Thermodynamic Trap: Kinetic Energy Operated Steam Trap

Thermodynamic Trap:

Thermodynamic trap is classified as kinetic energy operated traps and it’s the most widely used trap in the oil and gas industry amongst all types. This thermodynamic trap has an extremely robust design and works on a simple mode of operation. It can operate at very high temperature and pressure. The means of operation for the trap is the dynamic effect of steam as it passes through the trap as shown in the sketch 1 below. The disc above the flat face inside the control chamber or cap is the only moving part in the trap. 

Construction features of Thermodynamic Trap:

This is the commonest type of Trap relying on the fact that hot Condensate released under pressure will generate flash steam. Thermodynamic trap is fairly simple in construction, with only disc as the moving part inside the control chamber fitted on the top of the Trap body. The central inlet port in the body provides a flat seating surface with an annular groove around it to connect it to the discharge port.

Thermodynamic Steam Trap Working

Sketch 1: Working of Thermodynamic Steam Trap

Thermodynamic steam trap

Sketch 2: Thermodynamic steam trap

Sketch 2: Thermodynamic steam trap

Sketch 3: Thermodynamic steam trap

Working principle of thermodynamic trap:

The functioning of the thermodynamic trap can be understood from the accompanying sketch above. During the initial startup cold air and Condensate lift the disc from the seat to pass radially outwards through the annular groove to the exit port (refer Figure-1, Sketch 1).

As the Condensate approaches the accompanying steam temperature it tends to flash giving rise to the velocity of fluid passing under the disc and thereby causing a drop in the pressure. The flashed steam eventually filled in the control chamber, at the same time exerts pressure from above to push the disc down against the seat which snaps the flow passage shut to block the discharge of the condensate (refer Figure-2, Sketch 1).

The Disc remains closed under the steam pressure of the flash steam of the control chamber for a while (refer Figure-3, Sketch 1).

With lapse of time the flash steam in the control chamber condenses (by loosing heat to the surrounding atmosphere) to once again lift the disc under the upstream line pressure and allow the flow of Condensate.

Advantages of thermodynamic trap:

  • Compact and light weight design.
  • Owing to simple internal parts thermodynamic trap has a fairly high resistance to corrosion.
  • Less prone maintenance. As disc is the only moving part maintenance can be carried out without removing the trap from the line.
  • Efficient operation under varying steam pressure and condensate loads.
  • Thermodynamic trap allows high heat transfer efficiency by discharging condensate at the steam temperature.
  • Can be used for high and super heated steam and are not affected with water hammer and vibrations. As such no condensate banking.
  • Same model can be used for wide range of operating pressures.
  • Freezing has no damage effect and are unlikely to freeze if installed with the disc in a vertical plane and discharging freely to atmosphere. However, operation in this position may result in wear of the disc edge.

Disadvantages of thermodynamic trap:

  • On very low differential pressures, Thermodynamic steam traps will not work positively, as the velocity of flow across the underside of the disc is insufficient for lower pressure to occur. They are subjected to a minimum inlet pressure (typically 0.25 bar g) but can withstand a maximum backpressure of 80% of the inlet pressure.
  • If the inlet pressure builds up slowly, thermodynamic trap can discharge a large amount of air on ‘start-up’. However, rapid pressure build-up will cause high velocity air to shut the trap in the same way as steam, and it will ‘air-bind’. To avoid this case a separate thermostatic air vent can be fitted in parallel with the trap. Nowadays modern thermodynamic steam traps have an inbuilt anti-air-binding disc which prevents air pressure building up on top of the disc and allows air to escape.
  • The use of a thermodynamic trap in some locations may be prohibited because of the noise factor during the discharge of the trap. If this is a problem, it can easily be fitted with a diffuser which considerably reduces the discharge noise.
  • While sizing the thermodynamic trap care should be taken not to oversize the trap as this can increase cycle times and induce wear. Steam mains drainage applications often only need to be fitted with low capacity versions, providing proper consideration is given to sitting the drain pockets correctly.