Page 195 - Valve Selection Handbook

182 Valve Selection Handbook

The pressure relief valve shown in Figure 5-18 combines a vacuum
relief valve with a positive pressure relief. The valve has been designed
for sanitary application in the beverage, food processing, and pharmaceu-
tical industries.
Figure 5-19 shows a breather valve that carries separately a direct-
loaded vacuum relief valve and an overpressure relief valve. In deviation
from other vacuum relief valve designs, the disc swings open in an arc on
a point contact hinge. Valve seals consist of soft diaphragms that allow
the valves to reseat close to the set pressure. The valve is intended for
service on low-pressure storage tanks.
Direct-Loaded Pressure Relief Valves with Auxiliary Actuator

The need to improve the fluid tightness of spring-loaded pressure
relief valves that have to operate close to the set pressure, and the need to
inhibit valve chatter in difficult installations, led to the development of
direct-loaded pressure relief valves that are combined with an auxiliary
actuating device. The power to operate the auxiliary device is of an extra-
neous source, commonly compressed air in conjunction with electricity.
Should the power supply to the auxiliary actuating device fail, the valve
operates without interference as a spring-loaded pressure relief valve.
Three types of spring-loaded pressure relief valves with auxiliary
operating device have evolved over time to meet these needs:

• Assisted (anti-chatter) pressure relief valves 1
• Supplementary-loaded (and, simmer) pressure relief valves with
2
restricted loading
• Pressure relief valves with unrestricted supplementary loading and/or
3
lift assistance

Assisted pressure relief valves. Figure 5-20 shows the design of an
assisted pressure relief valve and Figure 5-21 the associated control pip-
ing diagram. Under normal operating conditions, the assist device is
deactivated. When the valve is called upon to open, the assist device is
activated and introduces an unrestricted supplementary opening force.
When the safe pressure is restored, the assist device is deactivated and

1
ASME Code (Sect. Ill), European Draft Standard EN 1268-1 (Safety Valves)
2
ASME Code (Sect. Ill), European Draft Standard EN 1268-1 (Safety Valves)
3
European Draft Standard EN 1268-5 (Controlled Safety Pressure Relief Systems)
German Code of Practice AD Merkblatt A 2 and TRD 421