Page 216 - Valve Selection Handbook

Pressure Relief Valves 203

Figure 5-35. Basic Layout of Pilot-Operated Pressure Relief Valve with Direct-Acting
Pilot. (Courtesy of Anderson, Greenwood & Co.)

the piston that grows with rising system pressure. When the system pres-
sure reaches the set pressure, the pilot vents the dome pressure sufficient-
ly to allow the pressure on the underside of the piston to force the valve
open. When receding overpressure reaches the predetermined closing
pressure, the pilot closes the vent and permits the system pressure to act
on the main valve piston in the closing direction.

Pilot types. Pilots may be of the flowing or non-flowing type.
Flowing type pilots permit the system fluid to continue to bleed
through the pilot to vent during the entire venting cycle.
Non-flowing pilots, on the other hand, stop the bleed of system fluid
into the dome volume when called upon to vent the dome. Stopping the
bleed of system fluid into the dome during the valve-opening cycle min-
imizes the possibility of impurities entering the pilot that could interfere
with the pilot operation. In general, the non-flowing pilot is the pre-
ferred pilot.

Opening and closing actions of pilots. Pilots may be designed for
either snap on/off or modulating opening and closing action.
On/off action pilots introduce the full system pressure to the dome
until the set pressure has been reached. Then the pilot vents the dome
pressure, causing the main valve to fully pop open. When the system
pressure has dropped to the blowdown pressure, the pilot reintroduces
the system pressure to the dome, causing the valve to drop closed.