Chapter - Steam Distribution and Utilization

STEAM DISTRIBUTION AND UTILIZATION 1. INTRODUCTION ................................................................................................................. 1 1.1 Why do we use steam? ...................................................................................................... 1 1.2 What is steam? .................................................................................................................... 3 1.3 Steam quality...................................................................................................................... 8

2. STEAM DISTRIBUTION SYSTEM ............................................................................... 8 2.1 What is the steam distribution system? ........................................................................... 8 2.2 Pipes................................ ............................................................. ................................................................................................. .................................................................... .. 10 2.3 Drain points....................................................................................................................... 16 2.4 Branch lines ...................................................................................................................... 17 2.5 Strainers ............................................................................................................................. 19 2.6 Filters ................................................................................................................................. 23 2.7 Separators .......................................................................................................................... 25 2.8 Steam traps ........................................................................................................................ 27 2.9 Air vents ............................................................................................................................ 36 2.10 Condensate recovery ..................................................................................................... 40 2.11 Insulation of steam pipelines and hot process equipments ....................................... 43

3. ASSESSMENT OF STEAM DISTRIBUTION SYSTEM ....................................... 47 3.1 Assessment of steam traps .............................................................................................. 47 3.2 Assessment of heat losses from un-insulated surfaces ................................................ 49 3.3 Assessment of savings from condensate recovery ....................................................... 52

4. ENERGY EFFICIENCY OPPORTUNITIES .............................................................. 54 5. OPTION CHECKLIST ...................................................................................................... 65 6. WORKSHEETS................................ ............................................................ ..................................................................... ....................................................... .............. 66 7. REFERENCES ..................................................................................................................... 67

1. INTRODUCTION This chapter explains what steam is, its properties and why we use steam.

1.1 Why do we use steam? Steam1 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, our food, textile, chemical, medical, power, heating and transport industries could not exist or perform as they do. Steam provides a means of transporting controllable amounts of energy from a central, automated boiler house, where it can be efficiently and 1

This section is a summary of Module 1.1 Steam – The Energy Fluid, In: Spirax Sarco Learning Centre, Block 1, ‘Introduction’. www.spiraxsarco.com www.spiraxsarco.com

economically generated, to the point of use. Therefore as steam moves around a plant it can equally be considered to be the transport and provision of 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 production to space heating and process applications. Reasons for using steam include: Steam is efficient and economic to generate Steam can easily and cost effectively be distributed to the po int of use Steam is easy to control Energy is easily transferred to the process The modern steam plant is easy to manage Steam is flexible § § § § § §

The alternatives to steam include water and thermal fluids such as high temperature oil. Each method has its advantages and disadvantages, as shown in Table 1. Table 1. Comparison of heating media with steam1 Steam High heat content Latent heat approximately 2 100 kJ/kg Inexpensive Some water treatment costs Good heat transfer coefficients High pressure required for high temperatures No circulating pumps required Small pipes

Easy to control with two way valves

Temperature breakdown is easy through a reducing valve Steam traps required Conden sate to be handled handle d Flash steam available Boiler blowdown necessary necessary Water treatment required to prevent corrosion Reasonable pipework required No fire risk System very flexible

Hot water wate r Moderate heat content Specific heat 4.19 kJ/kg°C Inexpensive Only occasional dosing Moderate coefficien ts

High temperature oils Poor heat content Specific heat often 1.69-2.93 kJ/kg°C Expensive

High pressure needed for high temperatures Circulating pumps required Large pipes

Relatively poor Coefficients Low pressures only to get high temperatures Circulating pumps required Even larger pipes

More complex to control three way valves or differential pressure valves may be required Temperature breakdown more difficult No steam traps required No condensate handling No flash steam No blowdown necessary Less corrosion

More complex to control three way valves or differential pressure valves may be required. Temperature breakdown more difficult No steam traps required requ ired No condensate handling No flash steam No blowdown necessary Negligible corrosion

Searching medium, Very searching medium, welded or flanged joints usual welded or flanged joints usual No fire risk Fire risk System less flexible System inflexible inflexibl e

1.2 What is steam? A better understanding of the properties of steam may be achieved by understanding the general molecular and atomic structure of matter and applying this knowledge to ice, water and steam. 2 A molecule is the smallest amount of any element or compound substance still poss essing all the chemical properties of that substance which can exist. Molecules are made up of even smaller particles called atoms, which define the basic elements such as hydrogen and oxygen. The specific combinations of these atomic elements provide comp ound substances. One such compound is represented by the chemical formula H2O, having molecules made up of two atoms of hydrogen and one atom of oxygen. The reason water is so plentiful on the earth is because hydrogen and oxygen are amongst the most abundant elements in the universe. Carbon is another element of significant abundance, and is a key component in all organic matter. Most mineral substances can exist in the three physical states (solid, liquid and vapour), which are referred to as phases. In the case of H2O, the terms ice, water and steam are used to denote the three phases respectively. The molecular structure of ice, water, and steam is still not fully understood, but it is convenient to consider the molecules as bonded together by electrical charges (referred to as the hydrogen bond). The degree of excitation of the molecules determines the physical state (or phase) of the substance 1.2.1 Triple point

All the three phases of a particular substance can only coexist in equilibrium at a certain temperature and pressure, and this is known as its triple point. The triple point of H2 O, where the three phases of ice, water and steam are in equilibrium, occurs at a temperature of 273.16 K and an absolute pressure pressure of 0.00 0. 0061 6112 12 bar. This pressure pressure is very close to a perfect vacuum. If the pressure is reduced further at this temperature, the ice, instead of melting, sublimates directly into steam. Ice In ice, the molecules are locked together in an orderly lattice type structure and can only vibrate. In the solid phase, the movement of molecules in the lattice is a vibration about a mean bonded position where the molecules are less than one molecular diameter apart. The continued addition of heat causes the vibration to increase to such an extent that some molecules will eventually break away from their neighbors, and the solid starts to melt to a liquid state (always at the same temperature of 0°C whatever the pressure). Heat that breaks the lattice bonds to produce the phase change while not increasing the temperature of the ice, is referred to as enthalpy of melting or heat of fusion. This phase change phenomenon is reversible when freezing occurs with the same amount of heat being released back to the surroundings. For most substances, the density decreases as it changes from the solid to the 2

This section is taken fromModule 2.2 What is Steam?, In: Spirax Sarco Learning Centre, Block 2, ‘ Steam Engineering Engineering Principles and Heat Transfer’. www.spiraxsarco.com

liquid phase. However, H2 O is an exception to this rule as its density increases upon melting, which is why ice floats on water. Water In the liquid phase, the molecules are free to move, but are still less than one molecular diameter apart due to mutual attraction, and collisions occur frequently. More heat increases molecular agitation and collision, raising the temperature of the liquid up to its boiling temperature. Steam As the temperature increases and the water approaches its boiling condition, some molecules attain enough kinetic energy to reach velocities that allow them to momentarily escape from the liquid into the space above the surface, before falling back into the liquid. Further heating causes greater excitation and the number of molecules with enough energy to leave the liquid increases. As the water is heated to its boiling point, bubbles of steam form within it and rise to break through the surface. Considering the molecular structure of liquids and vapours, it is logical that the density of steam is much less than that of water, because the steam molecules are further apart from one another. The space immediately above the water surface thus becomes filled with less dense steam molecules. When the number of molecules leaving the liquid surface is more than those re-entering, the water freely evaporates. At this point it has reached boiling point or its saturation temperature, as it is saturated with heat energy. If the pressure remains constant, adding more heat does not cause the temperature to rise any further but causes the water to form saturated steam. The temperature of the boiling water and saturated steam within the same system is the same, but the heat energy per unit mass is much greater in the steam. At atmospheric pressure the saturation temperature is 100°C. However, if the pressure is increased, this will allow the addition of more heat and an increase in temperature without a change of phase. Therefore, increasing the pressure effectively increases both the enthalpy of water, and the saturation temperature. The relationship between the saturation temperature and the pressure is known as the steam saturation curve (Figure 1).

Figure 1: Steam Saturation Curve

(Spirax Sarco)

Water and steam can coexist at any pressure on this curve, both being at the saturation temperature. Steam at a condition above the saturation curve is known as superheated steam: Temperature above saturation temperature is called the degree of superheat of the steam Water at a condition below the curve is called sub-saturated water. § §

If the steam is able to flow from the boiler at the same rate that it is produced, the addition of further heat simply increases the rate of production. If the steam is restrained from leaving the boiler, and the heat input rate is maintained, the energy flowing into the boiler will be greater than the energy flowing out. This excess energy raises the pressure, in turn allowing the saturation temperature to rise, as the temperature of saturated steam correlates to its pressure. 1.2.2 Enthalpy

Enthalpy of water, liquid enthalpy or sensible heat (h f ) of water This is the heat energy required to raise the temperature of water from a datum point of 0°C to its current temperature. At this reference state of 0°C, the enthalpy of water has been arbitrarily set to zero. The enthalpy of all other states can then be identified, relative to this easily accessible reference state. Sensible heat was the term once used, because the heat added to the water produced a change in temperature. However, the accepted terms these days are liquid enthalpy or enthalpy of water. At atmospheric pressure (0 bar g), water boils at 100°C, and 419 kJ of energy are required to heat 1 kg of water from 0°C to its boiling temperature of 100°C. It is from these figures that the value for the specific heat capacity of water (CP ) of 4.19 kJ/kg °C is derived for most calculations between 0°C and 100°C. Enthalpy of evaporation or latent heat (hf g ) This is the amount of heat required to change the state of water at its boiling temperature, into steam. It involves no change in the temperature of the steam/water mixture, and all the energy is used to change the state from liquid (water) to vapour (saturated steam). The old term latent heat is based on the fact that although heat was added, there was no change in temperature. However, the accepted term is now enthalpy of evaporation. Like the phase change from ice to water, the process of evaporation is also reversible. The same amount of heat that produced the steam is released back to its surroundings during condensation, when steam meets any surface at a lower temperature. This may be considered as the useful portion of heat in the steam for heating purposes, as it is that portion of the total heat in the steam that is extracted when the steam condenses back to water. Enthalpy of saturated steam, or total heat o f saturated steam This is the total energy in saturated steam, and is simply the sum of the enthalpy of water and the enthalpy of evaporation.

hg = hf + hf g

Where: hg = Total enthalpy of saturated steam (Total heat) (kJ/kg)

hf = Liquid enthalpy (Sensible heat) (kJ/kg) hfg = Enthalpy of evaporation (Latent heat) (kJ/kg) The enthalpy (and other properties) of saturated steam can easily be referenced using the tabulated results of previous experiments, known as steam tables. The steam tables list the properties of steam at varying pressures. They are the results of actual tests carried out on steam. 1.2.3 Dryness fraction

Steam with a temperature equal to the boiling point at that pressure is known as dry saturated steam. However, to produce 100 percent dry steam in an industrial boiler designed to produce saturated steam is rarely possible, and the steam will usually contain droplets of water. In practice, because of turbulence and splashing, as bubbles of steam break through the water surface, the steam space contains a mixture of water droplets and steam. If the water content of the steam is 5 percent by mass, then the steam is said to be 95 percent dry and has a dryness fraction of 0.95. The actual enthalpy of evaporation of wet steam is the product of the dryness fraction ( x) and the specific enthalpy (hf g) from the steam tables. Wet steam will have lower usable heat energy than dry saturated steam.

Actual enthalpy of evaporation = hfg x

Therefore:

Actual total enthalpy = hf

+ hfg x

Because the specific volume of water is several orders of magnitude lower than that of steam, the droplets of water in wet steam will occupy negligible space. Therefore the specific volume of wet steam will be less than dry steam:

Actual specific volume =

vg x

Where: vg is the specific volume of dry saturated steam

1.2.4 The Steam phase diagram

The data provided in the steam tables can also be expressed in a graphical form. Figure 2 illustrates the relationship between the enthalpy and the temperature at various different pressures, and is known as a phase diagram.

Figure 2. Temperature Enthalpy Phase Diagram

(Spirax Sarco) As water is heated from 0°C to its saturation temperature, its condition follows the saturated liquid line until it has received all of its liquid enthalpy, hf , (A - B).If further heat continues to be added, it then changes phase to saturated steam and continues to increase in enthalpy while remaining at saturation temperature, hfg, (B - C). As the steam/water mixture increases in dryness, its condition moves from the saturated liquid line to the saturated vapour line. Therefore at a point exactly halfway between these two states, the dryness fraction (x) is 0.5. Similarly, on the saturated vapour line the steam is 100 percent dry. Once it has received all of its enthalpy of evaporation, it reaches the saturated vapour line. If it continues to be heated after this point, the temperature of the steam will begin to rise as superheat is imparted (C - D). The saturated liquid and saturated vapour lines enclose a region in which a steam /water mixture exists - wet steam. In the region to the left of the saturated liquid line only water exists, and in the region to the right of the saturated vapour line only superheated steam exists. The point at which the saturated liquid and saturated vapour lines meet is known as the critical point. As the pressure increases towards the critical point the enthalpy of evaporation decreases, until it becomes zero at the critical point. This suggests that water changes directly into saturated steam at the critical point. Above the critical point only gas may exist. The gaseous state is the most diffuse state in which the molecules have an almost unrestricted motion, and the volume increases without limit as the pressure is reduced. The critical point is the highest temperature at which a liquid can exist. Any compression at constant temperature above the critical point will not produce a phase change. Compression at co nstant temperature below the critical point however, will result in liquefaction of the vapour as it passes from the superheated region to the wet steam region. The critical point occurs at 374.15 o C and 221.2 bara for steam. Above this pressure the steam is termed supercritical and no well-defined boiling point applies.

