body, depending upon its pressure and temperature, there may be condensate or
s t e a m within the bellows. T h e bellows action is a combination of temperature
and pressure, s i n c e lower pressures allow water to boil at lower temperature.
I f a bellows trap is taken apart while hot, the bellows may continue to expand
a n d d e s t r o y i t s e l f . I f a bellows trap is exposed to superheated water, the
f i l l w i l l c o m p l e t e l y v a p o r i z e , a c h i e v i n g much greater pressure inside the
b e l l o w s than it is designed for, causing the bellows element to distort or
I f bellows are subjected to water hammer, their ridges flatten from
a semicircle to a sharp crease, which will cause failure. When a bellows
breaks, it loses its vacuum and expands. T h i s pushes the valve into the seat
stopping any condensate flow. D i a p h r a g m capsules are similar in action to
Bellows and diaphragm steam traps normally fail closed.
7 . 4 Float and Thermostatic Trap. T h i s trap also depends for its operation on
t h e difference in density between the steam and the condensate. In operation,
c o n d e n s a t e that collects in the trap body gradually raises the float, thus
opening the discharge valve through a lever mechanism. A f t e r the condensate
i s discharged, the float drops, closing the valve and preventing the passage
of steam. Air and noncondensate gases are relieved through a thermostatic
vent. T h e thermostatic vent consists of a bellows and valve assembly. (Refer
to figure 8-7.) The chamber inside the bellows is filled with a liquid, or
h a s a small amount of volatile liquid, such as alcohol. In operation, the
liquid expands or vaporizes when steam contacts the expansive element. The
pressure developed expands the element and closes the valve, preventing the
passage of steam. W h e n relatively coo? condensate or air contacts the
e l e m e n t , the vapor condenses or the liquid contracts, thus decreasing the
pressure and opening the valve which permits the air and condensate to flow.
T h e discharge from this type of trap is intermittent. A cooling leg of 3 or 4
f e e t should be provided ahead of the trap. Some thermostatic elements employ
m e t a l diaphragms in place of bellows; but the operating principle is
F l o a t and thermostatic traps normally fail closed.
8 THERMODYNAMIC TRAPS. Thermodynamic traps operate using the differences in
t h e flow energy, velocity, and pressure of steam and condensate. Trap design
also takes into account the difference in the pressure drop between steam and
c o n d e n s a t e flowing through an orifice or venturi.
8 . 1 O r i f i c e P l a t e T r a p . W h e n a gas or vapor passes through a restriction, it
e x p a n d s to a lower pressure beyond the restriction. D r i l l i n g a s m a l l h o l e i n
a plate is the equivalent of slightly opening a valve.
( R e f e r to
figure 8-8.) An orifice trap operates on the principal of continuously
removing condensate from the steam line. This continual condensate removal
a l l o w s the orifice trap to use a smaller diameter outlet than other types of
s t e a m traps which operate on an open-close-open-close cycle. Thus, the
p o t e n t i a l loss of live steam during system startup, or when the trap has
f a i l e d open, is less for an orifice trap than for other types of steam traps.
The mass flow rate of steam is much less than that of condensate, cutting down
t h e potential loss of live steam during normal operation. Since they are
a l w a y s open, the mode of failure of an orifice trap would be closed if clogged
8 . 2 Venturi Nozzle Trap. P l a c i n g a short section of smaller inside diameter
p i p e between two sections of pipe creates a venturi nozzle. (Refer to
f i g u r e 8-9.) The nozzle trap is based on the two-phase flow principle. Steam