block 10 module 3

The Steam and Condensate Loop 10.3.1

Steam Mains and Drainage Module 10.3Block 10 Steam Distribution

Module 10.3Steam Mains and Drainage

The Steam and Condensate Loop

Steam Mains and Drainage Module 10.3

10.3.2

Block 10 Steam Distribution

Steam Mains and Drainage

Throughout the length of a hot steam main, an amount of heat will be transferred to theenvironment, and this will depend on the parameters identified in Block 2 - ‘Steam Engineeringand Heat Transfer’, and brought together in Equation 2.5.1.

Equation 2.5.1T

kA∆

=Q�

Where:Q = Heat transferred per unit time (W)k = Thermal conductivity of the material (W/m K or W/m °C)A = Heat transfer area (m²)∆T = Temperature difference across the material (K or °C)� = Material thickness (m)

With steam systems, this loss of energy represents inefficiency, and thus pipes are insulated tolimit these losses. Whatever the quality or thickness of insulation, there will always be a level ofheat loss, and this will cause steam to condense along the length of the main.

The effect of insulation is discussed in Module 10.5. This Module will concentrate on disposal ofthe inevitable condensate, which, unless removed, will accumulate and lead to problems suchas corrosion, erosion, and waterhammer.

In addition, the steam will become wet as it picks up water droplets, which reduces its heattransfer potential. If water is allowed to accumulate, the overall effective cross sectional area ofthe pipe is reduced, and steam velocity can increase above the recommended limits.

Piping layout

The subject of drainage from steam lines is covered in the UK British Standard BS 806:1993,Section 4.12.

BS 806 states that, whenever possible, the main should be installed with a fall of not less than1:100 (1 m fall for every 100 m run), in the direction of the steam flow. This slope will ensure thatgravity, as well as the flow of steam, will assist in moving the condensate towards drain pointswhere the condensate may be safely and effectively removed (See Figure 10.3.1).

Drain pointsThe drain point must ensure that the condensate can reach the steam trap. Careful considerationmust therefore be given to the design and location of drain points.

Consideration must also be given to condensate remaining in a steam main at shutdown, whensteam flow ceases. Gravity will ensure that the water (condensate) will run along sloping pipeworkand collect at low points in the system. Steam traps should therefore be fitted to these lowpoints.

Fig. 10.3.1 Typical steam main installation

Steam Trap setTrap set

Trap set

SteamGradient 1:100

Gradient 1:100

30 - 50 metre intervals

CondensateCondensate

Condensate



The amount of condensate formed in a large steam main under start-up conditions is sufficientto require the provision of drain points at intervals of 30 m to 50 m, as well as natural low pointssuch as at the bottom of rising pipework.

In normal operation, steam may flow along the main at speeds of up to 145 km/h, draggingcondensate along with it. Figure 10.3.2 shows a 15 mm drain pipe connected directly to thebottom of a main.

Table 10.3.1 Recomended drain pocket dimensionsMains diameter - D Pocket diameter - d1 Pocket depth - d2

Up to 100 mm nb d1 = D Minimum d2 = 100 mm125 - 200 mm nb d1 = 100 mm Minimum d2 = 150 mm

250 mm and above d1 �� D / 2 Minimum d2 = D

Steam main

Condensate return

Float trap within-built sensor

Steam D

d22222d11111

Fig. 10.3.4

Fig. 10.3.2 Trap pocket too small

Flow

Fig. 10.3.3 Trap pocket properly sized

Flow

Steam trap set

Pocket

Although the 15 mm pipe has sufficient capacity, it is unlikely to capture much of the condensatemoving along the main at high speed. This arrangement will be ineffective.

A more reliable solution for the removal of condensate is shown in Figure 10.3.3. The trap lineshould be at least 25 to 30 mm from the bottom of the pocket for steam mains up to 100 mm,and at least 50 mm for larger mains. This allows a space below for any dirt and scale to settle.

The bottom of the pocket may be fitted with a removable flange or blowdown valve for cleaningpurposes.

Recommended drain pocket dimensions are shown in Table 10.3.1 and in Figure 10.3.4.

Steam trap set

Steam

Condensate

Condensate

Steam



10.3.4


Waterhammer and its effects

Waterhammer is the noise caused by slugs of condensate colliding at high velocity into pipeworkfittings, plant, and equipment. This has a number of implications:o Because the condensate velocity is higher than normal, the dissipation of kinetic energy is

higher than would normally be expected.o Water is dense and incompressible, so the ‘cushioning’ effect experienced when gases encounter

obstructions is absent.o The energy in the water is dissipated against the obstructions in the piping system such as

valves and fittings.

