Technical Reference Guide on Steam Distribution

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TECHNICAL REFERENCE GUIDE

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Introduction

Steam distribution

Steam system basics

The steam distribution system is an important link between the central steam source and the steam user. The central steam source may be a boiler house or a cogeneration plant. The source must supply good quality steam at the required rate and pressure, and it must do this with the minimum of heat loss and maintenance attention.

This guide will look at the distribution of dry saturated steam as a conveyor of heat energy to the point of use, for either process heat exchange applications, or space heating, and will cover the issues associated with the implementation of an efficient steam distribution system.

From the outset, an understanding of the basic steam circuit, or 'steam and condensate loop' is required. The steam flow in a circuit is due to condensation of steam which causes a pressure drop. This induces the flow of steam 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 crown valve (the steam outlet from the boiler) is opened, steam immediately passes from the boiler into and along the steam mains. The pipework is cold initially so heat is transferred to it by the steam. The air surrounding the pipes is cooler than the steam, so the pipework will begin to lose heat to the air.

As the steam is flowing to a cooler environment, it will begin to condense immediately. On start-up of the system, the amount of condensate will be greatest as the steam will be used in heating up the cold pipework - this is known as the 'starting load'. Once the pipework has warmed up, condensation will still occur as the pipework loses heat to the surrounding air - this is known as the 'running load'.

The resulting condensate falls to the bottom of the pipe and is carried along with the steam flow and by gravity, due to the gradient in the steam main which should normally fall in the direction of steam flow. The condensate will then have to be drained from the lowest points in the steam main.

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When the valve on the steam pipe serving an item of steam using plant is opened, steam flow from the distribution system enters the plant and again comes into contact with surfaces cooler than itself. The steam then gives up its energy in warming up the equipment (starting load), and continues to transfer heat to the process (running load) when it will condense into water (condensate).

There is now a continuous flow of steam from the boiler to satisfy this connected load, and to maintain this supply more steam must be generated. In order to do this, more fuel is fed to the boiler and more water is pumped into it to make-up for the water which has already been evaporated into steam.

The condensate formed in both the steam distribution pipework and in the process equipment is a ready made supply of useable hot boiler feedwater. Although it is important to remove this condensate from the steam space, it is a far too valuable commodity to be allowed to run to waste. The basic steam circuit should be completed by returning all condensate to the boiler feedtank, wherever practicable. Feedtank Make-up water Boiler Steam Vat Pan Pan Steam Vat Condensate Process vessel Space heating system Condensate Steam Feedpump

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The pressure at which the steam is to be distributed is partially determined by the point of use on the plant needing the highest pressure.

It should be remembered that as the steam passes through the distribution pipework, it will lose some of its pressure due to resistance to flow, and condensation from loss of heat to the pipework. Therefore allowance should be made for this pressure loss when deciding upon the initial distribution pressure.

To summarise these points, the following need to be considered when selecting the working pressure:

Pressure required at the point of use.

Pressure drop along the pipe due to resistance of flow (friction). Pipe heat losses.

Steam at a higher pressure occupies less volume per kilogram than steam at a lower pressure. It therefore follows that if steam is generated in the boiler at a higher pressure than that needed by its application, and is distributed at this higher pressure, the size of the distribution mains will be smaller for any given mass flowrate. Figure 2 illustrates this point.

Working pressure

Determining the working pressure

Fig. 2 Dry saturated steam – pressure/specific volume relationship

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Pressure bar g Specific volume m ³/kg

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Steam generation and distribution at a higher pressure will have the following advantages:

Smaller bore steam mains are required, resulting in lower capital cost of steam mains, for materials such as pipes, flanges, support work, and labour.

Lower capital cost of pipe insulation.

Drier steam at the point of use due to the drying effect of pressure reduction.

The thermal storage capacity of the boiler is increased, helping it to cope more efficiently with fluctuating loads, reducing the risk of priming and carryover at maximum conditions.

Having distributed at a higher 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 pressure required by the application.

Please note, it is often thought that running a steam boiler at a lower pressure than its design rated pressure will save fuel. This logic is based on more fuel being needed to raise steam to a higher pressure and thus temperature.

Whilst this is marginally so, ultimately, the rate at which energy is used is determined by the connected load not the boiler. Hence the same energy is used (say in kJs) by the load wether the boiler delivers at 4 bar g, 10 bar g or 100 bar g. Hence the energy supplied by the burner is exactly the same.

Standing losses and flue losses increase, but these can be reduced by insulation and heat recovery technology, and can be considered marginal when compared to the advantages of distributing steam at high pressure.

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Pressure reduction

A separator is used before the reducing valve to remove water from incoming wet steam, therefore allowing only dry saturated steam to pass through the reducing valve. This will be looked at in more detail later.

If a pressure reducing valve is used, it is appropriate to fit a safety valve downstream to protect the steam using equipment. Should the reducing valve fail, and allow the downstream pressure to increase, the steam using equipment may be permanently damaged, and the possibility of danger to personnel may result. With a safety valve fitted, any excess pressure is bled off through the valve, to prevent this from happening.

Other items completing the pressure reducing valve station are: The first isolating valve - to shut the system down for

maintenance.

The first pressure gauge - to monitor the integrity of supply. The strainer - to keep the system clean.

The second pressure gauge - to set and monitor the downstream pressure.

The second isolating valve - to set the downstream pressure on no load conditions.

Fig. 3 A typical pressure reducing valve station DP17 Separator Strainer Reducing valve Safety valve Trap set Steam Condensate Steam The most common method for pressure reduction is to use a pressure reducing station, similar to the one shown in Figure 3.

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Pipeline sizing

A natural tendency exists, when choosing pipe sizes, to be guided by the size of connections on equipment to which they will be connected. If the pipework is sized in this way, then the desired volumetric flowrate may not be achieved. The use of concentric and eccentric reducers can be used to correct this, enabling pipework to be properly sized.

Pipe sizes may be chosen on the basis of either: Fluid velocity.

Pressure drop.

In each case it is wise to check using both methods to ensure that the alternative limits are not being exceeded.

Oversizing of pipework means:

The pipes will be more expensive than necessary.

A greater volume of condensate will be formed due to greater heat loss.

Poorer steam quality and ultimate heat transfer due to the greater volume of condensate formed.

Higher installation costs.

In a particular example, the cost of installing 80 mm pipework was found to be 44 % more than the cost of 50 mm pipework which would have had adequate capacity. The heat lost by the insulated pipework was some 21 % more from the 80 mm line than it would have been from the 50 mm. Any uninsulated parts would have lost some 50 % more from the 80 mm, than from 50 mm size. This is due to the extra surface area available.

Undersizing of pipework means:

Higher steam velocity and pressure drop creating a lower pressure than required at point of use.

Risk of steam starvation at point of use. Effects of oversizing

and undersizing pipework

Fig. 4 Concentric and eccentric reducers

Steam

Eccentric Concentric

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Probably the most common pipe standard in global use is that derived from the American Petroleum Institute (API), where pipes are categorised in schedule numbers.

These schedules bear a relation to the pressure rating of the piping and are eleven in number ranging from the lowest at 5 through 10, 20, 30, 40, 60, 80, 100, 120, 140 to schedule no. 160. For piping 150 mm nominal size and smaller, schedule 40 (sometimes called 'standard weight') is the lightest which is specified. Only schedules 40 and 80 cover the full range from 15 mm up to 600 mm nominal sizes and are the most commonly used schedule for steam pipe installations. For the purposes of this guide, reference will be to pipework of schedule 80 (sometimes called 'extra strong').

Tables of schedule numbers can be obtained from BS 1600 which are used as a reference for the nominal pipe size and wall thickness in millimetres. Table 1 is an example of the bore sizes of different sized pipes, for different schedule numbers. In Europe, pipe is manufactured to DIN standards and DIN 2448 pipe is included in the table.

Pipeline standards and wall thickness

For a 25 mm schedule 80 pipe, the internal bore diameter of the pipe is 24.3 mm, likewise a schedule 40 pipe has an internal bore diameter of 26.6 mm.

Pipes most commonly used are heavy grade carbon steel (standard length 6 m) for steam mains and condensate lines.

