Relief Valve Orifice Sizes
This page on Relief Valve Orifice Sizes is an excerpt from the original
Relief Valve Orifice Sizes webpage, published on the
Control And Instrumentation.com website, and is published here with their permission.
The American Petroleum Institute 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 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.
The full list of letters and corresponding effective area is shown in our Relief Valve Orifice Size table below;
| Relief Valve Orifice Sizes |
| Letter | Bore Dimensions |
| in2 | cm2 |
| D | 0.110 | 0.71 |
| E | 0.196 | 1.26 |
| F | 0.307 | 1.98 |
| G | 0.503 | 3.24 |
| H | 0.785 | 5.06 |
| J | 1.287 | 8.30 |
| K | 1.838 | 11.85 |
| L | 2.853 | 18.40 |
| M | 3.600 | 23.23 |
| N | 4.340 | 28.00 |
| P | 6.380 | 41.16 |
| Q | 11.050 | 71.29 |
| R | 16.000 | 103.22 |
| T | 26.000 | 167.74 |
It is worth noting that the relief valve manufacturer is not required to produce a valve with a bore area equal to the effective area. He is however obliged to produce a valve which will have a
flow rate equal to or greater than that determined by the API formulas.
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