Most validation temperature alarms can be traced to two root causes. They are booth related to condensate back-up upstream of the steam trap. The first most common cause is High Subcooling Trap Operation.
Balanced port thermostatic traps fail when the SS bellows develops a leak and the alcohol fill escapes. Without its volatile alcohol fill, the bellows can never expand and close when exposed to steam temperature. The trap will remain open at all times allowing clean steam to pass through the trap. During Temperature Hold (low condensate creation), this can be problematic as clean steam will blow through the trap into the condensate header to back up condensate. Back up will occur because the differential pressure across all of the traps tied into that header will be reduced. (DP = P1 -P2). Reduced differential pressure will result in reduced flow in one or more of the adjacent traps. In smaller volume condensate headers, this flow reduction through the traps can cause condensate back up significant enough to wet the sensor and cause a temperature alarm.
Assume the hot water Cv of one of the clean steams shown below is 2. Using our sizing program and solving for flow, that trap will allow flow of about 2423 Kg/hour with a differential pressure of 1.965 bar (assume P1 – 2 bar, and P2 0.035). If one of the traps on the header below fails, and the pressure in the condensate header (P2) increase to 0.14 bar, it will decrease the differential pressure across all of the adjacent traps to 1.86 bar. That small decrease in differential pressure from 1.965 to 1.86 will cuase a dcrease in flow of about 65 Kg/hour. With the same amount of condensate being produced by the system, and less flow out of each trap, more condensate will accumulate upstream of the traps. This will cause condensate to back up in the tubing and increase the probability that one or more of the temperature sensors will be wetted and cooled.
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