1.3 Steam quality Steam should be available at the point of use:3 In the correct quantity to ensure that a sufficient heat flow is provided for heat transfer At the correct temperature and pressure, or performance will be affected Free from air and incondensable gases which act as a barrier to heat transfer Clean, as scale (e.g. rust or carbonate deposit) or dirt have the effect of increasing the rate of erosion in pipe bends and the small orifices of steam traps and valves Dry, as the presence of water droplets in steam reduces the actual enthalpy of evaporation, and also leads to the formation of scale on the pipe walls and heat transfer surface. § § § §

§

2. STEAM DISTRIBUTION SYSTEM This section describes the steam distribution system and its various components.

2.1 What is the steam distribution system? The steam distribution system4 is the essential link between the steam generator and the steam user. There are various methods to carry steam from a central source to the point of use. The central source might be a boiler house or the discharge from a co-generation plant. The boilers may burn primary fuel, or be waste heat boilers using exhaust gases from high temperature processes, engines or even incinerators. Whatever the source, an efficient steam distribution system is essential if steam of the right quality and pressure is to be supplied, in the right quantity, to the steam using equipment. Installation and maintenance of the steam system are important issues, and must be considered at the design stage. An understanding of the basic steam circuit or ‘steam and condensate loop’ is required (see Figure 3). As steam condenses in a process, flow is induced in the supply pipe. Condensate has a very small volume compared to the steam, and this causes a pressure drop, which causes the steam to flow through the pipes. The steam generated in the boiler must be conveyed through pipework to the point where its heat energy is required. Initially there will be one or more main pipes, or 'steam mains', which carry steam from the boiler in the general direction of the steam using plant. Smaller branch pipes can then carry the steam to the individual pieces of equipment. When the boiler main isolating valve (sometimes referred to as the ‘crown’ valve) is opened, steam immediately passes from the boiler into and along the steam mains to the points at lower pressure. The pipework is initially cooler than the steam, so heat is transferred from the steam to the pipe. The air surrounding the pipes is also cooler than the steam, so the pipework will begin to transfer heat to the air. 3

More details on steam quality criteria can be found in Module 2.4 Steam Quality, In: Spirax Sarco Learning Centre, Block 2, ‘Steam Engineering Principles and Heat Transfer’. ww w.spiraxsarco.com 4 Section 2.1 is a summary of Module 10.1 Introduction to Steam Distribution, In: Spirax Sarco Learning Centre, Block 10, Steam Distribution. www.spiraxsarco.com

Figure 3. A Typical Steam Circuit (Spirax Sarco)

Steam on contact with the cooler pipes will begin to condense immediately. On start-up of the system, the condensing rate will be at its maximum, as this is the time where there is maximum temperature difference between the steam and the pipework. This condensing rate is commonly called the ‘starting load’. Once the pipework has warmed up, the temperature difference between the steam and pipework is minimal, but some condensation will occur as the pipework still continues to transfer heat to the surrounding air. This condensing rate is commonly called the ‘running load’. The resulting condensation (condensate) falls to the bottom of the pipe and is carried along by the steam flow and assisted by gravity, due to the gradient in the steam main that should be arranged to fall in the direction of steam flow. The condensate will then have to be drained from various strategic points in the steam main. When the valve on the steam pipe serving an item of steam using plant is opened, steam flowing from the distribution system enters the plant and again comes in contact with cooler surfaces. The steam then transfers its energy in warming up an equipment and product (starting load), and, when up to temperature, continues to transfer heat to the process (running load). There is now a continuous supply of steam from the boiler to satisfy the connected load and to maintain this supply more steam must be generated. In order to do this, more water (and fuel to heat this water) is supplied to the boiler to make up for the water which has previously been evaporated into steam. The condensate formed in both the steam distribution pipework and in the process equipment is a convenient supply of useable hot boiler feedwater. Although it is important to remove this condensate from the steam space, it is a valuable commodity and should not be allowed to run to waste. Returning all condensate to the boiler feedtank closes the steam energy loop, and should be practiced wherever practical.

The distribution pressure of steam is influenced by a number of factors, but is limited by: The maximum safe working pressure of the boiler The minimum pressure required at the plant § §

As steam passes through the distribution pipework, it will inevitably lose pressure due to: Frictional resistance within the pipework Condensation within the pipework as heat is transferred to the environment. Therefore allowance should be made for this pressure loss when deciding upon the initial distribution pressure. § §

A kilogram of steam at a higher pressure occupies less volume than at a lower pressure. It follows that, if steam is generated in the boiler at high pressure and also distributed at a high pressure, then the size of the distribution mains will be smaller than that for the same heat load. Generating and distributing steam at higher pressure offers three important advantages: The thermal storage capacit y of the boiler is increased, helping it to cope more efficiently with fluctuating loads, minimizing the risk of producing wet and dirty steam. Smaller bore steam mains are required, resulting in lower capital cost, for materials such as pipes, flanges, supports, insulation and labour. Smaller bore steam mains cost less to insulate. §

§

§

Having distributed at a high pressure, it will be necessary to reduce the steam pressure to each zone or point of use in the system in order to correspond with the maximum pressure required by the application. Local pressure reduction to suit individual plant will also result in drier steam at the point of use. The most important components of a steam distribution system are described in the next sections: Pipes (2.2) Drain points (2.3) Branch lines (2.4) Strainers (2.5) Filters (2.6) Separators (2.7) Steam traps (2.8) Air vents (2.9) § § § § § § § §

2.2 Pipes This section describes the pipework of a steam system. 5 2.2.1 Pipe material

Pipes for steam systems are commonly manufactured from carbon steel to ANSI B 16.9 Al06. The same material may be used for condensate lines, although copper tubing is preferred in some industries. For high temperature superheated steam mains, additional alloying elements, such as

5

Section 2.2 is a summary of information in Module 10.2 Pipes and Pipe Sizing, and Module 10.3 Steam Mains and Drainage. In: Spirax Sarco Learning Centre, Block 10, ‘ Steam Distribution’. www.spiraxsarco.com

chromium and molybdenum, are included to improve tensile strength and creep resistance at high temperatures. Typically, pipes are supplied in 6- meter lengths. 2.2.2 Pipeline sizing

The objective of the steam distribution system is to supply steam at the correct pressure to the point of use. Pipeline sizing is an important factor. Oversized pipework means: Pipes, valves, fittings, etc. will be more expensive than necessary. Higher installation costs will be incurred, including support work, insulation, etc. For steam pipes a greater vo lume of condensate will be formed due to the greater hea t loss. This in turn means that either more steam trapping is required or wet steam is delivered to the point of use. § § §

Undersized pipework means: A lower pressure may only be available at the point of use. This may hinder equipment performance due to only lower pressure steam being available. There is a risk of steam starvation. There is a greater risk of erosion, water hammer and noise due to the inherent increase in steam velocity. §

§ §

The require pipeline size can be calculated based on pressure drop and velocity described below. a) Pipeline sizing based on pressure drop Pressure drop through the distribution system is an important feature. In practice whether for water pipes or steam pipes, a balance is drawn between pipe size and pressure loss. Pressure drop as a general rule, should not exceed 0.1 bar/50 m. Pipe sizing can be computed using the chart in Figure 4. Those who prefer tables instead of graphs can use Table 2 to determine the pipeline size. An example calculation is as follows: Given: § § § §

Inlet pressure P1 = 7 bar g Steam flowrate = 286 kg/h Minimum allowable P2 = 6.6 bar g Length of pipeline = 165 m

Calculate the maximum pressure drop per 100 m

Maximum pressure drop per 100 m = P1 – P 2 x 100 L = (7.0 – 6.6) x 100 165 = 0.24 bar

Determine the pipeline size based on the pressure drop using the nomogram in Figure 4: Select the point on the saturated steam line at 7 bar g, and mark Point A. From point A, draw a horizontal line to the steam flowrate of 286 kg/h, and mark Point B. From point B, draw a vertical line towards the top of the nomogram (Point C). Draw a horizontal line from 0.24 bar/100 m on the pressure loss scale (Line DE). The point at which lines DE and BC cross will indicate the pipe size required. In this case, a § § § § §

40 mm pipe is too small, and a 50 mm pipe would be used.

Figure 4. Steam Pipeline Sizing Chart – pressure drop approach (Spirax Sarco)

Table 2. Saturated Steam Pipeline Capacities in kg/h for Different Velocities, schedule 40 pipe (Spirax Sarco)

Thermal Equipment: Steam distribution and utilization

Figure 5. Steam Pipeline Sizing Chart – velocity approach(Spirax Sarco)

b) Pipeline sizing based on velocity Velocity is an important factor in sizing pipes. As a general rule, a velocity of 25 to 40 m/s is used when saturated steam is the medium. 40 m/s should be considered an extreme limit, as above this, noise and erosion will take place particularly if the steam is wet. Even these velocities can be high in terms of their effect on pressure drop. In longer supply lines, it is often necessary to restrict velocities to 15 m/s to avoid high pressure drops. It is recommended that pipelines over 50 m long are always checked for pressure drop, no matter what the velocity.

Energy Efficiency Guide for Industry in Asia – www.energyefficiencyasia.org

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Superheated steam can be considered as a dry gas and therefore carries no moisture. Consequently there is no chance of pipe erosion due to suspended water droplets, and steam velocities can be as high as 50 to 70 m/s if the pressure drop permits this. Pipe sizing based upon the velocity approach for saturated and superheated steam can be done using the nomogram as shown in Figure 5. Those who prefer tables instead of graphs can use Table 2 to determine the pipeline size. An example calculation is as follows: Given: § § §

Inlet pressure: 7 bar g Steam flowrate: 5000 kg/h Maximum velocity: 25 m/s

Calculate the pipeline size based on velocity using the nomogram in Figure 5 §

§

§

Draw a horizontal line from the saturation temperature line at 7 bar g (Point A) on the pressure scale to the steam mass flowrate of 5 000 kg/h (Point B). From point B, draw a vertical line to the steam velocity of 25 m/s (Point C). From point C, draw a horizontal line across the pipe diameter scale (Point D). A pipe with a bore of 130 mm is required; the nearest commercially available size, 150 mm, would be selected.

2.2.3 Piping layout

The European Standard EN45510, Section 4.12 states that whenever possible, steam mains should be installed with a fall of not less than 1:100 (1 m fall for every 100 m run), in the direction of the steam flo w. This slope will ensure that gravity, as well as the flow of steam, will assist in moving the condensate towards drain points where the condensate may be safely and effectively removed (Figure 6).

Figure 6. Typical Steam Piping Installation (Spirax Sarco)


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2.3 Drain points The drain point6 must ensure that the condensate can reach the steam trap. Careful consideration must therefore be given to the design and location of the drain points. Consideration must also be given to condensate remaining in a steam main at shutdown, when steam flow ceases. Gravity will ensure that the water (condensate) will run along sloping pipework and collect at low points in the system. Steam traps should therefore be fitted to these low points. The amount of condensate formed in a large steam main under start-up conditions is sufficient to require the provision of drain points at intervals of 30m to 50m, as well as natural low points such as at the bottom of rising pipework. In normal operation, steam may flow along the main at speeds of up to 145 km/h, dragging condensate along with it. Figure 7 shows a 15 mm drain pipe connected directly to the bottom of a main.

Figure 7. Trap Pocket Too Small (Spirax Sarco)

Although the 15 mm pipe has sufficient capacity, it is unlikely to capture much of the condensate moving along the main at high speed. This arrangement will be ineffective. A more reliable solution for the removal of condensate is shown in Figure 8. The trap line should be at least 25 to 30 mm from the bottom of the pocket for steam mains up to 100 mm, and at least 50 mm for larger mains. This allows a space below for any dirt and scale to settle. Such dirt and scale can easily be removed if the bottom of the pocket is fitted with a removable flange or blowdown valve.

Figure 8. Properly Sized Trap Pocket(Spirax Sarco) 6

Section 2.3 is taken fromModule 10.3 Steam Mains and Drainage. In: Spirax Sarco Learning Centre, Block 10, ‘ Steam Distribution’. www.spiraxsarco.com


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Recommended drain pocket dimensions are shown in Figure 9 below.

