Steam Mains and Drainage

Steam Mains and Drainage Introduction

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

With steam systems, this loss of energy represents inefficiency, and thus pipes are insulated to limit these losses. Whatever the quality or thickness of insulation, there will always be a level of heat 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 of the inevitable condensate, which, unless removed, will accumulate and lead to problems such as corrosion, erosion, and waterhammer.

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

Piping layout

It is good engineering practice that, whenever possible, the main should be installed with a fall of not less than 1:100 (1 m fall for every 100 m run), in the direction of the steam flow. This slope will ensure that gravity, as well as the flow of steam, will assist in moving the condensate towards drain points where the condensate may be safely and effectively removed (See Figure 10.3.1).

Drain points

The drain point must ensure that the condensate can reach the steam trap. Careful consideration must 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, when steam flow ceases. Gravity will ensure that the water (condensate) will run along sloping pipework and collect at low points in the system. Steam traps should therefore be fitted to these low points.

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

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

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

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

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

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

Table 10.3.1 Recommended drain pocket dimensions

Mains diameter - D	Pocket diameter - d1	Pocket depth - d2
Up to 100 mm nb	d1 = D	Minimum d2 = 100 mm
125 - 200 mm nb	d1 = 100 mm	Minimum d2 = 150 mm
250 mm and above	d1 > D / 2	Minimum d2 = D

Waterhammer and its effects

Waterhammer is the noise caused by slugs of condensate colliding at high velocity into pipework fittings, plant, and equipment. This has a number of implications:

Because the condensate velocity is higher than normal, the dissipation of kinetic energy is higher than would normally be expected.
Water is dense and incompressible, so the ‘cushioning’ effect experienced when gases encounter obstructions is absent.
The energy in the water is dissipated against the obstructions in the piping system such as valves and fittings.

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

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

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

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

Sagging in the line, perhaps due to failure of supports.
Incorrect use of concentric reducers (see Figure 10.3.7) - Always use eccentric reducers with the flat at the bottom.
Incorrect strainer installation - They should be fitted with the basket on the side.
Inadequate drainage of steam lines.
Incorrect operation - Opening valves too quickly at start-up when pipes are cold.

To summarise, the possibility of waterhammer is minimised by:

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

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

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

Branch line connections

Branch line connections taken from the top of the main carry the driest steam (Figure 10.3.8). If connections 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 dirty steam 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 minimise condensate lying in the branch line, if the plant is likely to be shutdown for any extended periods.

Drop Leg

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

Rising ground and drainage

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

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

Steam separators

Modern packaged steam boilers have a large evaporating capacity for their size and have limited capacity to cope with rapidly changing loads. In addition, as discussed in Block 3 ‘The Boiler House’, other circumstances, such as . . .

Incorrect chemical feedwater treatment and/or TDS control
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.

As a general rule, providing the velocities in the pipework are within reasonable limits, separators will 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 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 less expensive than increasing the pipe size and fabricating drain pockets.

A separator is recommended before control valves and flowmeters. It is also wise to fit a separator where a steam main enters a building from outside. This will ensure that any condensate produced in the external distribution system is removed and the building always receives dry steam. This is equally 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 the pipe. In the case of older pipework, there will be rust, and in hard water districts, a carbonate deposit. Occasionally, pieces will break loose and pass along the pipework with the steam to rest inside 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. Once wiredrawing 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 valve and regulating valve. The illustration shown in Figure 10.3.13 shows a cut section through a typical strainer.

10.3.13 Cut section through a Y-type 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, dirt cannot. 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 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.

How to drain steam mains

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

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

Pressure rating
Capacity
Suitability

Pressure rating

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

Capacity

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

Warm-up load

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

The table shows the amount of condensate generated when bringing 50 m of steam main up to working temperature; 50 m being the maximum recommended distance between trapping points.
The values shown are in kilograms. To determine the average condensing rate, the time taken for the process must be considered. For example, if the warm-up process required 50 kg of steam, and was to take 20 minutes, then the average condensing rate would be:

When using these capacities to size a steam trap, it is worth remembering that the initial pressure 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 ‘low capacity’ steam trap. Only in rare applications at very high pressures (above 70 bar g), combined with large pipe sizes, will greater trap capacity be needed.

Running Load

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

For accurate means of calculating running losses from steam mains, refer to Module 2.12 ‘Steam consumption of pipes and air heaters’. Alternatively, for quick approximations of running load, Table 10.3.3 can be used which shows typical amounts of steam condensed each hour per 50 m of insulated steam main at various pressures.

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 20c, and an insulation efficiency of 80%

Steam pressure bar g	Steam main size (mm)														-18 °C correction factor
Steam pressure bar g	50	65	80	100	125	150	200	250	300	350	400	450	500	600	-18 °C correction 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	1.2
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

Table 10.3.3

Condensing rate of steam in 50 m of schedule 40 pipe - at working temperature (kg/h)

Note: Figures are based on an ambient temperature of 20c, and in insulations efficiency of 80%

Steam pressure bar g	Steam main size (mm)														-18 °C correction factor
Steam pressure bar g	50	65	80	100	125	150	200	250	300	350	400	450	500	600	-18 °C correction factor
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	1.5
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	1.4
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	58	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
120	52	63	77	99	122	145	189	236	280	308	352	395	440	527	1.22

Suitability

A mains drain trap should consider the following constraints:

Discharge temperature - The steam trap should discharge at, or very close to saturation temperature, unless cooling legs are used between the drain point and the trap. This means that the choice is a mechanical type trap (such as a float, inverted bucket type, or thermodynamic traps).
Frost damage - Where the steam main is located outside a building and there is a possibility of sub-zero ambient temperature, the thermodynamic steam trap is ideal, as it not damaged by frost. Even if the installation causes water to be left in the trap at shutdown and freezing occurs, the thermodynamic trap may be thawed out without suffering damage when brought back into use.
Waterhammer - In the past, on poorly laid out installations where waterhammer was a common occurrence, float traps were not always 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 as high 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. The thermostatic trap is included because it is ideal where there is no choice but to discharge condensate into a flooded return pipe.

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

10.3.14 Steam traps suitable for mains drainage

Steam Leaks

Steam leaking from pipework is often ignored. Leaks can be costly in both the economic and environmental sense and therefore need prompt attention to ensure the steam system is working at 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 can be readily translated into a fuel saving based on the annual hours of operation.

10.3.15 Steam leakage rate through holes

Summary

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 (1:100). Steam lines rising in the direction of flow should slope at not less than 250 mm per 10 metres of pipe (1:40).
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 from where the driest steam is taken.
Separators should be considered before any piece of steam using equipment ensuring that dry steam is used.
Traps selected should be robust enough to avoid waterhammer damage and frost damage.

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Steam Distribution

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