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Liquid expansion steam trap
This is one of the simplest thermostatic traps and is shown in Figure 11.2.1. An oil filled element expands when heated to close the valve against the seat. The adjustment allows the temperature of the trap discharge to be altered between 60C and 100C, which makes it ideally suited as a device to get rid of large quantities of air and cold condensate at start-up.
Typical application
Because of its fixed temperature discharge characteristic, the liquid expansion trap may be usefully employed as a 'shutdown drain trap'. Here, its outlet must always point upwards, as illustrated in Figure 11.2.3, to enable continuous immersion of the oil filled element. As the trap can only discharge between 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 the liquid expansion steam trap:

Liquid expansion traps can be adjusted to discharge at low temperatures, giving an excellent 'cold drain' facility.

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.

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. It is able to withstand vibration and waterhammer conditions.

Disadvantages of the liquid expansion steam trap:

The flexible tubing of the element can be destroyed by corrosive condensate or superheat.

Since the liquid expansion trap discharges condensate at a temperature of 100°C or below, it should never be used on applications which demand immediate removal of condensate from the steam space.

If the trap is to be subjected to freezing conditions the trap and its associated pipework must be well insulated.

The liquid expansion trap is not normally a trapping solution on its own, as it usually requires another steam trap to operate in parallel. However, it can often be used where start-up rate is not an important consideration, such as when draining small tank heating coils.

Balanced pressure steam trap




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