Indications of waterhammer include a banging noise, and perhaps movement of the pipe.

In severe cases, waterhammer may fracture pipeline equipment with almost explosive effect,with consequent loss of live steam at the fracture, leading to an extremely hazardous situation.

Good engineering design, installation and maintenance will avoid waterhammer; this is far betterpractice than attempting to contain it by choice of materials and pressure ratings of equipment.

Commonly, sources of waterhammer occur at the low points in the pipework (See Figure 10.3.6).Such areas are due to:o Sagging in the line, perhaps due to failure of supports.

o Incorrect use of concentric reducers (see Figure 10.3.7) - Always use eccentric reducers withthe flat at the bottom.

o Incorrect strainer installation - They should be fitted with the basket on the side.

o Inadequate drainage of steam lines.

o Incorrect operation - Opening valves too quickly at start-up when pipes are cold.

Fig. 10.3.5 Formation of a ‘solid’ slug of water

Fig. 10.3.6 Potential sources of waterhammer

Steam

Steam

Steam

Slug

Condensate

Condensate

Condensate

Condensate

Steam

Steam

Steam

Concentricreducer

Strainer withhanging basket

Riser



Fig. 10.3.7 Eccentric and concentric pipe reducers

To summarise, the possibility of waterhammer is minimised by:

o Installing steam lines with a gradual fall in the direction of flow, and with drain points installedat regular intervals and at low points.

o Installing check valves after all steam traps which would otherwise allow condensate to runback into the steam line or plant during shutdown.

o Opening isolation valves slowly to allow any condensate which may be lying in the system toflow gently through the drain traps, before it is picked up by high velocity steam. This isespecially important at start-up.

Branch lines

Fig. 10.3.8 Branch line

Steam

Eccentric reducer

Steam

Concentric reducer

Condensate

Condensate

Correct

Incorrect

Steam Steam

Steam

Steam main

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

Table 10.2.4 ‘Saturated steam pipeline capacities for different velocities’ in Module 10.2 willprove useful in this exercise.

Branch line



10.3.6


Branch line connectionsBranch line connections taken from the top of the main carry the driest steam (Figure 10.3.8). Ifconnections are taken from the side, or even worse from the bottom (as in Figure 10.3.9 (a)),they can accept the condensate and debris from the steam main. The result is very wet and dirtysteam reaching the equipment, which will affect performance in both the short and long term.

The valve in Figure 10.3.9 (b) should be positioned as near to the off-take as possible to minimisecondensate lying in the branch line, if the plant is likely to be shutdown for any extended periods.

Fig. 10.3.9 Steam off- take

Drop legLow points will also occur in branch lines. The most common is a drop leg close to an isolatingvalve or a control valve (Figure 10.3.10). Condensate can accumulate on the upstream side ofthe closed valve, and then be propelled forward with the steam when the valve opens again -consequently a drain point with a steam trap set is good practice just prior to the strainer andcontrol valve.

Fig. 10.3.10 Diagram of a drop leg supplying a unit heater

Steam

Drop leg

Strainer

Controlvalve

Isolation valve

Trap setCondensate Condensate

Isolation valve

Trap set

Isolationvalve

(b) Correct

(a) Incorrect

Unitheater



Rising ground and drainageThere are many occasions when a steam main must run across rising ground, or applicationswhere the contours of the site make it impractical to lay the pipe with the 1:100 fall proposedearlier. In these situations, the condensate must be encouraged to run downhill and against thesteam flow. Good practice is to size the pipe on a low steam velocity of not more than 15 m/s, torun the line at a slope of no less than 1:40, and install the drain points at not more than 15 metreintervals (see Figure 10.3.11).

The objective is to prevent the condensate film on the bottom of the pipe increasing in thicknessto the point where droplets can be picked up by the steam flow.

Fig. 10.3.12 Cut section through a separator

Air and incondensable gases vented

Dry steam out

Wet steam in

Moisture to trap set

Steam separatorsModern packaged steam boilers have a large evaporating capacity for their size and have limitedcapacity to cope with rapidly changing loads. In addition, as discussed in Block 3 ‘The BoilerHouse’, other circumstances, such as . . .o Incorrect chemical feedwater treatment and /or TDS controlo Transient peak loads in other parts of the plant

. . . can cause priming and carryover of boiler water into the steam mains.

Separators, as shown by the cut section in Figure 10.3.12, may be installed to remove this water.

