Practical Fabrication Tips for Steam Piping_Very Useful
Practical Fabrication Tips for Steam Piping_Very Useful
Piping Layout
The subject of drainage from steam lines is covered in the European Standard EN 45510, Section 4.12.
EN 45510 states 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.
Water hammer 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.
ndications 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.
o 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
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 Tutorial 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.
Fig. 10.3.9 Steam off-take
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.
Fig. 10.3.10 Diagram of a drop leg supplying a unit heater
Rising ground and drainage
Steam seperator
Strainer
How to drain steam mains
Still to come---------------------
Last edited by mkhurram79; 02-18-2010 at 07:07 PM.
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Muhammad Khurram
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