Page 209 - Valve Selection Handbook

196 Valve Selection Handbook

On receding over pressure, the valve retains full life until reaching the
operating point S, where lifting and closing forces are again in balance.
As the system pressure recedes further, the valve modulates closed until
reaching the operating point E.
At this point, the difference between the set pressure and the pressure at
the valve inlet is the sum of the pressure loss in the inlet line to the valve
and the blowdown at valve reseating. This pressure difference is referred
to in the following as the blowdown setting of the valve. As the operating
pressure commences to recede further, the closing force begins to exceed
the lifting force, causing the valve to reseat in one rapid stroke while the
pressure loss in the inlet line to the valve drops progressively to zero.
If the blowdown setting of the valve at operating point E were less
than the pressure loss in the inlet line to the valve, the valve could not
close completely. The valve would then reopen and reclose in rapid suc-
cession. Acoustic interaction between pipeline and valve could severely
aggravate valve chatter. Chatter would continue until the operating pres-
sure has dropped sufficiently or the valve has destroyed itself.
The diagram below the force lift diagram shows the corresponding
relationships between valve lift and operating pressure. The diagram,
referred to as the lift/pressure diagram, shows the periods of rapid and
modulating movements of the disc over the operating cycle from the set
pressure to the reseating pressure.

Operational behavior of spring-loaded pressure relief valves that
pop open partially only and then continue to modulate open (typical
only). The force lift diagram shown in Figure 5-33 is based on the
lift/pressure relationship shown in Figure 22 of API RP 520 Part I (1993).
Valves with this operating characteristic pop open over a portion of the
full lift only and then continue to modulate open until the full lift is
achieved. As the operating pressure rises still further, the valve achieves
its rated capacity at operating point D. This opening characteristic is
common to a wide range of spring-loaded pressure relief valves.
The difference between the lifting force at operating point D and the
closing force at operating point S is a measure of the tolerance of a par-
ticular valve to back pressure. If the force/lift diagrams (Figure 5-32 and
Figure 5-33) apply to different types of valves but identical operating
conditions, the valve with a force/lift characteristic of Figure 5-32 will
display a higher resistance to back pressure than the valve with a
force/lift characteristic of Figure 5-33. The back pressure tolerance of the
latter valve can be improved within limits by lowering the incline of the