If the upstream side of a trap has a lower pressure because the steam
v a l v e is modulated closed, Install a vacuum breaker on the trap to
a l l o w atmospheric pressure into the body of the trap, which allows a
p o s i t i v e pressure to cause flow through the trap.
F l o a t and thermostatic traps are widely used in low-pressure heating
systems because they are effective in removing all condensate.
R a t i n g s of float and thermostatic traps have a considerable safety
Bimetallic traps are resistant to damage by water hammer and allow
u n r e s t r i c t e d discharge of air on startup, but are poor in handling air
and noncondensables after startup.
T h e r m o s t a t i c traps are inexpensive for low-pressure applications and
allow air and noncondensables to escape on startup. These
c h a r a c t e r i s t i c s , along with typical sizes from 1/2 to 2 inches, make
t h e thermostatic trap useful for comfort heating systems.
I n v e r t e d bucket traps are especially suited to situations where water
hammer may be a problem. S i n c e they require a water seal to operate,
i n v e r t e d buckets are subject to freezing in outdoor applications.
B e c a u s e they have a moderate air handling ability, they are
recommended for unit heaters where there is no modulation of steam
T r a p s discharging into relatively high-pressure areas, such as a
r e t u r n line higher than the trap level, may reverse and destroy weaker
parts of the mechanism.
C o n t r a r y to manufacturer's claims, bucket, float, and thermostatic
t r a p s may freeze in outdoor installations when the steam is throttled
and insufficient heat is available.
D o not insulate thermostatic and disk traps because they must give off
h e a t to function properly. To ensure adequate opportunity for heat
t r a n s f e r , do not insulate 18 inches of pipe adjacent to the trap
i n l e t . Inverted bucket traps may be insulated since they do not need
t o lose heat to function.
Do not introduce condensate from a high-pressure steam system into the
r e t u r n lines of a lower-pressure system unless the return lines are of
an adequate size. The high-pressure condensate will flash and occupy
more volume than it did as condensate. T r a p s associated with the
l o w - p r e s s u r e system will be prevented from operating properly.
D i s c h a r g i n g high-pressure condensate into a cooler low-pressure return
system may cause water hammer.
A lifting steam trap is nonexistent. Condensate rises to a return
s y s t e m only if there is sufficient pressure behind it to overcome the
s t a t i c head.
B a l a n c e pressure thermostatic traps may be destroyed if there are long
runs of pipe between the source of the condensate and the trap.