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XII. Recommended Installation Practices (Cont.)
B. Inlet Piping
The inlet piping (Figure 6) to the valve should be short and
direct from the vessel, or equipment, being protected. The
radius of the connection to the vessel should permit smooth
flow to the valve. Avoid sharp corners. If this is not practical,
then the inlet should be at least one additional pipe diameter
larger. The pressure drop from the vessel to the valve shall not
exceed 3% of valve set pressure when the valve is allowing
full capacity flow. The inlet piping should never be smaller in
diameter than the inlet connection of the valve. Excessive
pressure drop in gas, vapor, or flashing- liquid service at the
inlet of the POSRV will cause extremely rapid opening and
closing of the valve, which is known as "chattering". Chattering
will result in lowered capacity and damage to the seating
surfaces. The most desirable installation is that in which the
nominal size of the inlet piping is the same as, or greater
than, the nominal size of the valve inlet flange; and in which
the length does not exceed the face-to-face dimensions of a
standard tee of the required pressure class.
Do not locate POSRV inlets 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 the inlet connection design to consider stress
conditions during valve operation, caused by external loading,
vibration, and loads due to thermal expansion of the discharge
piping.
The determination of reaction forces during valve discharge is
the responsibility of the vessel and/or piping designer. BHGE
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.
External loading, by poorly designed discharge piping and
support systems, and forced alignment of discharge piping
P.D.
Vessel
22 | BHGE
P.D.
Stop
Valve
Vessel
Figure 6: Pressure Drop on the Inlet Piping
can cause excessive stresses and distortions in the valve as
well as the inlet piping. The stresses in the valve may cause
a malfunction or leak. Therefore, discharge piping must be
independently supported and carefully aligned.
Vibrations in the inlet piping systems may cause valve seat
leakage and/or fatigue failure. These vibrations may cause
the disc seat to slide back and forth across the nozzle seat
and may result in damage to the seating surfaces. Also,
vibration may cause separation of the seating surfaces and
premature wear to valve parts. High-frequency vibrations are
more detrimental to POSRV tightness than low-frequency
vibrations. 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.
Temperature changes in the discharge piping may be caused
by fluid flowing from the discharge of the valve or by prolonged
exposure to the sun or heat radiated from nearby equipment.
A change in the discharge piping temperature will cause a
change in the length of the piping, which may cause stresses
to be transmitted to the POSRV and its inlet piping. Proper
support, anchoring or provision for flexibility of the discharge
piping can prevent stresses caused by thermal changes. Do
not use fixe supports.
C. Outlet Piping
Alignment of the internal parts of the POSRV is important
to ensure proper operation (see Figure 7). Although the
valve body will withstand a considerable mechanical load,
unsupported discharge piping consisting of more than a
companion flange long-radius elbow, and a short vertical pipe
is not recommended. Use spring supports to connect outlet
piping to prevent thermal expansion from creating strains on
the valve. The discharge piping should be designed to allow for
vessel expansion as well as expansion of the discharge pipe
The pressure
drop (P.D.)
between the
source of
pressure in the
protected
equipment and
the pressure relief
valve inlet is not
to exceed 3% of
the valve set
pressure.
P.D.
Vessel
© 2019 Baker Hughes, a GE company. All Rights Reserved.
P.D.
From Protected Equipment
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