Fig. 10.3.11 Reverse gradient on steam main

Steam

velocity

30 m/s

1:100 Fall

1:40 FallSteam

velocity

15 m/s

Increase

in pipe

diameter Fall

15 m15 m30 - 50 m

30 m/s



10.3.8


A

C

B

D

As a general rule, providing the velocities in the pipework are within reasonable limits, separatorswill be line sized. (Separators are discussed in detail in Module 12.5)

A separator will remove both droplets of water from pipe walls and suspended mist entrained inthe steam itself. The presence and effect of waterhammer can be eradicated by fitting a separatorin a steam main, and can often be less expensive than increasing the pipe size and fabricatingdrain pockets.

A separator is recommended before control valves and flowmeters. It is also wise to fit a separatorwhere a steam main enters a building from outside. This will ensure that any condensate producedin the external distribution system is removed and the building always receives dry steam. This isequally important where steam usage in the building is monitored and charged for.

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 accidentally deposited inside thepipe. In the case of older pipework, there will be rust, and in hard water districts, a carbonatedeposit. Occasionally, pieces will break loose and pass along the pipework with the steam to restinside a piece of steam using equipment. This may, for example, prevent a valve from opening /closing correctly. Steam using equipment may also suffer permanent damage through wiredrawing- the cutting action of high velocity steam and water passing through a partly open valve. Oncewiredrawing has occurred, the valve will never give a tight shut-off, even if the dirt is removed.

It is therefore wise to fit a line-size strainer in front of every steam trap, flowmeter, reducing valveand regulating valve. The illustration shown in Figure 10.3.13 shows a cut section through atypical strainer.

Steam flows from the inlet ‘A’ through the perforated screen ‘B’ to the outlet ‘C’. While steamand water will pass readily through the screen, dirt cannot. The cap ‘D’, can be removed, allowingthe screen to be withdrawn and cleaned at regular intervals. A blowdown valve can also be fittedto cap ‘D’ to facilitate regular cleaning.

Strainers can however, be a source of wet steam as previously mentioned. To avoid this situation,strainers should always be installed in steam lines with their baskets to the side.

Strainers and screen details are discussed in Module 12.4.

Fig. 10.3.13 Cut section through a Y-type strainer.



How to drain steam mains

Steam traps are the most effective and efficient method of draining condensate from a steamdistribution system.

The steam traps selected must suit the system in terms of:

o Pressure rating

o Capacity

o Suitability

Pressure ratingPressure rating is easily dealt with; the maximum possible working pressure at the steam trap willeither be known or should be established.

CapacityCapacity, that is, the quantity of condensate to be discharged, which needs to be divided intotwo categories; warm-up load and running load.

Warm-up load - In the first instance, the pipework needs to be brought up to operatingtemperature. This can be determined by calculation, knowing the mass and specific heat of thepipework and fittings. Alternatively, Table 10.3.2 may be used.

o The table shows the amount of condensate generated when bringing 50 m of steam main upto working temperature; 50 m being the maximum recommended distance between trappingpoints.

o The values shown are in kilograms. To determine the average condensing rate, the time takenfor the process must be considered. For example, if the warm-up process required 50 kg ofsteam, and was to take 20 minutes, then the average condensing rate would be:

o When using these capacities to size a steam trap, it is worth remembering that the initialpressure in the main will be little more than atmospheric when the warm-up process begins.However, the condensate loads will still generally be well within the capacity of a DN15 ‘lowcapacity’ steam trap. Only in rare applications at very high pressures (above 70 bar g), combinedwith large pipe sizes, will greater trap capacity be needed.

Running load - Once the steam main is up to operating temperature, the rate of condensation ismainly a function of the pipe size and the quality and thickness of the insulation.

Again, with sufficient data, the heat losses can be determined. Alternatively Table 10.3.3 can beused which shows typical amounts of steam condensed per 50 m of steam main at variouspressures. The average condensing rate is determined in the same way as that shown above for‘warm-up load’.

