Properly Protect Against Overpressure

Understand the appropriate use of different pressure relief devices.

By Amin Almasi, rotating equipment consultant

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Many processes require a device to control or limit the pressure that can build up due to an upset, instrument or equipment failure, or fire. Pressure relief valves (PRV) most commonly provide this protection against overpressure but backflow preventers (such as check valves and others) and rupture disks also play useful roles in many applications. In addition, special classes of relief valves, usually known as vacuum relief valves, safeguard against excessive vacuum. Such devices actuate when pressure (or vacuum) exceeds a specified design value.

A conventional PRV is a self-actuated spring-loaded valve that opens at a designated pressure to allow the pressurized fluid to exit. (Some small valves commonly handle thermal relief valve applications.) The basic elements of a spring-loaded PRV include an inlet nozzle connected to the vessel or system to be protected, a movable disk that regulates flow through the nozzle, and a spring that controls the position of the disk.

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Under normal system operating conditions, the pressure at the inlet is below the set pressure — so, the disk is seated on the nozzle, preventing flow through it.
However, when the set pressure is exceeded, the relief valve opens and becomes the “path of least resistance” for flow. A portion of the fluid exits the PRV, usually going through a piping system known as a flare header or relief header to a central, elevated gas flare (or similar) for combustion. The loss of fluid lowers the pressure inside the machinery or system being protected. When the pressure declines enough, the PRV will close. How much the pressure must drop before the valve reseats is termed blow-down and often is stated as a percentage of set pressure. Roughly, this can range from 3% to 20%; some valves have adjustable blow-downs.

In systems where the outlet is connected to piping, the PRV’s opening will prompt a pressure build up in the downstream piping. Other PRVs connected to the outlet pipe system may open and the pressure in the exhaust pipe system may increase. This may cause undesired operation. So, such systems frequently need an alternative solution. They often require “differential” relief valves — in these, the pressure works on an area much smaller than the opening area of the valve. The pressure must decrease enormously before the valve closes; also the outlet pressure of the valve can easily keep the valve open.

In some cases, a bypass valve protects a pump or gas compressor and any associated equipment from excessive pressure. It acts as a relief valve by returning all or part of the fluid discharged by the pump or compressor back to either a storage reservoir or the inlet of the machine. The bypass valve and bypass path can be internal (an integral part of the pump or compressor) or external (installed as a component in the fluid path).

Conventional Spring-Loaded PRV

The operation of this device is based on a force balance. The spring load is preset to equal the force exerted on the closed disk by the inlet fluid when the system pressure is at the PRV’s set pressure. When the inlet pressure is below the set pressure, as it should be for normal operations, the disk remains seated on the nozzle in the closed position. As the system pressure approaches the set pressure of the valve, the seating force between the disk and the nozzle approaches zero.

When the inlet pressure exceeds set pressure, the pressure force on the disk overcomes the spring force and the valve opens. Then when the inlet pressure falls to the closing pressure, the valve recloses.

Historically, many liquid applications used PRVs (or pressure safety valves) designed for compressible (or vapor) service. Many of these valves when used in liquid service required high overpressure (say, 20%) to achieve full lift and stable operation because liquids don’t provide the expansive forces that vapors do. Where liquid PRVs had to operate with a 10% overpressure limit, a conservative factor (say, 0.6) usually was applied to the valve capacity during sizing. Consequently, many installations were oversized and instability sometimes resulted.

Codes now incorporate rules that address performance of liquid service valves at 10% overpressure and that require a capacity certification. Vendors have developed PRVs for liquid services that achieve full lift, stable operation and rated capacity at 10% overpressure in compliance with the requirements; some designs feature adjustable blow-down. Valves that can operate on liquid and gas are available. However, such valves may exhibit different operational characteristics depending upon whether the flow stream is liquid, gas or a combination of the two. Many PRVs designed for liquid service, for example, will have a much longer blow-down (typically 20%) on gas than on liquid service. Additionally, some variation in set pressure may occur if the valve is set on liquid and required to operate on gas or vice versa.

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