Pressure Relief Valve (PRV) Sizing - API 520

This industrial-grade calculator sizes Pressure Relief Valves (PRV/PSV) according to API Standard 520 Part I. It supports Gas/Vapor (Critical Flow), Steam (Napier's), and Liquid service with iterative viscosity correction. The tool selects the standard API 526 Orifice, estimates reaction forces, and checks against valve type limits.

1. Service & Process Conditions

2. Fluid Properties

3. Valve Factors (API Standard)

Professional Insights: The "Industrial Grade" Approach

Selection: Conventional vs. Bellows vs. Pilot

Selecting the right valve type is just as important as the orifice sizing. The key constraint is Back Pressure.

  • Conventional (Spring-Loaded): Simple and reliable. However, the set pressure increases directly with back pressure. Use only when total back pressure is < 10% of the set pressure.
  • Balanced Bellows: Uses a metal bellows to shield the top of the disc from back pressure. Essential for variable back pressure or when total back pressure is between 10% and 30-50% of set pressure. Also protects moving parts from corrosive fluids.
  • Pilot Operated: Uses process pressure to seal the valve. The higher the pressure, the tighter the seal. Ideal for back pressures > 50% or when the operating pressure is very close to the set pressure (95%+).

Viscosity Correction ($K_v$): The Iterative Trap

Many simple calculators ignore viscosity. For thick liquids (oil, polymer), the flow becomes laminar, drastically reducing valve capacity. API 520 requires an iterative calculation:

  1. Assume turbulent flow ($K_v = 1.0$) and calculate a preliminary Area ($A$).
  2. Use this $A$ to calculate the Reynolds Number ($Re$) for the valve.
  3. Find the new $K_v$ from the API 520 chart/formula based on $Re$.
  4. Recalculate Area using the new $K_v$.
  5. Repeat until the area converges.

This tool performs this iteration automatically for you.

Reaction Forces & Piping Support

When a relief valve opens, it acts like a rocket engine. The high-velocity gas exiting the tailpipe creates a massive backward thrust called Reaction Force.

For open discharge systems, API 520 Part II provides the formula to calculate this force. The outlet piping support must be designed to withstand this dynamic load, or the pipe could shear off during a relief event.

API 526 Flange Ratings

Standard API 526 valves have specific inlet/outlet flange combinations based on the orifice size and pressure rating. For example, a 'J' Orifice valve typically comes in 2" Inlet x 3" Outlet or 3" Inlet x 4" Outlet configurations.

Always verify the flange rating (e.g., 150#, 300#, 600#) against your detailed pipe spec. The sizing here gives you the internal orifice; the body size is the next step in specification.