Skip to main content
Shops & Outbuildings Free Pro Features Available

Relief Valve Calculator - API 520 Orifice Sizing for Gas, Steam & Liquid Overpressure Protection

Calculate required orifice area and select API letter designation for pressure relief valves and safety valves

Size pressure relief valves per API Standard 520 Part I for gas/vapor, steam, and liquid service. Enter set pressure, relieving conditions, fluid properties, and required relieving capacity to calculate the minimum required orifice area and recommended API orifice letter designation (D through T). Supports conventional, balanced bellows, and pilot-operated relief valves. Includes backpressure correction, superheat correction for steam, viscosity correction for liquids, and capacity certification (ASME Code stamped vs. non-stamped) factors. Covers ASME Section VIII Division 1 sizing requirements.

Pro Tip: The number one relief valve sizing mistake is ignoring built-up backpressure in a closed discharge header. A conventional spring-loaded PRV loses 1% of capacity for every 1% of gauge backpressure up to 10%, and above 10% it becomes unpredictable. If your total backpressure (superimposed plus built-up) exceeds 10% of set pressure, you need a balanced bellows or pilot-operated valve. Always model the discharge header pressure drop at full relieving flow before finalizing orifice size.

Look up flange bolt patterns for PRV inlet and outlet

Flange Bolt Reference →

Calculate bolt torque for PRV flange connections

Torque-Tension Calculator →

Plan rigging for PRV removal and installation

Crane Rigging Calculator →

Check motor efficiency on overpressure protection systems

Motor Efficiency Calculator →
Start Using This Tool See How It Works

PREVIEW All Pro features are currently free for a limited time. No license key required.

Relief Valve Sizing Calculator
Pop Out

How It Works

  1. Select Service Type

    Choose gas/vapor, steam, or liquid service. Each service type uses a different API 520 sizing equation. Gas service uses the compressibility-corrected ideal gas formula, steam service uses the Napier equation with superheat correction, and liquid service uses a volumetric flow formula with viscosity correction.

  2. Enter Relieving Conditions

    Input set pressure (psig), allowable overpressure (typically 10% for single valve, 16% for fire case, 21% for multiple valves), backpressure (superimposed and built-up), and relieving temperature. The calculator determines the actual relieving pressure.

  3. Specify Fluid Properties

    For gas service, enter molecular weight, compressibility factor (Z), and ratio of specific heats (k). For steam, enter steam quality or superheat degrees. For liquid, enter specific gravity and viscosity at relieving temperature.

  4. Enter Required Capacity

    Input the required relieving capacity in SCFM, lb/hr, or GPM depending on service. This is the maximum flow the valve must pass during the worst-case overpressure scenario, determined from your process hazard analysis.

  5. Review Orifice Selection

    See the calculated minimum required effective orifice area (square inches) and the recommended API standard orifice letter designation. The tool selects the smallest standard orifice that meets or exceeds the required area, plus shows the next size up for margin.

Built For

  • Process engineers sizing pressure relief valves for new vessel and piping designs
  • Instrument technicians verifying existing PRV capacity against changed process conditions
  • Inspectors performing API 510 or API 570 overpressure protection reviews during turnarounds
  • Safety engineers conducting relief system capacity studies for Management of Change reviews
  • Piping designers selecting PRV inlet and outlet piping sizes based on API 520 Part II guidelines
  • Insurance auditors verifying that installed PRV capacity meets ASME Section VIII requirements
  • Plant operators understanding the difference between set pressure, accumulation, and blowdown

Features & Capabilities

API 520 Sizing Equations

Implements the complete API 520 Part I sizing methodology for gas/vapor (compressible flow with real gas correction), steam (Napier equation with Ksh superheat factor), and liquid (volumetric flow with Kv viscosity correction and Kw backpressure correction).

Standard Orifice Selection

Automatically selects the API standard orifice letter designation (D through T) whose effective area meets or exceeds the calculated requirement. Displays the effective discharge area, rated capacity, and percent utilization for the selected orifice.

Backpressure Analysis

Evaluates the impact of superimposed and built-up backpressure on valve capacity. Applies the correct correction factor (Kb for balanced bellows, Kw for liquids) and warns when backpressure exceeds conventional valve limits.

Valve Type Recommendation

Recommends conventional, balanced bellows, or pilot-operated valve type based on backpressure ratio, service conditions, and process fluid characteristics. Includes notes on when rupture disc upstream of the PRV should be considered.

Multiple Scenario Comparison

Size relief valves for multiple overpressure scenarios (blocked outlet, fire case, control valve failure, power loss) and determine the governing case that dictates the required orifice size.

