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Bimetallic steam trap
bimetallic steam traps are constructed using two strips of dissimilar metals welded together into one element. The element deflects when heated.There are two important points to consider regarding this simple element:

Operation of the steam trap takes place at a certain fixed temperature, which may not satisfy the requirements of a steam system possibly operating at varying pressures and temperatures

Because the power exerted by a single bimetal strip is small, a large mass would have be used which would be slow to react to temperature changes in the steam system.

Advantages of the bimetallic steam trap: Bimetallic steam traps are usually compact, yet can have a large condensate capacity.

The valve is wide open when the steam trap is cold, giving good air venting capability and maximum condensate discharge capacity under 'start-up' conditions.

As condensate tends to drain freely from the outlet, this type of steam trap will not freeze up when working in an exposed position. The bodies of some bimetallic steam traps are designed in such a way that they will not receive any damage even if freezing does occur.

Bimetallic steam traps are usually able to withstand waterhammer, corrosive condensate, and high steam pressures.

The bimetal elements can work over a wide range of steam pressures without any need for a change in the size of the valve orifice.

If the valve is on the downstream side of the seat, it will tend to resist reverse flow through the steam trap. However, if there is any possibility of reverse flow, a separate check valve should be fitted downstream of the trap.

As condensate is discharged at varying temperatures below saturation temperature and, provided waterlogging of the steam space can be tolerated, some of the enthalpy of saturated water can be transferred to the plant. This extracts the maximum energy from the condensate before it drains to waste, and explains why these traps are used on tracer lines where condensate is often dumped to waste.

Maintenance of this type of steam trap presents few problems, as the internals can be replaced without removing the trap body from the line.

The flash steam produced whenever condensate is discharged from a higher to a lower pressure will tend to cause an increase in backpressure in the condensate line. The cooling leg allows the condensate to cool down, producing less flash steam in the condensate line and thus helping to reduce the backpressure.

Disadvantages of the bimetallic steam trap:

As condensate is discharged below steam temperature, waterlogging of the steam space will occur unless the steam trap is fitted at the end of a long cooling leg, typically 1 - 3 m of unlagged pipe (see Fig. 11.2.14). Bimetallic steam traps are not suitable for fitting to process plants where immediate condensate removal is vital for maximum output to be achieved. This is particularly relevant on temperature controlled plants.

Some bimetallic steam traps are vulnerable to blockage from pipe dirt due to low internal flow velocities. However, some bimetallic traps have specially shaped valve trims that capture the discharge energy to open the valve more. These tend to give an intermittent blast discharge characteristic rather than a continual dribble discharge, and as such tend to be self-cleaning. These valve trims are sometimes referred to as dynamic clacks.

If the bimetallic steam trap has to discharge against a significant backpressure, the condensate must cool to a lower temperature than is normally required before the valve will open. A 50% backpressure may cause up to a 50°C drop in discharge temperature. It may be necessary to increase the length of cooling leg to meet this condition.

Bimetallic steam traps do not respond quickly to changes in load or pressure because the element is slow to react.





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