60 minutesAverage condensing rate = x 50 kg

20 minutesAverage condensing rate = 150 kg h



10.3.10


Table 10.3.2 Amount of steam condensed to warm-up 50 m of schedule 40 pipe (kg)

Note: Figures are based on an ambient temperature of 20°C, and an insulation efficiency of 80%Steam

Steam main size (mm)-18°C

pressure correctionbar 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 1.392 6 10 13 19 25 33 49 69 92 108 142 179 210 296 1.353 7 11 14 20 25 36 54 79 101 120 156 197 232 324 1.324 8 12 16 22 30 39 59 83 110 131 170 215 254 353 1.295 8 13 17 24 33 42 63 70 119 142 185 233 275 382 1.286 9 13 18 25 34 43 66 93 124 147 198 242 285 396 1.277 9 14 18 26 35 45 68 97 128 151 197 250 294 410 1.268 9 14 19 27 37 47 71 101 134 158 207 261 307 428 1.259 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 1.2412 10 17 22 31 42 54 84 115 152 180 236 298 350 488 1.2314 11 17 23 32 44 57 85 120 160 189 247 311 366 510 1.2216 12 19 24 35 47 61 91 128 172 203 265 334 393 548 1.2118 17 23 31 45 62 84 127 187 355 305 393 492 596 708 1.2120 17 26 35 51 71 97 148 220 302 362 465 582 712 806 1.2025 19 29 39 56 78 108 164 243 333 400 533 642 786 978 1.1930 21 32 41 62 86 117 179 265 364 437 571 702 859 1 150 1.1840 22 34 46 67 93 127 194 287 395 473 608 762 834 1 322 1.1650 24 37 50 73 101 139 212 214 432 518 665 834 1 020 1 450 1.1560 27 41 54 79 135 181 305 445 626 752 960 1 218 1 480 2 140 1.1570 29 44 59 86 156 208 346 510 717 861 1 100 1 396 1 694 2 455 1.1580 32 49 65 95 172 232 386 568 800 960 1 220 1 550 1 890 2 730 1.1490 34 51 69 100 181 245 409 598 842 1 011 1 288 1 635 1 990 2 880 1.14

100 35 54 72 106 190 257 427 628 884 1 062 1 355 1 720 2 690 3 030 1.14120 42 64 86 126 227 305 508 748 1 052 1 265 1 610 2 050 2 490 3 600 1.13

Table 10.3.3 Amount of steam condensed during operation of 50 m of schedule 40 pipe (kg)

Note: Figures are based on an ambient temperature of 20°C, and an insulation efficiency of 80%Steam

Steam main size (mm)-18°C

pressure correctionbar g 50 65 80 100 125 150 200 250 300 350 400 450 500 600 factor

1 5 5 7 9 10 13 16 19 23 25 28 31 35 41 1.542 5 6 8 10 12 14 18 22 26 28 32 35 39 46 1.503 6 7 9 11 14 16 20 25 30 32 37 40 45 54 1.484 7 9 10 12 16 18 23 28 33 37 42 46 51 61 1.455 7 9 11 13 17 20 24 30 36 40 46 49 55 66 1.436 8 10 11 14 18 21 26 33 39 43 49 53 59 71 1.427 8 10 12 15 19 23 28 35 42 46 52 56 63 76 1.418 9 11 14 16 20 24 30 37 44 49 57 61 68 82 1.409 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 1.3812 11 13 16 18 23 26 36 45 53 59 67 73 81 97 1.3814 12 14 17 20 26 30 39 49 58 64 73 79 93 106 1.3716 12 15 18 23 29 34 42 52 62 68 78 85 95 114 1.3618 14 16 19 24 30 36 44 55 66 72 82 90 100 120 1.3620 15 17 21 25 31 37 46 58 69 76 86 94 105 125 1.3525 15 19 23 28 35 42 52 66 78 86 97 106 119 141 1.3430 17 21 25 31 39 47 51 73 87 96 108 118 132 157 1.3340 20 25 30 38 46 56 70 87 104 114 130 142 158 189 1.3150 24 29 34 44 54 65 82 102 121 133 151 165 184 220 1.2960 27 32 39 50 62 74 95 119 140 155 177 199 222 265 1.2870 29 35 43 56 70 82 106 133 157 173 198 222 248 296 1.2780 34 42 51 66 81 97 126 156 187 205 234 263 293 350 1.2690 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 1.25120 52 63 77 99 122 145 189 236 280 308 352 395 440 527 1.22



SuitabilityA mains drain trap should consider the following constraints:

o Discharge temperature - The steam trap should discharge at, or very close to saturationtemperature, unless cooling legs are used between the drain point and the trap. This meansthat the choice is a mechanical type trap (such as a float, inverted bucket type, or thermodynamictraps).

o Frost damage - Where the steam main is located outside a building and there is a possibilityof sub-zero ambient temperature, the thermodynamic steam trap is ideal, as it not damagedby frost. Even if the installation causes water to be left in the trap at shutdown and freezingoccurs, the thermodynamic trap may be thawed out without suffering damage when broughtback into use.

o Waterhammer - In the past, on poorly laid out installations where waterhammer was a commonoccurrence, float traps were not always ideal due to their susceptibility to float damage.Contemporary design and manufacturing techniques now produce extremely robust units formains drainage purposes. Float traps are certainly the first choice for proprietary separators ashigh capacities are readily achieved, and they are able to respond quickly to rapid load increases.

