PSV Sizing Fundamentals: API 520 Part I Guidelines & Orifice Selection

Introduction to Pressure Safety Valves (PSVs)

Pressure Safety Valves (PSVs) are the last line of defense in process facilities to prevent equipment overpressure and catastrophic failure. Sizing these valves correctly ensures they can discharge the required relief load without exceeding the allowable accumulation pressure of the protected vessel.

The industry standard for sizing PSVs in the refining and chemical processing industries is API Standard 520, Part I (Sizing and Selection).

Overpressure and Accumulation

Before sizing, it is critical to understand the operating pressure profile:

• Maximum Allowable Working Pressure (MAWP): The maximum gauge pressure permissible at the top of a completed vessel in its normal operating position.
• Set Pressure ($P_{set}$): The pressure at which the PSV begins to open. Typically set at or below the MAWP.
• Overpressure: The pressure increase over the set pressure during relieving, expressed as a percentage.
• Accumulation: The pressure increase over the MAWP during relieving.

Common allowable overpressures per ASME Section VIII / API 520:

Vapor / Gas Relief Sizing (Critical Flow)

When relieving gases or vapors, the flow through the PSV orifice often reaches the speed of sound (sonic flow). This occurs when the downstream pressure is lower than the critical choking pressure ($P_{cf}$). In critical flow, the flow rate depends only on the upstream relieving pressure ($P_1$), not the backpressure.

The required effective discharge area ($A$) for critical flow (in $mm^2$) is given by API 520:

Where:

• $W$ = Required relieving mass flow rate (kg/h)
• $C$ = Coefficient determined from the ratio of specific heats ($k = C_p/C_v$). Typical values range from 315 to 377. (For $k=1.18$, $C \approx 330$)
• $K_d$ = Effective coefficient of discharge (typically 0.975 for gas/vapor)
• $P_1$ = Upstream relieving pressure (Set Pressure + Overpressure + Atmospheric) (bar a)
• $K_b$ = Capacity correction factor due to backpressure (1.0 for conventional valves discharging to atmosphere)
• $K_c$ = Combination correction factor for rupture disc upstream (typically 0.9 if disc is present, 1.0 otherwise)
• $Z$ = Compressibility factor at relieving conditions
• $T$ = Relieving temperature (K)
• $MW$ = Molecular weight of the gas (kg/kmol)

Liquid Relief Sizing

For incompressible liquids, the flow is driven by the differential pressure across the valve. The area required (in $mm^2$) is:

Where:

• $Q$ = Volumetric flow rate (L/min)
• $K_d$ = Effective coefficient of discharge for liquid (typically 0.65)
• $K_w$ = Capacity correction factor for backpressure on balanced bellows valves in liquid service
• $K_v$ = Viscosity correction factor
• $G$ = Specific gravity of the liquid at relieving temperature
• $P_1$ = Upstream relieving pressure (bar a)
• $P_2$ = Total backpressure (bar a)

API 526 Orifice Letters

Once the required effective area ($A_{req}$) is calculated, the designer selects the next largest standard orifice size from API Standard 526. These are denoted by letters D through T.

The selected API letter establishes the valve's rated capacity ($W_{rated}$), which will always be greater than the required capacity. The downstream piping must be sized based on the rated capacity, not the required capacity.

Try the Calculator

Use the PSV Sizing Calculator on this page to compute required effective area, select the appropriate API 526 orifice letter, and verify critical sonic flow for your specific relief scenarios.