Another term which is commonly used for pipe thickness is 'Blue band and Red band'. These are referred to from BS 1387, (Steel tubes and tubulars suitable for screwing to BS 21 threads), and apply to particular grades of pipe, Red being heavy, commonly used for steam pipe applications, and Blue being used as a medium grade, commonly used for air distribution systems. The coloured bands are 50 mm wide, and their positions on the pipe denote its length. Pipes less than 4 metres in length only have a coloured band at one end, while pipes of 4 to 7 metres in length have a coloured band at either end.

Example Pipe size (mm) 15 20 25 32 40 50 65 80 100 125 150 Schedule 40 15.8 21.0 26.6 35.1 40.9 52.5 62.7 77.9 102.3 128.2 154.1 Bore (mm) Schedule 80 13.8 18.9 24.3 32.5 38.1 49.2 59.0 73.7 97.2 122.3 146.4 Schedule 160 11.7 15.6 20.7 29.5 34.0 42.8 53.9 66.6 87.3 109.5 131.8 DIN 2448 17.3 22.3 28.5 37.2 43.1 60.3 70.3 82.5 107.1 131.7 159.3 Table 1

Fig. 5 Pipe band locations Single band.

Up to 4 m in length

Double band. Between 4 m - 7 m in length

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If pipework is sized on the basis of velocity, then calculations are based on the volume of steam being carried in relation to the cross sectional area of the pipe.

For dry saturated steam mains, practical experience shows that reasonable velocities are 25 - 40 m/s, but these should be regarded as the maxima above which 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 if high pressure drops are to be avoided. By using Table 2 (page 13) as a guide, it is possible to select pipe sizes from the steam pressure, velocity and flowrate.

Alternatively the pipe size can be calculated by following the mathematical procedure as outlined below. In order to do this, we need to define the following information:

Flow velocity (m/s) C

Specific volume (m3_/kg) _v

Mass flowrate (kg/s) m

Volumetric flowrate (m³/s) V = m(kg/s) x v(m3_/kg) From this information, the cross sectional area (A) of the pipe can be calculated:

Volumetric flowrate (V) Cross sectional area (A) =

Flow velocity m/sec (C)

i.e: p x D2 = V

4 C

This formula can be rearranged to give the diameter of the pipe:

D² = 4 x V

p x C

\ D = 4 x V

p x C

This will produce the diameter of the pipe in metres. It can easily be converted into millimetres by multiplying by 1 000.

Pipeline sizing on steam velocity ● ● ● ● ● ● ●

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Example It is required to size a pipeline to handle 5 000 kg/h of dry saturated steam a 7 bar g, and 25 m/s required flow velocity.

- Flow velocity (C) = 25 m/s

- Specific volume (v) = 0.24 m³/kg (from steam tables)

- Mass flowrate (m) = 5 000 kg/h = 1.389 kg/s 3 600 s/h - Volumetric flowrate (V) = m x v = 1.389 kg/s x 0.24 m³/kg = 0.333 m³/s Therefore, using:

Cross sectional area (A) = Volumetric flowrate (V) Flow velocity (C) p x D² = 0.333 4 25 D = 4 x 0.333 p x 25 D = 0.130 m or 130 mm

An alternative method is to use Figure 6 (page 14) for calculating pipe sizes by velocity. This method will work if you know the following requirements; Steam pressure, temperature (if superheated), flowrate and velocity. The example below will help to explain how this method works.

Using the above example, it is required to size a pipeline to handle 5 000 kg/h of saturated steam at 7 bar g. The maximum acceptable steam velocity is 25 m/s.

Method refer to Figure 6, page 14.

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). Now draw a vertical line to the steam velocity of 25 m/s (point C). From C, draw a horizontal line across the pipe diameter scale (point D). A pipe with a bore of 130 mm will suffice in this case.

Example

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Sometimes it is essential that the steam pressure feeding an item of plant is not allowed to fall below a specified minimum, in order to maintain temperature, thus ensuring that plant heat transfer factors are maintained under full load conditions. Here, it is appropriate to size the pipe on the 'pressure drop' method, by using the known pressure at the supply end of the pipe and the required pressure at the point of use.

There are numerous graphs, tables and even slide rules available for relating pipe size to pressure drop. One method which has proved satisfactory, is the use of utilizing pressure drop factors. An example of this method is shown in the appendix at the end of this guide.

An alternative and quicker method to sizing pipelines on the basis of pressure drop, is to use Figure 7 (page 15) if the following variables are known: steam temperature, pressure, flowrate and pressure drop requirements.

It is required to size a pipeline to handle 20 000 kg/h of superheated steam at 15 bar g pressure at 300°C, and a pressure drop of 0.3 bar/100 m.

Draw a vertical line from 300°C (point A) on the temperature scale to 15 bar g (point B) on the pressure scale. From B, draw a horizontal line to the steam flowrate of 20 000 kg/h (Point C). Now draw a vertical line to the top of the graph. Draw a horizontal line from 0.3 bar/100 m on the pressure loss scale (point D). The point at which this line crosses the vertical line from point C (point E), will determine the pipe size required. In this case 200 mm.

Example Pipeline sizing on pressure drop

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These pipelines should be sized using the pressure drop method. Calculations usually consider higher pressures and flowrates and superheated steam. The calculation uses a pressure ratio between the total pressure drop and inlet pressures, which may be utilised in Figure 8 (page 16).

It is required to size a pipe to handle 20 tonnes of steam per hour at a pressure of 14 bar gauge and a temperature of 325°C. The length of the pipe is 300 metres and the permissible pressure drop over this length is 0.675 bar.

Note that the chart is in absolute pressure and for an exercise of this kind, it is sufficiently accurate to approximate that 14 bar gauge equals 15 bar absolute.

First find the pressure ratio:

Ratio = Pressure drop

Inlet pressure (absolute)

= 0.675

15

= 0.045

From this point on the left hand scale, read horizontally to the right and at the intersection (A) with the curved line, read vertically upwards to meet the length line of 300 metres (B). At this point, extend the horizontal line across the chart to point C.

Now read from the base temperature line at 325°C and extend vertically upwards to meet the 15 bar abs. pressure line (D). Read horizontally to the right to meet the line of 20 tonnes/h (E) and from this point, extend a line vertically upwards. The pipe size is indicated where this line intersects line B - C at point F. This shows a pipe size of 200 mm.

This procedure can also be reversed to find the pressure drop in a known pipe size.

