Technical Reference Guide on Steam Distribution

TECHNICAL REFERENCE GUIDE

Steam distribution

Contents Introduction Steam distribution Steam system basics

2 2 2

Working pressure Determining the working pressure Pressure reduction

4 4 6

Pipeline sizing Effects of oversizing and undersizing pipework Pipeline standards and wall thickness Pipeline sizing on steam velocity Pipeline sizing on pressure drop Pipeline sizing for larger and longer steam mains

7 7 8 9 11 12

Steam mains and drainage Drain points Waterhammer and its effects Branchlines Branch connections Drop leg Rising ground and drainage Steam separators Strainers Mains drainage method Steam trap selection Steam leaks Summary

17 18 19 21 22 23 23 24 26 27 28 29 30

Pipe expansion and support Allowance for expansion Pipework flexibility Expansion fittings Pipe support spacing

32 32 33 36 40

Air venting

44

Reduction of heat losses Calculation of heat transfer

46 47

Relevant UK and international standards

49

Summary

51

Appendix 1 - Sizing on pipeline capacity and pressure drop

52

Further information

57

Appendix 2 - Steam tables

58

Appendix 3 - Conversion tables

60

1

Introduction Steam distribution

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.

Steam system basics

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.

2

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.

Pan

Steam Pan

Steam

Process vessel

Space heating system

Condensate Vat

Vat

Make-up water Condensate

Feedtank

Steam

Boiler Feedpump

Fig. 1 A typical steam circuit 3

Working pressure Determining the working pressure

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.

Specific volume m³/kg

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. 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 0

1

2

3

4

5

6

7

8

9

10

11

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

4

12 13 14 Pressure bar g

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.

5

Pressure reduction

The most common method for pressure reduction is to use a pressure reducing station, similar to the one shown in Figure 3. Safety valve Reducing valve DP17

Separator Steam

Steam Strainer

Trap set

Condensate

Fig. 3 A typical pressure reducing valve station 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.

6

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.

Steam

Steam

Concentric

Eccentric

Fig. 4 Concentric and eccentric reducers 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. Effects of oversizing and undersizing pipework

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. Greater risk of erosion, waterhammer and noise due to increase in steam velocity. 7

Pipeline standards and wall thickness

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.

Table 1 Pipe size (mm)

Bore (mm)

15

20

25

32

40

50

65

Schedule 40

15.8

21.0

26.6

35.1

40.9

52.5

62.7

77.9 102.3

128.2

154.1

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

Example

80

100

125

150

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.

Single band. Up to 4 m in length

Fig. 5 Pipe band locations 8

Double band. Between 4 m - 7 m in length

Pipeline sizing on steam velocity

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: ●

Cross sectional area (A)

=

Volumetric flowrate (V) Flow velocity m/sec (C)

p x D2 4

=

V C

i.e:

●

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

D² \ D

4xV pxC

= =

Ö

●

4xV pxC

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

9

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)

=

5 000 kg/h 3 600 s/h

●

- Mass flowrate (m) ●

=

1.389 kg/s

●

- 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 4

=

0.333 25

D

=

Ö 4pxx0.333 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. Example

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.

10

Pipeline sizing on pressure drop

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.

Example

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. Method refer to Figure 7, page 15. 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.

11

Pipeline sizing for larger and longer steam mains

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).

Example

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

Method refer to Figure 8, page 16. 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.

12

Table 2 Saturated steam pipeline capacities at specific velocities (schedule 80 pipe) Pressure bar

Velocity kg/h m/s 15mm 20mm 25mm 32mm 40mm 50mm 65mm 80mm 100mm 125mm 150mm 200mm 250mm 300mm

