What the letters are
API 526 standardises flanged steel safety valves so that valves from different makers are interchangeable. It defines fourteen orifice sizes, lettered D, E, F, G, H, J, K, L, M, N, P, Q, R and T from smallest to largest, and fixes the inlet and outlet flange sizes and the centre-to-face dimensions that go with each. A valve is described by its inlet size, orifice letter and outlet size, for example 2 J 3: a 2″ inlet, J orifice and 3″ outlet.
Effective area and actual area
Each letter has two areas, and mixing them up is the most common sizing error.
- The effective area is a nominal figure published in API 526. It is used with the nominal discharge coefficient of API 520 (0.975 for gas) for preliminary sizing, before a manufacturer is chosen.
- The actual area is the real flow area of a particular manufacturer's valve. It is used with that valve's certified, derated coefficient to give the capacity stamped on the nameplate.
Always pair an effective area with the effective coefficient, or an actual area with the certified coefficient. Never take one from each.
Orifice table
| Letter | API 526 effective area (in²) | API 526 effective area (mm²) | Flammer 66 Series actual area (mm²) | Typical inlet × outlet | Flammer max set pressure (bar g) |
|---|---|---|---|---|---|
| D | 0.110 | 71 | 86.5 | 1″ × 2″ | 414 |
| E | 0.196 | 126 | 176.0 | 1″ × 2″ | 414 |
| F | 0.307 | 198 | 240.6 | 1½″ × 2″ | 345 |
| G | 0.503 | 325 | 380.0 | 1½″ × 3″ | 255 |
| H | 0.785 | 506 | 568.4 | 1½″ × 3″ | 190 |
| J | 1.287 | 830 | 940.0 | 2″ × 3″ | 186 |
| K | 1.838 | 1186 | 1352 | 3″ × 4″ | 153 |
| L | 2.853 | 1841 | 2142 | 3″ × 4″ | 103 |
| M | 3.60 | 2323 | 2642 | 4″ × 6″ | 76 |
| N | 4.34 | 2800 | 3216 | 4″ × 6″ | 69 |
| P | 6.38 | 4116 | 4655 | 4″ × 6″ | 69 |
| Q | 11.05 | 7129 | 8353 | 6″ × 8″ | 41 |
| R | 16.00 | 10323 | 11490 | 6″ × 8″ | 21 |
| T | 26.00 | 16774 | 18620 | 8″ × 10″ | 21 |
Larger orifices are limited to lower pressures because of the force on the disc and the flange ratings available in those sizes. Other inlet and outlet combinations exist for most letters.
How to choose the letter
- Calculate the required area for the worst-case relief flow, using the equation for your fluid from API 520 Part I.
- Pick the smallest letter whose area is at least the required area and whose pressure rating covers your set pressure.
- Check the rated capacity of that orifice against the required flow. A large margin is not a benefit: a heavily oversized valve can chatter, opening and closing rapidly and damaging its seat. If the next letter up is far too big, consider two smaller valves.
- Check the inlet pressure drop (normally no more than 3 % of set pressure) and the back pressure for the valve type you have chosen.
Worked example
Air at 50 °C must be relieved at 5000 kg/h by a conventional valve set at 10 bar g, with 10 % overpressure and no back pressure.
| Step | Calculation | Result |
|---|---|---|
| Relieving pressure | 1000 × 1.10 + 101.325 | 1201.3 kPa abs |
| Gas constant C (SI) | 0.03948 · √(1.40 × 0.83336) | 0.02703 |
| Coefficient | 0.9 × 0.975 | 0.8775 |
| Required area | 5000 / (0.02703 × 0.8775 × 1201.3) × √(323.15 / 28.97) | 586 mm² |
| Orifice | H gives 568.4 mm², too small; J gives 940 mm² | J |
| Rated capacity of J | 940 / 586 × 5000 | 8021 kg/h |
The required area falls just above an H orifice, so the J is needed and it has 60 % more capacity than required. This is a case where checking the relief load again, or splitting it between two valves, is worth a few minutes. The quick estimate on the safety valve sizing page reproduces these figures.
Small valves outside API 526
Threaded and small flanged valves for low flows, thermal relief and portable equipment are not covered by API 526. They often use smaller orifices such as B and C, and their areas differ between manufacturers. The Flammer 9 Series, for example, covers orifices B to G in sizes from ½″ to 3″. For these valves, always size on the manufacturer's actual areas.