SpiraxSarco-B1-Introduction

8/14/2019 SpiraxSarco-B1-Introduction

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The Steam and Condensate Loop 1.1.1

Steam - The Energy Fluid Module 1.1Block 1 Introduction

Module 1.1

Steam - The Energy Fluid


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The Steam and Condensate Loop

Steam - The Energy Fluid Module 1.1

1.1.2

Block 1 Introduction

Fig. 1.1.1 An 18th century steam engine.Photography courtesy of

Kew Bridge Steam Museum, London

Fig. 1.1.2 A modern packaged steam heatexchange system used for producing hot water

It is useful to introduce the topic of steam by considering its many uses and benefits, beforeentering an overview of the steam plant or any technical explanations.

Steam has come a long way from its traditional associations with locomotives and the Industrial

Revolution. Steam today is an integral and essential part of modern technology. Without it, ourfood, textile, chemical, medical, power, heating and transport industries could not exist or performas they do.

Steam provides a means of transporting controllable amounts of energy from a central, automatedboiler house, where it can be efficiently and economically generated, to the point of use. Thereforeas steam moves around a plant it can equally be considered to be the transport and provisionof energy.

For many reasons, steam is one of the most widely used commodities for conveying heat energy.Its use is popular throughout industry for a broad range of tasks from mechanical power productionto space heating and process applications.

Steam - The Energy Fluid

Steam is efficient and economic to generate

Water is plentiful and inexpensive. It is non-hazardous to health and environmentally sound. In itsgaseous form, it is a safe and efficient energy carrier. Steam can hold five or six times as muchpotential energy as an equivalent mass of water.

When water is heated in a boiler, it begins to absorb energy. Depending on the pressure in theboiler, the water will evaporate at a certain temperature to form steam. The steam contains a

large quantity of stored energy which will eventually be transferred to the process or the spaceto be heated.


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Fig. 1.1.3

Steam can easily and cost effectively

be distributed to the point of use

Steam is one of the most widely used media to convey heat over distances. Because steam flowsin response to the pressure drop along the line, expensive circulating pumps are not needed.

Due to the high heat content of steam, only relatively small bore pipework is required to distributethe steam at high pressure. The pressure is then reduced at the point of use, if necessary. Thisarrangement makes installation easier and less expensive than for some other heat transfer fluids.

Overall, the lower capital and running costs of steam generation, distribution and condensatereturn systems mean that many users choose to install new steam systems in preference to otherenergy media, such as gas fired, hot water, electric and thermal oil systems.

It can be generated at high pressures to give high steam temperatures. The higher the pressure,the higher the temperature. More heat energy is contained within high temperature steam so itspotential to do work is greater.

o Modern shell boilers are compact and efficient in their design, using multiple passes andefficient burner technology to transfer a very high proportion of the energy contained in thefuel to the water, with minimum emissions.

o The boiler fuel may be chosen from a variety of options, including combustible waste, whichmakes the steam boiler an environmentally sound option amongst the choices available forproviding heat. Centralised boiler plant can take advantage of low interruptible gas tariffs,because any suitable standby fuel can be stored for use when the gas supply is interrupted.

o Highly effective heat recovery systems can virtually eliminate blowdown costs, return valuablecondensate to the boiler house and add to the overall efficiency of the steam and condensateloop.

The increasing popularity of Combined Heat and Power (CHP) systems demonstrates the highregard for steam systems in todays environment and energy-conscious industries.


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1.1.4


Fig. 1.1.4 Typical two port control valve with a pneumatic actuator and positioner

Energy is easily transferred to the process

Steam provides excellent heat transfer. When the steam reaches the plant, the condensationprocess efficiently transfers the heat to the product being heated.

Steam can surround or be injected into the product being heated. It can fill any space at auniform temperature and will supply heat by condensing at a constant temperature; this eliminatestemperature gradients which may be found along any heat transfer surface - a problem which isso often a feature of high temperature oils or hot water heating, and may result in quality problems,such as distortion of materials being dried.

Because the heat transfer properties of steam are so high, the required heat transfer area is

relatively small. This enables the use of more compact plant, which is easier to install and takesup less space in the plant. A modern packaged unit for steam heated hot water, rated to1 200 kW and incorporating a steam plate heat exchanger and all the controls, requires only0.7 m floor space. In comparison, a packaged unit incorporating a shell and tube heatexchanger would typically cover an area of two to three times that size.

The modern steam plant is easy to manage

Increasingly, industrial energy users are looking to maximise energy efficiency and minimiseproduction costs and overheads. The Kyoto Agreement for climate protection is a major externalinfluence driving the energy efficiency trend, and has led to various measures around the globe,such as the Climate Change Levy in the UK. Also, in todays competitive markets, the organisation

with the lowest costs can often achieve an important advantage over rivals. Production costs canmean the difference between survival and failure in the marketplace.

Steam is easy to control

Because of the direct relationship between the pressure and temperature of saturated steam, theamount of energy input to the process is easy to control, simply by controlling the saturated steampressure. Modern steam controls are designed to respond very rapidly to process changes.

The item shown in Figure 1.1.4 is a typical two port control valve and pneumatic actuator assembly,

designed for use on steam. Its accuracy is enhanced by the use of a pneumatic valve positioner.The use of two port valves, rather than the three port valves often necessary in liquid systems,simplifies control and installation, and may reduce equipment costs.


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Fig. 1.1.5 A modern boiler house package

Ways of increasing energy efficiency include monitoring and charging energy consumption torelevant departments. This builds an awareness of costs and focuses management on meetingtargets. Variable overhead costs can also be minimised by ensuring planned, systematicmaintenance; this will maximise process efficiency, improve quality and cut downtime.

Most steam controls are able to interface with modern networked instrumentation and controlsystems to allow centralised control, such as in the case of a SCADA system or a Building /Energy

Management System. If the user wishes, the components of the steam system can also operateindependently (standalone).