Steam traps used to drain condensate from steam mains, are shown in Figure 10.3.14. Thethermostatic trap is included because it is ideal where there is no choice but to dischargecondensate into a flooded return pipe.

The subject of steam trapping is dealt with in detail in the Block 11, ‘Steam Trapping’.

Steam leaks

Steam leaking from pipework is often ignored. Leaks can be costly in both the economic andenvironmental sense and therefore need prompt attention to ensure the steam system is workingat its optimum efficiency with a minimum impact on the environment.

Figure 10.3.15 illustrates the steam loss for various sizes of hole at various pressures. This loss canbe readily translated into a fuel saving based on the annual hours of operation.

Fig. 10.3.14 Steam traps suitable for steam mains drainageBall float type Thermodynamic type Thermostatic type Inverted bucket type

Fig. 10.3.15 Steam leakage rate through holes

1 2 3 4 5 10

500

400

300

200

100

03 mm

5 mm

7.5 mm

12.5 mm

Orifice size

Ste

am le

ak r

ate

kg/h

10 mm

Steam pressure bar g



10.3.12


Summary

Proper pipe alignment and drainage means observing a few simple rules:

o Steam lines should be arranged to fall in the direction of flow, at not less than 100 mm per10 metres of pipe (1:100).

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

o Where drainage has to be provided in straight lengths of pipe, then a large bore pocket shouldbe used to collect condensate.

o If strainers are to be fitted, then they should be fitted on their sides.

o Branch connections should always be taken from the top of the main from where the drieststeam is taken.

o Separators should be considered before any piece of steam using equipment ensuring that drysteam is used.

o Traps selected should be robust enough to avoid waterhammer damage and frost damage.



Questions

1. Which of the following is true of wet steam?

a| It can cause waterhammer if allowed to build up ¨

b| It can corrode pipes if allowed to continue ¨

c| It causes erosion of bends ¨

d| All of the above ¨

2. What is the effect of installing a steam main horizontally level?

a| None, provided the pipe is drained at 30 - 50 m intervals ¨

b| Complete drainage will be less effective, and waterhammer could result ¨

c| Larger diameter drain points should be fitted ¨

d| Condensate will not reach the drain points ¨

3. Steam pipeline strainers should be fitted with their baskets on the side to:

a| Prevent condensate filling the body and being carried overto the equipment being protected ¨

b| Provide a greater screening area ¨

c| Extend the periods between cleaning the strainer ¨

d| Provide more effective removal of the debris ¨

4. Using the velocity method, what size pipe is required to carry 500 kg /h of steam at6 bar g over a 40 m run with a rising slope? (The specific volume of steam at 6 bar g is0.272 m³ /kg

a| 40 mm ¨

b| 80 mm ¨

c| 50 mm ¨

d| 65 mm ¨



10.3.14


1: d, 2: b, 3: a, 4: d, 5: d, 6: d Answers

5. A correctly sized pilot operated reducing valve has been installed in a pressure reducingstation supplying an autoclave, as shown in Figure 10.3.16. What is wrong with theinstallation?

Steam at7 bar g

280 kg /h ofsteam at 5 bar g

DN25separator

DN25stop valve

DN25strainer

DN20pressurereducing

valve

DN32stop valve

Safetyvalve

Steam trap setCondensate

Fig. 10.3.16

a| The pipe after the PRV is at a lower pressure, and steam has a higher volume,so the pipe should be larger than 32 mm ¨

b| The upstream strainer and isolation valve should bethe same size as the reducing valve ¨

c| The separator should be one size larger than the pipeworkto avoid excessive pressure drop ¨

d| There is no downstream pressure gauge before the DN32 stop valve ¨

6. As a minimum, horizontal runs of 150 mm steam main should be drained at intervals of:

a| Every 15 metres via 100 mm bore drain pockets, 100 mm deep ¨

b| Every 30 - 50 metres via 150 mm bore drain pockets, 100 mm deep ¨

c| Every 15 metres via 100 mm bore drain pockets, 150 mm deep ¨

d| Every 30 - 50 metres via 100 mm bore drain pockets, 150 mm deep ¨

block 10 module 3

Documents

flow of steam

drainagethe steam

steam distributionmodule

steam lines

steam distributionthe

steam velocity

steam traps

large steam main