Relief Valve Sizing Calculator (API 520/521)
Service Type
psig
psig
°F
Required Orifice Area110.8829 in²
Selected OrificeT (26.000 in²)
Rated Capacity11,724.1
Kd (Discharge)0.975
Kb (Back Pressure)1.000
Kc (Combination)1.000
Overpressure10%
API 526 Standard Orifices
| Orifice | Area (in²)↕ |
|---|---|
| D | 0.110 |
| E | 0.196 |
| F | 0.307 |
| G | 0.503 |
| H | 0.785 |
| J | 1.287 |
| K | 1.838 |
| L | 2.853 |
| M | 3.600 |
| N | 4.340 |
| P | 6.380 |
| Q | 11.050 |
| R | 16.000 |
| T | 26.000 |
This pressure relief valve calculator sizes a PSV by computing the required effective orifice area from the relieving conditions, then selecting the next-larger API 526 standard orifice (D through T). It applies the API 520 sizing equations for gas/vapor, liquid, and steam service, plus a conservative two-phase estimate. The result tells you which lettered orifice to specify and the rated capacity that orifice delivers.
Formula
Gas/vapor: A = W × √(T·Z·M) / (C × Kd × P1 × Kb × Kc)
- A
- Required effective discharge area in square inches
- W
- Required relief mass flow rate (lb/hr)
- T
- Relieving temperature in degrees Rankine (°F + 460)
- Z
- Gas compressibility factor (≈1.0 for ideal gas)
- M
- Molecular weight of the gas
- C
- Coefficient from the specific-heat ratio k, C = 520·√(k·(2/(k+1))^((k+1)/(k-1)))
- Kd / Kb / Kc
- Discharge (0.975 gas), back-pressure, and combination correction factors
- P1
- Relieving pressure in psia = set × (1 + overpressure) + 14.7
How it works
- Pick the service type (gas/vapor, liquid, steam, or two-phase) and enter the set pressure, back pressure, and relieving temperature, plus the required relief rate (lb/hr for gas and steam, GPM for liquid).
- For gas service, supply molecular weight, compressibility Z, and the specific-heat ratio k; for liquid service, supply specific gravity and viscosity. Toggle the fire case to switch from 10% to 21% overpressure.
- The calculator builds the relieving pressure P1, applies the discharge coefficient Kd, back-pressure correction Kb, and combination factor Kc, computes the required area, then picks the smallest API 526 orifice whose area meets or exceeds it and reports the rated capacity.
Worked example
Air relief: W = 5,000 lb/hr, set pressure 100 psig, 0 back pressure, 200 °F, M = 29, Z = 1.0, k = 1.4, non-fire case.
- Overpressure 10%, so P1 = 100 × 1.10 + 14.7 = 124.7 psia.
- Temperature in Rankine: 200 + 460 = 660 °R.
- C for k = 1.4 is 356.06, Kd = 0.975, Kb = 1.0, Kc = 1.0.
- A = 5000 × √(660 × 1.0 × 29) / (356.06 × 0.975 × 124.7 × 1.0 × 1.0) = 15.98 in².
Required area ≈ 15.98 in², so the calculator selects API 526 orifice R (16.0 in²) with a rated capacity near 5,007 lb/hr.
Frequently asked questions
- What is API 526 and how are orifices chosen?
- API 526 defines a series of standard relief-valve orifice sizes lettered D through T, from 0.110 in² up to 26.0 in². This calculator computes the required area and then selects the smallest lettered orifice whose area meets or exceeds it.
- How does the fire case change the sizing?
- A fire-relief scenario permits higher accumulation, so the calculator uses 21% overpressure instead of the 10% used for non-fire process relief. The higher allowable pressure raises P1 and slightly reduces the required orifice area.
- Why do gas, liquid, and steam use different equations?
- Each fluid behaves differently through an orifice. Gas/vapor sizing uses the critical-flow coefficient C, liquid uses a √(specific gravity) form divided by the pressure drop, and steam uses the Napier-based relation, so the calculator switches formulas with the selected service type.
- Is this a substitute for a manufacturer or engineering review?
- No. This tool implements simplified API 520 equations for preliminary sizing and education. Final valve selection must be verified against the manufacturer's certified capacities and a qualified engineer's review.