Structure and Parameter:
||Name of part
||50CrVA / Inconel
||50CrVA / Inconel
||O Type Ring
Smaller package on the larger pipe sizes.
More options for control.
Seals more tightly as the system pressure approaches but does not reach set pressure.
Control pilot can be mounted remotely.
Some designs allow for changes in orifice size within the main valve.
Codes and Standards:
ASME (American Society of Mechanical Engineers) Boiler & Pressure Vessel Code, Section I
ASME (American Society of Mechanical Engineers) Boiler & Pressure Vessel Code, Section VIII, Division 1
API (American Petroleum Institute) Recommended Practice 520 and API Standard 526, API Standard 2000 (low pressure - Storage tank
European standard steam boiler safety valveISO 4126 (harmonized with European Union directives)
EN 764-7 (former CEN standard, harmonized with European Union directives, replaced with EN ISO 4126-1)
AD Merkblatt (German)
PED 97/23/EC (Pressure Equipment Directive - European Union)
JB/T308-2004 <Valves Code>
GB/T12241-2005 <Safety Valves’ Design, Manufacture and Test>
GB/T12243-2005 <Safety Valves’ Design, Manufacture and Test>
The pressure is supplied from the upstream side (the system being protected) to the dome often by a small pitot tube. The downstream side is the pipe or open air where the PORV directs its exhaust. The outlet pipe is typically larger than the inlet. 2”x3”, 3”x4”, 4”x6”, 6”x8”, 8x10” are some common sizes.
The upstream pressure tries to push the piston open but it is opposed by that same pressure because the pressure is routed around to the dome above the piston. The area of the piston exposed to pressure is larger in the dome than it is on the upstream side; the result is a net sealing force. F = PA (Force = Pressure * Area)
The pressure from the pilot tube to the dome is routed through the actual control pilot valve. There are many designs but the control pilot is essentially a conventional PRV with the special job of controlling pressure to the main valve dome. When the pilot valve reaches set pressure it opens and releases the pressure from the dome. The piston is then free to open and the main valve exhausts the system fluid. The control pilot opens either to the main valve exhaust pipe or to atmosphere.
Snap Acting: At set pressure the valve snaps to full lift, it can be quite violent on large pipes with significant pressure. The pressure has to drop below the set pressure in order for the piston to reseat (see blow down in relief valve article).
Modulating: The pilot is designed to open gradually, so that less of the system fluid is lost during each relief event. The piston lifts in proportion to the over pressure. Blow down is typically short.