THERMOSTATIC TRAPS (bellows & bimetallic) operate on a difference in temperature between condensate and
steam. They build up condensate so that the temperature of condensate drops down to a certain level below
saturation temperature in order for the trap to open. By backing up condensate, the trap will tend to modulate
open or closed depending on load.
In a bellows trap, should the bellows become compressed by water hammer, it will not function properly. The
occurrence of a leak will prevent the balanced pressure action of these traps. When either condition occurs, the trap
will fail in its natural position either opened or closed. If the trap fails closed, condensate will back up and no
sound will be heard. If the trap fails open, a continous rushing of live steam will be heard.
With bimetallic traps, as the bimetallic plates set due to the heat they sense and the cooling effect on the plates, they
may not set properly which will prevent the plates from closing completely and allow steam to pass through. This
will be heard as a constant rushing sound.
THERMODYNAMIC (DISC) traps work on the difference in dynamic reponse to velocity change in the flow of
compressible and incompressible fluids. As steam enters, static pressure above the disc forces the disc against the
valve seat. The static pressure over a large area overcomes the high inlet pressure of the steam. As the steam starts
to condense, the pressure against the disc lessens and the trap cycles. A good disc trap should cycle (hold-dis-
charge-hold) 4-10 times per minute. When it fails, it usually fails in the open position, allowing continuous blow-
through of steam.
INVERTED BUCKET TRAPS normally fail in the open position because the trap loses its prime. This condition
means a complete blow-through, not a partial loss. The trap will no longer operate intermittently. Aside from a
continuous rushing sound, another clue for steam blow-through is the sound of the bucket clanging against the side
of the trap.
A FLOAT AND THERMOSTATIC trap normally fails in the "closed" position. A pinhole leak produced in the
ball float will cause the float to be weighted down or water hammer will collapse the ball float. Since the trap is
totally closed - no sound will be heard. In addition, check the thermostatic element in the float and thermostatic
trap. If the trap is operating correctly, this element is usually quiet; if a rushing sound is heard, this will indicate
either steam or gas is blowing through the air vent. This indicates that the vent has failed in the open position and
is wasting energy.
-12-NOTE: A complimentary Steam Trap Trouble Shooting Guide is available. Contact UE Systems
directly by phone or fax.
bucket trap drawing