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 CHAPTER 8.
STEAM TRAPS
S e c t i o n 1.
TRAPS DESCRIPTION
1 PURPOSE OF TRAPS. Steam traps are automatic mechanical devices which drain
condensate, remove air and noncondensable gases from the system, and stop the
p a s s a g e of live steam through the drain lines. Accumulation of condensate
s e r i o u s l y affects the performance of steam distribution lines by decreasing
t h e capacity for heat transmission. Also, condensate may produce water hammer
w h i c h can result in burst pipes, blown gaskets, or personal injury. Water
hammer is the result of a slug of condensate being pushed by the steam and
forcefully hitting the walls of the steam conduit. A layer of air or gas is
c o m p a r a b l e to a layer of ground cork insulation. A i r in steam considerably
l o w e r s the temperature of the mixture, thus reducing the capacity for heat
t r a n s f e r ; also, air slows the rate at which steam can get into the equipment.
Air and some noncondensable gases, mainly carbon dioxide, may produce
c o r r o s i o n of the ferrous parts of the system.
2 CONSTRUCTION. Steam traps should conform to Federal Specification WW-T-696
( L a t e s t Revision), Traps, Steam, and Air. Traps have the following components:
( a ) A vessel to collect condensate
(b) An opening through which condensate is discharged
( c ) A valve to close the opening
(d) Adequate mechanisms to operate the valve
( e ) Inlet opening to the vessel for condensate admission
( f ) Outlet opening for condensate discharge
3 PIPING FOR TRAPS. Drip piping to traps should be of the same weight and
m a t e r i a l as the drain piping. Traps may be discharged through a check valve
i n t o a pumped condensate line if the pressure differential through the trap is
adequate.
P r e f e r a b l y , however, a discharge line from a trap should run
s e p a r a t e l y to a gravity condensate return main or to a nearby flash tank. The
discharge piping from a trap should be pitched down, at a minimum of 3 inches
p e r 100 feet (0.25 percent), to the collection tank of a condensate pump set,
o r to a gravity return. T h i s slope is not required, however, when there is
s u f f i c i e n t pressure in the steam line to overcome the friction and static head
i n the discharge line. I f it is impractical to return drips to a condensate
system, they may be drained as waste to a sewer. When the temperature of the
drains exceeds sewer limitations, the condensate must be cooled in a sump or
by other means. S o m e traps are provided with a built-in strainer. When this
i s not the case, a strainer should be installed ahead of the trap to act as a
c a t c h - p o c k e t of pipe scale, sediment, and foreign materials. F o r testing
c o n v e n i e n c e , trap installations should include a tee and test valve in the
d i s c h a r g e line to check trap action. W h e n it is required to ensure continuity
o f service, a three-valved bypass should be installed around the trap. This
p e r m i t s drainage of the line when the trap is out of service for repairs.
8-1
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