Pilot Operated Valves

Pilot Operated Valves

Pilot Operated Relief Valve

In order to overcome the various problems encountered when using springoperated SRVs and also at NASA’s request, the industry started looking for ideal SRV characteristics and tried to design a valve that came as close as possible to these characteristics. Thus, the POSRV was born.

The main objectives achieved with some pilot-operated valves on the market were:

■ Compensation for variable backpressure up to 90% to 100% without the use of vulnerable bellows

■ Tightness up to set pressure (98%)

■ Rated capacity at set pressure, not requiring any overpressure

■ Opening and blowdown independently adjustable

■ Large range of blowdown adjustment to overcome inlet piping losses

■ Possibility of installing the valve further away from the process

■ Easy maintenance

■ In-line functionality testing possibilities

■ Reducing the unnecessary loss of material during opening (cost and environmental considerations)

■ Reducing the weight of the valves in order to reduce mechanical supports and effects of reaction forces on the valve during opening

■ Reducing the noise during the opening cycle

■ Stable and reliable operation in case of dual flow (flashing)applications

VIRCO pilot operated valve









Another advantage in some pilot-operated designs is the possibility of having a customized orifice so that the valve fits the required flow exactly. By adjusting the bolt on top of the piston, the curtain area of the valve can be adjusted to accommodate for the perfect flow.

Also take a look at our pilot operated valves that we make in VIRCO.

Pilot operated valve function

The POSRV consists of two basic components: a main valve, which provides the capacity, and a pilot, which controls the main valve. In normal operation, the pilot allows the system pressure (P) to act upon the top of the piston. The unbalanced piston assembly is increasingly forced down onto the nozzle with increasing system pressure due to the piston seal area ( A D ) being larger than the seat seal area ( A N ). The surface ( A D ) on top of the piston on which system pressure acts is approximately 30% greater than the seat area. So the closing force ( F _ PA D ) increases as pressure rises ( Figure 5.33 ).

There are three main types of PILOT-OPERATED SAFETY RELIEF VALVES(POSRV):

■ Low-pressure diaphragm types

■ Snap-acting high-pressure types

■ Modulating high-pressure types

Where the pilot operated pressure relief valves are used?

  • Where large relief areas at high set pressure are required. Many POSRVs can be set to the full rating of the inlet flange.
  • Where the differential between normal operating pressure and set pressure of the valve is small.
  • On large low-pressure storage tanks to prevent icing and sticking
  • Where short or long adjustable blowdown is required
  • On cryogenic applications for preventing icing (no simmer)
  • Where backpressure is very high and balanced design is required. A pilot-operated valve with the pilot vented to atmosphere is fully balanced
  • Dual flow or flashing applications.
  • When fast acting (opening and closing) is required. At this application the pop action pilot is good choice.
  • where process conditions require sensing of pressure at one location and relief of fluid at another location
  • where inlet or outlet piping frictional pressure losses are high; and
  • where in situ, in service, set pressure verification is desired
  • Limitation
  • In operations with very high temperatures (especially in the case of POSRVs with soft goods).
  • In highly viscous liquid service. Pilot-operated valves have relatively small orifices which can become plugged by viscous liquids.
  • Where chemical compatibility of the process fluid with the diaphragms or seals of the valve is questionable or where corrosion build-up can impede the actuation of the pilot.

Pilot Operated Relief Valve are used primarily in the following services:

■ Where large relief areas at high set pressure are required. Many POSRVs can be set to the full rating of the inlet flange

■ Where the differential between normal operating pressure and set pressure of the valve is small

■ On large low-pressure storage tanks to prevent icing and sticking

■ Where short or long adjustable blowdown is required

■ On cryogenic applications for preventing icing (no simmer)

■ Where backpressure is very high and balanced design is required.

A pilot-operated valve with the pilot vented to atmosphere is fully balanced.

■ Dual flow or flashing applications.

pilot operated valves series 5100

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