Its operating temperature is affected by the surrounding steam pressure.
However, it can often be used where start-up rate is not an important consideration, such as when draining small tank heating coils.Ī large improvement on the liquid expansion trap is the balanced pressure trap, shown in Figure 11.2.4. The liquid expansion trap is not normally a trapping solution on its own, as it usually requires another steam trap to operate in parallel.If the trap is to be subjected to freezing conditions the trap and its associated pipework must be well insulated.Since the liquid expansion trap discharges condensate at a temperature of 100☌ or below, it should never be used on applications which demand immediate removal of condensate from the steam space.The flexible tubing of the element can be destroyed by corrosive condensate or superheat.It is able to withstand vibration and waterhammer conditions.ĭisadvantages of the liquid expansion steam trap: The liquid expansion trap can be used as a start-up drain trap on low pressure superheated steam mains where a long cooling leg is guaranteed to flood with cooler condensate.Like the balanced pressure trap, the liquid expansion trap is fully open when cold, giving good air discharge and maximum condensate capacity on ‘start-up’ loads.Liquid expansion traps can be adjusted to discharge at low temperatures, giving an excellent ‘cold drain’ facility.It can be installed alongside a mains drain trap which would normally be piped to a condensate return line.Īdvantages of the liquid expansion steam trap: As the trap can only discharge between 60☌ - 100☌ it will only normally open during start-up. Here, its outlet must always point upwards, as illustrated in Figure 11.2.3, to enable continuous immersion of the oil filled element. This cooling can only occur in the pipe between the process and trap, and if the trap discharge temperature remains constant, the process will waterlog.īecause of its fixed temperature discharge characteristic, the liquid expansion trap may be usefully employed as a ‘shutdown drain trap’. However, if pressure is increased to P 2 then condensate has to cool more (ΔT 2) to pass through the steam trap. It can be seen from Figure 11.2.2 that when the pressure is at pressure P 1, condensate would have to cool by only a small amount (ΔT 1), and trapping would be acceptable. The adjustment allows the temperature of the trap discharge to be altered between 60☌ and 100☌, which makes it ideally suited as a device to get rid of large quantities of air and cold condensate at start-up.Īs discussed in Module 2.2, the temperature of saturated steam varies with pressure.įigure 11.2.2 shows the saturation curve for steam, together with the fixed temperature response line (X - X) of the liquid expansion trap, set at 90☌. An oil filled element expands when heated to close the valve against the seat. This is one of the simplest thermostatic traps and is shown in Figure 11.2.1.
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