F l o a t traps usually consist of the following parts (refer
6.2 Float Traps.
to figure 8-3):
A s condensate enters the inlet of a float trap, the condensate level in the
t r a p r i s e s . When condensate rises above the outlet opening, the preadjusted
f l o a t rises and opens the trap valve releasing the condensate. The trap is
d e s i g n e d so that the condensate level never drops below the level of the
o u t l e t valve while the trap is operating. T h i s ensures that the lighter
d e n s i t y live steam can never escape through the outlet.
7 THERMOSTATIC TRAPS. Since steam contains more heat energy than the
condensate, the heat of the steam can be used to control the operation of a
steam trap. T h e r m o s t a t i c traps are most useful for removal of air or
noncondensable gases, e s p e c i a l l y during startup.
7 . 1 B i m e t a l l i c T r a p . The operation of a bimetallic steam trap is based on a
bimetallic element which changes shape with changes in temperature. Movement
o f the bimetallic element controls a valve that releases air and condensate.
T h e basic bimetallic trap is only sensitive to changes in temperature and
n e e d s to be adjusted to the pressure range in which it will be operating. To
p r e v e n t the loss of live steam, or a buildup of condensate, manufacturers use
s e v e r a l different valve and bimetallic element shapes and sizes. These
d e s i g n s allow the bimetallic steam trap to respond better to changes in its
o p e r a t i n g conditions. B i m e t a l l i c steam traps normally fall closed. Refer to
figure 8-4 for an illustration of a bimetallic steam trap.
7.2 Thermal Expansion Steam Trap. The thermal expansion type steam trap
o p e r a t e s over a specific temperature range without regard to changes in
p r e s s u r e . The thermal element may be a wax, a p l a s t i c , o r a s p e c i a l l i q u i d .
This thermal element is used because it has a high expansion rate when
s u b j e c t e d to a small increase in temperature. The thermal element is sealed
o f f from direct contact with the condensate and steam. When condensate is
f l o w i n g through the trap the valve would be fully open. With a slight rise in
t e m p e r a t u r e up to the saturation temperature, the thermal element would
d r a m a t i c a l l y expand closing the valve and preventing loss of live steam.
Almost any operating pressure can be selected over which to open and close the
v a l v e by selecting the corresponding saturation temperature at which the
t h e r m a l element will dramatically expand. Thermal expansion steam traps
n o r m a l l y fail open. ( R e f e r to figure 8-5.)
7 . 3 B e l l o w s . The more common design for low-pressure heating systems is the
b e l l o w s or diaphragm trap (refer to figure 8-6). The bellows element has
c o r r u g a t i o n s and may be filled with a liquid, such as alcohol, water, or a
m i x t u r e of both. When heated by steam around the bellows, the liquid inside
t h e bellows begins to vaporize. T h i s forces the bellows to expand since there
i s a greater pressure inside than outside the bellows. Bellows can be used at
varying pressures because when there is steam in the trap body outside the
b e l l o w s , there is steam within the bellows. When there is condensate in the