Both use the difference in temperature between live steam and condensate or air to control the release of water and noncondensable gases from the steam line. The element deflects against the line pressure with a variable increasing force in relation to rising steam pressure and temperature. The force exerted by the bimetallic element against line pressure closely parallels the steam curve, which permits the trap to operate throughout a wide pressure range without adjustment. The heart of this design is the bimetallic elements that consists of two dissimilar metals bonded together to form a composite strip. Due to the differing coefficients of expansion of the two metals, the strip will bend or deflect when subjected to a change in temperature.
Most designs have the ball valve wide open when the trap is cold.
This allows air and condensate to discharge rapidly during initial pressurization of the steam line. As warmer condensate enters the trap, the bimetallic element deflects slowly to close the valve. By the time live steam reaches the trap, the element has deflected so as to completly close the ball valve on the exit side of the trap. When condensate or air enters the trap, their temperature is below that of saturated steam and the consequent reaction of the element is insufficient to keep the valve tightly closed allowing line pressure on the valve to overcome the pull of the element and open the valve. When live steam again enters the trap, the element deflects to once again tightly close the exit side ball valve preventing the loss of live steam.