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API 526 Relief Valve Orifice Designation and Effective Area Chart

Pressure relief valves are sized by calculating a required effective discharge area and then picking the next standard orifice letter that is equal or larger. API 526 lists those standard letters from D to T. This table gives the effective area of each, in square inches and square centimeters, so you can complete a datasheet or a quote quickly.

Download:
OrderOrifice letterEffective area (in2)Effective area (cm2)Equivalent diameter (in)
1
D
0.11
0.71
0.374
2
E
0.196
1.265
0.5
3
F
0.307
1.981
0.625
4
G
0.503
3.245
0.8
5
H
0.785
5.065
1
6
J
1.287
8.303
1.28
7
K
1.838
11.858
1.53
8
L
2.853
18.406
1.906
9
M
3.6
23.226
2.141
10
N
4.34
28
2.351
11
P
6.38
41.161
2.85
12
Q
11.05
71.29
3.751
13
R
16
103.226
4.514
14
T
26
167.742
5.754

Areas are the standard effective areas used for sizing and selection, not the actual physical nozzle areas. The letter I and the letters after T are not used. Check the current API 526 edition for flange size and pressure class combinations.

Valve datasheet data in your ERP

Optional review of how orifice, set pressure and service data are held on valve items and quotes.

Optional. The tool and every export work without this. No spam.

How to use the table

First calculate the required area with the sizing method for your service, usually API 520 Part 1 for gas, vapor, steam or liquid. Then find the first letter in the table whose effective area is at least the required area. Never round down. If the required area is larger than T, use multiple valves or a different valve design. Export the table to Excel if you want to build a selection lookup into a quoting sheet.

Effective area versus actual area

API 526 effective areas are standardized values used in sizing equations together with a certified or nominal discharge coefficient. They are intentionally not the same as the measured physical flow area of a specific manufacturer's valve. Certified capacity comes from the manufacturer's tested coefficient of discharge and actual area, so a quote should always use the manufacturer's rated capacity for the final check.

  • Required area from API 520 sizing, then pick the next larger letter.
  • Equivalent diameter column helps sanity check scale.
  • Letter I is skipped, the sequence goes H, J.

Worked example

Suppose sizing gives a required effective area of 1.05 square inches. Reading down the table, H is 0.785 and too small, and J at 1.287 square inches is the first letter that covers the requirement. The datasheet lists orifice J, and the manufacturer then confirms the rated capacity at the set pressure and overpressure for the actual valve model.

Common mistakes

Common errors include selecting the closest area rather than the next larger, using the effective area as the physical bore for a drawing, ignoring backpressure effects on capacity, and forgetting that a valve selected for gas service is not automatically valid for liquid service. Store the orifice letter as a structured attribute on valve items so quotes, nameplates and datasheets stay consistent.

Frequently Asked Questions

Why is there no orifice letter I?

The API 526 sequence goes D, E, F, G, H, J, K, L, M, N, P, Q, R, T. The letters I, O and S are not used, which avoids confusion with numbers and with other designations.

Is the effective area the real nozzle area?

No. It is a standardized area used in the sizing equations with a nominal discharge coefficient. The manufacturer's certified capacity uses its own tested coefficient and actual area. Use manufacturer data for the final capacity confirmation.

What if the required area exceeds orifice T?

Use more than one relief valve, a larger valve design outside the standard letter series, or a different relief device. The multiple valve case has its own set pressure staggering rules in the sizing standards.

Does this replace API 520 sizing?

No. This table only lists orifice areas for selection after you have calculated the required area. Sizing, inlet and outlet line losses, and installation rules come from API 520 and API 521 and the applicable design code.

If valve datasheets and quotes carry orifice data in free text, we can help structure it inside your ERP or quoting system.

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