Example Pipeline sizing for larger and longer steam mains

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Pressure Velocity kg/h bar m/s 15mm 20mm 25mm 32mm 40mm 50mm 65mm 80mm 100mm 125mm 150mm 200mm 250mm 300mm 15 7 14 24 37 52 99 145 213 394 648 917 1606 2590 3678 0.4 25 10 25 40 62 92 162 265 384 675 972 1457 2806 4101 5936 40 17 35 64 102 142 265 403 576 1037 1670 2303 4318 6909 9500 15 7 16 25 40 59 109 166 250 431 680 1006 1708 27911 3852 0.7 25 12 25 45 72 100 182 287 430 716 1145 1575 2816 4629 6204 40 18 37 68 106 167 298 428 630 1108 1712 2417 4532 7251 10323 15 8 17 29 43 65 112 182 260 470 694 1020 1864 2814 4045 1.0 25 12 26 48 72 100 193 300 445 730 1160 1660 3099 4869 6751 40 19 39 71 112 172 311 465 640 1150 1800 2500 4815 7333 10370 15 12 25 45 70 100 182 280 410 715 1125 1580 2814 4845 6277 2.0 25 19 43 70 112 162 295 428 656 1215 1755 2520 4815 7525 10575 40 30 64 115 178 275 475 745 1010 1895 2925 4175 7678 11997 16796 15 16 37 60 93 127 245 385 535 925 1505 2040 3983 6217 8743 3.0 25 26 56 100 152 225 425 632 910 1580 2480 3440 6779 10269 14316 40 41 87 157 250 375 595 1025 1460 2540 4050 5940 10476 16470 22950 15 19 42 70 108 156 281 432 635 1166 1685 2460 4816 7121 10358 4.0 25 30 63 115 180 270 450 742 1080 1980 2925 4225 7866 12225 17304 40 49 116 197 295 456 796 1247 1825 3120 4940 7050 12661 19663 27816 15 22 49 87 128 187 352 526 770 1295 2105 2835 5548 8586 11947 5.0 25 36 81 135 211 308 548 885 1265 2110 3540 5150 8865 14268 20051 40 59 131 225 338 495 855 1350 1890 3510 5400 7870 13761 23205 32244 15 26 59 105 153 225 425 632 925 1555 2525 3400 6654 10297 14328 6.0 25 43 97 162 253 370 658 1065 1520 2530 4250 6175 10629 17108 24042 40 71 157 270 405 595 1025 1620 2270 4210 6475 9445 16515 27849 38697 15 29 63 110 165 260 445 705 952 1815 2765 3990 7390 12015 16096 7.0 25 49 114 190 288 450 785 1205 1750 3025 4815 6900 12288 19377 27080 40 76 177 303 455 690 1210 1865 2520 4585 7560 10880 19141 30978 43470 15 32 70 126 190 285 475 800 1125 1990 3025 4540 8042 12625 17728 8.0 25 54 122 205 320 465 810 1260 1870 3240 5220 7120 13140 21600 33210 40 84 192 327 510 730 1370 2065 3120 5135 8395 12470 21247 33669 46858 15 41 95 155 250 372 626 1012 1465 2495 3995 5860 9994 16172 22713 10.0 25 66 145 257 405 562 990 1530 2205 3825 6295 8995 15966 25860 35890 40 104 216 408 615 910 1635 2545 3600 6230 9880 14390 26621 41011 57560 15 50 121 205 310 465 810 1270 1870 3220 5215 7390 12921 20538 29016 14.0 25 85 195 331 520 740 1375 2080 3120 5200 8500 12560 21720 34139 47218 40 126 305 555 825 1210 2195 3425 4735 8510 13050 18630 35548 54883 76534 15 60 145 246 372 558 972 1524 2244 3864 6258 8868 15505 24646 34819 17.0 25 102 234 397 624 888 1650 2496 3744 6240 10200 15072 26064 40967 56662 40 151 366 666 990 1452 2634 4110 5682 10212 15660 22356 42658 65860 91841

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Fig. 6 Superheated and saturated steam pipeline sizing chart (velocity method) B C D Steam velocity m/s Steam flowrate kg/h 50 % Vacuum

Steam pressure bar g 600 500 15 10 25 20 30 400 150 175 200 250 300 125 100 80 70 60 50 40 150 5 100 10 20 30 50 100 200 500 1 000 2 000 3 000 ₂₀ 30 Steam temperature °C 100 200 300 400 500 200 000 100 000 50 000 20 000 30 000 10 000 5 000 100 75 50 20 30 10 50 5 3 2 1 0.5 0 bar g 7 10 A Pipe diameter mm

The dotted line A, B, C, D refers to the example on page 10

Saturation temperature

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Fig. 7 Steam pipeline sizing chart (pressure drop method) 400 300 200 100 500 18 10 5 3

Pressure loss bar/100 m

2 1 0.5 0.2 0.3 0.1 0.05 0.03 0.02 0.01 50 % Vacuum 0 bar g 0.5 1 2 3 5 7 A 10 B 20 30 50 100 75 Steam temperature °C Steam flowrate kg/h 200 000 100 000 C 50 000 30 000 5 000 3 000 2 000 1 000 500 300 200 100 50 30 20 10 20 000 10 000

Inside pipe diameter mm 600 500 400 300 250 200 150 125 100 80 70 60 50 40 30 25 20 15 10

The dotted line A, B, C, D, E refers to the example on page 11 Steam pressure bar g

Saturation temperature

curve

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Figure 8 Pipe sizing chart for larger steam mains 0.8 0.6 0.7 0.5 0.4 0.3 0.2 0.09 0.1 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.009 0.008 0.007 0.006 0.005 0.004 0.003 Ratio D P =

Pressure drop bar

Inlet pressure bar abs

100 200 300 400 500 110 120100 80 70 60 50 40 30 25 15 20 10 8 6 5 4 3 2 1

Steam inlet pressure bar abs

10 20 40 70 150 300 500 1 000 2 000 4 000 Pipe length m 7 000 15 30 50 100 200 400 700 1500 10000 50 70 100 150 200 300 400 500 750 Pipe diameter mm F C 600 450 350 250 175 125 80 60 300 150 70 40 20 10 6 4 2 1 200 100 50 30 15 8 1.5 3 5 5000 3000

G = Steam mass flowrate tonne/h

E D Steam temperature °C B A 0.9 ₄ 6 8 10 15 20 30 40 60 80 150 Steam velocity m/s

The dotted line A, B, C, D, E refers to the example on page 12

100

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In any steam main, some steam will condense due to radiation losses. For example, a well lagged 100 mm line 50 m long carrying steam at 7 bar, with surrounding air at 20°C, will condense approximately 26 kg of steam per hour, when heated from cold. This is probably less than 1 % of the carrying capacity of the main. Nevertheless it means, that at the end of 1 hour if not drained, the main would contain not only steam, but at least 26 litres of water and progressively more with time.

So some provision must be made for draining off this water. If this is not done effectively, problems such as corrosion and waterhammer will set in, which will be covered later. In addition, the steam will become wet as it picks up water droplets, thereby reducing its heat transfer potential. Under extreme conditions if water is allowed to build up, the overall effective cross sectional area of the pipe is reduced, hence increasing steam velocity above recommended limits.

Whenever possible the main should be run with a fall of not less than 100 mm in 10 m, in the direction of the steam flow. If the steam main rises in the direction of flow, then the condensate will tend to be dragged uphill with the steam flow. Instead relay points may be installed allowing the pipe to fall in the direction of flow between the points. Refer to the Figure 9 for further details. By installing the pipework with a fall in the direction of steam flow, both steam and condensate will run in the same direction. A drain point is needed at the foot of each relay, and the steam and condensate will run in the same direction towards the drain points. The subject of drainage from steam lines is covered in the UK British Standard BS 806, section 4.12.

Steam mains and drainage

Fig. 9 Diagram of rising ground pipework Rising ground

Trap set

Condensate Steam

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The benefits of selecting the most appropriate type of steam trap for a particular application will be wasted if condensate cannot easily find its way to the trap. For this reason, careful consideration should always be given to the size and situation of the drain point. Consideration should also be given to what happens to condensate in a steam main at shut-down when all flow ceases. Due to gravity the water will run along falling pipework and collect at the lower points in the system. Steam traps should therefore be fitted to these low points.

However, 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 30 m to 50 m, as well as at natural low points.

In normal operation steam may flow along the main at speeds of up to 145 km/h, dragging condensate along with it. Figure 10 shows a 15 mm drain pipe connected from the bottom of a main to a steam trap. Although the 15 mm pipe has sufficient capacity, it is unlikely to catch much of the condensate moving along the main at high speed. Such an arrangement will be ineffective. A more reliable solution for the removal of condensate is shown in Figure 11. The drain line off-take should be at least 25 to 30 mm from the bottom of the pocket for steam mains up to 100 mm, and roughly a third to centre of the pocket for larger mains, allowing a space below for any dirt and scale to settle. The bottom of the pocket may be fitted with a removable flange or blowdown valve for cleaning purposes.

Steam mains diameters Drain pocket

up to 100mm Bore same as main

depth at least 100 mm

125, 150, 200 mm Bore 100 mm; depth at least 150 mm

250 mm and above Bore half that of main

depth at least diameter of main Drain points

Fig. 10 Incorrect Fig. 11 Correct

Steam trap Pocket

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Waterhammer may occur when condensate is pushed along a pipe by the steam instead of being drained away at the low points, and is suddenly stopped by impacting on an obstacle in the system. The build up of droplets of condensate along a length of steam pipework, as shown in Figure 12 eventually forms a 'solid' slug which will be carried at steam velocity along the pipework. Such velocities can be of 30 m/s or more. This slug of water is dense and incompressible, and, when travelling at high velocity, has a considerable amount of kinetic energy.

