Liquid Expansion Trap – A Temperature Operated Trap

Liquid Expansion Trap:

As discussed in my previous post liquid expansion trap is classified under temperature operated traps.

Liquid Expansion Trap one of the simplest designs amongst Thermostatic type of Traps. The accompanying sketch (refer Figure-1, Exhibit 34.6) below shows the constructional features of a Liquid Expansion Trap.

Liquid expansion steam trap

Liquid expansion steam trap

Bimetallic steam trap

Bimetallic steam trap

Figure-1 (Exhibit 34.6)

It consists of an oil filled flexible thermostatic element which expands when heated by the fluid(i.e. steam/ condensate) surrounding it to exert the necessary amount of force to close the valve disc against the seat. It is possible by altering the gap between the disc and the seat (by adjusting the spring fitted at the end of the thermostatic element), to effectively alter the maximum temperature of the fluid (condensate) which is allowed to discharge through the seat.

That means that the moment the fluid entering the trap body is higher than the set temperature it will be stopped by the flexible thermostatic element (i.e. by closing the disc).

Depending upon the anticipated condensation temperature of the steam, the above setting could be initially adjusted to allow the discharge of the condensate just a little below the steam temperature. The problem with the above trap is that the discharge temperature of the condensate could be adjusted initially only once and the Trap as such is incapable of self-adjusting to the varying steam pressure (hence temperature).

The above phenomena can be easily explained in the sketch (refer Figure-1, Exhibit 34.7) below showing the saturation curve of the steam. Assuming that the Trap is set to discharge the Condensate at temperature slightly below T1 (i.e. the saturation temperature of steam atpressure P1), it will operate satisfactorily as long as the operating steam pressure remains close to P1. However the same trap on seeing a higher steam pressure P2 which would result into Condensate formation at temperature T2 (i.e. well above T1) will allow the Condensate to discharge only when it is cooled down to a temperature close to (below) T1, resulting into serious water logging problem. Similarly if the trap sees steam at lower pressure P3 it will allow blowing of live steam.

Liquid expansion trap response curve

Liquid expnasion trap response curve

Figure-1 (Exhibit 34.7)


Liquid expansion steam trap

Liquid expansion steam trap


The liquid expansion trap may be usefully employed as a ‘shutdown drain trap because of its fixed temperature discharge characteristic. In this case, its outlet must always point upwards, as illustrated in above figure to enable continuous immersion of the oil filled element. As the trap can only discharge between fixed temperature range (generally 60°C – 100°C) it will only normally open during start-up. It can be installed alongside a mains drain trap which would normally be piped to a condensate return line.

Advantages of Liquid Expansion Trap:

  • Since the Condensate discharge temperature can be manipulated, the above trap allows attaining high thermal efficiency (i.e. by utilizing the latent heat of steam as well sensibleheat of Condensate).
  • Does not require protection against freezing due to low ambient temperature when connected through a free draining discharge system.
  • Normally lower Condensate discharge temperature eliminates concerns of flashing steam.

Disadvantages of Liquid Expansion Trap:

  • In terms of heat transfer efficiency the above trap is a poor performer.
  • The use is limited to the applications where banking of condensate can be accepted.
  • It cannot self-adjust to varying steam pressure conditions.
  • The Bronze Bellows of the thermostatic element may become corroded when handling corrosive Condensate.