Boiler

Fig. 1.1.6 Just some of the products

manufactured using steam as an essential

part of the process

With proper maintenance a steam plant will last for many years, and the condition of manyaspects of the system is easy to monitor on an automatic basis. When compared with othersystems, the planned management and monitoring of steam traps is easy to achieve with a trap

monitoring system, where any leaks or blockages are automatically pinpointed and immediatelybrought to the attention of the engineer.

This can be contrasted with the costly equipment required for gas leak monitoring, or the time-consuming manual monitoring associated with oil or water systems.

In addition to this, when a steam system requiresmaintenance, the relevant part of the system is easy toisolate and can drain rapidly, meaning that repairs maybe carried out quickly.

In numerous instances, it has been shown that it is farless expensive to bring a long established steam plantup to date with sophisticated control and monitoringsystems, than to replace it with an alternative methodof energy provision, such as a decentralised gas system.The case studies refered to in Module 1.2 provide reallife examples.

Todays state-of-the-art technology is a far cry from thetraditional perception of steam as the stuff of steamengines and the Industrial Revolution. Indeed, steamis the preferred choice for industry today. Name anywell known consumer brand, and in nine cases out often, steam will have played an important part inproduction.


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1.1.6


Steam is flexible

Not only is steam an excellent carrier of heat, it is alsosterile, and thus popular for process use in the food,pharmaceutical and health industries. It is also widelyused in hospitals for sterilisation purposes.

The industries within which steam is used range fromhuge oil and petrochemical plants to small locallaundries. Further uses include the production ofpaper, textiles, brewing, food production, curingrubber, and heating and humidification of buildings.

Many users find it convenient to use steam as the sameworking fluid for both space heating and for processapplications. For example, in the brewing industry,steam is used in a variety of ways during different stagesof the process, from direct injection to coil heating.

Steam is also intrinsically safe - it cannot cause sparks and presents no fire risk. Many petrochemicalplants utilise steam fire-extinguishing systems. It is therefore ideal for use in hazardous areas orexplosive atmospheres.

Other methods of distributing energyThe alternatives to steam include water and thermal fluids such as high temperature oil. Eachmethod has its advantages and disadvantages, and will be best suited to certain applications ortemperature bands.

Compared to steam, water has a lower potential to carry heat, consequently large amounts ofwater must be pumped around the system to satisfy process or space heating requirements.However, water is popular for general space heating applications and for low temperature processes(up to 120C) where some temperature variation can be tolerated.

Thermal fluids, such as mineral oils, may be used where high temperatures (up to 400 C) arerequired, but where steam cannot be used. An example would include the heating of certain

chemicals in batch processes. However thermal fluids are expensive, and need replacing everyfew years - they are not suited to large systems. They are also very searching and high qualityconnections and joints are essential to avoid leakage.

Different media are compared in Table 1.1.1, which follows. The final choice of heating mediumdepends on achieving a balance between technical, practical and financial factors, which will bedifferent for each user.

Broadly speaking, for commercial heating and ventilation, and industrial systems, steam remainsthe most practical and economic choice.

Fig. 1.1.8 These brewing processes all use steam

Fig. 1.1.7 Clean steam pipeline equipment

used in pharmaceutical process plant


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Table 1.1.1 Comparison of heating media with steam

Steam Hot water High temperature oils

High heat content Moderate heat content Poor heat content

Latent heat approximately Specific heat Specific heat often

2 100 kJ/kg 4.19 kJ/kgC 1.69-2.93 kJ/kgC

Inexpensive Inexpensive

Some water treatment costs Only occasional dosing

Expensive

Good heat transfer Relatively poor

coefficientsModerate coefficients

coefficients

High pressure required High pressure needed Low pressures only

for high temperatures for high temperatures to get high temperatures

No circulating pumps required Circulating pumps required Circulating pumps required

Small pipes Large pipes Even larger pipes

More complex to control - More complex to control -

Easy to control with three way valves or three way valves ortwo way valves differential pressure valves differential pressure valves

may be required may be required.

Temperature breakdown is Temperature breakdown Temperature breakdown

easy through a reducing valve more difficult more difficult

Steam traps required No steam traps required No steam traps required

Condensate to be handled No condensate handling No condensate handling

Flash steam available No flash steam No flash steam

Boiler blowdown necessary No blowdown necessary No blowdown necessary

Water treatment requiredLess corrosion Negligible corrosion

to prevent corrosion

Reasonable pipework Searching medium, Very searching medium,

required welded or flanged joints usual welded or flanged joints usual

No fire risk No fire risk Fire risk

System very flexible System less flexible System inflexible


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1.1.8


System benefits

Small bore pipework, compact size

and less weight

No pumps, no balancingTwo port valves - cheaper

Maintenance costs lower than

for dispersed plant

Capital cost is lower than for

dispersed plant

SCADA compatible products

Automation; fully automated boiler houses

fulfil requirements such as PM5 and

PM60 in the UK

Low noise

Reduced plant size

(as opposed to water)

Longevity of equipment

Boilers enjoy flexible fuel

choice and tariff

Systems are flexible and

easy to add to

The benefits of steam - a summary:

Table 1.1.2 Steam benefits

Inherent benefits

Water is readily available

Water is inexpensive

Steam is clean and pure

Steam is inherently safe

Steam has a high heat content

Steam is easy to control due to the

pressure/temperature relationship

Steam gives up its heat at a

constant temperature

Environmental factors

Fuel efficiency of boilers

Condensate management and heat recovery

Steam can be metered and managed

Links with CHP/waste heat

Steam makes environmental and

economic sense

Uses

Steam has many uses -

chillers, pumps, fans, humidification

Sterilisation

Space heating

Range of industries


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Questions

1. How does the heat carrying capacity of steam compare with water ?

a| It is about the same

b| It is less than water

c| More than water

d| It depends on the temperature

2. Which of the following is true of steam ?

a| It carries much more heat than water

b| Its heat transfer coefficient is more than thermal oil and water

c| Pumps are not required for distribution

d| All of the above

3. The amount of energy carried by steam is adjusted by

a| Controlling steam pressure

b| Controlling steam flow

c| Controlling condensation

d| Controlling boiler feeedwater temperature

4. Approximately how much potential energy will steam hold compared to an equivalentmass of water?

a| Approximately the same

b| Half as much

c| 5 to 6 times as much

d| Twice as much

5. How does steam give up its heat ?

a| By cooling

b| By radiation

c| By conduction

d| By condensation

6. Which of the following statements is not true ?

a| Steam is less searching than high temperature oil or water

b| Steam pipes will be smaller than water or high temperature oil pipes

c| Temperature breakdown of water and oil is easier than steam

d| Steam plant is smaller than water plant.

