If back pressure in steam condensate return header is more than the steam pressure in the heat exchanger or process equipment, a phenomenon, known as a ‘stall’ condition occurs.
It causes steam condensate to build up and sub-cool in the heat exchanger, reducing heat transfer rate. Due to stalling, steam condensate will not be recovered into steam condensate header. Stalling is mainly due to negative pressure drop across the steam trap.
During stall situations, it is common for the plant operator to open steam trap bypass valve and/or drain steam condensate to atmosphere; this usually results in the loss of live steam and/or steam condensate.
Note that installing vacuum breaker on the exchanger is the mistake remedy operator could take. This causes introduction of atmospheric gases into the steam system.
Stall situation typically arises when LPS pressure is modulated by a control valve based on process temperature/duty control. Heat exchangers are typically oversized due to the consideration of fouling and design allowances for future duty additions.
Such heat exchangers may cause stalling problem even at 100% design duty. The only reliable solution to recover steam condensate in such stall cases is to use a condensate pump.
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