Page 186 - Valve Selection Handbook

Pressure Relief Valves 173

able for variable superimposed back pressure and should not normally be
manifolded.
Upon valve opening, built-up back pressure introduces a closing force
on the unbalanced disc area. This force restricts application of these
valves commonly to installations in which the valve discharges through a
short pipeline that limits the developing built-up back pressure common-
ly to 10% of the set pressure. In the case of valves with a high built-in
blowdown or which can be adjusted for a high blowdown, permissible
built-up back pressures of 15% and higher are being quoted (consult
manufacturer).
If the back pressure is superimposed and constant, the spring force can
be adjusted for the differential pressure across the nozzle so that the valve
may be employed for back pressures of up to 50% of the set pressure.
Figure 5-9 shows a safety relief valve that has been converted for
steam duty by replacing the closed bonnet with an open bonnet as in

Figure 5-9. Safety Relief Valve with Open
Bonnet. [Courtesy of Crosby Valve &
Gage Company, Wrentham, MA.)