Pressure Relief Valve

What are Pressure Relief Valves?

Pressure relief valves are safety devices used in a variety of machines, especially within hydraulics and liquid service machines [1]. These devices protect a pressurized vessel or system during an overpressure event. An overpressure event occurs when pressure in a vessel or system is increased beyond the specified design pressure or maximum allowable working pressure (MAWP). Their primary purpose is to vent or divert fluid from an over-pressurized vessel. 

Generally, pressure relief valves can be categorized in two ways. A standard relief valve is a pressure relief device that opens proportionally to the increase in vessel pressure. As a result, they do not open completely when the system is slightly overpressure. Instead, they open gradually, allowing the system to return to the predetermined pressure level, in which case they shut again. However, a safety relief valve is designed primarily with safety in mind. As a result, instead of controlling the pressure in a system, they immediately release pressure in the event of an emergency. Unlike relief valves, safety valves open immediately and completely to avoid an emergency, rather than to control the system pressure [2].

Applications

Pressure relief valves are incorporated to vessels or equipment to counter an increase in pressure that can result from the following [5]:

• Blocked outlet

• External fire exposure ("Fire Case")

• Thermal expansion of fluid

• Abnormal process conditions (e.g. chemical reaction)

• Cooling system failure

• Heat exchanger tube rupture

• Pipework component failure

• Control valve failure

Pressure relief valves are used in a wide range of applications, including [6]:

• Oil and gas

• Petrochemical

• Power generation (steam, air, gas or liquid)

• Multi-phase applications (refining and chemical processing systems)

Types of PRVs

Conventional Pressure Relief Valves

Programmed to open and close (typically via spring mechanism) when pressure reaches an unacceptable level. They can controlled remotely with a computer program, control panel, or other means [6].

Pilot-Operated Pressure Relief Valves

Hand-operated and are often opened and closed through the use of a wheel, crank, or similar device. The operator will open the valve when a readout for the system displays that the pressure is at unacceptable levels [6].

Temperature-Actuated Pressure Relief Valves

Actuated directly by external or internal temperature or by pressure on the inlet side [6].

Balanced Bellows Pressure Relief Valves

A bellow is incorporated in the design of a conventional PRV, which offsets the effect of back-pressure (which can affect relief of pressure). The bellow isolates the spring, bonnet, and guiding surfaces from direct contact with the process fluid [4].

The Effects of Back Pressure

The effects of back pressure must be taken into consideration when selecting a pressure relief valve (PRV) for any application. There are two main types of back pressure: superimposed back pressure (pressure in valve discharge header before the valve opens) and built-up back pressure (pressure from flow after valve opens). These two pressures coexist as combined back pressure.

Back pressure can affect a valve performance and capability by reducing both the set pressure and the capacity. Back pressure can result in chatter (rapid opening and closing), which can ultimately damage the valve. [7]

The effects of back pressure vary depending on the type of PRV:

Conventional PRV

Back pressure has the greatest effect on these types of valve. For set pressure, back pressure reduces it on a one-to-one basis. For example, a conventional PRV set to 50 psig with 5 psig of back pressure will not release until the valve experiences 55 psig. Therefore, the valve must be adjusted to consider back pressure, or the system can run higher than maximum allowable working pressure (MWAP). Higher back pressures can decrease a conventional PRV’s capacity by approximately 50% [7].

Balanced Bellows PRV

Balanced Bellows PRVs are better equipped to deal with back pressure, as the incorporated bellows mitigate the effects of back pressure up to a certain point. Usually, the valve is unaffected unless the back pressure exceeds approximately 35% of set pressure. However, they can fail altogether at higher pressures [7].

Pilot-Operated PRV

The effects of back pressure are least significant with these valves. Pilot-operated PRVs can typically handle back pressure of up to 50% set pressure without affecting valve capacity [7].