| 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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