LOGOS Learning · 16
Relief valve sizing (API 520 liquid): when the relief valve opens
Relief valve sizing for liquids per API 520 gives the required orifice area A = 11.78 * Q / (Kd * Kw * Kc * Kv) * sqrt(G / (p1 - p2)), evaluated at the relieving pressure, set pressure plus the allowed overpressure (10 % here). The next larger API 526 standard orifice is then selected.
Close the process valve until the line is blocked. Pressure climbs along the pump curve up to the set pressure; the relief valve opens and returns the excess to the tank.
Quick test
Three questions about this lesson. Got one wrong? The explanation shows right away.
1. The PSV is sized to pass the relieving flow at…
2. The orifice chosen per API 526 is…
3. If the PSV opens often in normal operation, the likely cause is…
Why this happens
Arrows compare with the process asking for 60 m³/h.
Set pressure and overpressure
The PSV starts to open at set pressure and must pass the full relieving flow by the allowed overpressure, here 10 %. It is sized at that relieving pressure.
Continuity at the discharge
With the PSV open, the pump sees two paths: the process and the return to the tank. With the process blocked, all pump flow goes through the PSV.
Orifice area (API 520, liquid)
The engine computes the required area, corrects for viscosity (Kv) and back pressure (Kw) and picks the next larger API 526 standard orifice.
Why protect
Without the PSV, with the line blocked, pressure would climb to the pump shut-off. If piping, instruments or equipment cannot take that pressure, the PSV is the last barrier. The pump keeps running, so the relief goes back to the tank.
Teaching pump with an illustrative curve.
Formulas in plain text
- Relieving pressure
P_relief = P_set * (1 + overpressure)- P_relief = relieving pressure (bar g) · P_set = set pressure (bar g) · overpressure = allowed pressure rise above set (fraction; 10 % in the lesson)
- Continuity at the pump discharge
Q_pump = Q_process + Q_PSV- Q_pump = pump flow (m3/h) · Q_process = flow to the process (m3/h) · Q_PSV = flow relieved back to the tank (m3/h)
- Required area, API 520 liquid (SI units)
A = 11.78 * Q / (Kd * Kw * Kc * Kv) * sqrt(G / (p1 - p2))- A = required effective orifice area (mm2) · Q = relieving flow (L/min) · Kd = effective discharge coefficient (0.65 for preliminary sizing) · Kw = back pressure correction (1 for a conventional valve) · Kc = combination correction (1 without rupture disk, 0.9 with) · Kv = viscosity correction (-) · G = specific gravity at flowing temperature (-) · p1 = relieving pressure (kPa g) · p2 = back pressure (kPa g)
- Protection condition
p_PSV <= P_relief < P_shutoff- p_PSV = pressure at the PSV inlet (bar g) · P_shutoff = pump shut-off pressure with the line blocked (bar g)
Frequently asked questions
What overpressure is allowed for a liquid relief valve?
For a single valve in a non-fire case the usual limit is 10 % above the set pressure (at MAWP), and the valve must pass the full relieving flow at that pressure. Fire cases allow 21 %.
What discharge coefficient Kd does API 520 use for liquids?
For preliminary sizing of a liquid relief valve with certified liquid trim, API 520 uses Kd = 0.65. The final check uses the certified coefficient of the selected valve.
How do I pick the API 526 orifice letter?
Compute the required area with the API 520 equation and take the next larger standard orifice in API 526, lettered D through T. Never round down.
Why does a pump discharge need a relief valve?
With the line blocked, a centrifugal pump climbs its curve to the shut-off pressure. If piping, instruments or equipment cannot take that pressure, the relief valve opens and returns the excess flow to the tank.