Balanced Pressure Thermostatic Trap

The trap is operated by a thermostatic fluid filled capsule. When steam is turned on, air discharges through the open valve orifice. Cold condensates enter & passes out. At a temperature t below the saturated steam temperature corresponding to the pressure at the trap, the mixture boils & exerts a vapour pressure. While the outside of the capsule is at steam pressure, the inside, because of the temperature head, is at a pressure higher by p . This excess pressure p closes the valve on its seat before steam temperature is reached. Condensate help in the trap, cools by t below steam temperature, until the vapour pressure in the capsule is below the steam pressure. The capsule contracts & the valve opens.
Since the curves of the fluid inside the capsule & water are parallel, it does not matter what the steam pressure is in the trap. At any pressure p there will be an unbalanced force p holding the valve shut, after a temperature t below saturated steam temperature is reached.
If superheat is present an internal pressure higher than p, is taken up by the flexible diaphragms in the capsule.