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If stall conditions are inevitable, potential problems can be overcome by designing the installation around one of three basic solutions:
1. Ensure the steam pressure in the steam space can never drop below atmospheric pressure, and that the condensate can drain by gravity to and from a ball float steam trap.
2. Accept that the pressure in the steam space may be less than the backpressure, and provide an alternative means of removing condensate, by installing a pump-trap.
3. Ensure the pressure in the steam space is stable and higher than the backpressure. This will entail having the temperature control system on the secondary side of the system.
Taking these three options in turn:
The steam trap cannot be subject to any backpressure higher than atmospheric, and must drain condensate either to an open end (which may be wasteful), or to a nearby vented receiver and pump, enabling the energy contained in the condensate to be reclaimed.
There are two criteria that must be satisfied:
• A vacuum breaker must be fitted to the steam inlet to the heat exchanger after the control valve.
• The trap must be installed at a discreet distance below the heat exchanger outlet such that sufficient static head is created to pass the requisite amount of condensate when stall occurs. A distance of between 0.5 to 1 m is usually sufficient; however, smaller distances can be accommodated with larger traps, if less head is available.
A standard float trap set is installed with condensate returning to a condensate system, which is either pressurised and/or elevated above the trap. An auxiliary float trap may be fitted, discharging condensate via an open end to drain.
When there is sufficient steam pressure to overcome the backpressure, the main float trap will function, but when stall occurs, condensate will back-up and drain through the auxiliary float trap thus preventing condensate flooding back into the heat exchanger.
As this condensate will drain to waste, this method should only be used if stall occurs infrequently. The auxiliary trap should be sized on static head to pass the stall load as in method 1a, and the ‘main’ trap should be the same size, but fitted at least 150 mm below the auxiliary take-off tee-piece.
Apart from the obvious disadvantage of energy loss, this method also requires available head between the trap inlets and the heat exchanger outlet.
Spirax Sarco Limited
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