While selecting steam traps, the differential pressure available across the steam trap should also be considered to ensure effective condensate evacuation.

The discharge capacity of the steam trap depends not only on its orifice size but also on the differential pressure that is created during its operation. The traps essentially being orifice based devices; their performance depends on the differential pressure created.

The back pressure applied to any steam trap by the return to which it is connected is the sum of at least three components. These are –

• The pressure at the end of the return line, either atmospheric or that of the vessel into which the line discharges.
• The hydrostatic head needed to lift the condensate up any risers in the line, as typically from a trap at a low level to an overhead return line. A lift of 10.6m means a back pressure of 1 bar, so with a margin, a lift of 1m imposes 0.1bar backpressure.
• Any frictional resistance to the flow of condensate, air or flash steam.

Thus while selecting steam traps it is worthwhile to calculate the differential pressure that will be available for the operation of the steam trap. As the differential pressure available reduces the discharge capacity of the trap also reduces, impacting effective condensate evacuation.