Figure 9. Recommended Drain Pocket Dimensions (Spirax Sarco)

2.4 Branch lines Branch lines 7 are normally much shorter than steam mains. As a general rule, therefore, provided the branch line is not more than 10 metres in length, and the pressure in the main is adequate, it is possible to size the pipe on a velocity of 25 to 40 m/s, and not to worry about the pressure drop.

Figure 10. A branch line (Spirax Sarco) 7

Section 2.4 is taken fromModule 10.3 Steam Mains and Drainage. In: Spirax Sarco Learning Centre, Block 10, Steam Distribution. www.spiraxsarco.com


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2.4.1 Branch line connections

Branch line connections taken from the top of the main line, carry the driest steam (Figure 10). If connections are taken from the side, or even worse from the bottom (as in Figure 11a), they can accept the condensate and debris from the steam main. The result is very wet and dirty steam reaching the equipment, which will affect performance in both the short and long term. The valve in Figure 11b should be positioned as near to the off-take as possible to minimize condensate lying in the branch line, if the plant is likely to be shutdown for any extended periods.

Figure 11a. Incorrect Steam Off-take

(Spirax Sarco)

Figure 11b. Correct Steam Off-take

(Spirax Sarco) 2.4.2 Drop leg

Low points will also occur in branc h lines. The most common is a drop leg close to an isolating valve or a control valve (Figure 12). Condensate can accumulate on the upstream side of the closed valve, and then be propelled forward with the steam when the valve opens again consequently a drain point with a steam trap set is good practice just prior to the strainer and control valve.

Figure 12. Drop Leg Supplying Steam to a Heater(Spirax Sarco)


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2.4.3 Rising Ground and Drainage

There are many occasions when a steam main must run across rising ground, or applications where the contours of the site make it impractical to lay the pipe with the 1:100 fall. In these situations, the condensate must be encouraged to run downhill and against the steam flow. Good practice is to size the pipe on a low steam velocity of not more than 15 m/s, to run the line at a slope of no less than 1:40, and install the drain points at not more than 15 meter intervals (see Figure 13). The objective is to prevent the condensate film on the bottom of the pipe increasing in thickness to the point where droplets can be picked up by the steam flow.

Figure 13. Reverse Gradient on Steam Main (Spirax Sarco)

2.5 Strainers

This section provides an overview of strainers. 8 As the marketplace becomes increasingly competitive, more emphasis has been placed on reducing plant downtime and maintenance. In steam and condensate systems, damage to plant is frequently caused by pipeline debris such as scale, rust, jointing compound, weld metal and other solids, which may find their way into the pipeline system. Strainers are devices which arrest these solids in flowing liquids or gases, and protect equipment from their harmful effects, thus reducing downtime and maintenance. A strainer should be fitted upstream of every steam trap, flow meter and control valve. Strainers can be classified into two main types according to their body configuration; namely the Y-type and the basket type. Typical examples of these types of strainers can be seen in Figure 14.

8

Section 2.5 is taken fromModule 12.4 Strainers. In: Spirax Sarco Learning Centre, Block 12, ‘Pipeline Ancillaries’.

www.spiraxsarco.com


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A Figure 14a. Y-Type Strainer

(Spirax Sarco)

Figure 14b. Basket Type Strainer

(Spirax Sarco)

2.5.1 Y-type strainers

For steam, a Y-type strainer is the usual standard and is almost universa lly used. Its body has a compact cylindrical shape that is very strong and can handle high pressures. It is literally a pressure vessel, and it is not uncommon for Y-type strainers to be able to handle pressures of up to 400 bar g. The use of strainers at these pressures is however complicated by the high temperatures associated with the steam at this pressure; and subsequently exotic materials such as chromemolybdenum steel have to be used. Although there are exceptions, size for size, Y-type strainers have a lower dirt holding capacity than basket strainers, which means that they require more frequent cleaning. On steam systems, this is generally not a problem, except where high levels of rust are present, or immediately after commissioning when large amounts of debris can be introduced. On applications where significant amount of debris are expected, a blowdown valve can usually be fitted in a strainer cap, which enables the strainer to use the pressure of the steam to be cleaned, and without having to shut down the plant. Y-type strainers in horizontal steam or gas lines should be installed so that the pocket is in the horizontal plane (Figure 15a). This stops water collecting in the pocket, helping to prevent water droplets being carried over, which can cause erosion and affect heat transfer processes. On liquid systems however, the pockets should point vertically downwards (Figure 15c). Although it is advisable to install strainers in horizontal lines, this is not always possible, and they can be installed in vertical pipelines if the flow is downwards, in which case the debris is naturally directed into the pocket (Figure 15b). Installation is not possible with upward flow, as the strainer would have to be installed with the opening of the pocket pointing downwards and the debris would fall back down the pipe.


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(a). Steam or gas applications

(b) Flow vertically downwards

(c) Liquid applications

Figure 15. Correct Orientation of Strainers (Spirax Sarco)

2.5.2 Straight and angle type strainers

In addition to Y-type strainers, straight and angle type strainers are used when the geometry of the steam pipework does not suit a Y-type strainer being used. 2.5.3 Basket type strainers

The basket type or pot type strainer is characterized by a vertically orientated chamber, typically larger than that of a Y-type strainer. Size for size, the pressure drop across a basket strainer is less than that across the Y-type as it has a greater free straining area, which makes the basket type strainer the preferred type for liquid applications. As the dirt holding capacity is also greater than in Y-type strainers, the basket type strainer is also used on larger diameter steam pipelines. Basket type strainers can only be installed in horizontal pipelines, and for larger, heavier basket strainers, the base of the strainer needs to be supported. When basket type strainers are used on steam systems, a significant amount of condensate may be formed. Consequently, strainers designed for use in steam systems usually have a drain plug, which can be fitted with a steam trap to remove the condensate. Basket type strainers are commonly found in a duplex arrangement. A second strainer is placed in parallel with the primary strainer, and flow can be diverted through either of the two strainers. This facilitates cleaning of the strainer unit whilst the fluid system is still operating, reducing the downtime for maintenance.


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Figure 16. A Duplex Basket Strainer (Spirax Sarco) 2.5.4 Strainer Screens

There are two types of screens used in strainers: Perforated screens. These are formed by punching a large number of holes in a flat sheet of the required material using a multiple punch. The perforated sheet is then rolled into a tube and spot welded together. These are relatively coarse screens and hole sizes typically range from 0.8 mm to 3.2 mm. Consequently, perforated screens are only suitable for removing general pipe debris. Mesh screens. Fine wire is formed into a grid or mesh arrangement. This is then commonly layered over a perforated screen, which acts as a support cage for the mesh. By using a mesh screen, it is possible to produce much smaller hole sizes than with perforated screens. Hole sizes as small as 0.07 mm are achievable. Subsequently, they are used to remove smaller particles, which would otherwise pass through a perforated screen. Mesh screens are usually specified in terms of ‘mesh’; which represents the number of openings per linear inch of screen, measured from the centerline of the wire. Figure 17 shows a 3-mesh screen. §

§

Figure 17. Examples of a 3-mesh screen (Spirax Sarco)


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2.5.5 Strainer Options

In addition to standard strainers, there are several other options available. Magnetic Inserts A magnetic insert may be placed in a basket type strainer in order to remove small iron or steel debris. Small particles of iron or steel may be present in a fluid where there is wear of iron or steel parts. These particles will pass through even the finest mesh screens, and it is necessary to use a magnetic insert. The insert is designed so that all the fluid passes over the magne t at relatively low velocity and the magnetic element is powerful enough to catch and hold all the metal particles present. The magnetic material is usually encased in an inert material such as stainless steel to prevent corrosion. Self Cleaning Strainers There are number of different types of self-cleaning strainer, which enable the build up of debris on the screen to be removed without shutting down the plant. The cleaning process can be initiated either manually or automatically; furthermore, strainers that are automatically cleaned can usually be set to clean either on a periodic basis, or when the pressure drop across the strainer increases. The most common types are: Mechanical type self-cleaning strainers, which use some form of mechanical scraper or brush, which is raked over the screen surface. It dislodges any debris that is trapped in the screen, causing it to fall down into a collection area at the bottom of the strainer. Backwashing type strainers, which reverse the direction of flow through the screen. A set of valves is changed over so that water is directed across the screen in the reverse direction and out through a flush valve. The fluid dislodges any debris entrained in the screen and carries it out in the backwash fluid to a waste drain. §

§

Temporary Strainers Temporary strainers are designed for protection of equipment and instrumentation during startup periods. The strainer is usually installed between a set of flanges for an initial period after a new plant has been installed.

2.6 Filters Filters are used to remove smaller particles.9 Whilst strainers remove all visible particles in the steam, it is sometimes necessary to remove smaller particles, for example, in the following applications: When there is direct injection of steam into a process, which may cause contamination of the product. Example: In the food industry, and for the sterilization of process equipment in the pharmaceutical industry. Where dirty steam may cause rejection of a product or process batch due to staining or visible particle retention. Example: Sterilizers and paper/board machines. Where minimal particle emission is required from steam humidifiers. Example: Humidifiers used in a “clean” environment. §

§

§

9

Section 2.6 is taken fromModule 12.4 Strainers. In: Spirax Sarco Learning Centre, Block 12, ‘Pipeline Ancillaries’.

www.spiraxsarco.com


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Thermal Equipment: Steam distribution and utilization §

For the reduction of the steam water content, ensuring a dry, saturated supply.

In such 'clean steam' applications, strainers are not suitable and filters must be used. A filter used in a steam system typically consists of a sintered stainless steel filter element. The sintering process produces a fine porous structure in the stainless steel, which removes any particles from fluid passing through it. Filters capable of removing particles as small as 1 /gym are available, conforming to the good practice needs of culinary steam. The fine porous nature of the filter element will create a large pressure drop across the filter than that associated with the same size strainer; this must be given careful consideration when sizing such filters. In addition filters are easily damaged by excessive flow rates, and the manufacturers’ limits should not be exceeded. When the filter is used in steam applications, a separator should be fitted upstream of the filter to remove any droplets of condensate held in suspension. In addition to improving the quality of the steam, this will prolong the life of the filter. A Y-type strainer should also be fitted upstream of the filter to remove all larger particles which would otherwise rapidly block the filter, increase the amount of cleaning required and reduce the life of the filter element. By installing pressure gauges either side of the filter, the pressure drop across the filter can be measured, which can then be used to identify when the filter requires cleaning. An alternative to this is to install a pressure switch on the downstream side of the filter. When the downstream pressure decreases below a set level, an alarm light can be switched on in a control room alerting an operator who can then clean the filter.

Figure 18. A Horizontal In-line Filter (Spirax Sarco)


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2.7 Separators Separators 10 are used to remove suspended water droplets from steam. Wet steam is steam containing a degree of water, and is one of the main concerns in any steam system. It can reduce plant productivity and product quality, and can cause damage to most items of plant and equipment. Whilst careful drainage and trapping can remove most of the water, it will not deal with the water droplets suspended in the steam. To remove these suspended water droplets, separators are installed in steam pipelines. The steam produced in a boiler designed to generate saturated steam is inherently wet. Although the dryness fraction will vary according to the type of boiler, most shell type steam boilers will produce steam with a dryness fraction of between 95 and 98 percent. The water content of the steam produced by the boiler is further increased if priming and carryover occur. There is always a certain degree of heat loss from the distribution pipe, which causes steam to condense. The condensed water molecules will eventually gravitate towards the bottom of the pipe forming a film of water. Steam flowing over this water can raise ripples that can build up into waves. The tips of the waves tend to break off, throwing droplets of condensate into the steam flow. The presence of water in steam can cause a number of problems: As water is an extremely effective barrier to heat transfer, its presence can reduce plant productivity and product quality. Water droplets traveling at high steam velocities will erode valve seats and fittings, a condition known as wiredrawing. The water droplets will also increase the amount of corrosion. Increased scaling of pipework and heating surfaces from the impurities carried in the water droplets. Erratic operation of control valves and flow meters. Failure of valves and flow meters due to rapid wear or water hammer. §

§

§

§ §

Although there are a number of different designs of separator, they all attempt to remove the moisture that remains suspended in the steam flow, which cannot be removed by drainage and steam trapping. There are three types of separator in common use in steam systems: 2.7.1 Baffle type separators

A baffle or vane type separator consists of a number of baffle plates, which cause the flow to change direction a number of times as it passes through the separator body. The suspended water droplets have a greater mass and a greater inertia than the steam; thus, when there is a change in flow direction, the dry steam flows around the baffles and the water droplets collect on the baffles.

10

Section 2.7 is taken fromModule 12.5 Separators. In: Spirax Sarco Learning Centre, Block 12, ‘Pipeline Ancillaries’.

www.spiraxsarco.com


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Furthermore, as the separator has a large cross-sectional area, there is a resulting reduction in the speed of the fluid. This reduces the kinetic energy of the water droplets, and most of them will fall out of suspension. The Condensate collects in the bottom of the separator, where it is drained away through a steam trap.