1:c,2:d,3:a,4:c,5:d,6:cAnswers


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1.1.10



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Steam and the Organisation Module 1.2Block 1 Introduction


Module 1.2

Steam and the Organisation


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Steam and the Organisation Module 1.2

1.2.2


Steam and the Organisation

The benefits described are not of interest to all steam users. The benefits of steam, as a problemsolver, can be subdivided according to different viewpoints within a business. They are perceiveddifferently depending on whether you are a chief executive, a manager or at operating level.

The questions these people ask about steam are markedly different.

Chief executive

The highest level executive is concerned with the best energy transfer solution to meet the strategicand financial objectives of the organisation.

If a company installs a steam system or chooses to upgrade an existing system, a significant capitalinvestment is required, and the relationship with the system, and the system provider, will be longand involved.

Chief executives and senior management want answers to the following questions:

Q. What kind of capital investment does a steam system represent ?

A steam system requires only small bore pipes to satisfy a high heat requirement. It does notrequire costly pumps or balancing, and only two port valves are required.

This means the system is simpler and less expensive than,for example, a high temperature hot water system. Thehigh efficiency of steam plant means it is compact andmakes maximum use of space, something which is oftenat a premium within plant.

Furthermore, upgrading an existing steam system withthe latest boilers and controls typically represents 50%of the cost of removing it and replacing it with a

decentralised gas fired system.Q. How will the operating and maintenance costs ofa steam system affect overhead costs ?

Centralised boiler plant is highly efficient and can use low interruptible tariff fuel rates. The boilercan even be fuelled by waste, or form part of a state-of-the-art Combined Heat and Power plant.

Steam equipment typically enjoys a long life - figures of thirty years or more of low maintenancelife are quite usual.

Modern steam plant, from the boiler house to the steam using plant and back again, can be fullyautomated. This dramatically cuts the cost of manning the plant.

Sophisticated energy monitoring equipment will ensure that the plant remains energy efficientand has a low manning requirement.

All these factors in combination mean that a steam system enjoys a low lifetime cost.

Q. If a steam system is installed, how can the most use be made of it ?

Steam has a range of uses. It can be used for space heating of large areas, for complex processesand for sterilisation purposes.

Using a hospital as an example, steam is ideal because it can be generated centrally at highpressure, distributed over long distances and then reduced in pressure at the point of use. Thismeans that a single high pressure boiler can suit the needs of all applications around the hospital,for example, heating of wards, air humidification, cooking of food in large quantities and sterilisation

of equipment.

It is not as easy to cater for all these needs with a water system.

Fig. 1.2.1


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Q. What if needs change in the future ?

Steam systems are flexible and easy to add to. They can grow with the company and be altered tomeet changing business objectives.

Q. What does using steam say about the company ?

The use of steam is environmentally responsible. Companies continue to choose steam because it

is generated with high levels of fuel efficiency. Environmental controls are increasingly stringent,even to the extent that organisations have to consider the costs and methods of disposing of plantbefore it is installed. All these issues are considered during the design and manufacture of steamplant.

Management level

A manager will consider steam as something that will provide a solution to a management problem,as something that will benefit and add value to the business. The managers responsibility is toimplement initiatives ordered by senior executives. A manager would ask How will steamenable successful implementation of this task ?

Managers tend to be practical and focused on completing a task within a budget. They willchoose to use steam if they believe it will provide the greatest amount of practicality and expediency,at a reasonable cost.

They are less concerned with the mechanics of the steam system itself. A useful perspectivewould be that the manager is the person who wants the finished product, without necessarilywanting to know how the machinery that produces it is put together.

Managers need answers to the following questions:

Q. Will steam be right for the process ?

Steam serves many applications and uses. It has a high heat content and gives up its heat at aconstant temperature. It does not create a temperature gradient along the heat transfer surface,

unlike water and thermal oils, which means that it may provide more consistent product quality.As steam is a pure fluid, it can be injected directly into the product or made to surround theproduct being heated. The energy given to the process is easy to control using two port valves,due to the direct relationship between temperature and pressure.

Q. If a steam system is installed, how can the most use be made of it ?

Steam has a wide variety of uses. It can be used for space heating over large areas, and for manycomplex manufacturing processes.

On an operational level, condensate produced by a manufacturing process can be returned to

the boiler feedtank. This can significantly reduce the boiler fuel and water treatment costs, becausethe water is already treated and at a high temperature.

Lower pressure steam can also be produced from the condensate in a flash vessel, and used inlow pressure applications such as space heating.

Fig. 1.2.2


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1.2.4


Q. What does steam cost to produce ?

Water is plentiful and inexpensive, and steam boilers are highly efficient because they extract alarge proportion of the energy contained within the fuel. As mentioned previously, central boilerplant can take advantage of low interruptible fuel tariffs, something which is not possible fordecentralised gas systems which use a constant supply of premium rate fuel.

Flash steam and condensate can be recovered and returned to the boiler or used on low pressureapplications with minimal losses.

Steam use is easy to monitor using steam flowmeters and SCADA compatible products.

For real figures, see The cost of raising steam, later in this Module.

In terms of capital and operating costs, it was seen when answering the concerns of the chiefexecutive that steam plant can represent value for money in both areas.

