During startup, the trap is cold and the bimetal air vent ring is contracted, keeping the disc valve raised. During this time, large amounts of initial air and cold condensate can be discharged from the trap. The bimetal air vent ring holds the disc valve open, preventing air-flow obstacles (air binding).
As steam enters the trap, the bimetal air vent ring is heated and expands, quickly slipping down to the valve seat skirt, freeing the disc. The steam flows rapidly under the released disc and the jet creates a low-pressure region (Bernoulli's principle). The steam jet flows into the pressure chamber creating a high pressure region as the steam loses velocity and is compressed. This pressure pushes the disc valve down to close the valve seat.
When condensate enters the trap, the temperature in the pressure chamber drops, causing the steam to condense and the pressure to drop. If the pressure becomes lower than the inlet pressure, the disc valve opens to discharge condensate. Soon after the condensate is discharged, the valve closes utilizing the same principle. In this manner, the valve opens and closes automatically to intermittently discharge condensate that enters the trap.