Figure 19. Baffle Type Separator

(Spirax Sarco) 2.7.2 Cyclonic Type

The cyclonic or centrifugal type separator uses a series of fins to generate high-speed cyclonic flow. The velocity of the steam causes it to swirl around the body of the separator, throwing the heavier, suspended water to the wall, where it drains down to a steam trap installed under the unit.

Figure 20. Cyclonic Type Separator (Spirax Sarco)


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2.7.3 Coalescence Type

Coalescence type separators provide an obstruction in the steam path. The obstruction is typically a wire mesh pad (sometimes referred to as a demister pad), upon which water molecules become entrapped. These water molecules tend to coalesce, producing droplets that are too large to be carried further by the gas system. As the size of the droplets increases, they become too heavy and ultimately fall into the bottom of the separator. It is common to find separators, which combine both coalescence and cyclonic type operations. By combining the two methods, the overall efficiency of the separator is improved.

Figure 21. Coalescence Type Separato

(Spirax Sarco)

2.8 Steam traps 2.8.1 What are steam traps?

No steam system is complete without that crucial component 'the steam trap' (or trap) 11 . This is the most important link in the condensate loop because it connects steam usage with condensate return. A steam trap quite literally 'purges' condensate, (as well as air and other incondensable gases), out of the system, allowing steam to reach its destination in as dry a state/condition as possible to perform its task efficiently and economically. The quantity of condensate a steam trap has to deal with may vary considerably. It may have to discharge condensate at steam temperature (i.e. as soon as it forms in the steam space) or it may be required to discharge below steam temperature, giving up some of its 'sensible heat' in the process. 11

Section 2.8.1 is a summary of Module 11.1 Introduction – Why steam traps? In: Spirax Sarco Learning Centre, Block 11, ‘ Steam Traps and Steam Trapping’. www.spiraxsarco .com


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The pressures at which steam traps can operate may be anywhere from vacuum to well over a hundred bar. To suit these varied conditions there are many different types, each having their own advantages and disadvantages. One type of steam trap cannot possibly be the correct choice for all applications. Considerations for steam trap selection include the ability of the steam trap to: Vent air at 'start-up', i.e. the beginning of the process when the heater space is filled with air, which unless displaced, will reduce heat transfer and increase the warm-up time Remove condensate but not the steam Maximize plant performance. Simply put, unless specifically designed to waterlog, for a heat exchanger to operate at its best performance, the steam space must be filled with clean dry steam. The type of steam trap will influence this. §

§ §

There are three basic types of steam trap into which all variations fall. All three are classified by International Standard ISO 6704:1982. These are shown in Figure 22 and include: Thermostatic (operated by changes in fluid temperature). The temperature of saturated steam is determined by its pressure. In the steam space, steam gives up its enthalpy of evaporation (heat), producing condensate at steam temperature. As a result of any further heat loss, the temperature of the condensate will fall. A thermostatic trap will pass condensate when this lower temperature is sensed. As steam reaches the trap, the temperature increases and the trap closes. Mechanical (operated by changes in fluid density). This range of steam traps operates by sensing the difference in density between steam and condensate. These steam traps include 'ball float traps' and 'inverted bucket traps'. In the 'ball float trap', the ball rises in the presence of condensate, opening a valve, which passes the denser condensate. With the 'inverted bucket trap', the inverted bucket floats when steam reaches the trap and rises to shut the valve. Both are essentially 'mechanical' in their method of operation. Thermodynamic (operated by changes in fluid dynamics). Thermodynamic steam traps rely partly on the formation of flash steam from condensate. This group includes 'thermodynamic', 'disc', 'impulse' and 'labyrinth' steam traps. §

§

§

Steam Traps

Thermostatic 1. 2. 3.

Liquid expansion Balance pressure Bimetallic

Mechanical 1. 2.

Ball floating Inverted bucket

Thermodynamic 1. 2. 3.

Impulse Labyrinth Fixed orifice

Figure 22. Types of steam traps


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Also loosely included in this type are 'fixed orifice traps', which cannot be clearly defined as automatic devices as they are simply a fixed diameter hole set to pass a calculated amount of condensate under one set of conditions. All rely on the fact that hot condensate, released under dynamic pressure, will flash-off to give a mixture of steam and water. Because mechanical steam traps are most commonly used, only these will be described in more detail. For more details on all types of steam traps please refer to the Spirax Sarco Learning Centre (www.spiraxsarco.com). 2.8.2 Mechanical steam traps

Mechanical steam traps include ball float steam traps and inverted steam traps, which are described below.12 a) Ball float steam trap The ball float type trap operates by sensing the difference in density between steam and condensate. In the case of the trap shown in Figure 23a, condensate reaching the trap will cause the ball float to rise, lifting the valve off its seat and releasing condensate. As can be seen, the valve is always flooded and neither steam nor air will pass through it. Modern traps use a thermostatic air vent, as shown in Figure 23b. This allows the initial air to pass whilst the trap is also handling condensate.

Figure 23a. Float Trap with Air Cock

(Spirax Sarco)

Figure 23b. Float Trap with Thermostatic Air Vent (Spirax Sarco)

The automatic air vent uses the same balanced pressure capsule element as a thermostatic steam trap, and is located in the steam space above the condensate level. After releasing the initial air, it remains closed until air or other non-condensable gases accumulate during normal running and cause it to open by reducing the temperature of the air/steam mixture. The thermostatic air vent offers the added benefit of significantly increasing condensate capacity on cold start- up.

12

Section 2.8.2 is taken from Module 11.3 Mechanical Steam Traps. In: Spirax Sarco Learning Centre, Block 11, ‘ Steam Traps and Steam Trapping’. www.spiraxsarco.com


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In the past, the thermostatic air vent was a point of weakness if water hammer was present in the system. Even the ball could be damaged if the water hammer was severe. However, in modern float traps the air vent is a compact, very robust, all stainless steel capsule, and the modern welding techniques used on the ball makes the complete float-thermostatic steam trap very robust and reliable in waterhammer situations (for information on waterhammer see item 8 in section 4). In many ways the float-thermostatic trap is the closest to an ideal steam trap. It will discharge condensate as soon as it is formed, regardless of changes in steam pressure. Advantages of the float-thermostatic steam trap The trap continuously discharges condensate at steam temperature. This makes it the first choice for applications where the rate of heat transfer is high for the area of heating surface available. It is able to handle heavy or light condensate loads equally well and is not affected by wide and sudden fluctuations of pressure or flowrate. As long as an automatic air vent is fitted, the trap is able to discharge air freely. It has a large capacity for its size. The versions which have a steam lock release valve are the only type of trap entirely suitable for use where steam locking can occur. It is resistant to water hammer. §

§

§ § §

§

Disadvantages of the float-thermostatic steam trap Although less susceptible than the inverted bucket trap, the float type trap can be damaged by severe freezing and the body should be well lagged, and /or complemented with a small supplementary thermostatic drain trap, if it is to be fitted in an exposed position. As with all mechanical type traps, different internals are required to allow operation over varying pressure ranges. Traps operating on higher differential pressures have smaller orifices to balance the buoyancy of the float. §

§

b) Inverted bucket steam trap The inverted bucket steam trap is shown in Figure 24. As its name implies, the mechanism consists of an inverted bucket, which is attached by a lever to a valve. An essential part of the trap is the small air vent hole in the top of the bucket. Figure 24 shows the method of operation. In (i) the bucket hangs down, pulling the valve off its seat. Condensate flows under the bottom of the bucket filling the body and flowing away through the outlet. In (ii) the arrival of steam causes the bucket to become buoyant, it then rises and shuts the outlet. In (iii) the trap remains shut until the steam in the bucket has condensed or bubbled through the vent hole to the top of the trap body. It will then sink, pulling the main valve off its seat. Accumulated condensate is released and the cycle is repeated. In (ii), air reaching the trap at start-up will also give the bucket buoyancy and close the valve. The bucket vent hole is essential to allow air to escape into the top of the trap for eventual discharge through the main valve seat. The hole, and the pressure differential, is small so the trap is relatively slow at venting air. At the same time it must pass (and therefore waste) a certain amount of steam for the trap to operate once the air has cleared. A parallel air vent fitted outside the trap will reduce start-up times.


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Figure 24. Operation of an Inverted Bucket Steam trap

(Spirax Sarco)

Advantages of the inverted bucket steam trap The inverted bucket steam trap can be made to withstand high pressures. Like a float-thermostatic steam trap, it has a good tolerance to waterhammer conditions. Can be used on superheated steam lines with the addition of a check valve on the inlet. Failure mode is usually open, so it is safer on those applications that require this feature, for example turbine drains. § § § §

Disadvantages of the inverted bucket steam trap The small size of the hole in the top of the bucket means that this type of trap can only discharge air very slowly. The hole cannot be enlarged, as steam would pass through too quickly during normal operation. There should always be enough water in the trap body to act as a seal around the lip of the bucket. If the trap loses this water seal, steam can be wasted through the outlet valve. This can often happen on applications where there is a sudden drop in steam pressure, causing some of the condensate in the trap body to 'flash' into steam. The bucket loses its buoyancy §

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§

§

§

and sinks, allowing live steam to pass through the trap orifice. Only if sufficient condensate reaches the trap will the water seal form again, and prevent steam wastage. If an inverted bucket trap is used on an application where pressure fluctuation of the plant can be expected, a check valve should be fitted on the inlet line in front of the trap. Steam and water are free to flow in the direction indicated, while reverse flow is impossible, as the check valve would be forced onto its seat. The higher temperature of superheated steam is likely to cause an inverted bucket trap to lose its water seal. A check valve in front of the trap should be regarded as essential under such conditions. Some inverted bucket traps are manufactured with an integral check valve as standard. The inverted bucket trap is likely to suffer damage from freezing if installed in an exposed position with sub-zero ambient conditions. As with other types of mechanical traps, suitable lagging can overcome this problem if conditions are not too severe. If ambient conditions well below zero are to be expected, then it may be prudent to consider a more robust type of trap to do the job. In the case of mains drainage, a thermodynamic trap would be the first choice.

2.8.3 Steam trap selection and installation

Table 3 summarizes the selection of steam traps for different applications. Table 3. Selection of suitable steam traps for different process applications (BEE, 2004) Application Steam mains

Feature Open to atmosphere, small capacity Frequent change in pressure Low pressure - high pressure Large capacity Variation in pressure and temperature is undesirable Efficiency of the equipment is a problem Reliability with no over heating

§

§

Suitable trap Thermodynamic, Mechanical: Float

§ § § § § § § §

Equipment Reboiler Heater Dryer Heat exchanger etc. Tracer line Instrumentation

§ §

§

§

Mechanical: Float Bucket Inverted bucket Thermodynamic, Thermostatic: Bimetallic

When selecting and installing a steam trap, the following sho uld be considered:13 a) Waterhammer Waterhammer is condensate in the steam system that is picked up by moving steam and can cause damage to pipelines, fittings and steam traps. Symptoms of waterhammer are often attributed to malfunction of the steam trap. A more likely explanation is that a faulty steam trap has been damaged by waterhammer. Waterhammer can be caused in a number of ways, including: Failure to remove condensate from the path of high velocity steam in the pipework. §

13

Section 2.8.3 is a summary of (a) Module 11.5 Considerations for Selecting Steam Traps. In: Spirax Sarco Learning Centre, Block 11, ‘Steam Traps and Steam Trapping’. www.spiraxsarco.com, and (b) Energy Efficiency in Thermal Utilities. Book 2, by the India Bureau of Energy Efficiency, 2004


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§

From an application, which is temperature controlled and where condensate has to lift to a return line, or return to a pressurized system. The inability of condensate to properly enter or travel along an undersized return line, due to either (a) flooding, or (b) overpressurisation with the throttling effects of flash steam.

The problem of waterhammer can be eliminated by positioning the pipes so that there is a continuous slope in the direction of flow. A slope of at least 12 mm in every 3 metres is necessary, as also an adequate number of drain points every 30 to 50 metres. For more information on waterhammer see item 8 in section 4. b) Dirt Dirt is another major factor which must be considered when selecting traps. Although steam condenses to distilled water, it can sometimes contain trace products of boiler feed treatment compound and natural minerals found in water. Pipe dirt created during installation and the products of corrosion also need to be considered. c) Strainers These devices are frequently forgotten about in steam systems, often, it seems, in an effort to reduce installation costs. Pipe scale and dirt can affect control valves and steam traps, and reduce heat transfer rates. It is extremely easy and inexpensive to fit a strainer in a pipe, and the low cost of doing so will pay dividends throughout the life of the installation. Scale and dirt are arrested, and maintenance is usually reduced as a result. Selection is simple. The strainer material is selected to match the type of installation and the system pressure up to which it is expected to operate. Different filter screen sizes may be considered for differing degrees of protection. The finer the filter, the more often it may need cleaning. One thing is certain, strainers are far easier and cheaper to buy and maintain than control valves or steam traps. d) Steam locking The possibility of steam locking can sometimes be a deciding factor in the selection of steam traps. It can occur whenever a steam trap is fitted remotely from the plant being drained. It can become acute when condensate is removed through a syphon or dip pipe. To relieve this problem a trap is needed with a 'steam lock release' valve. This is an internal needle valve which allows the steam locked in the syphon pipe to be bled away past the main valve. The float trap is the only type of trap with this facility and is the correct choice on rotating machinery such as drying cylinders. e) Group trapping Group trapping describes the use of one trap serving more than one application (Figure 25). The original reason for group trapping was that there used to be only one kind of steam trap. It was the forerunner of the present day bucket trap, and was very large and expensive. Steam traps today are considerably smaller and cost effective, allowing individual heat exchangers to be properly drained. It is always better for steam using equipment to be trapped on an individual basis rather than on a group basis.