Q. Is there enough installation space ?

The high rates of heat transfer enjoyed by steam means that the plant is smaller and more compactthan water or thermal oil plant. A typical modern steam to hot water heat exchanger packagerated to 1 200 kW occupies only 0.7 m floor space. Compare this to a hot water calorifier whichmay take up a large part of a plant room.

Q. Not wishing to think too much about this part of the process, can a total solution beprovided ?

Steam plant can be provided in the form of compact ready-to-install packages which are installed,commissioned and ready to operate within a very short period of time. They offer many years oftrouble-free operation and have a low lifetime cost.

Technical personnel /operatorsAt the operating level, the day-to-day efficiency and working life of individuals can be directlyaffected by the steam plant and the way in which it operates. These individuals want to know

that the plant is going to work, how well it will work, and the effect this will have on their timeand resources.

Technical personal/operators need answers to the following questions:

Q. Will it break down ?

A well designed and maintained steam plant should have no cause to break down. The mechanicsof the system are simple to understand and designed to minimise maintenance. It is not unusualfor items of steam plant to enjoy 30 or 40 years of trouble-free life.

Q. When maintenance is required, how easy is it ?

Modern steam plant is designed to facilitate rapid easy maintenance with minimum downtime.

The modern design of components is a benefit in this respect. For example, swivel connectorsteam traps can be replaced by undoing two bolts and slotting a new trap unit into place. Modernforged steam and condensate manifolds incorporate piston valves which can be maintainedin-line with a simple handheld tool.

Sophisticated monitoring systems target the components that really need maintenance, ratherthan allowing preventative maintenance to be carried out unnecessarily on working items ofplant. Control valve internals can simply be lifted out and changed in-line, and actuators can bereversed in the field. Mechanical pumps can be serviced, simply by removing a cover, which hasall the internals attached to it. Universal pipeline connectors allow steam traps to be replaced inminutes.


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An important point to note is that when maintenance of the system is required, a steam system iseasy to isolate and will drain rapidly, meaning that repairs can be quickly actioned. Any minorleaks that do occur are non-toxic. This is not always the case with liquid systems, which areslower and more costly to drain, and may include toxic or difficult to handle thermal fluids.

Q. Will it look after itself ?

A steam system requires maintenance just like any other important part of the plant, but thanksto todays modern steam plant design, manning and maintenance requirements and the lifetimecosts of the system are low. For example, modern boiler houses are fully automated. Feedwatertreatment and heating burner control, boiler water level, blowdown and alarm systems are allcarried out by automatic systems. The boiler can be left unmanned and only requires testing inaccordance with local regulations.

Similarly, the steam plant can be managed centrally using automatic controls, flowmetering andmonitoring systems. These can be integrated with a SCADA system.

Manning requirements are thus minimised.

Industries and processes which use steam:

Table 1.2.1 Steam users

Heavy users Medium users Light users

Food and drinks Heating and ventilating Electronics

Pharmaceuticals Cooking Horticulture

Oil refining Curing Air conditioning

Chemicals Chilling Humidifying

Plastics Fermenting

Pulp and paper Treating

Sugar refining Cleaning

Textiles Melting

Metal processing Baking

Rubber and tyres Drying

Shipbuilding

Power generation


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1.2.6


Interesting uses for steam:

o Shrink-wrapping meat.

o Depressing the caps on food jars.

o Exploding corn to make cornflakes.

o Dyeing tennis balls.

o Repairing underground pipes (steam is used to expand and seal a foam which has been pumpedinto the pipe. This forms a new lining for the pipe and seals any cracks).

o Keeping chocolate soft, so it can be pumped and moulded.

o Making drinks bottles look attractive but safe, for example tamper-proof, by heat shrinking afilm wrapper.

o Drying glue (heating both glue and materials to dry on a roll).

o Making condoms.

o Making bubble wrap.

o Peeling potatoes by the tonne (high pressure steam is injected into a vessel full of potatoes.Then it is quickly depressurised, drawing the skins off).

o Heating swimming pools.

o Making instant coffee, milk or cocoa powder.

o Moulding tyres.

o Ironing clothes.

o Making carpets.

o Corrugating cardboard.

o Ensuring a high quality paint finish on cars.

o Washing milk bottles.

o Washing beer kegs.

o Drying paper.

o Ensuring medicines and medical equipment are sterile.

o Cooking potato chips.

o Sterilising wheelchairs.

o Cooking pieces of food, for example seafood, evenly in a basket using injected steam for

heat, moisture and turbulence at the same time.

o Cooking large vats of food by direct injection or jacket heating.

and hundreds more.


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The cost of raising steam

In todays industry, the cost of supplying energy is of enormous interest. Table 1.2.2 showsprovisional industrial fuel prices for the United Kingdom, obtained from a recent Digest of UKEnergy Statistics, which were available in 2001.

Table 1.2.2 UK fuel prices - 2001 (provisional)

Fuel Size of consumer 2001Small 55.49

Coal ( per tonne) Medium 46.04

Large 33.85

Small 142.73

Heavy fuel oil ( per tonne) Medium 136.15

Large 119.54

Small 230.48

Gas oil ( per tonne) Medium 224.61

Large 204.30

Small 4.89

Electricity (pence per kWh) Medium 3.61

Large 2.76

Small 1.10

Gas (pence per kWh) Medium 0.98

Large 0.78

The cost of raising steam based on the above costsAll figures exclude the Climate Change Levy (which came into force in April 2001) although theoil prices do include hydrocarbon oil duty.

The cost of raising steam is based on the cost of raising one tonne (1 000 kg) of steam using thefuel types listed and average fuel cost figures.

