Liquid Relief Valve Sizing Calculator (API 520)
Size a pressure relief valve for liquid service using the API 520 Part I equation for certified valves, including the viscosity correction, and select the API 526 orifice letter.
- P1 = set × (1 + overpressure) = 1,100 kPag; ΔP = P1 − back pressure = 1,100 kPa
- A = 11.78 × Q ÷ (Kd × Kw × Kc × Kv) × √(G ÷ ΔP)
- Kv = (1 + 170/Re)−0.5, Re = Q × 18800 × G ÷ (µ√A), iterated: A = 273.3 mm²
API 520 Liquid Sizing (SI)
Kv = (1 + 170 ÷ Re)−0.5, Re = Q × 18,800 × G ÷ (µ × √A)
A is the effective area in mm², Q the flow in L/min, G the specific gravity, P1 the relieving pressure and P2 the back pressure in kPa gauge, µ the viscosity in cP, Kd 0.65 for a preliminary size of a capacity certified valve, Kw the balanced bellows back pressure correction (1.0 for conventional valves discharging to atmosphere) and Kc 0.9 with a rupture disc.
Worked example
500 L/min of water set at 10 barg with 10 % overpressure discharging to atmosphere: ΔP = 1100 kPa, A ≈ 273 mm² after viscosity correction, so a G orifice (324.5 mm²) is selected.
Frequently Asked Questions
Why is Kd lower for liquids?
Liquids do not expand through the nozzle, so the effective discharge coefficient for certified liquid valves is about 0.65 against 0.975 for gas.
When does viscosity matter?
For viscous liquids the Reynolds number falls and Kv drops below 1, needing a larger orifice. For water Kv is close to 1.
What is Kw?
The back pressure correction for balanced bellows valves. Conventional valves discharging to atmosphere use Kw = 1.
Learn more on the blog
Instrumentation and control standards
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
Read Full Article