Pressure Relief Valve (PRV) Types – part 3

This is the third part of our description of the types of pressure relief valve (PRV) types. You can catch up here on part one and part two.

Balanced Pressure Relief Valves

Balanced pressure relief valves can be used in all of the services mentioned for the pressure relief valve where the backpressure is either constant or variable. Balanced bellows pressure relief valves are used in corrosive or dirty services. The bellows help to keep the guiding surfaces of the valve free from contaminants thus prevent sticking as a result of the ingress of dirt or corrosion on the guiding surface.

Limitations

Balanced pressure relief valves should not be used:

  • On superheaters or steam boiler drums.
  • As pressure control or bypass valves.

Balanced bellows relief valves must have their bonnets vented to atmosphere, the need for venting the pipe to a safe location must be determined. A discharge will only occur from the bonnet of a balanced bellows pressure relief valve if the bellows fail. Sometimes a supplementary balancing piston is fitted as a back up device, if the bellows fail the balancing piston will make sure that the valve reliefs at the correct set pressure. Removal of a balancing piston (if the valve is designed to have one) would cause an adverse affect on the set point of the PRV. This could result in the valve not relieving it’s full capacity or the valve may not achieve full lift within 10% overpressure.

Pilot Operated Pressure Relief Valves

Pilot operated pressure relief valves are valves that have a major relieving chamber that is controlled by a self actuated pressure relief valve. Basically, pilot operated relief valves consist of two basic units; the pilot valve or pilot assembly/controller and the main valve.

Pilot operated relief valves are used primarily in the following services:

  • Where high set pressures and large relief areas are required. Pilot relief valves can be often set to the full rating of the inlet flange.
  • On large low pressure storage tanks to prevent icing and sticking.
  • Where back pressures are high and a balanced design is required. A pilot operated valve with the pilot vented to atmosphere is fully vented.
  • Where the differential pressure between normal operating pressure and valve set pressure is low.
  • Where there is a requirement for a short blowdown.

Limitations

Pilot operated relief valves are generally not used:

  • In general refinery process service where very high temperatures and dirty fluid exists.
  • Pilot relief valves have relatively small orifices which can be plugged by viscous liquids.
  • In high temperature services where the temperature exceeds the safe limits for the diaphragms or seals selected.
  • Processes where the chemical being processes in incompatible with the diaphragms or seals selected.