Waterhammer and its effects

When obstructed, perhaps by a bend or tee in the pipe, the kinetic energy of the water is converted into pressure energy and a pressure shock is applied to the obstruction. (The laws of thermodynamics, state that energy cannot be created or destroyed, but is simply converted into a different form). Commonly there is a banging noise, and perhaps movement of the pipe. In severe cases the fitting may fracture with almost explosive effect, with consequent loss of live steam at the fracture, providing a hazardous situation.

Fortunately, waterhammer may be avoided if steps are taken to ensure that the condensate in the pipework is not allowed to collect along the pipework.

Avoiding waterhammer is a better alternative than attempting to contain it by choice of materials, and pressure ratings of equipment.

Common sources of waterhammer trouble occur at the low points in the pipework (See Figure 13). Such areas are:

Sags in the line.

Incorrect use of concentric reducers and strainers. For this Fig. 12 The formation of a 'solid' slug of water

Steam

Steam Steam

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To summarise, in order to minimise the possibility of waterhammer; Steam lines should be arranged with a gradual fall in the direction of flow, with drain points installed at regular intervals and at low points.

Check valves should be fitted after all traps which would otherwise allow condensate to run back into the steam line or plant during shut-down.

Isolation valves should be opened slowly to allow any condensate which may be lying in the system to flow gently towards, and through, the drain traps before it is picked up by high velocity steam. This is especially important at start-up.

Steam

Fig. 13 Potential sources of waterhammer trouble Steam

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Branch

Branchlines It is important to remember that branch lines are normally much

shorter in length than the steam mains. Sizing branches on the basis of a given pressure drop is accordingly less convenient on short lengths of pipe. With a main of 250 m length, a pressure drop limitation of 0.5 bar may be perfectly valid, even though it leads to the use of lower velocities than might be expected. In a branch line of only 5 m or 10 m length, the same velocity would lead to values of only 0.01 or 0.02 bar. Clearly these are insignificant, and it is usual to size branch lines on a higher steam velocity. This may create a higher pressure drop, but with a shorter pipe length, this pressure drop will be acceptable.

Sizes are often selected from a table, like the 'Pipeline capacities at specific velocities' table (Table 2). When using steam velocities of 25 to 35 m/s where short branch connections to equipment are being considered, it should be noted that the accompanying rate of pressure loss per unit length can be relatively high. A large pressure drop can be created if the pipeline contains several fittings like connections and elbows. Longer branch lines should be restricted to a velocity below 15 m/s unless the pressure drop is also calculated.

Fig. 14 Branchline

Steam main

Steam Steam

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Steam

Branch connections Branch connections taken from the top of the main carry the

driest steam. If taken from the side, or even worse from the bottom as Figure 15, they can carry the condensate from the main and in effect become a drain pocket. The result is very wet steam reaching the equipment. The valve in Figure 16 should be positioned as near to the off-take as possible to minimize condensate laying in the branch line, if shut-down for extended periods.

Fig. 15 Incorrect

Fig. 16 Correct

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Low points will also occur in branch lines. The most common is a drop leg near to an isolating valve or a control valve. Condensate builds up in front of the closed valve, and will be entrained with the steam when the valve opens again - consequently a drain point with a steam trap set is required at this point.

Drop leg

It is not uncommon for a steam main to run across rising ground, where the contours of the site make it quite impractical to lay the pipe with a natural fall, therefore the condensate must be induced to run downhill against the steam flow. It is then wise to make sure that the pipe size is large enough, over the rising section, to lower the steam velocity to not more than 15 m/s. Equally, the spacing between the drain points should be reduced, to not more than 15 m. The aim is to prevent the condensate film on the bottom of the pipe increasing in thickness to a point where droplets are picked up by the steam flow, Figure 18 below.

Rising ground and drainage

Fig. 17 Diagram of a drop leg

30 - 50 m Fall Drop leg Trap set Steam main Control valve Isolation valve Isolation valve Steam Steam Condensate Steam velocity 40 m/s Fall Increase in pipe diameter _Fall 15 m 15 m Steam velocity 15 m/s

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Modern packaged steam boilers have a high duty for their size and lack any reserve capacity to cope with overload conditions. Incorrect chemical feedwater treatment, TDS control and transient peak loads can cause serious priming and carryover of boiler water into the steam mains. The use of a separator to remove this water is shown in Figure 20. Selection is not difficult when using a sizing chart. See Figure 19.

Determine the size of separator required for a flowrate of 500 kg/h at 13 bar g pressure.

1. Taking the pressure and flowrate, draw line A - B. 2. Draw a horizontal line B - C.

3. Any separator size curve that is bisected by the line B - C within the shaded area will operate at near 100 % efficiency. 4. Additionally, line velocity for any size can be determined by

dropping a vertical line D - E. (e.g. 18 m/s for a size DN32 unit). 5. Also, pressure drop can be determined by plotting lines E - F

and A - F. The point of intersection is the pressure drop across the separator, i.e.: 0.037 bar approximately.

Steam separators

Separator sizing chart example

Fig. 19 Separator sizing chart

C

B D

A

E F

Steam pressure bar g

Separator size

Pressure drop across separator bar

DN150 DN125 DN100 DN80 DN65 DN50 DN40 _DN32 DN25 DN20 DN15 1 2 3 4 5 6 7 8 9 101112 16 18 20 22 24 25 5 10 15 20 25 30 35 40 10 000 5 000 2 000 1 000 500 200 100 50 20 10 0.002 0.02 0.01 0.05 0.1 0.2 Flow velocity m/s Steam flowrate kg/h

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Separators should be selected on the basis of the best compromise between line size, velocity and pressure drop for each application.

As soon as steam has left the boiler, some of it must condense to replace the heat being lost through the pipe wall. Insulation will naturally reduce the heat loss, but the heat flow and the condensation rate remain as small but finite amounts and if appropriate action is not taken these amounts will accumulate. The condensate will form droplets on the inside of the pipe wall, and these can merge into a film as they are swept along by the steam flow.

The water will also gravitate towards the bottom of the pipe, and so the thickness of the film will be greatest there. Steam flowing over this water film can raise ripples which can build up into waves. If this build up continues, the tips of the waves will break off, throwing droplets of condensate into the steam flow. The result is that the heat exchange equipment receives very wet steam, which reduces heat transfer efficiency and the working life of control valves. Anything that will reduce the propensity for wet steam in mains or branch lines will prove beneficial.

A separator will remove both droplets of water from pipe walls and suspended mist entrained in the steam itself. The presence and effect of waterhammer can be eradicated by fitting a separator in a steam main, and can often be a cheaper alternative than altering pipework to overcome this phenomenon.

Fig. 20 A typical cut section through a separator

Condensate to steam trap

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When new pipework is installed, it is not uncommon for fragments of casting sand, packing, jointing, swarf, welding rods and even nuts and bolts to be left inside. In the case of older pipework, there will be rust and in hard water districts, a carbonate deposit. From time to time, pieces will break loose and pass along the pipework with the steam, to rest inside a piece of steam using equipment, which could prevent a valve from opening/closing correctly The steam using equipment may also suffer permanent damage through wire drawing - the cutting action of high velocity steam and water passing through a partly open valve. Once wire drawing has occurred, the valve will never give a tight shut-off, even if the dirt is removed.

Therefore, it is sensible practice to fit a simple pipeline strainer in front of every steam trap, meter, reducing valve and regulating valve. The diagram shown in Figure 21 shows a typical strainer in section. Strainers

Fig. 21 A typical cut section through a strainer

Steam flows from the inlet 'A' through the perforated screen 'B' to the outlet 'C'. While steam and water will pass readily through the screen, the progress of dirt will be arrested. The cap 'D', can be removed, allowing the screen to be withdrawn and cleaned at regular intervals. A blowdown valve can also be fitted to the cap 'D' to facilitate regular cleaning.

Strainers however, can be a source of waterhammer trouble as previously mentioned. To avoid this problem strainers should be installed on their sides when they are part of a steam line.

C A

B

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The use of steam traps is the most efficient method of draining condensate from a steam distribution system.

