Page 213 - Valve Selection Handbook

200 Valve Selection Handbook

Keeping the pressure loss below 3% becomes progressively more dif-
ficult as the value of the area ratio of valve entry to valve nozzle A E/A N
decreases. 47 For a ratio of 0.9 A E/KA N = 4 for a full lift liquid relief
valve, for example, the value of the permissible resistance coefficient is
only about 0.5 (see sizing diagram Figure 7-8 on page 271). This is
already the resistance coefficient of a sharp-edged pipe inlet. In this par-
ticular case, either a larger inlet pipe should be considered or, where this
is not possible, a liquid relief valve with oscillation damper. Consult
manufacturer in the latter case.

Instability due to valve oversizing. If a pop action pressure relief valve
is grossly oversized, the discharging fluid may not be able to hold the
valve in the open position. The valve will then reopen and close in suc-
cession. According to one manufacturer's catalog, 48 this may happen if
the rate of flow is less than approximately 25% of the valve capacity.
The frequency of valve opening and closing for a given mass flow
depends on the volume of the system and the valve blowdown setting.
Instability can be overcome entirely by limiting the valve lift in consulta-
tion with the valve manufacturer.

Verification of Operating Data of Spring-Loaded Pressure
Relief Valves Prior to and after Installation

Verification of operating data of pressure relief valves prior to installa-
tion consists commonly of a set pressure test only. The test is normally
carried out on a test bench as instructed by the valve manufacturer.
Under bench conditions, the set pressure point can readily be registered
by observing first steady flow or lift.
However, only verification testing in situ can register the plant specific
influences on set pressure and operation of the valve. But because plant
noise interferes with the audible detection of the set pressure, the pop-
ping pressure is commonly taken as the set pressure. This assessment
method is of little consequence for high pressure valves in which the
opening pressure difference amounts often to not more than 1% to 2% of
the set pressure. In low pressure valves, however, the popping pressure
may be higher than the actual set pressure by as much as 5%. This devia-
tion must be taken into account when correlating the thus determined set
pressure to the normal operating pressure, the reseating pressure, and the
assessment of the achieved blowdown. Thus, if in addition, the opera-