Table 1.2.3 UK steam costs - 2001 (provisional)

FuelAverage unit

Unit of supplyCost of raising

cost () 1 000 kg of steam ()

Heavy (3 500 s) 0.074 0 Per litre 9.12

OilMedium oil (950 s) 0.091 8 Per litre 11.31

Light oil (210 s) 0.100 0 Per litre 12.32

Gas oil (35 s) 0.105 4 Per litre 12.99

Natural gasFirm 0.006 3 Per kWh 6.99

Interruptible 0.005 0 Per kWh 5.55

Coal 35.160 0 Per Tonne 3.72

Electricity 0.036 7 Per kWh 25.26


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1.2.8


Fig. 1.2.3

Boiler efficiency

A modern steam boiler will generally operate at an efficiency ofbetween 80 and 85%. Some distribution losses will be incurredin the pipework between the boiler and the process plantequipment, but for a system insulated to current standards, thisloss should not exceed 5% of the total heat content of the steam.

Heat can be recovered from blowdown, flash steam can be usedfor low pressure applications, and condensate is returned to theboiler feedtank. If an economiser is fitted in the boiler flue, theoverall efficiency of a centralised steam plant will be around 87%.

This is lower than the 100% efficiency realised with an electricheating system at the point of use, but the typical running costsfor the two systems should be compared. It is clear that thecheapest option is the centralised boiler plant, which can use alower, interruptible gas tariff rather than the full tariff gas orelectricity, essential for a point of use heating system. The overallefficiency of electricity generation at a power station is

approximately 30 to 35%, and this is reflected in the unit charges.

Components within the steam plant are also highly efficient. For example, steam traps only allowcondensate to drain from the plant, retaining valuable steam for the process. Flash steam fromthe condensate can be utilised for lower pressure processes with the assistance of a flash vessel.

The following pages introduce some real life examples of situations in which a steam userhad, initially, been poorly advised and/or had access to only poor quality or incompleteinformation relating to steam plant. In both cases, they almost made decisions which wouldhave been costly and certainly not in the best interests of their organisation.

Some identification details have been altered.

Case study: UK West Country hospital considers replacing their steam systemIn one real life situation in the mid 1990s, a hospital in the West of England considered replacingtheir aged steam system with a high temperature hot water system, using additional gas firedboilers to handle some loads. Although new steam systems are extremely modern and efficientin their design, older, neglected systems are sometimes encountered and this user needed totake a decision either to update or replace the system.

The financial allocation to the project was 2.57 million over three years, covering professionalfees plus VAT.

It was shown, in consultation with the hospital, that only 1.2 million spent over ten yearswould provide renewal of the steam boilers, pipework and a large number of calorifiers. It was

also clear that renewal of the steam system would require a much reduced professional input.In fact, moving to high temperature hot water (HTHW) would cost over 1.2 million morethan renewing the steam system.

The reasons the hospital initially gave for replacing the steam system were:

o With a HTHW system, it was thought that maintenance and operating costs would be lower.

o The existing steam plant, boilers and pipework needed replacing anyway.

Maintenance costs for the steam system were said to include insurance of calorifiers, steam trapmaintenance, reducing valves and water treatment plant, also replacement of condensate pipework.

Operating costs were said to include water treatment, make-up water, manning of the boilerhouse, and heat losses from calorifiers, blowdown and traps.

The approximate annual operating costs the hospital was using for HTHW versus steam, aregiven in the Table 1.2.4.


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Table 1.2.4 Operating costs

Utility Steam () HTHW ()

Fuel245 000 180 000

0 37 500

Attendance 57 000 0

Maintenance 77 000 40 000

Water treatment 8 000 0

Water 400 100Electricity 9 000 12 000

Spares 10 000 5 000

Total 406 400 274 600

Additional claims in favour of individual gas fired boilers were given as:

o No primary mains losses.

o Smaller replacement boilers.

o No stand-by fuel requirement.

The costings set out above made the HTHW system look like the more favourable option interms of operating costs.

The new HTHW system would cost 1 953 000 plus 274 600 per annum in operating andmaintenance costs. This, in effect, meant decommissioning a plant and replacing it at a cost inexcess of 2 million, to save just over 130 000 a year.

The following factors needed to be taken into account:

o The 130 000 saving using HTHW is derived from 406 400 - 274 600. The steam fuel costcan be reduced to the same level as for HTHW by using condensate return and flash steamrecovery. This would reduce the total by 65 000 to 341 400.

o The largest savings claimed were due to the elimination of manned boilers. However, modern

boiler houses are fully automated and there is no manning requirement.

o The 37 000 reduction in maintenance costs looked very optimistic considering that the HTHWsolution included the introduction of 16 new gas fired boilers, 4 new steam generators and9 new humidifiers. This would have brought a significant maintenance requirement.

o The steam generators and humidifiers had unaccounted for fuel requirements and watertreatment costs. The fuel would have been supplied at a premium rate to satisfy the claim thatstand-by fuel was not needed. In contrast, centralised steam boilers can utilise low costalternatives at interruptible tariff.

o The savings from lower mains heat losses (eliminated from mains-free gas fired boilers) wereminimal against the total costs involved, and actually offset by the need for fuel at premiumtariff.

o The proposal to change appeared entirely motivated by weariness with the supposed lowefficiency calorifiers however on closer inspection it can be demonstrated that steam towater calorifiers are 84% efficient, and the remaining 16% of heat contained in the condensatecan almost all be returned to the boiler house. Gas fired hot water boilers struggle to reach the84% efficiency level even at full- load. Unused heat is just sent up the stack. Hot water calorifiersare also much larger and more complicated, and the existing plant rooms were unlikely tohave much spare room.

o A fact given in favour of replacing the steam system was the high cost of condensate pipereplacement. This statement tells us that corrosion was taking place, of which the commonest

cause is dissolved gases, which can be removed physically or by chemical treatment. Removingthe system because of this is like replacing a car because the ashtrays are full !

o A disadvantage given for steam systems was the need for insurance inspection of steam/water


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In 1998, a steam trace heating system was installed at one of the UKs largest oil refineries.

BackgroundThe oil company in question is involved in the export of a type of wax product. The wax hasmany uses, such as insulation in electric cabling, as a resin in corrugated paper and as a coatingused to protect fresh fruit.

