Safety Valve Liquid Orifice Calculator (API 520)
Calculate liquid safety relief valve required orifice area per API Standard 520 Part I with viscosity correction factor Kv, overpressure Kp, and API 526 selection.
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Pressures & Derating Factors
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Frequently Asked Questions
Why is the liquid discharge coefficient Kd (0.65) lower than gas Kd (0.975)?
Gas expansion through a convergent nozzle accelerates efficiently with minimal boundary-layer separation, giving an effective discharge coefficient of 0.975. Incompressible liquids suffer significant vena contracta contraction and turbulent shear in the nozzle throat, reducing certified Kd to 0.62 - 0.65.
How does high viscosity affect safety relief valve sizing?
Viscous liquids drag heavily against the nozzle wall, lowering fluid velocity and throat discharge rate. API Standard 520 Part I requires calculating the Reynolds number Re and dividing preliminary area by viscosity correction factor Kv, which can increase required orifice size by 20% to 150%.
What is thermal liquid expansion relief and why is 25% overpressure allowed?
When liquid trapped inside a block-valved pipe run or heat exchanger is heated by solar radiation or steam tracing, hydraulic pressure skyrockets rapidly with negligible volume expansion. ASME Section VIII allows up to 25% accumulation for liquid thermal relief valves because volumetric demand is minuscule.