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Impulse steam trap
The impulse trap consists of a hollow piston (A) with a piston disc (B) working inside a tapered piston (C) which acts as a guide. At 'start-up' the main valve (D) rests on the seat (E) leaving a passage of flow through the clearance between piston and cylinder and hole (F) at the top of the piston. Increasing flow of air and condensate will act on the piston disc and lift the main valve off its seat to give increased flow. Some condensate will also flow through the gap between the piston and disc, through E and away to the trap outlet.

As the condensate approaches steam temperature some of it flashes to steam as it passes through the gap. Although this is bled away through hole F it does create an intermediate pressure over the piston, which effectively positions the main valve to meet the load. The trap can be adjusted by moving the position of piston (B) relative to the seat, but the trap is affected by significant backpressure. It has a substantial capacity, bearing in mind its small size. Conversely, the trap is unable to give complete shut-off and will pass steam on very light loads. The main problem however is the fine clearance between the piston and cylinder. This is readily affected by the dirt normally found in a steam system. The use of impulse traps is relatively limited so they are not considered in some subsequent sections of this Module.

Advantages of the impulse steam trap

Impulse traps have a substantial condensate handling capacity for their size.

They will work over a wide range of steam pressures without any change in valve size and can be used on high pressure and superheated steam.

They are good at venting air and cannot 'air-bind'.

Disadvantages of the impulse steam trap

Impulse traps cannot give a dead tight shut-off and will blow steam on very light loads.

They are easily affected by any dirt which enters the trap body due to the extremely small clearance between the piston and the cylinder.

The traps can pulsate on light load causing noise, waterhammer and even mechanical damage to the valve itself.

They will not work against a backpressure which exceeds 40% of the inlet pressure





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