Xylem Bell & Gossett Hoffman Speciality Series Engineering Data Manual page 18

The use of bypass piping around steam traps
is not recommended. Bypass valves, if
opened, may cause pressurization of conden-
sate receivers and cause a safety hazard.
Where stand-by protection is desired the use
of a stand-by trap in parallel to the normal
trap is recommended.
Where the trap drains into a pressurized
return line or to an overhead return, a check
valve should be installed after the trap to pre-
vent backflow through the trap when the
steam is off. The check valve also helps pro-
tect the trap from cavitation (water hammer)
that may occur when traps discharge high
temperature condensate into wet return lines.
Water hammer occurs when high temperature
condensate under pressure ahead of the trap
discharges into a lower pressure return line.
The high temperature condensate flashes,
causing steam pockets to form. When these
steam pockets give up their heat they implode
and cause water hammer.
Differential Pressure = P
P (Inlet)
1
Trap Installation
Trap draining to overhead return line or pressurized return line
EQUIPMENT DRAIN POINT
STATIC
HEAD
SHUT-OFF
VALVE
STRAINER
DIRT
POCKET
Differential Pressure
The differential pressure across the trap will
be the sum of the minimum operating pres-
sure, plus the positive static head at the trap
inlet minus any back-pressure in the return
line minus static head in the discharge piping.
Trap capacities should be calculated at the
minimum differential pressure to assure com-
plete condensate drainage.
Lifts in the return piping should be avoided
wherever possible. High temperature conden-
sate discharging from the trap may flash at
the lower return line pressure. The flashing
into a wet return line will cause steam pock-
ets. As these steam pockets lose their latent
heat they implode, causing water hammer.
Water hammer can damage traps, pipe and
fittings.
Lifts in the discharge piping after a trap will
cause back-pressure. A 2.4 ft. Iift is equal to
a 1 psig pressure. This is especially important
on low pressure operation or where a modu-
lating control valve is used to control the flow
of steam. Reduced flow will cause pressure
drops. A positive differential must be assured
minus P
1 2
under all possible conditions to assure com-
plete condensate drainage.
P (Outlet)
2
TRAP
TRAP
"Y"
TO DRAIN
RETURN
LINE
STATIC
CHECK
HEAD
VALVE
AGAINST
SHUT-OFF
DISCHARGE
VALVE
/ PSI/FT.
1
2
PRESSURIZED
TEST &
RETURN LINE
PRESSURE
RELIEF
19