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 (see Figure 3.1).
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.
The performance of any steam trap can be measured by its response to the steam saturation curve. The ideal response would closely follow the curve and be just below it. A simple bimetal element tends to react to temperature changes in a linear fashion.
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| FIG:- 3 Simple bimetallic element |
Figure 3.1 shows the straight line characteristic of a simple bimetal element relative to the steam saturation curve. As steam pressure increases above P1, the difference between steam saturation temperature and trap operating temperature would increase. Waterlogging increases with system pressure, highlighting the trap's inability to respond to changing pressure conditions.
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| FIG:- 3.1 Typical response of a single element bimetal steam trap |
It needs to be noted that at pressures below P1, the steam trap operating temperature is actually above the saturation temperature. This would cause the steam trap to pass steam at these lower pressures. It may be possible to ensure the steam trap is adjusted during manufacture to ensure that this portion of the saturation curve is always above the operating line. However, due to the linear action of the element, the difference between the two would increase even more with system pressure, increasing the waterlogging effect.
Clearly, this is not a satisfactory operation for any steam trap, and various attempts have been made by manufacturers to improve upon the situation. Some use combinations of two different sets of bimetal leaves in a single stack, which operate at different temperatures (Figure 3.2 ).
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| FIG:- 3.2 Operation of a bimetel steam trap with two leaf element |
The typical result is the split response line similar to that shown in Figure 3.3. This is an improvement on Figure 3.1, but still does not exactly follow the saturation curve. One set of bimetal leaves deflect to give the response P1 to P2. At a higher temperature a second set of bimetal leaves contributes to give response P2 to P3. Clearly, although an improvement from the former design, this is still unsatisfactory in terms of following the saturation curve.
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| FIG:- 3.3 Typical response of a two leaf element Z - Z |
Advantages of the bimetallic steam trap:-
1.Bimetallic steam traps are usually compact, yet can have a large condensate capacity.
2. 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.
3. 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.
4. Bimetallic steam traps are usually able to withstand water hammer, corrosive condensate, and high steam pressures.
5. 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.
6. 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.
7. 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.
8 .Maintenance of this type of steam trap presents few problems, as the internals can be replaced without removing the trap body from the line.
9. 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.
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| FIG:- 3.4 Bimetallic steam trap with cooling leg |
Disadvantages of the bimetallic steam trap:-
1. 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.( 3.4). 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.
2. 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.
3. 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.
4. Bimetallic steam traps do not respond quickly to changes in load or pressure because the element is slow to react.
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