Thermodynamic Steam Traps Introduction
THERMODYNAMIC TRAPS Thermodynamic traps use only one moving part, the valve disc, which allows condensate to be discharged when present and closes tightly upon the arrival of steam. These traps have an inherently rugged design and are commonly used as drip traps on steam mains and supply lines. Their solid construction and single moving part make them resistant to waterhammer and are freeze-proof when installed vertically. Thermodynamic traps will only discharge small amounts of air and therefore are typically not used in process applications. Since Thermodynamic traps rely on steam velocity to operate, they are not intended for low pressure service (below 30 PSI). Operation: The inlet pressure to the trap pushes the disc off the seat and allows unwanted condensate to be discharged through the peripheral outlet surrounding the inlet ( Figure A ). As hot condensate reaches the disc chamber, flash steam is created that travels at high velocity from the inlet to the outlet creating a low pressure area under the disc and higher pressure above the disc ( Figure B ). This differential pressure causes the disc to close against the seat and trap the steam in the system ( Figure C ). The steam pressure above the disc creates a force holding the disc closed. Heat transfer takes place through the cap and the steam pressure above the disc begins to reduce. When the downward force created by the steam pressure above the disc falls below the force created by the incoming condensate, the disc is pushed off its seat and the process repeats itself ( Figure A ). Cycle time is dependent on steam temperature, and more importantly, ambient temperature outside the trap. Since the amount of time the valve remains closed is primarily dependent on the heat transfer from the steam above the disc to the ambient environment, frequent cycling of the valve can occur in cold or wet environments. Applying an insulating cap over the cover of the trap will reduce the cycle rate.
A) Valve Disc (Open)
B) Valve Disc (Starting to Close)
Disc (Closing) Inlet
Peripheral Outlet
C) Valve Disc (Closed)
Flash Steam
Condensate
Control Chamber
Steam
A) When condensate is present, trap remains in the open position allowing condensate to discharge.
B) When steam enters the trap, it creates an internal pressure above the disc that instantly forces the disc and seat to close tightly, preventing steam from escaping. C) Trap will remain closed, trapping steam in the system until the steam above the disc condenses, due to heat loss through the cap.
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