The wax has similar properties to candle wax. To enable it to be transported any distance in theform of a liquid, it needs to be maintained at a certain temperature. The refinery therefore requireda pipeline with critical tracing.

The project required the installation of a 200 mm diameter product pipeline, which would runfrom a tank farm to a marine terminal out at sea a pipeline of some 4 km in length.

The project began in April 1997, installation was completed in August 1998, and the first successfulexport of wax took place a month later.

Although the refinery management team was originally committed to an electric trace solution,they were persuaded to look at comparative design proposals and costings for both electric andsteam trace options.

The wax applicationThe key parameter for this critical tracing application was to provide tight temperature control ofthe product at 80C, but to have the ability to raise the temperature to 90C for start-up orre-flow conditions. Other critical factors included the fact that the product would solidify attemperatures below 60C, and spoil if subjected to temperatures above 120C.

Steam was available on site at 9 bar g and 180 C, which immediately presented problems ofexcessive surface temperatures if conventional schedule 80 carbon steel trace pipework were tobe used. This had been proposed by the contractor as a traditional steam trace solution for the oilcompany.

The total tracer tube length required was 11.5 km, meaning that the installation of carbon steel

pipework would be very labour intensive, expensive and impractical. With all the joints involvedit was not an attractive option.

However, todays steam tracing systems are highly advanced technologically. Spirax Sarcoandtheir partner on the project, a specialist tracing firm, were able to propose two parallel runs ofinsulated copper tracer tube, which effectively put a layer of insulation between the product pipeand the steam tracer. This enabled the use of steam supply at 9 bar g, without the potential forhot spots which could exceed the critical 120C product limitation.

The installation benefit was that as the annealed ductile steam tracer tubing used was available incontinuous drum lengths, the proposed 50 m runs would have a limited number of joints, reducingthe potential for future leaks from connectors.

This provided a reliable, low maintenance solution.

After comprehensive energy audit calculations, and the production of schematic installationdrawings for costing purposes, together with some careful engineering, the proposal was to usethe existing 9 bar g distribution system with 15 mm carbon steel pipework to feed the tracingsystem, together with strainers and temperature controls. Carbon steel condensate pipework wasused together with lightweight tracing traps which minimised the need for substantial fabricatedsupports.

The typical tracer runs would be 50 m of twin isolated copper tracer tubing, installed at the 4 and8 oclock positions around the product pipe, held to the product pipeline with stainless steelstrap banding at 300 mm intervals.

The material and installation costs for steam trace heating were about 30% less than the electric


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1.2.12


tracing option. In addition, ongoing running costs for the steam system would be a fraction ofthose for the electrical option.

Before the oil company management would commit themselves to a steam tracing system, theynot only required an extended product warranty and a plant performance guarantee, but alsoinsisted that a test rig should be built to prove the suitability of the self- acting controlled tracer forsuch an arduous application.

Spirax Sarco were able to assure them of the suitability of the design by referral to an existinginstallation elsewhere on their plant, where ten self-acting controllers were already installed andsuccessfully working on the trace heating of pump transfer lines.

The oil company was then convinced of the benefits of steam tracing the wax product line andwent on to install a steam tracing system.

Further in-depth surveys of the 4 km pipeline route were undertaken to enable full installationdrawings to be produced. The company was also provided with on-site training for personnel oncorrect practices and installation procedures.

After installation the heat load design was confirmed and the product was maintained at the

Fig. 1.2.4

Lagging

Wax

Steam

required 80C.

The oil company executives were impressed with the success of the project and chose to installsteam tracing for another 300 m long wax product line in preference to electric tracing, eventhough they were initially convinced that electric tracing was the only solution for criticalapplications.


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Questions

1. How does the cost of upgrading a steam system compare with installing a decentralisedgas fired system ?

a| It costs the same to upgrade the steam system.

b| It costs twice as much to upgrade the steam system.

c| It costs 75% as much to upgrade the steam system.

d| It costs half as much to upgrade the steam system.

2. Which of the following uses for steam could be found in a hospital ?

a| Space heating.

b| Sterilisation.

c| Cooking.

d| All of the above.

3. Which of the following statements is true ?

a| Steam creates a temperature gradient along the heat transfer surface,ensuring consistent product quality.

b| Steam gives up its heat at a constant temperature without a gradient along theheat transfer surface, ensuring consistent product quality.

c| High temperature oils offer a constant temperature along theheat transfer surface, which leads to poor product quality.

d| High temperature oils can be directly injected into the product to be heated.

4. A hot water calorifier can occupy much of a plant room. How much floor space does amodern steam to hot water packaged unit need if it is rated at 1200 kW ?

a| 0.7 m

b| 7.0 m

c| 1.2 m

d| 12 m

5. Why is steam inexpensive to produce ?

a| Steam boilers can use a variety of fuels.

b| Steam boilers can utilise the heat from returned condensate.

c| Steam boilers can be automated.

d| All of the above.

6. Which of the following statements best describes steam tracing ?

a| Steam is injected into the process pipe to keep the contents moving.

b| An electric jacket is used to heat the process piping.

c| A steam tracer is a small steam pipe which runs along the outside of a process pipe.

d| A tracer is a small water filled pipe which runs along the outside of a process pipe.

1:c,2:d,3:b,4:a,5:d,6:cAnswers


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1.2.14



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The Steam and Condensate Loop Module 1.3Block 1 Introduction


Module 1.3



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The Steam and Condensate Loop Module 1.3

1.3.2



This Module ofThe Steam and Condensate Loop is intended to give a brief, non-technical overviewof the steam plant. It offers an overall explanation of how the different parts of the steam plantrelate to each other - and represents useful reading for anyone who is unfamiliar with the topic,prior to progressing to the next Block, or, indeed, before undertaking any form of detailed study

of steam theory or steam plant equipment.

The boiler house

The boilerThe boiler is the heart of the steam system. The typical modern packaged boiler is powered by aburner which sends heat into the boiler tubes.