0.4

15 25 40

7 10 17

14 25 35

24 40 64

37 62 102

52 92 142

99 162 265

145 265 403

213 384 576

394 675 1037

648 972 1670

917 1457 2303

1606 2590 2806 4101 4318 6909

3678 5936 9500

0.7

15 25 40

7 12 18

16 25 37

25 45 68

40 72 106

59 100 167

109 182 298

166 287 428

250 430 630

431 716 1108

680 1145 1712

1006 1575 2417

1708 27911 3852 2816 4629 6204 4532 7251 10323

1.0

15 25 40

8 12 19

17 26 39

29 48 71

43 72 112

65 100 172

112 193 311

182 300 465

260 445 640

470 730 1150

694 1160 1800

1020 1660 2500

1864 2814 4045 3099 4869 6751 4815 7333 10370

2.0

15 25 40

12 19 30

25 43 64

45 70 115

70 112 178

100 162 275

182 295 475

280 428 745

410 656 1010

715 1215 1895

1125 1755 2925

1580 2520 4175

2814 4845 6277 4815 7525 10575 7678 11997 16796

3.0

15 25 40

16 26 41

37 56 87

60 100 157

93 152 250

127 225 375

245 425 595

385 632 1025

535 910 1460

925 1580 2540

1505 2480 4050

2040 3983 6217 8743 3440 6779 10269 14316 5940 10476 16470 22950

4.0

15 25 40

19 30 49

42 63 116

70 115 197

108 180 295

156 270 456

281 450 796

432 742 1247

635 1080 1825

1166 1980 3120

1685 2925 4940

2460 4816 7121 10358 4225 7866 12225 17304 7050 12661 19663 27816

5.0

15 25 40

22 36 59

49 81 131

87 135 225

128 211 338

187 308 495

352 548 855

526 885 1350

770 1265 1890

1295 2110 3510

2105 3540 5400

2835 5548 8586 11947 5150 8865 14268 20051 7870 13761 23205 32244

6.0

15 25 40

26 43 71

59 97 157

105 162 270

153 253 405

225 370 595

425 658 1025

632 1065 1620

925 1520 2270

1555 2530 4210

2525 4250 6475

3400 6654 10297 14328 6175 10629 17108 24042 9445 16515 27849 38697

7.0

15 25 40

29 49 76

63 114 177

110 190 303

165 288 455

260 450 690

445 785 1210

705 1205 1865

952 1750 2520

1815 3025 4585

2765 3990 7390 12015 16096 4815 6900 12288 19377 27080 7560 10880 19141 30978 43470

8.0

15 25 40

32 54 84

70 122 192

126 205 327

190 320 510

285 465 730

475 810 1370

800 1260 2065

1125 1870 3120

1990 3240 5135

3025 4540 8042 12625 17728 5220 7120 13140 21600 33210 8395 12470 21247 33669 46858

10.0

15 25 40

41 66 104

95 145 216

155 257 408

250 405 615

372 562 910

626 990 1635

1012 1530 2545

1465 2205 3600

2495 3825 6230

3995 5860 9994 16172 22713 6295 8995 15966 25860 35890 9880 14390 26621 41011 57560

14.0

15 25 40

50 85 126

121 195 305

205 331 555

310 520 825

465 740 1210

810 1375 2195

1270 2080 3425

1870 3120 4735

3220 5200 8510

5215 7390 12921 20538 29016 8500 12560 21720 34139 47218 13050 18630 35548 54883 76534

17.0

15 25 40

60 102 151

145 234 366

246 397 666

372 624 990

558 888 1452

972 1650 2634

1524 2496 4110

2244 3864 3744 6240 5682 10212

6258 8868 15505 24646 34819 10200 15072 26064 40967 56662 15660 22356 42658 65860 91841

13

Fig. 6 Superheated and saturated steam pipeline sizing chart (velocity method) 600 500 400

e St

am

l ve

ity

m

/s

C

oc 5

10

D

200 175 150 125 100

20

30 0 5

0 10 50 1

80 70 60 50 40

Pipe diameter mm

300 250

30 25 20 15 Steam pressure bar g

10

m ea t S

te ra w flo 10

/h kg

50 % 20 0 3

50 0 10

A Sa tur e mp n te atio curve

50 75 100

ure

14

20 30

rat

100

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

um

g 0 bar 0.5 1 2 3 5 7 10

0 20 0 50 00 B 0 0 1 0 0 0 2 00 3 00 5 0 000 10 000 0 20 0 00 0 3 00 0 50 0 00 0 10 0 00 20

Vacu

300 200 400 Steam temperature °C

500

Fig. 7 Steam pipeline sizing chart (pressure drop method) 18 10

15

1 0.5 0.3 0.2

E

D

0.1 0.05

400 5 0 0 Insi de p ipe 600 diam ete rm m

Pressure loss bar/100 m

3 2

20 25 30 40 50 60 70 80 100 125 150 200 250 300

10

5

0.03 0.02 0.01

m

Sa

g 0 bar 0.5 1 2 3 5 7 10

tura per tem tion rve cu

B

100 20 3000 500 10 00 20 3 0 00 0 50 0 00 10 000 20 30 000 000 50 000 100 000 Ste 20 am 0 00 flow 0 rate kg/ h