The steam traps used to drain the main must be suitable for the system, and have sufficient capacity to pass the amounts of condensate reaching them with the pressure differentials which are present at any given time.

The first requirement is easily dealt with, the maximum working pressure at the steam trap will either be known or can readily be found. The second requirement covering the amounts of condensate reaching the trap under working conditions, when only the heat losses from the line are leading to condensation of the steam, may be calculated, or read with sufficient accuracy from Table 3 (page 31).

It should be remembered, that traps draining a boiler header may at times be required to discharge water carried over from the boiler with the steam. A total capacity of up to 10 % of the boiler rating is usually thought reasonable. In the case of the other traps further along the system, Table 3 page 31, shows that providing the drain points are not further apart than the recommended 50 m, the condensate loads will normally be well within the capacity of a 15 mm low capacity trap. Only in those rare applications of very high pressures (above 70 bar), combined with large pipe sizes, will greater trap capacity be needed.

A little more care is sometimes needed when steam lines are frequently shut-down and started up. Amounts of steam condensed while the pipes are being warmed from cold to working temperature are also listed in Table 3 page 31. Since these are steam masses rather than steam flowrates, the time allowed for the heating process must also be taken into account. For example, if a pipe is brought to working pressure in 20 minutes, then the hourly rate will be 60/20, or 3 times the load shown in the table.

During the first part of the heating up process, the condensing rate will be at least equal to the average rate. However, the pressure within the pipe will be only a little above atmospheric pressure, perhaps by 0.05 bar. This means that the capacity of the trap will be correspondingly reduced. In those cases where start-up loads are frequent, the DN15 steam trap with normal capacity may be a more appropriate choice

This also highlights another benefit of the large pipe-sized drain pocket, which, at start-up, can fill up with condensate when steam pressure may not be high enough to push it away through the trap.

Mains drainage method

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The specification for a mains drain trap should give due consideration to a number of aspects.

The steam trap should discharge at, or very close to saturation temperature, unless long cooling legs are used between the drain point and the trap. This means that the choice is often between mechanical traps like float and inverted bucket patterns, or thermodynamic traps.

Where mains are outside buildings and the possibility of frost damage arises, the thermodynamic steam trap is pre-eminent. Even if the installation is such that water is left in the trap at shut-down and freezing occurs, the thermodynamic trap may be thawed out without suffering any damage when it is to be brought back into use.

Historically, on poorly laid out installations where waterhammer may be prevalent, float traps may not have been ideal due to their susceptibility to float damage. Contemporary design and manufacturing techniques, now produce extremely robust units for mains drainage purposes. Float traps are certainly the first choice for proprietary separators. The high capacities which are readily achieved, and the near instantaneous response to rapid load increases, are desirable features.

Thermodynamic steam traps are also suitable, for draining longer runs of large diameter mains, especially where lines are in continuous service. Frost damage is then less likely. Typical steam traps which are used to drain condensate from mains are shown in Figure 22.

The subject of steam trapping is dealt with in more detail in the technical reference guide 'Steam Trapping and Air Venting'. Steam trap

selection

Fig. 22 Steam traps

Thermodynamic type Inverted bucket type

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Leaking steam is all too often ignored. However, leaks can be costly in both financial and environmental senses and therefore need prompt attention to ensure the steam system is working at its optimum efficiency with a minimum impact on the environment. For example, for each litre of heavy fuel oil burned unnecessarily to compensate for a steam leak, approximately 3 kg of carbon dioxide are emitted to the atmosphere.

Figure 23 illustrates the steam loss for various sizes of hole and this loss can be readily translated into an annual fuel saving based on either 8 400 or 2 000 hours of operation per year. Steam leaks

Fig. 23 Steam loss through leaks 1 000 500 400 300 200 100 50 40 30 20 10 5 4 3 1 2 3 4 5 10 14 12.5 mm 10 mm 7.5 mm 5 mm 3 mm Leaking hole

Steam leak rate kg/h

Coal tonnes/year

Heavy fuel oil x 1 000 litres/year Gas x 1 000 kWh/year 1 000 500 400 300 200 100 50 40 30 20 10 5 4 8 400 2 000

Hours per day Hours per year 8 400 2 000 1 2 3 4 5 10 20 30 40 50 100 200 500 400 300 200 100 50 40 30 20 10 5 4 3 2 1 2 3 4 5 10 20 30 40 50 100 0.5

Hours per year 8 400 2 000 1 000 500 400 300 200 100 50 40 30 20 10 5 5 000 4 000 3 000 2 000 1 000 500 400 300 200 100 50 40 30 20 Steam pressure bar (x 100 = kPa)

24 hour day, 7 day week, 50 week year = 8 400 hours 8 hour day, 5 day week, 50 week year = 2 000 hours

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To summarise this section, proper pipe alignment and drainage means observing a few simple rules:

Steam lines should be arranged to fall in the direction of flow, at not less than 100 mm per 10 metres of pipe.

Steam lines should be drained at regular intervals of 30-50 m and at any low points in the system.

Where drainage has to be provided in straight lengths of pipe, then a large bore pocket should be used to collect condensate. If strainers are to be fitted, then they should be fitted on their sides.

Branch connections should always be taken from the top of the main so the driest steam is taken.

Separators should be considered before any piece of steam using equipment ensuring that dry steam is obtained.

Traps selected should be robust for the job to avoid the risk of waterhammer damage, and appropriate for their environment. (i.e. frost damage).

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Steam Main size - mm -18°C pressure correction bar g 50 65 80 100 125 150 200 250 300 350 400 450 500 600 factor 1 5 9 11 16 22 28 44 60 79 94 123 155 182 254 1.39 2 6 10 13 19 25 33 49 69 92 108 142 179 210 296 1.35 3 7 11 14 20 25 36 54 79 101 120 156 197 232 324 1.32 4 8 12 16 22 30 39 59 83 110 131 170 215 254 353 1.29 5 8 13 17 24 33 42 63 70 119 142 185 233 275 382 1.28 6 9 13 18 25 34 43 66 93 124 147 198 242 285 396 1.27 7 9 14 18 26 35 45 68 97 128 151 197 250 294 410 1.26 8 9 14 19 27 37 47 71 101 134 158 207 261 307 428 1.25 9 10 15 20 28 38 50 74 105 139 164 216 272 320 436 1.24 10 10 16 20 29 40 51 77 109 144 171 224 282 332 463 1.24 12 10 17 22 31 42 54 84 115 152 180 236 298 350 488 1.23 14 11 17 23 32 44 57 85 120 160 189 247 311 366 510 1.22 16 12 19 24 35 47 61 91 128 172 203 265 334 393 548 1.21 18 17 23 31 45 62 84 127 187 355 305 393 492 596 708 1.21 20 17 26 35 51 71 97 148 220 302 362 465 582 712 806 1.20 25 19 29 39 56 78 108 164 243 333 400 533 642 786 978 1.19 30 21 32 41 62 86 117 179 265 364 437 571 702 859 1150 1.18 40 22 34 46 67 93 127 194 287 395 473 608 762 834 1322 1.16 50 24 37 50 73 101 139 212 214 432 518 665 834 1020 1450 1.15 60 27 41 54 79 135 181 305 445 626 752 960 1218 1480 2140 1.15 70 29 44 59 86 156 208 346 510 717 861 1100 1396 1694 2455 1.15 80 32 49 65 95 172 232 386 568 800 960 1220 1550 1890 2730 1.14 90 34 51 69 100 181 245 409 598 842 1011 1288 1635 1990 2880 1.14 100 35 54 72 106 190 257 427 628 884 1062 1355 1720 2690 3030 1.14 120 42 64 86 126 227 305 508 748 1052 1265 1610 2050 2490 3600 1.13 50 65 80 100 125 150 200 250 300 350 400 450 500 600 1 5 5 7 9 10 13 16 19 23 25 28 31 35 41 1.54 2 5 6 8 10 12 14 18 22 26 28 32 35 39 46 1.50 3 6 7 9 11 14 16 20 25 30 32 37 40 45 54 1.48 4 7 9 10 12 16 18 23 28 33 37 42 46 51 61 1.45 5 7 9 11 13 17 20 24 30 36 40 46 49 55 66 1.43 6 8 10 11 14 18 21 26 33 39 43 49 53 59 71 1.42 7 8 10 12 15 19 23 28 35 42 46 52 56 63 76 1.41 8 9 11 14 16 20 24 30 37 44 49 57 61 68 82 1.40 9 9 11 14 17 21 25 32 39 47 52 60 64 72 88 1.39 10 10 12 15 17 21 25 33 41 49 54 62 67 75 90 1.38 12 11 13 16 18 23 26 36 45 53 59 67 73 81 97 1.38 14 12 14 17 20 26 30 39 49 58 64 73 79 93 106 1.37 16 12 15 18 23 29 34 42 52 62 68 78 85 95 114 1.36 18 14 16 19 24 30 36 44 55 66 72 82 90 100 120 1.36 20 15 17 21 25 31 37 46 58 69 76 86 94 105 125 1.35 25 15 19 23 28 35 42 52 66 78 86 97 106 119 141 1.34 30 17 21 25 31 39 47 51 73 87 96 108 118 132 157 1.33 40 20 25 30 38 46 56 70 87 104 114 130 142 158 189 1.31 50 24 29 34 44 54 65 82 102 121 133 151 165 184 220 1.29 60 27 32 39 50 62 74 95 119 140 155 177 199 222 265 1.28 70 29 35 43 56 70 82 106 133 157 173 198 222 248 296 1.27 80 34 42 51 66 81 97 126 156 187 205 234 263 293 350 1.26 90 38 46 56 72 89 106 134 171 204 224 265 287 320 284 1.26 100 41 50 61 78 96 114 149 186 220 242 277 311 347 416 1.25

