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In the previous article we discussed the basics of a stream trap and learnt about two types of steam traps - mechanical and thermostatic steam traps. In this article we will learn about thermodynamic and vacuum traps. We will also have a brief look at the maintenance procedures of the same.
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Thermodynamic Steam Trap
The thermodynamic steam trap consists of a simple metal disc type valve which opens and closes over a seat. Let's take a look how this is done.
- The whole opening and closing procedure depends on the pressure energy of the steam. As shown in the figure(i),
- the disc A is raised off from the seat C when the incoming pressure acting on the disc increases. Air and condensate flows through B when the disc is raised.
- Now when the condensate approaches the trap orifice the temperature increases and it flashes to steam.
- Due to this the fluid that is flowing through B increases because of an increase in dynamic pressure and reduction in static pressure.
- Thus the disc is pulled towards the seat but it will never sit completely.
- The steam will continue to escape from around the disc and will accumulate in the control chamber D as shown in the figure (ii).
- When the pressure in the chamber D becomes more than the incoming condensate pressure, it acts on the full surface area of the disc and pushes the disc towards the seat.
- This makes the disc snap shut over the orifice. The advantage and importance of this snapping action is that it removes an foreign particle that might got stuck to the seat and also ensures that the valve is closely shut without any leakage
- After sometime the condensate pressure will again increase, lifting the disc and the cycle will be repeated.
- Generally the trap will remain open for around 15-20 seconds, but then it usually depends on the steam pressure and ambient air temperature inside the trap.
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The performance of the drain systems of the engine room can be improved with the help of vacuum traps. It would be better to allow the condensate to drain to a low level reservoir under gravity and then pump it back to the engine room as and when required. Using this low level hot wells will reduce the problem of back pressure. The condensate can also be transferred to an intermediate hot well and then pumped to the engine room from there. This will enable the pumps to handle the warm condensate in a better way and also improve the overall efficiency of the plant. Let's take a look as to how this vacuum trap works.The arrangement of the trap is shown in the figure.
Let's assume that the valve is in the open position. i.e the exhaust valve C is open and the steam valve D is closed.
- Water enters the trap through a non-return valve A. When the level of water in the trap rises, it raises the float E which compresses the spring H.
- The compression of the spring pulls the spindle J away from the magnet G.
- This closes the exhaust valve C and opens the steam valve D.
- The stream enters the trap and pushes the float E downwards.
- This forces the water out through the non return valve B. Along with the float the spindle J is also pushed down which engages the collar F. This opens the exhaust valve and the steam valve closes and the cycle is repeated.
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McGeorge, H.D (1995) Marine Auxiliary Machinery, 7th ed.