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VI.B. (Continued)
Safety relief valve
inlets should not be
located at the end
of a long, horizontal
inlet pipe through
which there is
normally no flow.
Foreign matter
may accumulate,
or liquid may be
trapped, and may
interfere with the
operation of the
valve or be the
cause of more
frequent valve
maintenance.
Safety relief valve
inlets should not be located where excessive
turbulence is present such as near elbows, tees,
bends, orifice plates, or throttling valves.
Section VIII of the ASME Boiler and Pressure
Vessel Code requires that the design of the inlet
connection consider stress conditions induced
by reaction forces during valve operation, by
external loading, by vibration and by loads due to
thermal expansion of discharge piping.
1.
The determination of reaction forces during
valve discharge is the responsibility of the
vessel and/or piping designer. DVCD
publishes certain technical information
about reaction forces under various fluid
flow conditions, but assumes no liability for
the calculations and design of the inlet
piping.
2.
External loading by poorly designed
discharge piping and support systems can
be the cause of excessive stresses and
distortions in the valve as well as the inlet
piping. The stresses set up in the valve
may cause malfunctioning or leakage of
the valve. Forced alignment of the
discharge piping will also induce such
stresses. Discharge piping should be
independently supported and carefully
aligned.
Do not locate valve at
end of pipe thru which
there is normally no
flow, or near elbows,
tees, bends, etc.
WARNING
3.
Vibrations in the inlet piping systems may
cause valve seat leakage and/or fatigue
failure of the piping. These vibrations may
cause the disc seat to slide back and forth
across the nozzle seat and result in damage
to the seating surfaces. Vibration may also
cause separation of the seating surfaces
and premature wear to certain valve parts.
High-frequency vibrations are more
detrimental to pressure relief valve tightness
than low-frequency movements. This effect
can be minimized by providing a larger
difference between the operating pressure
of the system and the set pressure of the
valve, particularly under high-frequency
conditions.
4.
Temperature changes in the discharge
piping may be caused by fluid flowing from
the discharge of the valve, prolonged
exposure to the sun, or heat radiated from
nearby equipment. Any change in
temperature of the discharge piping will
cause a change in the length of the piping.
The resulting change in length may cause
stresses which will be transmitted to the
pressure relief valve and its inlet piping.
Stresses caused by thermal changes in the
discharge piping can be avoided by proper
support, anchoring, or provision for flexibility
of the discharge piping. Fixed supports
should not be used.
