14 | BHGE
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Safety relief valve discharges should be located or piped in such a manner that the
discharged fluid is carried clear of man ways. Referring to Figure 4, a drain should be
provided at the low point of the discharge elbow where condensate may discharge. An
open gravity drain is also supplied on the backside of the valve body casting which is
below the level of the valve seat to remove condensate in the body bowl.
Exhaust piping and drain lines must be installed so that they will not impose undue
stress on the safety relief valve. Stresses set up in the body from any source may cause
distortion and leakage at pressures below the set point. If standard drip pans are used
(see Figure 4), sufficient clearance must be provided to allow for movement of the valve
due to expansion and contraction of the unit on which it is installed. If blow back around
the drip pan occurs, the vent piping is inadequate and its design should be investigated.
If the valves are to exhaust through horizontal piping then the distance from the valve
center line to the end of the horizontal pipe must not exceed the dimension shown in
Table 1.
If discharge piping is required beyond the length limit specified in Table 1, then a
length of larger diameter piping should be allowed for the expansion and contraction
of the unit on which the valve is installed. If the larger discharge piping extends
through a wall to the outside it must be adequately fastened to the building to
prevent breaking free when the valve is blown.
Cold air blowing into the valve body through the discharge pipe can cause the valve to leak.
To prevent this from happening the discharge pipe should point downwind. If this is not
possible, then a 90
0
elbow, or a “T”, should be installed.
CAUTION
All discharge piping shall have sufficient area to accommodate the full capacity of the
safety valve without causing steam to escape backward into the drip pan.
Flexible metal hoses, if used to connect safety relief valve outlets to discharge stacks,
must have sufficient length and must be designed and installed in such a manner that
they will not become "solid" in any position of the valve. Better results are obtained if
the hoses are installed so that they will permit movement by bending rather than by
stressing and compressing along their length.
Steam flowing vertically out of the discharge elbow produces a downward reaction on
the elbow, depending on the quantity of steam flowing and its velocity. In large, high
capacity valves this force can equal several thousand pounds and can produce severe
stresses in the valve neck. The bending stresses are determined by the amount of the
reactive force, combined with the moment arm or horizontal distance between the
vertical centerlines of the outlet elbow and the valve. The pilot operated 13900 Safety
Relief Valve is designed so that as long as the moment arm distance from the centerline
of the valve to centerline of the vent piping is held within the limits stated in Table 1,
neck stresses will be within code allowable values.
When installing the valve, the inlet flange bolts should be pulled down evenly to prevent
inlet flange distortion and effect a leak tight flange seal (See Figure 6 and Table 2). For
outlet flanges, refer to Figure 7.
It is recommended that the valve be insulated with a minimum of 2" (50.8 mm) insulation.
XI. Recommended Installation Practices
Do not drop or
strike the valve.
Prevent dirt from entering
outlet or inlet port.
