XII. Recommended Installation Practices (Contd.)
Vessel
Vessel
Vessel
Stop
Valve
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.
P.D.
From Protected Equipment
P.D.
P.D.
Figure 12: Pressure Drop on the Inlet Piping
B. Inlet Piping
The inlet piping (Figure 12) 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 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.
26 | BHGE
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