Table 3 Warm-up / running loads per 50 m of steam main

Running loads per 50 m of steam main (kg/h) Warm-up loads per 50 m of steam main (kg/h)

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Material Temperature range °C

< 0 0 - 100 0 - 200 0 - 315 0 - 400 0 - 485 0 - 600 0 - 700

Mild steel 0.1-0.2 % C 12.8 14.0 15.0 15.6 16.2 17.8 17.5 —

Alloy steel 1 % Cr 0.5 % Mo 13.8 14.4 15.1 15.8 16.6 17.3 17.6 — Stainless steel 18 % Cr 8 % Ni 9.4 20.0 20.9 21.2 21.8 22.3 22.7 23.0

Pipe expansion and support

Allowance for expansion

All pipes will be installed at ambient temperature. Pipes carrying hot fluids, whether water, or steam, operate at higher temperatures. It follows that they expand, especially in length, with an increase from ambient to working temperatures. This may create stresses upon certain areas within the distribution system, such as a pipe joints which could be fractured. The amount of the expansion is readily calculated using the following equation, or read from appropriate charts.

Expansion = L x D_t x a (mm)

where: L = Length of pipe between anchors (m)

D_t = Temperature difference °C

a = Expansion coefficient (mm/m°C) x 10-_³ Table 4 Expansion coefficients (a)

Find the expansion of 30 m of pipe from ambient (10°C) to 152°C (steam at 4 bar g) L = 30 m D_t = 152°C - 10°C = 142°C a = 15.0 x 10-_{³ mm/m°C} \ Expansion = 30 x 142 x 15.0 x 10-_{³ mm} i.e. expansion = 64 mm

Alternatively, the amount of pipe expansion can be determined by using Table 6 (page 40) to calculate the amount of expansion over 10 m of pipe for the different pipe materials. Expansion charts like Figure 34 (page 41) are also an easy method for determining the amount of expansion.

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The pipework must be sufficiently flexible to accommodate the movements of the components as it heats up. In most cases the pipework has enough natural flexibility, by virtue of having reasonable lengths and plenty of bends, that no undue stresses are set up. In other installations, it will be necessary to build in some means of achieving the required flexibility. An example of building in flexibility is when condensate is drained from a steam main drain trap to a condensate main. In this case, the difference between the expansion of the two mains due to the change in temperature or the pipes' material expansion rates must be remembered.

The steam main may be at a temperature very much above that of the return main, and the two connection points can move in relation to each other during system warm up. Some flexibility should be incorporated in the steam trap piping so that branch connections do not become over stressed. (See Figure 24).

The amount of movement to be taken up by the piping and any device incorporated in it can be reduced by the use of 'cold draw'. The total amount of expansion is first calculated for each section between fixed anchor points. The pipes are left short by half this amount, and stretched cold, as by pulling up bolts at a flanged joint, so that at ambient temperature, the system is stressed in one direction. When warmed through half the total temperature rise, the piping is unstressed. At working temperature and having fully expanded, the piping is stressed in the opposite direction. The Pipework flexibility

Fig. 24 Flexibility in connection to condensate return line Steam main

Condensate main

Steam Steam

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In practical terms, the pipework is assembled with a spacer piece, of length equal to half the expansion, between two flanges. When the pipework is fully installed and anchored, the spacer is removed and the joint pulled up tight. (See Figure 25).

The remaining part of the expansion, if not accepted by the natural flexibility of the pipework will call for the use of an expansion fitting. Pipework expansion and support in practice, can therefore be classified into the following three areas as shown in Figure 26 below.

The fixed point support (A) provides a datum position from which expansion takes place.

The variable anchor point (B) will allow free movement for expansion of the pipework, while keeping the pipeline in alignment.

Fig. 25 Use of spacer for expansion when pipework is installed

Fig. 26 Diagram of pipeline with fixed point, variable anchor point and expansion fitting Position after cold draw

Hot position

Half calculated expansion over length L

Neutral position _{Spacer piece}

Point A Fixed Point B Variable anchor Point C Expansion fitting Point A Fixed Point B Variable anchor

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Fig. 27 Chair and roller

Roller supports are an ideal method for supporting pipes, while allowing them to move in two directions. For steel pipework, the rollers should be manufactured from ferrous material. For copper pipework, they should be manufactured from non-ferrous material. It is good practice for pipework supported on rollers to be fitted with a pipe saddle bolted to a support bracket at not more than 6 metre centres to keep the pipework in alignment while expansion and contraction occurs.

Where two pipes are to be supported, it is bad practice to carry the bottom pipe from the top pipe using a pipe clip. This will cause extra stress to be added to the top pipe whose thickness has been sized to take only the stress of its working pressure.

All pipe supports should be specifically designed to suit the outside diameter of the pipe concerned.

The expansion fitting (C) is one method of accommodating for the expansion. These fittings are placed within a line, and are designed to accommodate the expansion, without the total length of the line changing.

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Full loop (Figure 29)

This is simply one complete turn of the pipe and should preferably be fitted in a horizontal rather than a vertical position to prevent condensate building up. The downstream side passes below the upstream side and great care must be taken that it is not fitted the wrong way round. When full loops are to be fitted in a confined space, care must be taken in ordering, otherwise wrong handed loops may be supplied. The full loop does not produce a force in opposition to the expanding pipework as in some other types, but with steam pressure inside the loop, there is a slight tendency to unwind, which puts an additional stress on the flanges.

This design is rarely used today due to the space taken up by the pipework, and proprietory expansion bellows are now readily available. However large steam uses such as power stations or establishments with large outside distribution systems still tend to use loop type expansion devices, as space is usually available and cost is relatively low.

Horseshoe or lyre loop (Figure 30)

When space is available this type is sometimes used. It is best fitted horizontally so that the loop and the main are on the same plane. Pressure does not tend to blow the ends of the loop apart, but there is a very slight straightening out effect. This is due to the design but causes no misalignment of the flanges. In other cases, the 'loop' is fabricated from straight lengths of pipe and 90° bends. This may not be effective and requires more space, but it meets the same need. If any of these arrangements are fitted with the loop vertically above the pipe then a drain point must be provided on the upstream side.

Expansion fittings

Expansion loops (Figure 30)

A development of the Horse shoe loop, expansion loops are fabricated from lengths of straight pipes and elbows welded at the joins. The amounts of expansion which can be accommodated in such assemblies are shown in Figures 36 and 37 page 42.

