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Inverted bucket steam trap
As its name implies, the mechanism consists of an inverted bucket which is attached by a lever to a valve. An essential part of the trap is the small air vent hole in the top of the bucket. Figure 11.3.3 shows the method of operation. In (i) the bucket hangs down, pulling the valve off its seat. Condensate flows under the bottom of the bucket filling the body and flowing away through the outlet. In (ii) the arrival of steam causes the bucket to become buoyant, it then rises and shuts the outlet. In (iii) the trap remains shut until the steam in the bucket has condensed or bubbled through the vent hole to the top of the trap body. It will then sink, pulling the main valve off its seat. Accumulated condensate is released and the cycle is repeated.

In (ii), air reaching the trap at start-up will also give the bucket buoyancy and close the valve. The bucket vent hole is essential to allow air to escape into the top of the trap for eventual discharge through the main valve seat. The hole, and the pressure differential, are small so the trap is relatively slow at venting air. At the same time it must pass (and therefore waste) a certain amount of steam for the trap to operate once the air has cleared. A parallel air vent fitted outside the trap will reduce start-up times.

Advantages of the inverted bucket steam trap

The inverted bucket steam trap can be made to withstand high pressures.

Like a float-thermostatic steam trap, it has a good tolerance to waterhammer conditions.

Can be used on superheated steam lines with the addition of a check valve on the inlet.

Failure mode is usually open, so it’s safer on those applications that require this feature, for example turbine drains.

Disadvantages of the inverted bucket steam trap

The small size of the hole in the top of the bucket means that this type of trap can only discharge air very slowly. The hole cannot be enlarged, as steam would pass through too quickly during normal operation.

There should always be enough water in the trap body to act as a seal around the lip of the bucket. If the trap loses this water seal, steam can be wasted through the outlet valve. This can often happen on applications where there is a sudden drop in steam pressure, causing some of the condensate in the trap body to 'flash' into steam. The bucket loses its buoyancy and sinks, allowing live steam to pass through the trap orifice. Only if sufficient condensate reaches the trap will the water seal form again, and prevent steam wastage.

If an inverted bucket trap is used on an application where pressure fluctuation of the plant can be expected, a check valve should be fitted on the inlet line in front of the trap. Steam and water are free to flow in the direction indicated, while reverse flow is impossible as the check valve would be forced onto its seat.

The higher temperature of superheated steam is likely to cause an inverted bucket trap to lose its water seal. A check valve in front of the trap should be regarded as essential under such conditions. Some inverted bucket traps are manufactured with an integral check valve as standard.

The inverted bucket trap is likely to suffer damage from freezing if installed in an exposed position with sub-zero ambient conditions. As with other types of mechanical traps, suitable lagging can overcome this problem if conditions are not too severe. If ambient conditions well below zero are to be expected, then it may be prudent to consider a more robust type of trap to do the job. In the case of mains drainage, a thermodynamic trap would be the first choice.





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