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The only satisfactory arrangement is to drain each steam space with own trap and then connect the outlets of the various traps to the common condensate return main (Figure 25).

Figure 25. Group Trapping and Individual Trapping with Common Condensate Return (BEE, 2004)

f) Diffusers With steam traps draining to atmosphere from open ended pipes, it is possible to see the discharge of hot condensate. A certain amount of flash steam will also be present relative to the condensate pressure before the trap. This can present a hazard to passers by, but the risks can be minimized by reducing the severity of the discharge. This may be achieved by fitting a simple diffuser (Figure 25) to the end of the pipe which reduces the ferocity of discharge and sound. Typically, sound levels can be reduced by up to 80%.

Figure 26. Diffuser (Spirax Sarco)


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g) Drain point The drain point should be large enough and should be located to allow the condensate to flow into the trap easily. For example, a 150 mm steam main will require a drain of at least 100 mm diameter and 150 mm deep located at the bottom of the main. Table 4 can be used to select the drain point dimensions. Table 4. Drain Pocket Dimensions (Spirax Sarco)

h) Pipe sizing The pipes leading to and from steam traps should be of adequate size. This is particularly important in the case of thermodynamic traps, because their correct operation can be disturbed by excessive resistance to flow in the condensate pipework. Pipe fittings such as valves, bends and tees close to the trap could also cause excessive back pressures and should be avoided. i) Air venting When air is carried into the trap space by the steam, the trap function can be affected unless adequate provision is made for removing air either through the steam trap or a separate air vent. If air is not vented properly, the plant can take a long time to warm up and may operate below its potential output.


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2.9 Air vents This section explains the use of air vents in a steam distribution system. 14 2.9.1 Effect of air on steam

If air is mixed with steam and flows along with it, pockets of air will remain at the heat exchange surfaces where the steam condenses. Gradually, a thin layer builds up to form an insulating blanket, hindering heat transfer as shown in Figure 27. The thermal conductivity of air is 0.025 W/m °C, while the corresponding value for water is typically 0.6 W/m °C, for iron it is about 75 W/m °C and for copper about 390 W/m °C. A film of air only 1 mm thick offers about the same resistance to heat flow as a wall of copper some 15 metres thick! It is unlikely that the air exists as an even film inside the heat exchanger. More probably, the concentration of air close to the condensing surface is higher, and lower further away. It is convenient however, to deal with it as a homogenous layer when trying to show its resistance to heat flow. When air is added to steam, the heat content of a given volume of the mixture is lower than the same volume of pure steam, so the mix temperature is lowered. Hence, the presence of air has a double effect: It offers a resistance to heat transfer via its layering effect It reduces the temperature of the steam space thus reducing the temperature gradient across the heat transfer surface § §

Figure 27. Effect of Air on heat Transfer (Spirax Sarco)

14

Section 2.9 is a summary of Module 11.12 Air Venting Theory. In: Spirax Sarco Learning Centre, Block 11, ‘ Steam Traps and Steam Trapping’. www.spiraxsarco.com


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The overall effect is to reduce the heat transfer rate below that which may be required by a critical process, and in worst cases may even prevent a final required process temperature being reached. In many processes, a minimum temperature is needed to achieve a chemical or physical change in a product, just as a minimum temperature is essential in a sterilizer. The presence of air is particularly problematic because it will cause a pressure gauge to mislead. It follows that the temperature cannot be inferred from the pressure. 2.9.2 Air in the system

Air is present within steam pipes and steam equipment at start-up. Even if the system were filled with pure steam when used, the condensing steam would cause a vacuum and draw air into the pipes at shutdown. Air can also enter the system in solution in the feedwater. At 80°C, water can dissolve about 0.6 percent of its volume, of air. Signs of air are: A gradual fall off in the output of any steam heated equipment Air bubbles in the condensate Corrosion § § §

2.9.3 Air removal

The most efficient means of air venting is with an automatic device. Air mixed with steam lowers the mix temperature. This enables a thermostatic device (based on either the balanced pressure or bimetallic principle) to vent the steam system. An air vent fitted on the steam space of a vessel (Figure 28) or at the end of a steam main (Figure 29) will open when air is present. For maximum removal of air, the discharge should be as free as possible. A pipe is often fitted to carry the discharge to a safe location, preferably not a condensate return line, which could restrict the free release of air and may also encourage corrosion.

Figure 28. Jacketed Pan with Automatic Air Vent (Spirax Sarco)

Figure 29: End of Main Automatic Air Vent (Spirax Sarco)

When an air vent is fitted to bypass a steam trap (Figure 29), it will act as a steam trap after the air is vented, and may from time to time discharge condensate. In such cases it is necessary to


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reconnect the air vent to the condensate line after the trap. If the condensate discharge line from a trap rises to high level, the flooded line imposes a backpressure on the trap and the air vent. The ability of the air vent to discharge air is reduced, especially at start-up. This applies equally when the air vent is incorporated within a steam trap. When the shape of the application steam space and the location of the steam inlet mean that most of the air leaves through the condensate outlet, it is preferable if discharge lines from the steam trap and air vent do not rise to high level.

Figure 29. Inverted Bucket Trap with Parallel Air Vent

(Spirax Sarco)

2.9.4 The air vent location

When a coil or a vessel has a relatively small cross-section, the steam admitted to it will act like a piston, pushing the air to a point remote from the steam inlet. This 'remote point' is usually the best location for the air vent. In the case of a steam user of the shape shown in Figure 29, some of the air will pass through the condensate outlet, according to the provision made in the trap, or in a bypass, for handling air. The rest of the air might collect as indicated, forming a cold spot on the heating surface. The unit cannot warm up evenly, and distortion may be caused in some equipment, such as the beds of laundry ironers. As an air/steam mixture is denser than pure steam at the same pressure, it is usually sufficient to provide air venting capability within the low- lying steam trap. However, the mode of operation of the trap means that condensate forms a water seal at the trap inlet sometimes preventing air from reaching the trap. There may be the need to consider an automatic air vent connected to the steam space above the level of any condensate. Often it is convenient and sufficiently effective to connect it to the top of the steam space, as in Figure 30.


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Figure 30. Air Vent at the opposite end of the Steam Inlet(Spirax Sarco)

However, in the case of two steam spaces of the same size and shape but with different steam inlet positions, the location of the air vent could be different. In Figure 31 and Figure 32, condensate drains from the bottom of the vessel but with the bottom steam inlet, at start-up, air would tend to be pushed to the remote point which is at the top. It may be best to locate an air vent at the top whilst a float-thermostatic steam trap will handle any residual air which has collected at the bottom of the vessel. With top steam entry, the air will tend to be pushed to the bottom at start-up, and provision should be made for venting it at low level. Usually, a trap with a high air venting capability such as a float-thermostatic trap will do the job. However, in practice, to ensure complete removal of air during running conditions, a separate air vent fitted at the top of the vessel (as shown in Figure 32) may again often prove beneficial, especially on irregularly shaped vessels.

Figure 31. Air Vent located opposite low level steam inlet (Spirax Sarco)

Figure 32. Air Vent (in steam trap) located opposite high level steam inlet (Spirax Sarco)


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2.10 Condensate recovery This section explains condensate recovery in a steam system.

15

2.10.1 What is condensate recovery

When a kilogram of steam condenses completely, a kilogram of condensate is formed at the same pressure and temperature. An efficient steam system will reuse this condensate. Failure to reclaim and reuse condensate makes no financial, technical or environmental sense. Saturated steam used for heating gives up its latent heat (enthalpy of evaporation), which is a large proportion of the to tal heat it contains. The remainder of the heat in the steam is retained in the condensate as sensible heat (enthalpy of water) as shown in Figure 33.

Figure 33. After giving up its latent heat to heat the process, steam turns to water containing only sensible heat (Spirax Sarco)

As well as having heat content, the condensate is basically distilled water, which is ideal for use as boiler feedwater. An efficient steam system will collect this condensate and either return it to a deaerator, a boiler feedtank, or use it in another process. Only when there is a real risk of contamination should condensate not be returned to the boiler. Even then, it may be possible to collect the condensate and use it as hot process water or pass it through a heat exchanger where its heat content can be recovered before discharging the water mass to drain. Condensate is discharged from steam plant and equipment through steam traps from a higher to a lower pressure. As a result of this drop in pressure, some of the condensate will re-evaporate into ‘flash steam’. The proportion of steam that will ‘flash off’ in this way is determined by the amount of heat that can be held in the steam and condensate. A flash steam amount of 10% to 15% by mass is typical. However, the percentage volumetric change can be considerably more. Condensate at 7 bar g will lose about 13% of its mass when flashing to atmospheric pressure, but the steam produced will require a space some 200 times larger than the condensate from which it was formed. This can have the effect of choking undersized trap discharge lines, and must be taken into account when sizing these lines.

15

Section 2.10 is taken from Module 14.1 Introduction to Condensate Recovery. In: Spirax Sarco Learning Centre, Block 14, ‘Condensate Recovery’. www.spiraxsarco.com


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2.10.2 Condensate recovery system

An effective condensate recovery system, collecting the hot condensate from the steam using equipment and returning it to the boiler feed system, can pay for itself in a remarkably short period of time. Figure 34 shows a simple steam and condensate circuit, with condensate returning to the boiler feedtank.

Figure 34. A Typical Steam and Condensate Circuit (Spirax Sarco) 2.10.3 Reasons for condensate recovery

Reasons for condensate recovery include: Financial reasons: Condensate is a valuable resource and even the recovery of small quantities is often economically justifiable. The discharge from a single steam trap is often worth recovering. Un-recovered condensate must be replaced in the boiler house by cold make-up water with additional costs of water treatment and fuel to heat the water from a lower temperature. Water charges: Any condensate not returned needs to be replaced by make- up water, incurring further water charges from the local water supplier. Effluent restrictions: In the UK for example, water above 43°C cannot be returned to the public sewer by law, because it is detrimental to the environment and may damage earthenware pipes. Condensate above this temperature must be cooled before it is discharged, which may incur extra energy costs. Similar restrictions apply in most countries, and effluent charges and fines may be imposed by water suppliers for non-compliance. Maximizing boiler output: Colder boiler feedwater will reduce the steaming rate of the boiler. The lower the feedwater temperature, the more heat, and thus fuel needed to heat the water, thereby leaving less heat to raise steam. Boiler feedwater quality: Condensate is distilled water, which contains almost no total dissolved solids (TDS). Boilers need to be blown down to reduce their concentration of §

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dissolved solids in the boiler water. Returning more condensate to the feedtank reduces the need for blowdown and thus reduces the energy lost from the boiler. 2.10.4 Layout and sizing of condensate return lines

Layout and sizing of condensate return lines are important when installing a condensate recovery system. 16 No single set of recommendations can cover the layout of condensate pipework. Much depends on the application pressure, the steam trap characteristics, the position of the condensate return main relative to the plant, and the pressure in the condensate return main. For this reason it is best to start by considering what has to be achieved, and to design a layout which will ensure that basic good practice is met. The prime objectives are that: Condensate must not be allowed to accumulate in the plant, unless the steam using apparatus is specifically designed to operate in this way. Generally apparatus is designed to operate non- flooded, and where this is the case, accumulated condensate will inhibit performance, and encourage the corrosion of pipes, fittings and equipment. Condensate must not be allowed to accumulate in the steam main. Here it can be picked up by high velocity steam, leading to erosion and waterhammer in the pipework. §

§

The subject of condensate piping will divide naturally into four basic types where the requirements and considerations of each will differ. These four basic types are defined and illustrated in Figure 35. Sizing of all condensate lines is a function of: Pressure. The difference in pressure between one end of the pipe and the other. This pressure difference may either promote flow, or cause some of the condensate to flash to steam. Quantity. The amount of condensate to be handled. Condition. Is the condensate predominately liquid or flash steam? §

§ §

16

Section 2.10.4 is a summary of Module 14.2 Layout of Condensate Return Lines and of Module 14.3 Sizing of Condensate Return Lines. In: Spirax Sarco Learning Centre, Block 14, ‘Condensate Recovery’. www.spiraxsarco.com. More detailed explanations can be f ound in these modules.