Vacuu

10

50 %

20 30 50

Steam pressure bar g

C

20 30

atu

50 75 100

re

A 100

300 200 400 Steam temperature °C

500

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

15

Figure 8 Pipe sizing chart for larger steam mains

le

ng

th

m

ne/

h

150 300

1

2

40 70

abs

4 6 10 20

ar sure b t pres le in Steam

2

ton

3

rate ss f ma am

D

E

20 25

G=

Ste

15 30

1.5

40 50 60 70 80

3 5 8 15

10

200

1

low

Pressure drop bar Inlet pressure bar abs

600

450 350

250

200

100 110 120

16

80

150

8

100

30

100

70 40

4 5 6

Ratio DP =

15

60

0.004 0.003

10

C

175

0.01 0.009 0.008 0.007 0.006 0.005

20

8

40

125

10

F

80

0.02

6

20

60

0.03

500

A

0.04

400 300

15

200

70 00 40 00 20 00 10 00 50 0 30 0 15 0

B

0.05

150

50 30

0.1 0.09 0.08 0.07 0.06

100

20 0 10 0

0.2

70

0.3

50

70 0 40 0

30 50 100

0.4

pe

m rm ete iam ed 750 Pip

0.5

4

Pi

400 200 300 Steam temperature °C

500

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

Steam velocity m/s

10 0 50 00 30 00 00 15 00

0.9 0.8 0.7 0.6

Steam mains and drainage 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 Steam

Condensate Trap set Rising ground

Fig. 9 Diagram of rising ground pipework

17

Drain points

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

Steam trap

Fig. 10 Incorrect 18

Pocket

Fig. 11 Correct

Steam trap

Waterhammer and its effects

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.

Steam

Steam

Steam

Fig. 12 The formation of a 'solid' slug of water 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 reason it is better to fit strainers on their sides in steam lines. Inadequate drainage of steam lines. 19

Steam

Steam

Steam

Fig. 13 Potential sources of waterhammer trouble 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.

20

Steam main

Steam

Steam

Branch

Steam

Fig. 14 Branchline 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.

21

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.

Steam

Fig. 15 Incorrect

Steam

Fig. 16 Correct

22

Drop leg

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.

Steam

Steam Steam main

Drop leg Control valve Isolation valve Trap set

Isolation valve

Condensate

Fig. 17 Diagram of a drop leg Rising ground and drainage

Steam velocity 40 m/s

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.

Fall 30 - 50 m

Steam velocity 15 m/s

15 m

Increase in pipe diameter

Fall

Fall

15 m

Fig. 18 Reverse gradient on steam main 23

Separator size

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.

Steam flowrate kg/h

Steam separators

DN150 DN125

10 000

DN100 5 000

DN80 DN65 DN50 DN40

2 000 1 000

B

D

DN32 DN25

500

DN20

C

DN15

200 100

10

50

20

1 2 3 4 5 6 7 8 9 101112

A 16

18 20 22 24 25 5

0.002

10 15 20 25 30

35 40

Flow velocity m/s

Steam pressure bar g 0.01 0.02 0.05

F

E 0.1

0.2

Pressure drop across separator bar

Fig. 19 Separator sizing chart Separator sizing chart example

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.

24

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.

Wet steam

Dry steam

Condensate to steam trap

Fig. 20 A typical cut section through a separator 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.

25

Strainers

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.

A

C

B

D 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.

26

Mains drainage method

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.

27

Steam trap selection

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'.

Ball float type

Fig. 22 Steam traps 28

Thermodynamic type

Thermostatic type

Inverted bucket type

Steam leaks

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.

Steam leak rate kg/h

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.

1 000

Leaking hole 12.5 mm

500 400 300 200 100 50 40 30 20

10 mm 7.5 mm

5 mm 3 mm

10 5 4 3 1 2 3 4 5 10 14 Steam pressure bar (x 100 = kPa)

Fig. 23 Steam loss through leaks

Coal tonnes/year 1 000 200 500 400 300 200 100 50 40 30 20 10 5 4

100 50 40 30 20 10 5 4 3 2 1

8 400 2 000 Hours per day

Heavy fuel oil x 1 000 litres/year 500 400 300 200 100 50 40 30 20 10

100 50 40 30 20 10 5 4 3 2

5 1 4 3 0.5 2 8 400 2 000 Hours per year

Gas x 1 000 kWh/year 5 000 4 000 1 000 3 000 2 000 500 400 300 1 000 200 500 400 300 200

100 50 40 30

100

20

50 40 30

10

5 20 8 400 2 000 Hours per year

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

29

Summary

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).

