PRV Sizing - API 520
Professional sizing for Gas, Steam, and Liquid relief cases with iterative viscosity correction and API 526 orifice selection.
The Physics of Pressure Relief
Relief valves are the last line of defense in industrial process plants. API Standard 520 provides the formulas for sizing these critical components based on the relieving fluid's behavior.
Critical vs. Sub-critical Flow
For gas and vapor, the flow becomes "Choked" or "Critical" when the downstream pressure is less than or equal to the critical pressure ($P_{cf}$). At this point, the gas reaches sonic velocity ($Mach = 1$) at the nozzle throat.
Once choked, the mass flow rate becomes independent of the downstream (back) pressure. This is why API 520 Part I formulas are divided into critical and sub-critical cases.
Back Pressure Sensitivity
API 526 Orifice Capacities
Fire Relief & Thermal Expansion
One of the most common sizing scenarios is the Fire Case (API 521). When a vessel is engulfed in fire, the heat input causes the internal fluid to vaporize or expand.
API 521 Heat Input Formula
For an uninsulated vessel, the heat absorbed ($Q_f$) is typically calculated as:
$Q_f = 21,000 \cdot F \cdot A^{0.82}$ [BTU/hr]
Where $F$ is the environment factor and $A$ is the wetted surface area. The required relief rate is then $W = Q_f / \lambda$ (Latent heat of vaporization).
International Design Codes
Professional FAQs
Why is a Bellows valve used for back pressure?
A balanced bellows isolates the bonnet from the outlet pressure, ensuring the set pressure remains constant regardless of variable back pressure in the discharge header.
What is "Chatter" and why is it dangerous?
Chatter is the rapid opening and closing of a PRV. It can lead to mechanical failure, seat damage, and catastrophic loss of containment.
What is the difference between a PSV and a PRV?
A Pressure Safety Valve (PSV) typically pops wide open (gas/vapor), while a Pressure Relief Valve (PRV) opens proportionally to the pressure (liquid).
How often should a PRV be tested?
Most industrial standards recommend a "Pop Test" every 1 to 3 years depending on the service criticality and fluid cleanliness.
What is the 3% Rule?
API 520 recommends that the pressure drop in the inlet piping to the PRV should not exceed 3% of the set pressure to prevent valve instability.
Can I size a PRV for two-phase flow?
Yes, but it requires the Omega Method or HEM (Homogeneous Equilibrium Model), which is more complex than basic API 520 sizing.
What is the Overpressure limit for Fire cases?
Per ASME and API, the allowable overpressure for a single valve in a fire scenario is typically 21%.
Why is the discharge coefficient ($K_d$) 0.975?
0.975 is the effective discharge coefficient for a certified API nozzle design in vapor service, accounting for friction and entrance losses.