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Figure 35. Four Types of Condensate Lines (Spirax Sarco)

2.11 Insulation of steam pipelines and hot process equipments Insulation is an important part of conserving energy in the steam system. 17 2.11.1 Purpose of insulation

A thermal insulator is characterized by a low thermal conductivity and is therefore able to keep heat confined within or outside a system by preventing heat transfer to or from the external environment. Insulation materials are porous and contain a large number of dormant air cells. A large amount of heat energy may be lost without insulation or if insulation is inefficient or improperly installed. Thermal insulation, by reducing heat loss, delivers the following benefits: Reduction of fuel consumption Better process control by maintaining process temperatures at a constant level Corrosion prevention by keeping the exposed surface of a refrigerated system above dew point § § §

17

Section 2.11 is an edited version of Energy Efficiency in Thermal Utilities. Book 2, by the India Bureau of Energy Efficiency, 2004, page 97 -99


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Thermal Equipment: Steam distribution and utilization § §

Fire protection of equipme nt Absorbing of vibration

In addition, staff working conditions are improved because insulation protects them from exposure to hot surfaces and radiant heat and because insulation reduces noise levels. 2.11.2 Types of insulation

Insulators can be classified based on the three temperature ranges for which they are used: Low Temperature Insulators (up to 90o C), which are used for refrigerators, cold and hot water systems, storage tanks, etc. The most commonly used materials are cork, wood, 85 percent magnesia, mineral fibres, polyurethane and expanded polystyrene Medium Temperature Insulators (90 – 325o C), which are used in low-temperature heating and steam generating equipment, steam lines, flue ducts, etc. The most commonly used materials include 85 percent magnesia, asbestos, calcium silicate and mineral fibres High Temperature Insulators (325oC and above), which are typically used for boilers, super-heated steam systems, oven, driers and furnaces. The most commonly used materials are asbestos, calcium silicate, mineral fibre, mica, vermiculite, fireclay, silica and ceramic fibre. §

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The table below describes the applications, advantages and disadvantages of various insulating materials. Insulation materials can also be obtained in bulk in the form of moulded sections, for example, semi-cylindrical for pipes and slabs for vessels, flanges, valves etc. The main advantage of moulded sections is the ease of installation of new insulation and replacement or repair of existing insulation. 2.11.3 Selecting insula ting materials

Important factors that should be considered when choosing insulating materials are: Operating temperature of the system Type of fuel being fired Resistance of the materials to heat, weather and adverse conditions Thermal conductivity of the material Thermal diffusivity of the material Ability of the material to withstand the various conditions, such as thermal shock, vibration and chemical attack Resistance of the material to flames/fire Permeability of the material Total cost, including material purchase, installing and maintenance § § § § § §

§ § §

Table 6. Insulating Materials for Different Applications TYPE OF INSULATION

APPLICATION

Polystyrene An organic insulator made by polymerizing styrene

Suitable for low temperatures (-167o C to 82 oC). Mainly used in cool rooms, refrigeration piping and concrete retaining structures Suitable for low temperatures

Polyurethane

ADVANTAGES & DISADVANTAGES Advantages: rigid and lightweight Disadvantages: combustible, has a low melting point, is UV degradable, and susceptible to attacks by solvents Advantages: closed cell structure,


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Thermal Equipment: Steam distribution and utilization TYPE OF INSULATION

APPLICATION

Made by reacting isocyanides and alcohols. Made in continuous slab or foamed in situ

(-178o C to 4o C). Mainly used in cool rooms, refrigerated transports, deep freezing cabinets, refrigeration piping and floor and foundation insulation Suitable for temperatures up to o 820 C. Mainly used to insulate industrial ovens, heat exchangers, driers, boilers and high temperature pipes

Rockwool (mineral fibre) Manufactured by melting basalt and coke in a cupola at about o 1500 C. Phenol-based binders are used. Is available as mats, blankets, and loose form or preformed for pipe insulation Fibreglass Formed by bonding long glass fibres with a thermo setting resin to form blankets and bats, semirigid boards, high density rigid boards and preformed pipe sections Calcium Silicate Made from anhydrous calcium silicate material reinforced with a non-asbestos binder. Available in slab form of various sizes.

Ce ramic fibre Made from high purity alumina and silica grains, melted in an electric furnace and blasted by high velocity gases into light fluffy fibres. Made in various forms, including cloth, felt, tape, coating cements and variform castable (fire brick)

Suitable for temperatures up to o 540 C. Mainly used to insulate industrial ovens, heat exchangers, driers, boilers and pipework

Suitable for temperatures up to 1050o C. Mainly used to insulate furnace walls, fire boxes, back-up refractory, flue lining and boilers

Suitable for temperatures up to 1430o C. Mainly used to insulate furnace and kiln backup refractories, fire boxes, glass feeder bowls, furnace repair, induction coil insulation, high temperature gaskets and wrapping materials

ADVANTAGES & DISADVANTAGES low density and high mechanical strength Disadvantages: combustible, produces toxic vapors and has a tendency to smolder

Advantages: has a wide density range and is available in many forms. It is chemically inert, noncorrosive and maintains mechanical strength during handling

Advantages: will not settle or disintegrate with ageing. Disadvantages: Fibreglass products are slightly alkaline –pH9 (neutral is pH7). It should be protected against external contamination to avoid acceleration of steel corrosion Advantages: small air cell structure, a low thermal conductivity, and will retain its size and shape in its useable temperature range. It is lightweight, but has good structural strength so it can withstand mechanical abrasion. It will not burn or rot, is moisture resistant and non-corrosive Advantages: suitable for many applications because of the variety of forms

2.11.4 Insulation of steam and condensate lines

It is essential to insulate steam and condensate pipelines because they are a major source of heat loss through heat radiation from the pipelines. Suitable insulation materials are cork, glass wool, rock wool and asbestos. Flanges should also be insulated because an uncovered flange is equivalent to leaving 0.6 meter of pipeline un-insulated (SEAV, 2005). Flanges are often not insulated to make it easier to check the condition of the flange. A solution is to install detachable insulation covers, which can be removed when checking the flange.


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Figure 37 gives an indication of the amount of heat loss from un-insulated pipelines. An explanation of how to calculate the required thickness of insulation is given in section 3.

Figure 37. Heat loss from a 1-metre un-insulated pipe at various pipe diameters (SEAV, 2005)


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3. ASSESSMENT OF STEAM DISTRIBUTION SYSTEM This section explains how to carry out an assessment of steam traps, of heat losses from uninsulated surfaces and of savings from condensate recovery.

3.1 Assessment of steam traps Steam traps themselves do not use a lot of energy. But malfunctioning steam traps can result in large energy losses of the steam system. The assessment of steam trap performance is therefore concerned with answering the following two questions:18 Is the trap working correctly or not? If not, has the trap failed in the open or closed position? § §

Traps that fail in the ‘open’ position cause energy losses. Any condensate not returned into the steam system causes the boiler to heat up new water to make more steam. The steam heating capacity may also be reduced, resulting in indirect energy losses. Traps that have failed open will also pressurise the condensate discharge lines and affect the discharge efficiency of other traps. Traps that fail ‘closed’ do not result in energy or water losses, but can result in significantly reduced heating capacity and damage to steam heating equipment. Four steam trap performance tests exist: visual, sound, temperature and integrated. Worksheet 2 in section 6 of this chapter can be used to carry out a steam trap audit. 3.1.1 Visual testing

Visual testing of the flow or variation of flow of steam traps makes use of sight glasses (Figure 41), sight checks, test tees and three-way test valves. This method works well with traps that have an on/off cycle or low flows. This method becomes less viable when the flow, and thus the volume of water and flash steam, is high. One solution is to divert condensate before it reaches the trap and if the trap continues to flow then this is a sign of leakage.

18

Section 3.1 is an edited version of Energy Efficiency in Thermal Utilities. Book 2, by the India Bureau of Energy Efficiency, 2004, page 63-64


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For testing a steam trap, there should be an isolating valve provided downstream of the trap and a test valve in the trap discharge. When the test valve is in the open position, the following should be observed: Condensate discharge. Inverted bucket and thermodynamic disc traps should have intermittent condensate discharge. Float and thermostatic traps should have a continuous condensate discharge. Thermostatic traps can have either continuous or intermittent discharge depending upon the load. If inverted bucket traps are used for extremely small load, it will have a continuous condensate discharge. Flash steam. This should not be mistaken for a steam leak in the trap. It is difficult to visually identify whether a trap is blowing flash steam or live steam but, generally, if steam blows out continuously in a blue stream, it is leaking live steam. If steam floats out intermittently in a white cloud, it is flash steam. Continuous steam blow and no flow indicate that there is a problem in the trap. Whenever a trap fails to operate and the reasons are not immediately obvious, check for any lack of discharge from the trap, steam loss, continuous flow, sluggish heating, to find out whether it is a system problem or the mechanical problem in the steam trap. §

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3.1.2 Sound testing

Sound testing makes use of ultrasonic leak detectors (see Monitoring Equipment chapter), mechanics stethoscopes, screwdriver or a metal listening rod. These methods use the sound created by flow to determine if the trap is functioning properly. This method works well with traps that have an on/off cycle or low flows, but not so well with traps with varying and/or high flows. Similar to visual checking, trap leaks can be detected by diverting the flow before it reaches the trap 3.1.3 Temperature testing

Temperature testing makes use of infrared guns (see Monitoring Equip ment chapter), surface pyrometers, temperature tapes, and temperature crayons. These equipment measure the discharge temperature on the outlet side of the trap, with high temperatures indicating leaks and low temperatures indicating blocked, undersized, or faulty traps. Temperature tapes or crayons are set at a chosen temperature level and if the temperature on the trap outlet exceeds this level. Infrared guns and surface pyrometer can detect temperatures on both sides of the trap. To obtain reliable readings, the pipe must be bare, clean and without scale, as all of these reduce heat transfer and thus lower the recorded temperature. Some of the more expensive infrared guns can compensate for wall thickness and material differences. 3.1.4 Integrated testing

Spirax Sarco mentions the development of an integrated steam trap testing device due to the limitations of the above methods. 19 This consists of a sensor, fitted inside the steam trap, which is capable of detecting the physical state of the medium at that point by conductivity. This device has the following benefits: It is not affected by flash steam disturbance. The result is finite and not subject to interpretation. § §

19

Module 11.14 Testing and Maintenance of Steam Traps. In: Spirax Sarco Learning Centre, Block 11, ‘Steam Traps and Steam Trapping’. www.spiraxsarco.com


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Monitoring can be done locally, remotely, manually or automatically, and can detect immediate failure, thus minimizing waste and maximizing investment.

3.2 Assessment of heat losses from un-insulated surfaces Heat losses from un-insulated surfaces can be substantial and should therefore be assessed.20 3.2.1 Economic Thickness of Insulation (ETI)

The effectiveness of insulation follows the law of decreasing returns. This means that insulation results in energy and cost savings, but with increasing thickness of insulation the additional amount of energy and cost saved is going down. At a certain level, any additional insulation is no longer economically justifiable. The point where the amount of insulation gives the greatest return on investment is called the “economic thickness of insulation” (ETI) and is shown in Figure 40. The ETI is calculated based on the following factors, which differ between companies: Cost of fuel Annual hours of operation Heat content of fuel Boiler efficiency Operating surface temperature Pipe diameter/thickness of surface Estimated cost of insulation. Average exposure ambient still air temperature § § § § § § § §

I+H

Cost

I

H

M

Insulation Thickness I : Cost of Insulation I + H : Total Cost

H : Cost of Heat Loss M : Economic Thickness

Figure 40. Determination of the Economic Thickness of Insulation Material (BEE, 2004) 20

Section 3.2 is an edited version of Energy Efficiency in Thermal Utilities. Book 2, by the India Bureau of Energy Efficiency, 2004, page 99-102


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3.2.2 Calculating heat loss - methodology

Various charts, graphs and references are available to calculate the amount of heat loss. Worksheet 3 in section 6 of this chapter can be used to carry out an assessment of heat losses through insulation. The heat loss can be calculated using the following equations:

Total heat loss (Hs in kCal/hr) = S x A S = [10+(Ts-Ta)/20] (Ts-Ta) A (m2 ) = 3.14 x diameter (m) x length (m)

Where: S A Ts Ta

= Surface heat loss in kCal/hr m2 = Surface area in m2 = Hot surface temperature in o C = Ambient temperature in oC

Note: This equation can be used for surface temperatures up to 2000C. Factors like wind velocities, and conductivity of insulating material have not been considered. The additional energy costs associated with heat loss can be calculated with the following equations:

Equivalent fuel loss (Hf) (kg/year)

Annual costs of heat loss ($)

Hs x yearly hours of operation = -----------------------------------------GCV x ? b = Hf x Fuel cost ($/kg)

Where: GCV = Gross Calorific Value of fuel kCal/kg ?b = Boiler efficiency in percent 3.2.3 Calculating heat loss - example