The hot gases from the burner pass backwards and forwards up to 3 times through a series oftubes to gain the maximum transfer of heat through the tube surfaces to the surrounding boilerwater. Once the water reaches saturation temperature (the temperature at which it will boil at thatpressure) bubbles of steam are produced, which rise to the water surface and burst. The steam is

released into the space above, ready to enter the steam system. The stop or crown valve isolatesthe boiler and its steam pressure from the process or plant.

Fig. 1.3.1 Typical heat path through a smoke tube shell boiler

If steam is pressurised, it will occupy less space. Steam boilers are usually operated under pressure,

so that more steam can be produced by a smaller boiler and transferred to the point of use usingsmall bore pipework. When required, the steam pressure is reduced at the point of use.As long as the amount of steam being produced in the boiler is as great as that leaving the boiler,the boiler will remain pressurised. The burner will operate to maintain the correct pressure. Thisalso maintains the correct steam temperature, because the pressure and temperature of saturatedsteam are directly related.

The boiler has a number of fittings and controls to ensure that it operates safely, economically,efficiently and at a consistent pressure.

FeedwaterThe quality of water which is supplied into the boiler is important. It must be at the correcttemperature, usually around 80C, to avoid thermal shock to the boiler, and to keep it operatingefficiently. It must also be of the correct quality to avoid damage to the boiler.

Steam at 150C

3rd Pass (tubes)

2nd Pass (tubes)

1st Pass (furnace tube(s))

400C

600C

200C

350C


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Ordinary untreated potable water is not entirely suitable for boilers and can quickly cause themto foam and scale up. The boiler would become less efficient and the steam would become dirtyand wet. The life of the boiler would also be reduced.

The water must therefore be treated with chemicals to reduce the impurities it contains.

Both feedwater treatment and heating take place in the feedtank, which is usually situated highabove the boiler. The feedpump will add water to the boiler when required. Heating the water inthe feedtank also reduces the amount of dissolved oxygen in it. This is important, as oxygenatedwater is corrosive.

Blowdown

Chemical dosing of the boiler feedwater will lead to the presence of suspended solids in theboiler. These will inevitably collect in the bottom of the boiler in the form of sludge, and areremoved by a process known as bottom blowdown. This can be done manually - the boilerattendant will use a key to open a blowdown valve for a set period of time, usually twice a day.

Other impurities remain in the boiler water after treatment in the form of dissolved solids. Theirconcentration will increase as the boiler produces steam and consequently the boiler needs to beregularly purged of some of its contents to reduce the concentration. This is called control of totaldissolved solids (TDS control). This process can be carried out by an automatic system which useseither a probe inside the boiler, or a small sensor chamber containing a sample of boiler water, tomeasure the TDS level in the boiler. Once the TDS level reaches a set point, a controller signalsthe blowdown valve to open for a set period of time. The lost water is replaced by feedwater with

a lower TDS concentration, consequently the overall boiler TDS is reduced.

Level controlIf the water level inside the boiler were not carefully controlled, the consequences could becatastrophic. If the water level drops too low and the boiler tubes are exposed, the boiler tubescould overheat and fail, causing an explosion. If the water level becomes too high, water couldenter the steam system and upset the process.

For this reason, automatic level controls are used. To comply with legislation, level control systemsalso incorporate alarm functions which will operate to shut down the boiler and alert attention ifthere is a problem with the water level. A common method of level control is to use probes whichsense the level of water in the boiler. At a certain level, a controller will send a signal to the

feedpump which will operate to restore the water level, switching off when a predetermined levelis reached. The probe will incorporate levels at which the pump is switched on and off, and atwhich low or high level alarms are activated. Alternative systems use floats.

Fig. 1.3.2 A sophisticated feedtank system where

the water is being heated by steam injection


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1.3.4


It is a legal requirement in most countries to have two independent low level alarm systems.

The flow of steam to the plant

When steam condenses, its volume is dramatically reduced, which results in a localised reductionin pressure. This pressure drop through the system creates the flow of steam through the pipes.

The steam generated in the boiler must be conveyed through the pipework to the point where itsheat energy is required. Initially there will be one or more main pipes or steam mains which carrysteam from the boiler in the general direction of the steam using plant. Smaller branch pipes canthen distribute the steam to the individual pieces of equipment.

Steam at high pressure occupies a lower volume than at atmospheric pressure. The higher thepressure, the smaller the bore of pipework required for distribution of a given mass of steam.

Steam quality

It is important to ensure that the steam leavingthe boiler is delivered to the process in the rightcondition. To achieve this the pipework whichcarries the steam around the plant normallyincorporates strainers, separators and steamtraps.

A strainer is a form of sieve in the pipeline.It contains a mesh through which the steammust pass. Any passing debris will be retainedby the mesh. A strainer should regularly becleaned to avoid blockage. Debris should beremoved from the steam flow because it can bevery damaging to plant, and may alsocontaminate the final product.

High alarm

Controllers

Boiler shell

Second low alarmFirst low alarm

Protectiontubes

Pump on

Pump off

Fig. 1.3.3 Typical boiler level control/ alarm configuration

Fig. 1.3.4 Cut section of a strainer


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o Condensate does not transmit heat effectively. A film of condensate inside plant will reducethe efficiency with which heat is transferred.

o When air dissolves into condensate, it becomes corrosive.

o Accumulated condensate can cause noisy and damaging waterhammer.

o Inadequate drainage leads to leaking joints.

A device known as a steam trap is used to release condensate from the pipework whilst preventingthe steam from escaping from the system. It can do this in several ways:

o A float trap uses the difference in density between steam and condensate to operate a valve. Ascondensate enters the trap, a float is raised and the float lever mechanism opens the main valveto allow condensate to drain. When the condensate flow reduces the float falls and closes themain valve, thus preventing the escape of steam.

o Thermodynamic traps contain a disc which opens to condensate and closes to steam.

o In bimetallic thermostatic traps, a bimetallic element uses the difference in temperature betweensteam and condensate to operate the main valve.

o In balanced pressure thermostatic traps, a small liquid filled capsule which is sensitive to heatoperates the valve.