30

Table 3 Warm-up / running loads per 50 m of steam main Warm-up loads per 50 m of steam main (kg/h) Steam pressure bar g 1 2 3 4 5 6 7 8 9 10 12 14 16 18 20 25 30 40 50 60 70 80 90 100 120

Main size - mm 50 5 6 7 8 8 9 9 9 10 10 10 11 12 17 17 19 21 22 24 27 29 32 34 35 42

65 9 10 11 12 13 13 14 14 15 16 17 17 19 23 26 29 32 34 37 41 44 49 51 54 64

80 11 13 14 16 17 18 18 19 20 20 22 23 24 31 35 39 41 46 50 54 59 65 69 72 86

100 16 19 20 22 24 25 26 27 28 29 31 32 35 45 51 56 62 67 73 79 86 95 100 106 126

125 22 25 25 30 33 34 35 37 38 40 42 44 47 62 71 78 86 93 101 135 156 172 181 190 227

150 28 33 36 39 42 43 45 47 50 51 54 57 61 84 97 108 117 127 139 181 208 232 245 257 305

200 44 49 54 59 63 66 68 71 74 77 84 85 91 127 148 164 179 194 212 305 346 386 409 427 508

250 60 69 79 83 70 93 97 101 105 109 115 120 128 187 220 243 265 287 214 445 510 568 598 628 748

300 79 92 101 110 119 124 128 134 139 144 152 160 172 355 302 333 364 395 432 626 717 800 842 884 1052

350 94 108 120 131 142 147 151 158 164 171 180 189 203 305 362 400 437 473 518 752 861 960 1011 1062 1265

400 123 142 156 170 185 198 197 207 216 224 236 247 265 393 465 533 571 608 665 960 1100 1220 1288 1355 1610

450 155 179 197 215 233 242 250 261 272 282 298 311 334 492 582 642 702 762 834 1218 1396 1550 1635 1720 2050

500 182 210 232 254 275 285 294 307 320 332 350 366 393 596 712 786 859 834 1020 1480 1694 1890 1990 2690 2490

-18°C correction 600 factor 254 1.39 296 1.35 324 1.32 353 1.29 382 1.28 396 1.27 410 1.26 428 1.25 436 1.24 463 1.24 488 1.23 510 1.22 548 1.21 708 1.21 806 1.20 978 1.19 1150 1.18 1322 1.16 1450 1.15 2140 1.15 2455 1.15 2730 1.14 2880 1.14 3030 1.14 3600 1.13

150 13 14 16 18 20 21 23 24 25 25 26 30 34 36 37 42 47 56 65 74 82 97 106 114 145

200 16 18 20 23 24 26 28 30 32 33 36 39 42 44 46 52 51 70 82 95 106 126 134 149 189

250 19 22 25 28 30 33 35 37 39 41 45 49 52 55 58 66 73 87 102 119 133 156 171 186 236

300 23 26 30 33 36 39 42 44 47 49 53 58 62 66 69 78 87 104 121 140 157 187 204 220 280

350 25 28 32 37 40 43 46 49 52 54 59 64 68 72 76 86 96 114 133 155 173 205 224 242 308

400 28 32 37 42 46 49 52 57 60 62 67 73 78 82 86 97 108 130 151 177 198 234 265 277 352

450 31 35 40 46 49 53 56 61 64 67 73 79 85 90 94 106 118 142 165 199 222 263 287 311 395

500 35 39 45 51 55 59 63 68 72 75 81 93 95 100 105 119 132 158 184 222 248 293 320 347 440

600 41 46 54 61 66 71 76 82 88 90 97 106 114 120 125 141 157 189 220 265 296 350 284 416 527

Running loads per 50 m of steam main (kg/h) 1 2 3 4 5 6 7 8 9 10 12 14 16 18 20 25 30 40 50 60 70 80 90 100 120

50 5 5 6 7 7 8 8 9 9 10 11 12 12 14 15 15 17 20 24 27 29 34 38 41 52

65 5 6 7 9 9 10 10 11 11 12 13 14 15 16 17 19 21 25 29 32 35 42 46 50 63

80 7 8 9 10 11 11 12 14 14 15 16 17 18 19 21 23 25 30 34 39 43 51 56 61 77

100 9 10 11 12 13 14 15 16 17 17 18 20 23 24 25 28 31 38 44 50 56 66 72 78 99

125 10 12 14 16 17 18 19 20 21 21 23 26 29 30 31 35 39 46 54 62 70 81 89 96 122

1.54 1.50 1.48 1.45 1.43 1.42 1.41 1.40 1.39 1.38 1.38 1.37 1.36 1.36 1.35 1.34 1.33 1.31 1.29 1.28 1.27 1.26 1.26 1.25 1.22

Note: Warm-up and running loads based on an ambient temperature of 20°C, and an insulation efficiency of 80 %

31

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 = where:

L = Dt = a =

L x Dt x a (mm) Length of pipe between anchors (m) Temperature difference °C Expansion coefficient (mm/m°C) x 10-³

Table 4 Expansion coefficients (a) Material