Bottom Blowdown
Suspended solids can be kept in suspension as long as the boiler water is agitated, but as soon as the agitation stops, they will fall to the bottom of the boiler. If they are not removed, they will accumulate and, given time, will inhibit heat transfer from the boiler fire tubes, which will overheat and may even fail.
The recommended method of removing this sludge is via short, sharp blasts using a relatively large valve at the bottom of the boiler. The objective is to allow the sludge time to redistribute itself so that more can be removed on the next blowdown.
For this reason, a single four-second blowdown every eight hours is much more effective than one, twelve-second blowdown in the first eight hour shift period, and then nothing for the rest of the day.
Blowdown water will either pass into a brick-lined blowdown pit encased below ground, or a metal blowdown vessel situated above ground. The size of the vessel is determined by the flowrate of blowdown water and flash steam that enters the vessel when the blowdown valve is opened.
The major influences on blowdown rate are:
- The boiler pressure
- The size of the blowdown line.
- The length of the blowdown line between the boiler and the blowdown vessel.
In practice, a reasonable minimum length of blowdown line is 7.5 m, and most blowdown vessels are sized on this basis. Blowdown lines will contain bends, check valves and the blowdown valve itself; and these fittings will increase the pressure drop along the blowdown line. They may be thought of in terms of an ‘equivalent straight length of pipe’, and can be added to the pipe length to give an overall equivalent length. Table 3.14.1 gives equivalent lengths of various valves and fittings.