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Sliding joint (Figure 32)

These are sometimes used because they take up little room, but it is essential that the pipeline is rigidly anchored and guided, to the manufacturers' instructions, otherwise steam pressure acting on the cross sectional area of the sleeve part of the joint tends to blow the joint apart in opposition to the forces produced by the expanding pipework. Misalignment will cause the sliding sleeve to bend, while regular maintenance of the gland packing is also needed.

Bellows (Figure 33)

A simple bellows has the advantage that it is an in-line fitting and requires no packing as does the sliding joint type. But it does have the same disadvantages as the sliding joint in that pressure inside tends to extend the fitting so that anchors and guides must be able to withstand this force.

Fig. 32 Sliding joint Fig. 31 Expansion loop

Fig. 33 Bellows Welded bend radius = 1.5 dia Weld joint 2W W

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Bellows can incorporate limit rods which limit over-compression and over-extension of the element. These may have little function under normal operating conditions, as most simple bellows assemblies are able to withstand small lateral and angular movement. However, in the event of anchor failure, they behave as tie rods and contain the pressure thrust forces, preventing damage to the unit whilst reducing the possibility of further damage to piping, equipment and personnel.

Where larger forces are expected, some form of additional mechanical re-inforcement should be built into the device, such as hinged stay bars.

There is invariably more than one way to accomodate the relative movement between two laterally displaced pipes depending upon the relative positions of bellows anchors and guides, but generally, axial displacement is better than angular which, in turn, is better than lateral. Angular and lateral movement should be avoided wherever possible.

Figure 34 a, b, and c give a simple indication of the effects of these movements, but, under all circumstances, it is highly recommended that expert advice is sought from the bellows manufacturer regarding any installation.

Fig. 34a Axial movement of bellows Short distance Guides Fixing point Guides Axial movement Axial movement

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Fig. 34b Small lateral and angular movement of bellows Medium distance Guides Fixing point Guides limit rods Small lateral movement Small lateral movement Small angular movement Small angular movement Long distance Fixing point

hinged stay bars

large angular movement Axial movement large angular movement

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Nominal pipe size (mm) Interval of horizontal run Interval of vertical run

Steel / Copper metres metres

Bore Outside dia. Mild steel Copper Mild steel Copper

12 15 1.0 1.2 15 18 2.0 1.2 2.4 1.4 20 22 2.4 1.4 3.0 1.7 25 28 2.7 1.7 3.0 2.0 32 35 2.7 1.7 3.0 2.4 40 42 3.0 2.0 3.6 2.4 50 54 3.4 2.0 4.1 2.4 65 67 3.7 2.0 4.4 2.9 80 76 3.7 2.4 4.4 3.2 100 108 4.1 2.7 4.9 3.6 125 133 4.4 3.0 5.3 4.1 150 159 4.8 3.4 5.7 200 194 5.1 6.0 250 267 5.8 5.9

The frequency of pipe supports will vary according to the bore of the pipe; the actual pipe material (i.e. steel or copper); and whether the pipe is horizontal or vertical.

Generally, pipe supports should be provided which comply with BS 3974, Part 1, 1974: 'Pipe hangers, slider and roller type supports.'

Some of the important points are as follows:

Pipe supports should be provided at joints in the pipe, i.e. bends, tees, valves, flanges and at intervals not greater than shown in the next table, Recommended support spacing for steel pipes. The reason for supporting at joints is to eliminate the stresses in screwed or flanged joints.

Where two or more pipes are supported on a common bracket, the spacing between the supports should be that for the smallest pipe. When an appreciable movement will occur, i.e. where straight pipes are greater than 15 metres in length, the supports should be of the roller type as outlined previously.

The following table can be used as a guide when calculating the distance between pipe supports for steel and copper pipework. Pipe support spacing

Table 5 Recommended support for pipework

Vertical pipes should be adequately supported at the base, to withstand the total weight of the vertical pipe. Branches from vertical pipes should not be used as a means of support for the pipe, because this will place undue strain upon the tee joint. All pipe supports should be specifically designed to suit the outside diameter of the pipe concerned. The use of oversized pipe brackets is not good practice.

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Temperature Materials

°C _mm/10mC. steel 12 % Cr steel_mm/10m _mm/10m18/8 s.s Ductile iron_mm/10m _mm/10mCopper

-30 -4.99 -5.05 -7.79 -4.54 -7.16 -25 -4.44 -4.49 -6.92 -4.04 -6.38 -20 -3.90 -3.94 -6.05 -3.53 -5.59 -15 -3.35 -3.38 -5.19 -3.03 -4.79 -10 -2.80 -2.82 -4.32 -2.52 -4.00 -5 -2.24 -2.26 -3.46 -2.02 -3.20 0 -1.69 -1.69 -2.59 -1.51 -2.41 5 -1.13 -1.13 -1.73 -1.01 -1.61 10 -0.56 -0.57 -0.86 -0.50 -0.80 15 0.00 0.00 0.00 0.00 0.00 20 0.57 0.57 0.86 0.50 0.81 25 1.14 1.13 1.73 1.01 1.61 30 1.71 1.70 2.59 1.51 2.42 35 2.29 2.27 3.46 2.02 3.24 40 2.86 2.84 4.32 2.52 4.05 45 3.44 3.42 5.18 3.21 4.87 50 4.03 3.99 6.05 3.75 5.68 55 4.61 4.56 6.91 4.28 6.50 60 5.20 5.14 7.78 4.82 7.33 65 5.79 5.72 8.64 5.36 8.15 70 6.39 6.29 9.50 5.89 8.98 75 6.98 6.87 10.37 6.43 9.80 80 7.58 7.45 11.23 6.96 10.63 85 8.18 8.03 12.09 7.50 11.47 90 8.79 8.62 12.95 8.03 12.30 95 9.39 9.20 13.82 8.57 13.14 100 10.00 9.78 14.68 9.10 13.97 110 11.23 10.96 16.41 10.53 15.66 120 12.47 12.13 18.13 11.64 17.35 130 13.72 13.32 19.85 12.75 19.04 140 14.97 14.50 21.58 13.86 20.75 150 16.24 15.69 23.30 14.97 22.46 160 17.52 16.89 25.02 16.60 24.19 170 18.81 18.08 26.75 17.74 25.92 180 20.11 19.29 28.47 18.89 27.65 190 21.43 20.50 30.19 20.03 29.40 200 22.75 21.71 31.91 21.18 31.15 210 24.08 23.04 33.63 23.38 220 25.42 24.28 35.35 24.58 230 26.78 25.53 37.07 240 28.14 26.78 38.79 250 29.52 28.04 40.51 260 30.90 29.30 42.23 270 32.30 30.57 43.94 280 33.70 31.85 45.66 290 35.12 33.13 47.38 300 36.55 34.42 49.09 310 37.98 35.71 50.81 320 39.43 37.01 52.53 330 40.89 38.32 54.24 340 42.36 39.63 55.95 350 43.84 40.94 57.67 360 45.33 42.26 59.38 370 46.83 43.59 61.10 380 48.35 44.93 62.81 390 49.87 46.27 64.52 400 51.40 47.61 66.23 410 48.96 67.94 420 50.32 69.66 430 51.68 71.37 440 53.05 73.08 450 54.43 74.79 460 55.81 76.49 470 57.19 78.20

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Fig. 35 Expansion chart for mild steel pipe

bar g 1 2 3 4 5 7.5 10 15 20 25 30

°C 120 134 144 152 159 173 184 201 215 226 236 Temperature of saturated steam

500 400 300 200 50 200 100 50 40 30 20 10 5 10 20 30 40 50 100 200 300 500 1000 Temperature difference °C 2000 Expansion of pipe (mm) Length of pipe (m) 220 100

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Fig. 36 Copper expansion loop 200 175 150 125 100 75 50 25 200 100 90 80 70 60 50 40 30 20 0.5 1 1.5 2 2.5 2W W

Maximum pressure 10 bar

3.5 4

3

W. metres

Expansion from neutral position (mm)

Nominal pipe size (mm)

W. metres 25 2.5 3 2 1.5 1 0.5 25 30 40 50 60 70 80 90 100 200 300 400 50 75 100 125 150 175 200 3.5 4 4.5 5 W 2W

Maximum pressure 17 bar Maximum temperature 260°C

Welded bends radius = 1.5 dia. Expansion from neutral position (mm)

Nominal pipe size (mm)

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Air venting

It is often overlooked that when steam is first admitted to a pipe after a period of shut-down, 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. Nevertheless, these gases will accumulate within the pipe and in the steam spaces of heat exchangers when the steam condenses, unless steps are taken to discharge them. The warming up of the steam system will become a lengthy business which will contribute towards a fall in plant efficiency.