Question: A 100 m steam pipe line with a 100 mm diameter is not insulated and supplies steam at 10 kg/cm2 to the equipment. Calculate the fuel savings if the steam pipeline would be insulated with 65 mm glass wool with aluminum cladding. Assumptions: Boiler efficiency = 80 percent


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Fuel oil cost = US$ 300/tonne Gross calorific value of fuel oil = 10300 kCal/kg

Surface temperature without insulation (Ts) = 170o C Surface temperature after insulation (Ts) = 65o C Ambient temperature (Ta) = 25 o C Step 1: calculate the surface heat loss and total heat loss of the un-insulated pipeline (S1 and Hs1) S1 = [10+ (Ts-Ta)/20] x (Ts-Ta) Ts = 170oC Ta = 25o C S1 = [10+(170-25)/20] x (170-25) = 2500 kCal/hr m2 A (m2 ) = 3.14 x diameter (m) x length (m) Diameter = 0.1 m Length = 100 m A1 = 3.14 x 0.1 x 100 = 31.4 m2 Total heat loss (Hs1) = S1 x A1 = 2500 x 31.4 = 78850 kCal/hr Step 2: calculate the surface heat loss and total heat loss of the insulated pipeline (S2) S2 = [10+ (Ts-Ta)/20] x (Ts-Ta) Ts = 65o C Ta = 25o C S2 = [10+(65-25)/20] x (65-25) = 480 kCal/hr m2 Total heat loss/hr (Hs2) = S2 x A2 Diameter = 0.23 m (= 100 mm + 65 mm + 65 mm) Length = 100 m A2 = 3.14 x 0.23 x 100 = 31.4 m2 Total heat loss (Hs2) = S2 x A2 = 480 x 72.2 = 34656 kCal/hr

Step 3: calculate the yearly fuel savings and cost savings (Hf and US$) Total reduction in heat loss Hs = Hs1 – Hs2 = 78860 – 34656 = 44194 kCal/hr

Equivalent fuel loss (Hf) (kg/year) = Hs x yearly hours of operation GCV x ? b


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Annual hours of operation = 8400 hrs Total heat loss reduction = 44194 kCal/hr Gross calorific value of fuel oil = 10300 kCal/kg Boiler efficiency = 80 percent (0.8)

Fuel oil costs = US$ 300/tonne (US$ 0.3/kg) Hf = 44194 x 8400 = 45052 kg/year 10300 x 0.8 Annual costs of heat loss (US$) = Hf x Fuel cost (US$/kg) = 45052 x 0.3 = US$ 13516

3.3 Assessment of savings from condensate recovery The procedure for calculating the energy and cost savings that can be achieved by condensate recovery is explained in Figure 41.


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Figure 41. Calculations of Energy and Cost Savings from Condensate Recovery

(UNEP, 2004)


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4. ENERGY EFFICIENCY OPPORTUNITIES This section describes the main opportunities to improve the energy efficiency of the steam distribution system. 1. Manage steam traps

Energy losses can be reduced using steam traps, provided attention is giving to the following areas:21 Testing of steam traps is explained in section 3.1 Routine maintenance, which depends on the type of trap and its application. The balanced pressure steam trap for example, has an element which is designed for easy replacement. Changing these on a regular basis, maybe once every three years or so, might seem wasteful in time and materials. However, this practice reduces the need for trap checking and should ensure a trouble free system with minimal losses through defective traps. Routine maintenance maintenance which involves cleaning and re- using existing internals uses just as much labour labour but leaves an untrustworthy steam trap. It will have to be checked from time to time and will be prone to fatigue. Any routine maintenance should include the renewal of any suspect parts, if it is to be cost effective. Replacement of internals. The renewal of internal parts of a steam trap makes good sense. The body will generally have as long a life as the plant to which it is fitted and it is only the internal parts which wear, depending on system conditions. There are obvious advantages in replacing these internals from time to time. It depends on the ease with which the new parts can be fitted and the reliability and availability of the refurbished trap. The elements of thermostatic traps can generally be changed by removing a screwed in seat. Replacement is simple and the remade trap will be reliable assuming the maintenance instructions are correctly carried out. Always check with the manufacturer regarding the correct technique for any maintenance work required on steam traps. A reputable manufacturer will always be able to supply appropriate literature, advice, and spare parts. Replacement of traps. On occasions, it will be easier and cheaper to replace traps rather than repair them. In these cases it is essential that the traps themselves themselves can be changed easily. Flanged connections provide one solution, although the flanged trap is more expensive than the equivalent screwed trap. Mating flanges are an additional expense. § §

§

§

2. Avoid steam leaks

Steam leaks are a source of energy loss and must be avoided. It is estimated that a 3 mm diameter hole on a pipeline carrying 7kg/cm2 steam would waste 33 kL of fuel oil per year. Steam leaks on high-pressure mains are prohibitively costlier than on low pressure mains. Any steam leakage must be quickly attended to. In fact, the plant should consider a regular surveillance programme for identifying leaks at pipelines, valves, flanges and joints. Indeed, by plugging all leakages, one may be surprised at the extent of fuel savings, which may reach up to 5 percent of the steam consumption in a small or medium scale industry or even higher in installations having several process departments. To avoid leaks it may be worthwhile considering replacement of the flanged joints which are rarely opened in old plants by welded joints. Figure 42 provides a quick estimate for steam steam 21

Item 1 is a summary of Module 11.14 Testing and Maintenance of Steam Traps. In: Spirax Sarco Learning Centre, Block 11, ‘ Steam Traps and Steam Trapping’. www.spiraxsarco .com

Energy Efficiency Guide for Industry in Asia – www.energyeffici www.energyefficiencyasia encyasia.org .org

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leakage based on plume length. For example, if the plume length is 700 mm then the steam loss is 10 kg/hr.

Figure 42: Steam Losses VS Plume Length

(Spirax Sarco) 3. Provide dry steam for process

The best steam for industrial process heating is the dry saturated steam. Wet steam reduces total heat in the steam. Also water forms a wet film on heat transfer and overloads traps and condensate equipment. Superheated steam is not desirable for process heating because it gives up heat at a rate slower than the condensation heat transfer of saturated steam. It must be remembered that a boiler without a superheater cannot deliver perfectly dry saturated steam. At best, it can ca n deliver only o nly 95 to 98 percent dry steam. The dryness fraction of steam depends on various factors, such as the level of water in the boiler. Indeed, even as simple a thing as improper boiler water treatment can become a cause for wet steam. Since dry saturated steam is required for process equipment, due attention must be paid to the boiler operation and lagging of the pipelines. Wet steam can reduce plant productivity and product quality, and can cause damage to most items of plant and equipment. Whilst careful drainage and trapping can remove most of the water, it will not deal with the water droplets suspended in the steam. To remove these suspended water droplets, separators are installed in steam pipelines. The steam produced in a boiler designed to generate saturated steam is inherently wet. Although the dryness fraction will wil l vary according to the type of boiler, most shell type steam boilers will produce steam with a dryness fraction of between 95 and 98 percent. The water content of the steam produced by the boiler is further increased if priming and carryover occur. A steam separator may be installed on the steam main as well as on the branch lines to reduce wetness in steam and improve the quality of the steam going to the units.


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4. Utilize steam at the lowest acceptable pressure for the process

A study of the steam tables would indicate that the latent heat in steam reduces as the steam pressure increases. It is only the latent heat of steam, steam, which takes part in the heating process when applied to an indirect heating system, such as a heat exchanger. Thus, it is important that its value be kept as high as possible. This can only be achieved with lower steam pressures. As a guide, the steam should always be generated and distributed at the highest possible pressure, but utilized at as low a pressure as possible since it then has higher latent heat. However, it may also be seen from the steam tables that the lower the steam pressure, the lower will be its temperature. Since temperature is the driving force for the transfer of heat at lower steam pressures, the rate of heat transfer will be slower and the processing time greater. In equipment where fixed losses are high (e.g. big drying cylinders), there may even be an increase in steam consumption at lower pressures due to increased processing time. There are, however, several equipment in certain industries where one can profitably go in for lower pressures and realize economy in steam consumption without materially affecting production time. Therefore, there is a limit to the reduction of steam pressure. Depending on the equipment design, the lowest possible steam pressure with which the equipment can work should be selected without sacrificing either on production time or on steam consumption 5. Proper utilization of directly injected steam

The heating of a liquid by direct injection of steam is often desirable. The equipment required is relatively simple, cheap and easy to maintain. No condensate recovery system is necessary. The heating is quick, and the sensible heat of the steam is also used up along with the latent heat, making the process thermally efficient. In processes where dilution is not a problem, heating is done by blowing steam into the liquid, i.e. direct steam injection is applied. If the dilution of the tank contents and agitation are not acceptable in the process, i.e. direct steam agitation is not acceptable, indirect steam steam heating is the only answer. Ideally, the injected steam should be condensed completely as the bubbles rise through the liquid. This is possible only if the inlet steam pressures are kept very low—around 0.5kg/cm2 –and certainly not exceeding 1 kg/cm2 . If pressures are high, the velocity of the steam bubbles will also be high and they will not get sufficient time to condense before they reach the surface. Figure 46 shows a recommended arrangement for direct injection of steam by sparge pipe. A large number of small diameter holes (2 to 5mm), facing downwards, should be drilled on the separate pipe. This will help in dissipating the velocity of bubbles in the liquid. A thermostatic control of steam admitted is highly desirable. A more efficient way to inject steam into water is to use a properly manufactured steam injector. Good injectors are designed such that the steam creates a venturi action to coincidentally draw cool water through the injector. The heat in the steam is transferred transferred much faster and more completely than by a sparge pipe, ensuring that all injected steam is absorbed by the water before it is able to escape from the water surface, thus reducing heat losses.


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Figure 43: Typical Steam Injector Installation

(Spirax Sarco)

6. Minimize heat transfer barriers

22

The metal wall may not be the only barrier in a heat transfer process. There is likely to be a film of air, condensate and scale on the steam side. On the product side there may also be baked-on product or scale, and a stagnant film of product. Agitation of the product may eliminate the effect of the stagnant film, whilst regular cleaning on the product side should reduce the scale. Regular cleaning of the surface on the steam side may also increase the rate of heat transfer by reducing the thickness of any layer of scale, however, this may not always be possible. This layer may also be reduced by careful attention to the correct operation of the boiler, and the removal of water droplets carrying impurities from the boiler. Filmwise condensation The elimination of the condensate film is not quite as simple. As the steam condenses to give up its enthalpy of evaporation, droplets of water may form on the heat transfer surface. These may then merge together to form a continuous film of condensate. The condensate film may be between 100 and 150 times more resistant to heat transfer than a steel heating surface, and 500 to 600 times more resistant than copper.

22

Item 6 is taken from Module 2.5 Heat Transfer. In: Spirax Sarco Learning Centre, Block 2, ‘ Steam Engineering Principles and Heat Transfer’. www.spiraxsarco.com


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Dropwise condensation If the droplets of water on the heat transfer surface do not merge immediately and no continuous condensate film is formed, ‘dropwise’ condensation occurs. The heat transfer rates which can be achieved during dropwise condensation are generally much higher than those achie ved during filmwise condensation. As a larger proportion of the heat transfer surface is exposed during dropwise condensation, heat transfer coefficients may be up to ten times greater than those for filmwise condensation. In the design of heat exchangers where dropwise condensation is promoted, the thermal resistance it produces is often negligible in comparison to other heat transfer barriers. However, maintaining the appropriate conditions for dropwise condensation has proved to be very difficult to achieve. If the surface is coated with a substance that inhibits wetting, it may be possible to maintain dropwise condensation for a period of time. For this purpose, a range of surface coatings such as Silicones, PTFE and an assortment of waxes and fatty acids are sometimes applied to surfaces in a heat exchanger on which condensation is to be promoted. However, these coatings will gradually lose their effectiveness due to processes such as oxidation or fouling, and film condensation will eventually predominate. As air is such a good insulator, it provides even more resistance to heat transfer. Air may be between 1 500 and 3 000 times more resistant to heat flow than steel, and 8 000 to 16 000 more resistant than copper. This means that a film of air only 0.025 mm thick may resist as much heat transfer as a wall of copper 400 mm thick! Of course all of these comparative relationships depend on the temperature profiles across each layer. Figure 44 illustrates the effect this combination of layers has on the heat transfer process. These barriers to heat transfer not only increase the thickness of the entire conductive layer, but also greatly reduce the mean thermal conductivity of the layer. The more resistant the layer to heat flow, the larger the temperature gradient is likely to be. This means that to achieve the same desired product temperature, the steam pressure may need to be significantly higher. The presence of air and water films on the heat transfer surfaces of either process or space heating applications is not unusual. It occurs in all steam heated process units to some degree. To achieve the desired product output and minimize the cost of process steam operations, a high heating performance may be maintained by reducing the thickness of the films on the condensing surface. In practice, air will usually have the most significant effect on heat transfer efficiency, and its removal from the supply steam will increase heating performance


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. Figure 44. Temperature Gradients across Heat Transfer Barriers

(Spirax Sarco) 7. Proper air venting23

When steam is first admitted to a pipe after a period of shutdown, the pipe is full of air. Further amounts of air and other non-condensable gases will enter with the steam, although the proportions of these gases are normally very small compared with the steam. When the steam condenses, these gases will accumulate in pipes and heat exchangers. Precautions should be taken to discharge them. The consequence of not removing air is a lengthy warming up period, and a reduction in plant efficiency and process performance. Air in a steam system will also affect the system temperature. Air will exert its own pressure within the system, and will be added to the pressure of the steam to give a total pressure. Therefore, the actual steam pressure and temperature of the steam/air mixture will be lower than that suggested by a pressure gauge. Of more importance is the effect air has upon heat transfer. A layer of air only 1 mm thick can offer the same resistance to heat as a layer of water 25 µ m thick, a layer of iron 2 mm thick or a layer of copper 15 mm thick. It is very important therefore to remove air from any steam system. Automatic air vents for steam systems (which operate on the same principle as thermostatic steam traps) should be fitted above the condensate level so that only air or steam/air mixtures can reach them. The best location for them is at the end of the steam mains. The discharge from an air vent must be piped to a safe place. In practice, a condensate line falling towards a vented receiver can accept the discharge from an air vent. In addition to air venting at the end of a main, air vents should also be fitted: In parallel with an inverted bucket trap or, in some instances, a thermodynamic trap. These traps are sometimes slow to vent air on start-up. In awkward steam spaces (such as at the opposite side to where steam enters a jacketed pan). §

§

23

Item 7 is taken from Module 10.5 Air Venting, Heat Losses and Summary of Various Pipe Related Standards. In: Spirax Sarco Learning Centre, Block 10, ‘Steam Distribution’. www.spiraxsarco.com


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Where there is a large steam space (such as an autoclave), and a steam/air mixture could affect the process quality.