Comparison

API Orifice Effective Area (in²) Typical Inlet Typical Outlet Common Application
D 0.110 1" 2" Small vessels, instruments
F 0.307 1.5" 2.5" Process vessels, columns
J 0.785 2" 3" Medium vessels, heat exchangers
N 2.853 3" 4" Large vessels, reactors
Q 11.05 4"x6" 6"x8" Storage tanks, large columns
T 26.00 6"x8" 8"x10" Large-scale fire relief

Frequently Asked Questions

A relief valve opens proportionally as pressure exceeds the set point and is used primarily for liquid service. A safety valve opens fully (pops) at set pressure with a defined blowdown and is used for gas and steam service. A safety relief valve can function as either, depending on whether the fluid is compressible or incompressible. ASME Section VIII requires pressure relief devices on all pressure vessels, and the specific type depends on the service fluid and process conditions.
The required capacity comes from evaluating all credible overpressure scenarios: blocked outlet (full flow at pump or compressor capacity), fire case (heat input using API 521 formulas based on wetted surface area), control valve failure (full open flow at supply pressure), cooling water loss, power failure, and chemical reaction runaway. The worst-case scenario that produces the highest required flow rate is the governing case. API 521 provides detailed methods for calculating each scenario.
Allowable overpressure is the pressure increase above set pressure permitted during a relieving event. ASME Section VIII allows 10% overpressure for a single relief device, 16% when multiple devices protect a vessel, and 21% for fire contingencies. Higher overpressure increases the pressure differential driving flow through the valve, which increases capacity. This is why fire-case PRVs with 21% overpressure can sometimes use a smaller orifice than the same flow at 10% overpressure for process scenarios.
Backpressure is the pressure at the PRV outlet during relieving. Superimposed backpressure exists before the valve opens (from a pressurized header). Built-up backpressure develops as flow passes through the discharge piping. Conventional spring-loaded valves lose capacity as backpressure increases because it opposes the opening force. Balanced bellows valves use a bellows to isolate the spring from backpressure, maintaining capacity up to about 40% backpressure. Pilot-operated valves are unaffected by backpressure up to about 70% of set pressure.
API 526 standardizes relief valve orifice sizes using letter designations from D (smallest, 0.110 in²) through T (largest, 26.00 in²). Standard designations are D, E, F, G, H, J, K, L, M, N, P, Q, R, and T. Using standard orifices allows interchangeability between manufacturers - an API \"J\" orifice from any certified manufacturer has the same effective discharge area of 0.785 square inches. This standardization simplifies valve replacement and spare parts management.
A rupture disc upstream of a PRV is typically required when the process fluid is corrosive, toxic, or would damage the valve internals during normal operation, when zero leakage past the valve seat is required, or when the process operates within a few percent of the valve set pressure causing simmering. ASME Section VIII requires a capacity correction factor (typically 0.9) when a rupture disc is installed upstream of a PRV to account for potential partial flow restriction. The combination must be tested or certified together.
API Recommended Practice 576 provides guidelines for PRV inspection and testing intervals, typically every 3-5 years for process industry applications. Many jurisdictions have regulatory requirements, often tied to the process safety management (PSM) program. National Board Inspection Code (NBIC) NB-23 requires periodic testing. Some risk-based inspection programs extend intervals to 10 years for valves with demonstrated reliable performance history. Each valve should be tested for set pressure accuracy, seat tightness, and capacity.
API 520 Part I covers the sizing and selection of pressure relieving devices - calculating the required orifice area based on relieving conditions, fluid properties, and required capacity. Part II covers the installation of pressure relieving devices - inlet piping, discharge piping, header design, and reaction forces. Both parts must be considered for a complete relief system design. Common Part II issues include inlet pressure drop exceeding 3% of set pressure (which causes valve chatter) and discharge piping reaction forces that stress the vessel nozzle.
Disclaimer: This calculator provides preliminary relief valve sizing estimates based on API 520 Part I methods. Final PRV sizing must be verified by a qualified process engineer and confirmed against ASME Section VIII requirements. Relief valve sizing is safety-critical and errors can result in equipment failure, injury, or death. ToolGrit is not responsible for pressure relief system design, valve selection, or regulatory compliance.

Learn More

Shops & Outbuildings

Relief Valve Sizing: API 520 Orifice Selection & Set Pressure

How to size pressure relief valves per API 520. Required orifice area calculation, set pressure vs MAWP, backpressure effects, and orifice letter designations.

Read Guide →

Related Tools

Shops & Outbuildings Live

Shop Heater BTU Sizing Calculator

Calculate the exact BTU output your shop or garage heater needs. Factors in wall R-values, ceiling insulation, slab edge loss, overhead door infiltration, and air changes per hour to size propane, natural gas, and electric heaters correctly.

Launch Tool →
Shops & Outbuildings Live

Overhead Door Infiltration Loss Calculator

Calculate heat loss through overhead doors in shops, garages, and warehouses. Compares open-door vs closed-door losses, seal condition impact, and annual cost of infiltration with payback on door seals and high-speed doors.

Launch Tool →
Shops & Outbuildings Live

Long-Run Voltage Drop Calculator

Calculate voltage drop for long wire runs to detached shops, barns, garages, and outbuildings. Compares copper vs aluminum, shows motor starting voltage impact, and recommends the right wire size for your distance and load.

Launch Tool →