Once the steam has been employed in the process, the resulting condensate needs to be drainedfrom the plant and returned to the boiler house. This process will be considered later in this Module.

Pressure reductionAs mentioned before, steam is usually generated at high pressure, and the pressure may have to

be reduced at the point of use, either because of the pressure limitations of the plant, or thetemperature limitations of the process.

This is achieved using a pressure reducing valve.

Fig. 1.3.5 Cut section of a separatorshowing operation

Air to atmospherevia an air vent

The steam should be as dry as possible to ensureit is carrying heat effectively. A separator is a bodyin the pipeline which contains a series of platesor baffles which interrupt the path of the steam.The steam hits the plates, and any drops ofmoisture in the steam collect on them, beforedraining from the bottom of the separator.

Steam passes from the boiler into the steammains. Initially the pipework is cold and heatis transferred to it from the steam. The airsurrounding the pipes is also cooler than thesteam, so the pipework will begin to lose heat tothe air. Insulation fitted around the pipe willreduce this heat loss considerably.

When steam from the distribution system entersthe steam using equipment the steam will againgive up energy by: a) warming up the equipment

and b) continuing to transfer heat to the process. As steam loses heat, it turns back into water.Inevitably the steam begins to do this as soon asit leaves the boiler. The water which forms isknown as condensate, which tends to run to thebottom of the pipe and is carried along with thesteam flow. This must be removed from thelowest points in the distribution pipework forseveral reasons:

Steam out

Steam in

Condensate to drainvia a float trap


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1.3.6


Steam at the point of use

A large variety of steam using plant exists. A few examples are described below:

o Jacketed pan - Large steel or copper pans used in the food and other industries to boilsubstances - anything from prawns to jam. These large pans are surrounded by a jacket filledwith steam, which acts to heat up the contents.

o Autoclave -A steam-filled chamber used for sterilisation purposes, for example medicalequipment, or to carry out chemical reactions at high temperatures and pressures, for examplethe curing of rubber.

o Heater battery - For space heating, steam is supplied to the coils in a heater battery. The air tobe heated passes over the coils.

o Process tank heating -A steam filled coil in a tank of liquid used to heat the contents to thedesired temperature.

o Vulcaniser -A large receptacle filled with steam and used to cure rubber.

o Corrugator -A series of steam heated rollers used in the corrugation process in the productionof cardboard.

o Heat exchanger - For heating liquids for domestic/industrial use.

Control of the processAny steam using plant will require some method to control the flow of steam. A constant flow ofsteam at the same pressure and temperature is often not what is required a gradually increasingflow will be needed at start-up to gently warm the plant, and once the process reaches thedesired temperature, the flow must be reduced.

Control valves are used to control the flow of steam. The actuator, see Figure 1.3.6, is the devicethat applies the force to open or close the valve. A sensor monitors conditions in the process, andtransmits information to the controller. The controller compares the process condition with the

set value and sends a corrective signal to the actuator, which adjusts the valve setting.

Fig. 1.3.6 A pneumatically operated two port control valve

Valve stem

Valve plug

Actuator

Valve

Springs

Diaphragm

Movement


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A variety of control types exist:

o Pneumatically actuated valves - Compressed air is applied to a diaphragm in the actuator toopen or close the valve.

o Electrically actuated valves -An electric motor actuates the valve.

o Self-acting - There is no controller as such - the sensor has a liquid fill which expands and

contracts in response to a change in process temperature. This action applies force to open orclose the valve.

Condensate removal from plant

Often, the condensate which forms will drain easily out of the plant through a steam trap. Thecondensate enters the condensate drainage system. If it is contaminated, it will probably be sentto drain. If not, the valuable heat energy it contains can be retained by returning it to the boilerfeedtank. This also saves on water and water treatment costs.

Sometimes a vacuum may form inside the steam using plant. This hinders condensate drainage,but proper drainage from the steam space maintains the effectiveness of the plant. The condensate

may then have to be pumped out.Mechanical (steam powered) pumps are used for this purpose. These, or electric powered pumps,are used to lift the condensate back to the boiler feedtank.

A mechanical pump, see Figure 1.3.7, is shown draining an item of plant. As can be seen, thesteam and condensate system represents a continuous loop.

Once the condensate reaches the feedtank, it becomes available to the boiler for recycling.

Control valve

Steam

Condensate collecting receiver

Heated medium

Condensate returns to the feedtank

Plant

CondensateCondensate Steam

Mechanical pump

Fig. 1.3.7 Condensate recovery and return

Air

Energy monitoringIn todays energy conscious environment, it is common for customers to monitor the energyconsumption of their plant.

Steam flowmeters are used to monitor the consumption of steam, and used to allocate costs toindividual departments or items of plant.


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Questions

1. What is the purpose of the multi-flue passes in a boiler ?

a| To reduce the amount of flue gases exhausted

b| To help produce drier steam

c| To provide more even generation of steam bubbles

d| To give a greater heat transfer area to the water

2. What is the purpose of the boiler feedtank ?

a| To store chemically treated water for the boiler

b| To provide a reservoir of hot water for the boiler

c| To collect condensate returning from the plant

d| All of the above

3. The boiler feedtank is heated to approximately what temperature ?

a| 80C

b| 20C

c| Steam temperature

d| It isnt heated, all heating takes place in the boiler

4. What is the purpose of boiler bottom blowdown ?

a| To remove total dissolved solids in the boiler water

b| To remove separated out oxygen

c| To dilute the boiler water to reduce TDS

d| To remove solids which collect in the bottom of the boiler

5. What is used to remove suspended water particles in a steam main ?

a| A separator and steam trap

b| A strainer and steam trap

c| A strainer

d| A reducing valve

6. Which of the following is the purpose of a boiler automatic level control ?

a| To provide TDS control

b| To maintain a specified level of water

c| To comply with legislation

d| To take corrective action if the boiler alarms sound

Documents

SpiraxSarco-B1-Introduction