogen. This could expensive when purging large storage vessel. Siphon purging is used to minimize such expenses. The purging process start by filling the vessel with liquid (water or other). The purged gas is subsequently added to the vapor space as the liquid drained from the vessel. The volume of the purge gas is equal to the volume of the vessel, and the rate of purging is equivalent to the volumetric rate of liquid discharge.

Static Electricity

Static electricity is commonly produced when:

liquid flows through a pipe or hose, or through an opening in a pipe or hose spraying or coating blending or mixing filling tanks, drums, cans or pails dry powdered material passes through chutes or pneumatic conveyors non-conductive conveyor belts or drive belts are moving

Fundamentals of Static Charge• A common ignition source is sparks resulting from static

charge build-up and sudden discharge. • Electrostatic discharge occurs when two materials at different

potentials come close enough together to generate a charge transfer.

• For good conductors, charge build-up is small because the electrons are able to move between the surfaces.

• For poor conductors or insulators, the charge build-up is greater as electrons are not as mobile and are trapped on surfaces.

• Household examples which result in a buildup of a static charge are walking across a rug, combing hair, friction between fabrics, pumping of a non-conductive liquid through pipe, mixing immiscible liquids, pneumatically conveying solids.

Streaming CurrentA streaming current, Is is the flow of electricity produced by transferring electrons from one surface to another by a flowing fluid or solid.The streaming current is obtained by integrating the product of voltage gradient and the fluid velocity over the thickness of the double layer.

More equations

Energy of Charged Capacitors

Electrostatic Values

Capacitance of a BodyThe buildup of a charge on one surfaces relative to another produces a capacitor. In the chemical industry, the properties of the developed capacitor are estimated by assuming parallel flat plate or spherical geometrics.

Electrostatic Hazards

An electrostatic charge by itself does not necessarily represent an ignition hazard.

Such a hazard exists only when the charge is so high that discharges occur owing to the high electric field.

A discharge can be incentive when the energy released is equal to or greater than the minimum ignition energy.

In the petrochemical industry static electricity is a very serious problem for two reasons:

- Flammable vapors may be present in normal operations, and electrostatic discharges can initiate fires and explosions- The conductivity of refined petroleum liquids and bulk hydrocarbon chemicals is very low, permitting charge accumulation

Controlling Static Electricity

• Bonding & Grounding (B&G)- Voltage difference between two conductive materials is reduced to zero by bonding and grounding these two materials.- B&G reduced voltage of an entire system to ground level or zero voltage, thus eliminate charge build-up and potential static sparks.

• Relaxation- When pumping fluids into a vessel through pipe, the separation

process build-up charges- Reduce this hazard by adding an enlarged section of pipe just

before entering tank. This provides time for charge reduction by relaxation.

• Dip pipes- Prevent accumulation of charges when liquid is allowed to free fall

into tank by extending discharge pipe.

Increasing Conductivity with Additives

The conductivity of non conducting organics can sometimes be increased using additives called antistatic additives.

Examples of antis tats include water or polar solvents such as alcohols.

Water is only effective when it is soluble in the offending liquid, because an insoluble phase gives an additional source of separation and charge buildup.

Handling Solids

Explosion-Proof Equipment

• Process divided into two major environments: XP and non-XP.

• XP (Explosion proof) means flammable materials might be present at certain times.

• Non- XP means that flammable materials are not present even under abnormal conditions. For non-XP designated areas, open flames, heated elements and other sources of ignition may be present.

Explosion –Proof Housings

• At XP area, electrical equipment must have special housings designed to withstand internal explosion and prevent combustion from spreading beyond the inside of enclosure.

• Motor starter is enclosed in a heavy cast walled box with the strength needed to withstand explosive pressures.

Area and material classification

Class systems: The classes are related to the nature of flammable material Class I: Locations where flammable gases/vapors are presentClass II: Locations where flammable dusts are presentClass III: Hazard locations where combustible fibers or dust are present not likely to be in suspension

Group system: The groups designate the presence of specific chemical types. Chemicals that are grouped have equivalent hazards: Group A: AcetyleneGroup B: Hydrogen, EthyleneGroup C: Carbon monoxide, Hydrogen sulfideGroup D: Butane, Ethane, Ethyl alcoholGroup E: Aluminum dustGroup F: Carbon blackGroup G: Flour

Area and material classification

Division system: regions are categorized in relationship to the probability of the material being within the flammable or explosive region Division 1: Probability of ignition is high, flammable concentration are normally present Division 2: Hazardous only abnormal conditions. Flammable materials are normally contained in closed containers or system

Design of XP Area

When designing an XP area, all pieces of electrical equipment and instrumentation are specified for the class, group and divisions.

All pieces of equipment and instrumentation within an area must be appropriately specified and installed.

The overall classification is only as good as the piece of equipment in an area with the lowest classification.

Explosion Proof Equipment & Instruments Ventilation

The purpose of ventilation is to dilute the explosive vapors with air to prevent explosion and to confine hazardous flammable mixtures.

Open Air PlantsOpen air plants are recommended because the average wind velocities are high enough to safely dilute volatile chemical leaks which may exist within a plant.

Plants inside buildingsLocal ventilation is the most effective method for controlling flammable gas releases.Dilution ventilation, however is also used because the potential points of releases are usually numerous.

Design Criteria for designating ventilation system

Other MethodsSprinkler systems

- Consist of an array of sprinkler heads connected to a water supply.

- Heads are mounted at high location and disperse a fine spray of water over an area when activated.

- If a fire is detected, the entire sprinkler array within an area is activated, possibly in areas not even affected by the fire. This approach is called an open head area system.

- Sprinkler systems may cause considerable water damage when activated, depending upon the contents of the building or process structure..

- Sprinkler systems require maintenance to insure they remain in service and have an adequate and uninterrupted water supply.

Types of Sprinkler Systems Antifreeze sprinkler system: A wet pipe system that contains an antifreeze solution and that is connected to water supply. Deluge sprinkler system: Open sprinklers and an empty line that is connected to water supply line through a valve that is opened upon detection of heat or flammable material. Dry pipe sprinkler system: A system filled with nitrogen or air under pressure. When the sprinkler is opened by heat, the system is depressurized, allowing water to flow into the system and out the open sprinkler. Wet pipe sprinkler system: A system containing water that discharges through the opened sprinklers via heat.

Fire Protection for Chemical Plants

Closed Head Area Systems for small storage areas, laboratories, control rooms, and small pilot plants.

Open Head Area Systems for process areas including larger pilot plant.

Deluge Water Spray Systems for vessels, heat exchangers etc. These systems are similar to open head area systems.

Nominal Discharge Capacities of approved sprinklers having a nominal ½ inch orifice.

Miscellaneous Designs for Preventing Fires and Explosions (Feature)

Maintenance ProgramsFireproofingControl RoomsWater suppliesControl valves for delugeManual Fire ProtectionSeparate UnitsUtilitiesPersonnel areas Group Units Isolation Valves Railroads and FlaresOnline Analyzers Fail – Safe designs