Page 279 - Boiler plant and distribution system optimization manual

264 Boiler Plant and Distribution System Optimization Manual

the seat. This opens the trap and it begins the cy- reaches the trap, the element warms and changes
cle again. shape, exerting a closing force on the valve.
Reduced temperature from a steam and non-
condensible gas mixture will also cool the bellows Expansive Element Steam Traps
and fluid, allowing the discharge of these gases. Expansive element traps (Figure 14.19) are
characterized by a constant discharge tempera-
Diaphragm Traps ture for a given condensate load, regardless of the
Diaphragm traps are a modification of the steam system pressure.
bellows type trap. However, instead of a large Various elements are used and they may be
bellows with many convolutions or welded ele- liquid or solid. In all cases, the response to chang-
ments, a single element is used. ing conditions is relatively slow and dependent
Operation of this type steam trap closely upon the thermal mass of the element.
parallels the operation of bellows traps.

Bimetallic Traps
Bimetallic traps (Figure 14.18) use the tem-
perature of the condensate in the trap to bend
bimetallic elements against the force exerted by
the steam pressure on the valve. There are many Figure 14.19—An expansive element steam trap.
different configurations of bimetallic traps which
cause significant differences in operation. During start up the element is contracted,
allowing the discharge of cold condensate, air
and non condensable gases. Discharge continues
until warmer condensate heats and expands the
element, moving the valve towards the seat. The
trap will reach equilibrium condition, discharg-
ing condensate continuously at a nearly constant
temperature. Only rarely will these traps cycle
and then only under very light load conditions,
before returning to the modulating mode.
The operation of these traps is regulated
Figure 14.18—A bimetallic steam trap.
by the condensate temperature and are suitable
for applications where condensate backup and a
Since a bimetallic element is a nearly linear
slow response to load changes is acceptable.
device, most manufacturers use several types of
bimetal or special shapes to cause the trap to ap-
proximate the steam saturation curve over the op-
STEAM TRAP LOSSES
erating range.
Because of the relatively large thermal mass It is clear, that steam leaks are quite expen-
of the bimetallic element, response to system sive. The high and low estimates in Table 14.1
changes can be slow. Additionally, these traps are take into account the throttling effect of conden-
sometimes affected by the back pressure which sate choking the full flow of escaping steam with
works against the opening force on the valve pos- variations in condensate formation load.
sibly increasing the amount of sub-cooling. We can assume that the cost of steam leaks
During start up the bimetallic element is through failed traps will range from $2,000 to
relaxed, allowing the steam system pressure to $50,000 per trap over the course of a year.
open the valve discharging the cold condensate The economic incentive for eliminating failed
and non-condensible gases. As warm condensate steam traps clearly exists. A trap anywhere in a