Page 184 - Valve Selection Handbook
P. 184
Pressure Relief Valves 171
popping pressure. Raising the nozzle ring reduces the width of the sec-
ondary orifice that leads to an earlier popping of the valve but, in turn,
lengthens the blowdown by some small amount. Conversely, lowering
the nozzle ring increases the difference between set pressure and popping
pressure but, in turn, lengthens blowdown by some small amount.
The lower blowdown ring is commonly factory set and should not nor-
mally need readjustment when putting a new valve into service. No
attempt should be made to eliminate the difference between set pressure
and popping pressure altogether.
The valve shown in Figure 5-5 has done away with the nozzle ring.
The width of the secondary orifice is fixed in this case by the geometry
of seat and disc.
Safety valves with open bonnets are partially balanced by the stem
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guide. This may allow safety valves to operate against a built-up back
pressure of about 20%, but the manufacturer must be consulted. They
are, however, not suitable for superimposed back pressure because of
leakage around the stem to the bonnet. For this reason, safety valves
should not be manifolded.
Safety Relief Valves
Safety relief valves are general-purpose pressure relief valves for use
in either gas or liquid service as encountered in the process industry.
Because the fluids handled in these industries cannot normally be tolerat-
ed to escape around the valve, the bonnet of these valves is either vented
to the valve outlet or sealed against the valve chamber by means of bel-
lows. An exception is safety relief valves for process steam duty in which
the bonnet is provided with windows as in safety valves.
When used on gas or vapor service, the valves pop open. When used
on liquid service, the valves modulate open and reach the fully open
position at an overpressure of about 25% or, in newer designs, at an over-
pressure of 10%, as in the valves shown in Figure 5-7 and Figure 5-10.
The industry distinguishes between conventional safety relief valves
and balanced safety relief valves.
Conventional safety relief valves. Conventional safety relief valves are
provided with a closed bonnet that is vented to the valve outlet, as in the
valves shown in Figure 5-7 and Figure 5-8. By this design, back pressure
is allowed to act on the entire back of the disc.