Piston steam traps are in the thermodynamic trap category that are actuated by the principles of thermodynamics and fluid dynamics.

Thermodynamic steam traps are phase detectors in that they can discriminate between liquids and gases, but they do not discriminate between steam and air or other non-condensable gases. Therefore, they have a reduced ability to bleed-off those gases. Small amounts of steam may also be passed.

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Piston steam traps utilize the heat energy in hot condensate, and the kinetic energy in steam, to open and close a valve. Like disc steam traps, they are phase detectors sensing the difference between a liquid and gas or vapor.

During initial start-up, pressure created by the cold condensate lifts the piston valve, allowing discharge of condensate. During this phase, the control chamber pressure is low because the second or control orifice, can discharge more condensate than can be supplied to the control chamber through the first orifice. When the temperature of the discharging condensate is very close to steam temperature (i.e., saturation temperature), the condensate, experiencing the lower pressure of the control chamber, will change into flash steam (in accordance with the laws of thermodynamics). This flashing of the condensate in the control chamber chokes the flow through the control orifice, causing an increase in control chamber pressure. This increased pressure, acting on a larger effective area of the piston valve than the inlet pressure, causes it to snap shut, thus preventing steam flow through the trap. When cooler condensate reaches the trap, causing the control chamber pressure to drop, flashing ceases and the trap re-opens to repeat the cycle.

The control orifice provides a continuous discharge which is helpful in passing air or other non-condensable gases during start-up. The piston valve remains closed in the presence of steam because the pressure on top of the piston acts on a larger effective area than the inlet pressure under it. Steam loss through the control orifice is minimal.

Piston steam traps were introduced in the 1930's as the first thermodynamic trap. It is a "hot" trap, providing excellent service in high pressure applications.

Prior to selecting a steam trap for your application, review steam traps selection with its advantages versus disadvantages and additional steam trap types: Disc Steam Traps, Lever Steam Traps, Closed Float Steam Traps, Inverted Bucket Steam Traps, Open Bucket Steam Traps, Bimetallic Steam Traps, Bellows Steam Traps, Liquid or Solid Expansion Steam Traps (Wax Capsule Steam Trap), and Orifice Steam Traps.