29 March, 2024

The critical performance characteristics of a pressure relief valve

10 June, 2021

There are several factors that must be considered to ensure proper operation of a pressure relief valve within a system or pressure vessel. Neglecting to consider these factors can lead to reduced valve or system performance, damage to other components within the system, or a total system failure. The Lee Company explains the performance characteristics that should be defined when selecting a pressure relief valve.


System pressures 

There are four pressure ratings that should be considered for any pressure relief valve: operating pressure, system pressure, proof pressure, and burst pressure. Operating pressures are the pressures the valve will be subject to during normal operation throughout its life, both in the relief flow direction and The critical performance characteristics of a pressure relief valve the checked, or opposite, direction. System pressure is the maximum nominal pressure that the system the valve is installed into will achieve. Proof pressure is the pressure the valve should be able to withstand without permanent deformation or degradation of performance when the system returns to operating pressure. Burst pressure is the pressure at which the valve should survive without rupturing or bursting. All four pressure ratings must be considered during design to ensure the valve and its components are durable enough for the application.

Cracking pressures

A pressure relief valve’s set pressure, or cracking pressure, is the pressure at which the valve opens and begins to allow fluid to pass. The cracking pressure is based on the pressure vessel or system’s design criteria and is typically defined as a nominal pressure with a tolerance or as a minimum. The minimum cracking pressure is the lowest pressure that the valve may open and implies the actual crack will occur at a pressure between the minimum and the relief flow pressure as pressure increases. If the valve opens at a pressure that is too low, there is a risk of improper valve operation or a decrease in system efficiency. Alternately, if it does not crack open and pressure increases too much, there is a risk of experiencing the issues which the valve is supposed to prevent such as deformation to the system or its subcomponents.

Flow rate

To ensure that the system pressure does not reach a critical point, the relief valve must allow a certain volume of fluid to exit within a limited period of time. This flow rate is based on the potential rate of a pressure increase (also called a pressure rise rate), the volume of fluid in the system, and the volume of fluid that needs to be displaced to alleviate the pressure increase. For example, a pressure relief valve protecting a large pressure vessel from damage due to a powerful pump malfunction requires more flow capacity than a pressure relief valve protecting a small pressure vessel from the pressure increase created by thermal expansion on a cold day.

The flow rate the valve is designed to achieve is typically defined at a specific pressure known as the flow point pressure, overpressure point, or relief flow pressure. The flow point pressure is always higher than the cracking pressure and may signify when the valve is in its fully open state, at which point it will perform as a fixed restriction. This performance criteria ensures the pressure relief valve will relieve enough fluid at a low enough pressure to prevent further increases in pressure or to reduce system pressure back to safe levels

Reseat of shut off pressure

A pressure relief valve’s reseat pressure is the pressure at which the valve closes when decreasing from the open state. The shut off pressure is the pressure at which the valve completely shuts off and no longer allows excessive internal leakage past the valve’s seal. These pressures are typically defined as minimum pressures. These are the lowest pressures that the valve is allowed to close or stop leaking and implies the actual reseat or shut off will occur at a pressure between the relief flow pressure and the minimum as pressure decreases. The crack or overpressure points protect the vessel or system, while correctly specifying the reseat or shut off pressure ensures continued normal operation of the system. Like the cracking pressure, if a reseat or shut off pressure is too low, there is a risk of improper valve operation or a decrease in system efficiency

Leakage

Valve leakage can be broken down into two categories: external and internal. External leakage refers to fluid flowing around the exterior of the valve body, which may include threads, O-ring seals, or other external features. Internal leakage is any fluid flow through the valve’s body while the valve is in its closed position. Leakage allowances can be influenced by variables such as whether it is an open or closed system, total system volume, fluid transfer capability, and desired system efficiency.

Materials

A pressure relief valve is comprised of several sub-components. The materials of each component must be able to withstand the various forces that will be applied to them during the valve’s operating life. This includes the pressures applied internally and externally to the valve, along with the associated pressure rise rates. Materials must also be compatible with their environment including external fluids, temperatures, and the system fluid that will relieve through the relief valve. It is possible that a valve may be subject to extreme humidity or be incorporated in a system submerged in other liquids or gases. The valve’s materials should be considered when determining how the valve will be installed into the system. Failure to consider material compatibility may create issues related to thermal expansion and corrosion

Envelope

The envelope is an important factor to consider when selecting a relief valve. The first consideration is the location of the valve within the system and the desired flow path for the relieved fluid. The system may require the relief valve to be located within a specific area, limiting external dimensions or overall size. The location may also dictate the flow path of the relieved fluid based on existing lines. The envelope must also account for installation, retention, and maintenance requirements. For example, some valves incorporate threaded fitting ends, while others are installed directly into manifold housings. Next, determine whether the installation must be permanent or removable. Finally, evaluate if the valve may be used in a system in which weight is a factor, such as a portable system or when fuel efficiency is paramount.

An engineer’s guide to selecting a pressure relief valve The above is an extract from The Lee Company’s publication: An engineer’s guide to selecting a pressure relief valve. The full publication can be downloaded free of charge from the company’s website: https://www.theleeco.com/resources/ebo oks/an-engineers-guide-to-selecting-apressure-relief-valve/

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