A further effect of air in a steam system will be the effect upon pressure and temperature. Air will exert its own partial pressure within the steam space, and this pressure will be added to the partial pressure of the steam in producing the total pressure present. Therefore, the actual steam pressure will be lower than that shown by the total pressure on a pressure gauge. The overall temperature will also be lower than that suggested by the pressure gauge. In reality this is usually a marginal effect. Far more important is the effect air has upon heat transfer. A layer of air only 1 micron thick can offer the same resistance to heat as a layer of water 25 microns thick, a layer of iron 2 mm thick or a layer of copper 17 mm thick. Therefore it is of utmost importance that air is removed from the system.

Automatic air vents for steam systems are nothing more than thermostatic steam traps, fitted above the level of any condensate so that only steam, or air, or steam/air mixtures can reach them. They are usually best located at the ends of the steam mains and the larger diameter branches as can be seen in Figure 37.

The discharge from the air vent can be piped to any safe place. In practice, it is often taken into the condensate line, where it is a gravity line falling towards a vented receiver.

Fig. 38 Draining and venting at the end of a main Steam

Condensate

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In addition to air venting at the end of a main, other parts of the steam system which may require air venting are:

In parallel with an inverted bucket trap which is relatively slow to air vent on start-up.

In awkward steam spaces such as at the opposite side to where steam enters a jacketed pan.

Where there is a large steam space, and a steam/air mixture is to be avoided.

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Reduction of heat losses

Once a steam main has warmed up, condensation will continue as heat is lost by radiation, the rate depending upon the steam temperature, ambient temperature, and the efficiency of the system insulation.

If a steam distribution system is to be as efficient as possible, then all appropriate steps should be taken to ensure that any heat losses are reduced to the economic minimum. The most economical thickness of insulation will depend upon several factors:

Installation cost.

Value of the heat carried by the steam. Size of the pipework.

Pipework temperature.

If the pipework to be insulated is outside, then the air velocity and potential dampness of the insulation must be taken into account. Most insulation materials depend on minute air cells for their effectiveness, which are held in a matrix of inert material such as mineral wool, fibreglass or calcium silicate. Typical installations use aluminium clad fibreglass, aluminium clad mineral wool and calcium silicate. It is important that insulating material is not crushed or allowed to become waterlogged. Adequate mechanical protection and water proofing are essential, especially in outdoor locations.

The heat loss from a steam pipe to water, or to water saturated insulation, can be as much as 50 times greater than from the same pipe to air. Particular care should be taken to protect steam lines which must run through waterlogged ground, or in ducts which may be subjected to flooding.

The need to insulate all hot parts of the system must be kept in mind. This includes all flanged joints on the mains, and also the valves and other fittings. It was, at one time, common to cut back the insulation at each side of a flanged joint, to leave access to the bolts for maintenance purposes. This meant about 0.3 m of pipe was deliberately left bare, in addition to the surface of the flanges themselves. The total effect was as if some 0.6 m of pipe had been left uninsulated at each joint. Fortunately, the availability of prefabricated insulating covers for flanged joints and boxes to insulate valves is now more widely appreciated. These are usually provided with fasteners so that they can readily be detached to provide access for maintenance purposes.

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Temperature Pipe size difference 15 20 25 32 40 50 65 80 100 150 steam to air mm mm mm mm mm mm mm mm mm mm °C W/m 56 54 65 79 103 108 132 155 188 233 324 67 68 82 100 122 136 168 198 236 296 410 78 83 100 122 149 166 203 241 298 360 500 89 99 120 146 179 205 246 289 346 434 601 100 116 140 169 208 234 285 337 400 501 696 111 134 164 198 241 271 334 392 469 598 816 125 159 191 233 285 285 394 464 555 698 969 139 184 224 272 333 333 458 540 622 815 1133 153 210 255 312 382 382 528 623 747 939 1305 167 241 292 357 437 437 602 713 838 1093 1492 180 274 329 408 494 494 676 808 959 1190 1660 194 309 372 461 566 566 758 909 1080 1303 1852

Note: Heat emission from bare horizontal pipes with ambient temperatures between 10°C and 21°C and still air conditions

The calculation of heat losses from pipes can be very complex and time consuming, as heat transfer theory by conduction, convection and radiation need to be considered. The formulae for these factors are all different, and assume that obscure data concerning pipewall thickness, heat transfer coefficients and various derived constants are easily available.

The derivation of these formulae is outside the scope of this guide, but it may be said that further information can be readily found in any good thermodynamics textbook. To add to this, an abundance of contemporary computer software exists to provide this service for the more discerning engineer.

This being so, the commonplace solution to the problem can easily be found by reference to Table 7 and a simple equation. The table assumes ambient conditions of between 10 - 21°C, and considers heat losses from bare horizontal pipes of different sizes with steam contained at various pressures.

Calculation of heat transfer

Table 7 Heat emission from pipes

Other factors can be included in the equation, for instance, should the pipe be lagged with insulation providing a reduction in heat losses to 15 % of the uninsulated pipe, then M is simply multiplied by a factor of 0.15.

Where:

M = Rate of condensation (kg/h) Q = Heat emission (W/m) (as Table 7)

L = Effective length of pipe, allowing for flanges and fittings(m) hfg= Specific enthalpy of evaporation (kJ/kg)

M = Q x L x 3.6 x f hfg

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Relevant UK and International

standards

Symbols have been used to indicate harmonised standards, technically equivalent standards, and related standards - º ; = and ¹ respectively.

BS 10 Specification for flanges and bolting for pipes, valves and fittings.

BS 21 = ISO 7/1 ¹ ISO 7/2 Specification for pipe threads for tubes

and fittings where pressure tight joints are made on the threads. BS 806 Specification for design and construction of ferrous piping installations for and in connection with land boilers.

BS 1306 Specification for copper and copper alloy piping systems. BS 1387 Specification for screwed and socketed tubes and tubulars and for plain end steel tubes suitable for welding and screwing to BS 21 pipe threads.

BS 1560 Circular flanges for pipes, valves and fittings (Class designated); Part 3 Section 3.1 Specification for steel flanges

(¹ ISO 7005); Part 3 Section 3.2 Specification for cast iron

flanges (¹ ISO 7005-2); Part 3 Section 3.3 Specification for copper alloy and composite flanges (¹ ISO 7005-3)

BS 1600 Dimensions of steel pipe for the petroleum industry. BS 1965 Specification for butt welding pipe fittings for pressure purposes.

BS 1710 Specification for identification of pipelines.

BS 2779 = IS0 228/1 and ISO 228/2 Specification for pipe threads for tubes and fittings where pressure tight joints are not made on the threads.

BS 3600 Specification for dimensions and masses per unit length of welded and seamless steel pipes and tubes for pressure purposes.

BS 3601 Specification for steel pipes and tubes with specified room temperature properties for pressure purposes.

BS 3602 Specification for steel pipes and tubes for pressure purposes: carbon and carbon manganese steel with specified elevated temperature properties.

BS 3603 Specification for carbon and alloy steel pipes and tubes with specified low temperature properties for pressure purposes.

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BS 3604 Steel pipes and tubes for pressure purposes: ferritic alloy steel with specified elevated temperature properties.

BS 3605 Austenitic stainless steel pipes and tubes for pressure purposes.

BS 3799 Specification for steel pipe fittings, screwed and socket welded for the petroleum industry.

BS 3974 Specification for pipe supports.

BS 4504 Part 3 Section 3.1 Specification for steel flanges; Section 3.2 Specification for cast iron flanges (¹ ISO 7005-2); Section 3.3 Specification for copper alloy and composite flanges (¹ ISO 7005/3).

Technical Reference Guide on Steam Distribution