Steam traps are special types of valves which prevent the passage of steam but allow condensate through. It works automatically and is used in steam heating lines to drain condensate without passing any steam. The benefit gained with a steam trap, is that steam is contained in the heating line until it condenses, thus giving up all of its latent heat.

Thermodynamic steam trap use pressure energy of the steam to close the valve which consists of a simple metal disc. The sequence of operation is shown in figure below.

In (i), disc A is raised from seat rings C by incoming pressure allowing discharge of air and condensate through outlet B. As the condensate approaches steam temperature it flashes to steam at the trap orifice. This means that the rate of fluid flow radially outwards under the disc is greatly increased. There is thus an increase in velocity and a reduction in static pressure. The disc is therefore drawn towards the seat. Due to this alone the disc will never seat. However, steam can flow round the edge of the disc resulting in a pressure build up in the control chamber D as shown in (ii). When the steam pressure in chamber D acting over the full area of disc (iii) exceeds the incoming condensate / steam pressure acing on the much smaller inlet area, the disc snaps shut over the orifice. This snap action is important. It eliminates any possibility of wire-drawing the seat, while the seating itself is tight, ensuring no leakage. As shown in (iv) the incoming pressure will eventually exceed the control chamber pressure and the disc will be raised, starting the cycle all over again.

The rate of operation of a steam trap depends upon he steam pressure and ambient air temperature. In practice, the trap will usually open after 15 – 25 seconds; the length of time open depends on the amount of condensate to be discharged. If no condensate have been formed, then the trap snaps shut immediately. From the foregoing it will be seen that the trap is never closed for more than 15 – 25 seconds, so condensate is removed virtually as soon as it is formed.

 

References

“REED’S GENERAL ENGINEERING KNOWLEDGE FOR MARINE ENGINEERS”, by Leslie Jackson and Thomas D. Morton