PSV sizing and selection
How the required area leads to the orifice letter.
API 526 orifice sizes
API 526 standardizes flanged steel pressure relief valves. It fixes 14 orifice letters, D to T, each with an effective area. For each letter it also sets the inlet and outlet sizes, the pressure classes, and the center-to-face dimensions.
The short answer
The orifice letter names a standard effective area, from 0.110 in² for D to 26.00 in² for T. API 526 valves with the same letter, size, and pressure class have the same center-to-face dimensions. A valve from one maker can then fit the piping made for another, subject to a check of the drawing.
Effective orifice areas
These are the effective areas of API 526 as commonly tabulated in the industry, given here for reference. They serve preliminary sizing with the effective coefficients of discharge of API 520. The current edition of the standard governs.
| Orifice letter | Effective area, in² | Effective area, cm² |
|---|---|---|
| D | 0.110 | 0.71 |
| E | 0.196 | 1.26 |
| F | 0.307 | 1.98 |
| G | 0.503 | 3.25 |
| H | 0.785 | 5.06 |
| J | 1.287 | 8.30 |
| K | 1.838 | 11.86 |
| L | 2.853 | 18.41 |
| M | 3.60 | 23.23 |
| N | 4.34 | 28.00 |
| P | 6.38 | 41.16 |
| Q | 11.05 | 71.29 |
| R | 16.00 | 103.23 |
| T | 26.00 | 167.74 |
Square centimeters are converted from square inches (1 in² = 6.4516 cm²) and rounded to two decimals. The letters I, O, and S are not used.
The actual area of a real valve differs from the effective area. The final capacity check uses the actual area with the certified coefficient of discharge of that valve.
How a size code reads
A size code such as 3K4 gives the inlet size, the orifice letter, and the outlet size, in NPS. 3K4 is an NPS 3 inlet, a K orifice, and an NPS 4 outlet.
The outlet is usually larger than the inlet. Luofu writes the size range of each series the same way. For example, 1"D2" to 8"T10" starts at an NPS 1 inlet, a D orifice, and an NPS 2 outlet. It ends at an NPS 8 inlet, a T orifice, and an NPS 10 outlet.
Interchangeability
For each letter, size, and pressure class, API 526 fixes the center-to-face dimensions of the inlet and the outlet. Check the dimensions of a replacement valve against the drawing of the installed valve.
The standard also gives pressure and temperature limits by material. The drawing for each tag gives the values that apply to the valve quoted.
Luofu series with API 526 orifice letters
Four Luofu series use the API 526 letters within their size ranges:
| Product line | Series | Size range | Letters |
|---|---|---|---|
| Spring-loaded | LFO, LFB | 1"D2" to 8"T10" | D to T |
| Pilot-operated | LFXD | 1"D2" to 8"T10" | D to T |
| Steam service | LFGS | 1.5"F2" to 8"T10" | F to T |
| Lined | LFCL | 1"D2" to 8"T10" | D to T |
LFXY thermal relief valves use C and D orifices below this range, and LFNJ jacketed valves run up to a V orifice above it. The drawing for each tag confirms the center-to-face dimensions. Tell us in the inquiry if the valves must replace existing API 526 valves.
Questions engineers ask
Is the API 526 effective area the same as the actual orifice area?
No. The effective area is a nominal value for preliminary sizing with an effective coefficient of discharge. The rated capacity of the selected valve is confirmed with its actual area and its certified coefficient.
What if the required area is larger than a T orifice?
Two or more valves share the duty. The applicable code sets the accumulation allowed when several valves protect one vessel. Give the cases and the capacities, and the sizing proposes the split.
Does API 526 cover threaded valves?
No. API 526 covers flanged steel valves. Threaded valves, such as the LFXY series, are sized and quoted from their own data.
Related pages
ASME UV stamp and National Board
Code marking and capacity certification of relief valves.
All product lines
Temperature, size, and pressure limits of the seven Luofu product lines.