8. Minimize waterhammer24

Waterhammer is the noise caused by slugs of condensate colliding at high velocity into pipework fittings, plant, and equipment. This has a number of implications: Because the condensate velocity is higher than normal, the dissipation of kinetic energy is higher than would normally be expected. Water is dense and incompressible, so the ‘cushioning’ effect experienced when gases encounter obstructions is absent. The energy in the water is dissipated against the obstructions in the piping system such as valves and fittings. §

§

§

Figure 45. Formation of a ‘solid’ slug of water (Spirax Sarco)

Indications of waterhammer include a banging noise, and perhaps movement of the pipe. In severe cases, waterhammer may fracture pipeline equipment with almost explosive effect, with consequent loss of live steam at the fracture, leading to an extremely hazardous situation. Good engineering design, installation and maintenance will avoid waterhammer; this is far better practice than attempting to contain it by choice of materials and pressure ratings of equipment. Commonly, sources of waterhammer occur at the low points in the pipework (Figure 46). Such areas are due to: Sagging in the line, perhaps due to failure of supports. Incorrect use of concentric reducers (see Figure 10.3.7) - Always use eccentric reducers with the flat at the bottom. Incorrect strainer installation - They should be fitted with the basket on the side. Inadequate drainage of steam lines. Incorrect operation - Opening valves too quickly at start- up when pipes are cold. § §

§ § §

24

Item 8 is taken from Module 10.4 Steam Mains and Drainage. In: Spirax Sarco Learning Centre, Block 10, ‘ Steam Distribution’. www.spiraxsarco.com


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Figure 46. Possible Sources of Waterhammer (Spirax Sarco)

To summarize, the possibility of waterhammer is minimized by: Installing steam lines with a gradual fall in the direction of flow, and with drain points installed at regular intervals and at low points. Installing check valves after all steam traps, which would otherwise allow condensate to run back into the steam line or plant during shutdown. Opening isolation valves slowly to allow any condensate, which may be lying in the system to flow gently through the drain traps, before it is picked up by high velocity steam. This is especially important at start-up. §

§

§

9. Insulate steam pipelines and hot process equipments

Insulation is needed to avoid the loss of heat through radiation from steam pipes as explained in section 2.11. 10. Improve condensate recovery25

Condensate recovery was already explained in section 2.10. Figure 47 compares the amount of energy in a kilogram of steam and condensate at the same pressure. The percentage of energy in condensate to that in steam can vary from 18 percent at 1 bar g to 30 percent at 14 bar g. Clearly the liquid condensate is worth reclaiming. If this water is returned to the boiler house, it will reduce the fuel requirements of the boiler. For every 6o C rise in the feed water temperature, there will be approximately 1 percent saving of fuel in the boiler.

25

Item 10 is taken from Module 14.1 Introduction to Condensate Recovery. In: Spirax Sarco Learning Centre, Block 14, ‘Condensate Recovery’. www.spiraxsarco.com


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Figure 47. Heat Content of Steam and Condensate at the same Pressures (Spirax Sarco) 26

11. Recover flash steam

Flash steam is released from hot condensate when its pressure is reduced. As an example, when steam is taken from a boiler and the boiler pressure drops, some of the water content of the boiler will flash off to supplement the ‘live’ steam produced by the heat from the boiler fuel. If use is to be made of flash steam, it is helpful to know how much of it will be available. The quantity is readily determined by calculation, or can be read from simple tables or charts. Example: Calculate available flash steam The condensate enters the steam trap as saturated water, at a gauge pressure of 7 bar g and a temperature of 170°C. The specific amount of heat in the condensate at this pressure is 721 kJ/kg. After passing through the steam trap, the pressure in the condensate return line is 0 bar g. At this pressure, the maximum amount of heat each kilogram of condensate can hold is 419 kJ and the maximum temperature is 100°C. There is an excess of 302 kJ of heat which evaporates some of the condensate into steam. The heat needed to produce 1 kg of saturated steam from water at the same temperature, at 0 bar gauge, is 2257 kJ. The percentage of flash steam per kg of condensate is calculated as follows: Percentage of flash steam available =

S1 —S2

X 100%

L2 Where: S1 = the sensible heat of higher pressure condensate S2 = the sensible heat of the steam at lower pressure (at which it has been flashed) L2 = the latent heat of flash steam (at lower pressure). 26

Item 11 is a summary of part of Module 14.6 Flash Steam. In: Spirax Sarco Learning Centre, Block 14, ‘Condensate Recovery’. www.spiraxsarco.com


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In this example, the percentage of flash steam evaporated therefore is: = [(S1 —S2 ) / L2 ] X 100% = [(721 – 419)] / 2257] X 100% = 13.4% Example: Determine available flash steam using a chart Alternatively, the chart in Figure 48 can be read directly for the moderate and low pressures encountered in many plants. The example is depicted in Figure 48 and shows that 0.134 kg of flash steam is produced per kg of condensate passing through the trap.

Figure 48: Quantity of Flash Steam Graph (Spirax Sarco)

Typical applications of flash steam include heating (with air heater batteries and space heating installations using either radiant panels or unit heaters), steam heated hot water calorifier and a steam to water calorifier. Boiler blowdown can also be recovered as flash steam, as is explained in the separate Boiler Chapter. The Spirax Sarco Learning Centre provides more information on flash steam recovery vessels, requirements for successful recovery applications, and typical applications of flash steam.


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12. Use a thermo-compressor to reuse low pressure steam

In many cases, very low pressure steam is reused as water after condensation when there is no better reuse option. In many cases it becomes feasible to compress this low pressure steam with very high pressure steam and reuse it as a medium pressure steam. The major energy in steam is in its latent heat value and thus thermo-compression would give a large improvement in waste heat recovery. The thermo-compressor (Figure 50) is a simple equipment with a nozzle where HP steam is accelerated into a high velocity fluid. This entrains the LP steam by momentum transfer and then recompresses in a divergent venturi. It is typically used in evaporators where the boiling steam is recompressed and used as heating steam.

DISCHARGE STEAM M.P.

MOTIVE STEAM H.P.

SUCTION STEAM L.P.

Figure 49. Thermocompressor


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5. OPTION CHECKLIST This section includes most important energy efficiency options §

Fix steam leaks and condensate leaks (a 3 mm diameter hole on a pipe line carrying 7 kg/cm 2 steam would waste 33 kilolitres of fuel oil per year)

§

§ § § § § § § §

Accumulate work orders for repair of steam leaks that cannot be fixed unless the plant is shut down. Provide each leak with tag. Use back pressure steam turbines to produce lower steam pressures Use more efficient steam desuperheating methods Ensure process temperatures are correctly controlled Maintain lowest acceptable process steam pressures Reduce hot water wastage to drain Remove or isolate all redundant steam piping Ensure condensate is properly removed from process and heating equipment Ensure condensate is returned or reused in the process. (6 0 C raise in feed water temperature by economizer/condensate recovery corresponds to a 1 percent saving in fuel consumption, in boiler)

§

§ § § §

§ § § §

§

Preheat boiler feed-water with condensate and flash steam wherever possible. Where not possible, heat with live steam from the boiler by steam injection method. Recover the heat from continuous (TDS) boiler blowdown Check operation of steam traps Minimize waterhammer Remove air from indirect steam using equipment (a 0.25 mm thick air film offers the same resistance to heat transfer as a 330 mm thick copper wall ) Inspect steam traps regularly and repair malfunctioning traps promptly Consider recovery of vent steam (e.g. on large flash vessels Use waste steam for water heating Use an absorption chiller to condense exhaust steam before returning the condensate to the boiler Establish a steam efficiency- maintenance programme. Start with an energy audit and follow up, then make a steam efficiency-maintenance programme a part of your continuous energy management programme


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6. WORKSHEETS This section includes following worksheets: Technical Specifications of Steam Traps Steam Trap Audit Insulation Losses • • •

Worksheet 1: TECHNICAL SPECIFICATIONS OF STEAM TRAPS o N . n o i t c e S

. f e ) r o p N a ( r T

e p y t p a r T

e z i s p a r T

Location re f. (plant dept/block)

Type of discharge (continuous/ semi continuous/ intermittent)

Worksheet 2: STEAM TRAP AUDIT . o n n o i t c e S

) . o n ( . f e r p a r T

e p y t p a r T

e z i s p a r T

e r u s s 2 e m r / p c g p k a r T

n o p i a t r a T c o l

Worksheet 3: INSULATION LOSSES No. Location Equipment reference

p a r t f o n o i t a c i l p p A

l p a a t r n o f i t o c s n u u t a F t s

Outer diameter


n o i t a u t i s f o s i s o n g a i D

Surface temperature

Trap capacity (kg Condensate/hr)

s g n i t t i f p a r t f o s u t a t S

s k r a m e R

Insulation thickness (if any)

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7. REFERENCES Bureau of Energy Efficiency. Energy Efficiency in Thermal Utilities. Book 2 , 2004 US Department of Energy (DOE), Fedral Energy Management Programme. Steam Trap Performance Assessment, Doe/Ee-0193, Page 8. 1999. www.eren.doe.gov/femp Spirax Sarco. Learning Centre. www.spiraxsarco.com/learn Sustainable Energy Authority of Victoria (SEAV). Fact sheet: Boiler Optimisation. 2005. www.seav.sustainability.vic.gov.au/manufacturing/sustainable%5Fmanufacturing/ United Nations Environment Programme (UNEP). Cleaner Production – Energy Efficiency Manual – Guidelines for the integration of Cleaner Production and Energy Efficiency . 2004. www.uneptie.org Many sections of this chapter were taken from, based on or are a summary of modules featured in Spirax Sarco’s web-based Learning Centre with the kind permission of Spirax Sarco. For more detailed information please refer to www.spiraxsarco.com/learn for the following modules: 1. Introduction 2. Steam Engineering Principles and Heat Transfer 3. The Boiler House 4. Flow metering 5. Basic Control Theory 6. Control Hardware: Electric/Pneumatic Actuation 7. Control Hardware: Self-acting Actuation 8. Control Applications 9. Safety Valves 10. Steam Distribution 11. Steam Traps and Steam Trapping 12. Pipelines Ancillaries 13. Condensate Removal 14. Condensate Recovery 15. Desuperheating 16. Equations Copyright: Copyright © United Nations Environment Programme (year 2006) This publication may be reproduced in whole or in part and in any form for educational or non-profit purposes without special permission from the copyright holder, provided acknowledgement of the source is made. UNEP would appreciate receiving a copy of any publication that uses this publication as a source. No use of this publication may be made for resale or any other commercial purpose whatsoever without prior permission from the United Nations Environment Programme.

Disclaimer: This energy equipment module was prepared as part of the project “ Greenhouse Gas Emission Reduction from Industry in Asia and the Pacific” (GERIAP) by the National Productivity Council, India. While reasonable efforts have been made to ensure that the contents of this publication are factually correct and properly referenced, UNEP does not accept responsibility for the accuracy or completeness of the contents, and shall not be liable for any loss or damage that may be occasioned directly or indirectly through the use of, or reliance on, the contents of this publication.


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Chapter - Steam Distribution and Utilization

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