Relief Valve Orifice Sizes

What is a Relief Valve?

relief valve
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Relief Valves, also called pressure relief valves, are mechanical devices designed to operate if an over-pressure situation occurs - they are used to safeguard the plant.

The relief valve is designed to open at a predetermined pressure to protect vessels, tanks and piping systems from experiencing pressures in excess of their design limits. Generally, they are the last line of defence in protecting the plant; following on from an Emergency Shutdown (ESD) and operator intervention. See our page on Levels of Shutdown for more insight into shutdown level hierarchy.

In most countries, industries are legally required to protect pressurised systems by the use of relief valves.

So what is a Relief Valve orifice?

The American Petroleum Institute (API) has developed a series of inlet size, orifice, outlet size combinations for various pressure classes of flanged relief valves. These combinations have been widely adopted by engineers throughout the process, oil and gas, and allied industries.

Central to these combinations are a series of fourteen standard orifice sizes each denoted by a letter ranging from D through to T. Each letter refers to a specific effective orifice area, as shown below.



What do the orifices denote?

The valve sizing engineer (usually a process or instrument engineer) determines the controlling relieving rate from all possible scenarios, then the required relief valve orifice size is determined using the appropriate equation given in the API code. Knowing the required relief valve orifice size, an actual orifice size equal to or greater than the calculated orifice size is chosen from a the standard range. The maximum flow through this actual orifice will be the valve's capacity.

What are the standard orifice sizes?

relief valve orifice sizes

Technical Library

The following pages on Control and give more detail on relief valves, and related topics:


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For those who want to delve further into valves then the following books from Amazon will be of interest: