The thermodynamic trap is perhaps the simplest trap ever produced, which gives it an obvious maintenance advantage. It is usually constructed of stainless steel and has only one moving part - the flat disc. The trap is small but extremely robust. It can readily. be supported by the pipework and will withstand the worst waterhammer or freezing.
The trap closes initially when condensate under pressure reaching the inlet orifice approaches steam temperature. When this occurs, some of the condensate flashes to steam, produces a high velocity across the disc and closes the trap. The trap opens again when some of the flash steam left in the control chamber on closure condenses and the chamber pressure falls.
The point to note is that the trap closes as the condensate approaches steam temperature. Provided there is a reasonable supply of condensate, live steam will never reach the trap. Only when the flow of condensate approaches zero will live steam reach the trap and the resulting high velocity will shut it practically instantaneously.
The essential thing is the high velocity across the underside of the disc. To achieve this, the inlet pressure should not be less than 0.25 bar (3.5 psi) and the back pressure should not exceed about 80% of the inlet pressure. These figures vary a little in the case of some very high pressure traps.
The trap will stay shut until some of the steam in the control chamber has condensed. The traps normally associated with high pressures and temperatures are often fitted with an insulating shield in order to prevent a too rapid loss of temperature. This would cause the trap to operate more frequently and shorten its life.