Relief Valve Sizing Calculator
Calculate required orifice area and select API letter designation for pressure relief valves and safety valves
Source-aware relief-valve area screen for local gas/vapor, steam, and liquid prompts. Enter a pressure prompt, backpressure prompt, relieving load or flow, and local fluid properties to see cached D through T orifice-row prompts. The app preserves legacy API-style arithmetic but does not reproduce API 520, API 521, API 526, ASME BPVC, National Board, manufacturer, jurisdictional, PSM, or RMP requirements. Use it only as a review prompt before current source and qualified pressure-safety review.
Screen flange bolt planning rows for PRV connection review
Flange Bolt Reference →Check pipe pressure drop in discharge headers
Pipe Pressure Drop Calculator →Screen steam properties before relief-sizing review
Steam Properties Calculator →Read the relief valve sizing guide
Relief Valve Sizing Guide →How It Works
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Choose the Local Prompt Path
Choose gas/steam or liquid service. The app shows a local arithmetic prompt only; current API equations, correction factors, and manufacturer capacity data remain external checks.
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Enter Pressure Context
Enter the local pressure and backpressure prompts. The app does not determine MAWP, set-pressure allowance, accumulation, cold differential test pressure, inlet loss, outlet loss, or header behavior.
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Enter Fluid and Load Prompts
Enter mass flow for gas/steam or GPM for liquid plus the local fluid-property prompts. The required relief load still has to come from a process hazard and relief-system study.
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Review Cached Row Output
Review the required local effective-area prompt and the smallest cached D through T row meeting it. Treat the row as a source-gap prompt, not a purchase, installation, or compliance decision.
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Carry the Source Gaps Forward
Use current API 520/521/526, ASME BPVC, National Board, manufacturer, jurisdictional, and qualified engineering review before changing any pressure relief device.
Built For
- Process engineers doing a first-pass arithmetic check before a licensed relief-system calculation
- Maintenance teams collecting pressure, backpressure, and fluid-property prompts before manufacturer review
- Inspectors flagging missing relief-load, capacity-certification, or installation-basis records for follow-up
- Safety teams preparing Management of Change questions without replacing the relief-system design basis
- Piping designers noting that inlet loss, outlet loss, header pressure, and reaction forces still require API/manufacturer review
- Operators learning why set pressure, accumulation, backpressure, and certified capacity are separate review items
Features & Capabilities
Local Area Prompts
Preserves legacy gas/vapor, steam, and liquid area arithmetic with fixed local correction factors and visible source warnings.
Cached D Through T Rows
Shows the smallest cached effective-area row meeting the local prompt. Current API 526, manufacturer certified capacity, and National Board records control actual selection.
Backpressure Review Warnings
Flags elevated backpressure and low local area margin without approving conventional, balanced bellows, pilot-operated, or header arrangements.
Report and PDF Boundaries
Exported reports carry API 520/521/526, ASME, National Board, manufacturer, OSHA PSM, and qualified-review warnings.
Source Pointers
Includes source pointers for the relevant standards family without copying licensed tables, equations, or compliance text into the app.
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 | 1.287 | 3" | 4" | Medium vessels, heat exchangers |
| N | 4.340 | 4" | 6" | 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 |
Assumptions
- Local formulas use fixed legacy correction-factor prompts and do not reproduce licensed API tables or examples.
- The entered pressure is a local prompt, not a determination of MAWP, set pressure, accumulation, or relieving pressure.
- Gas prompts assume Z = 1 and known molecular weight and Cp/Cv at relieving conditions.
- Steam prompts use simplified local saturation and superheat factors, not current API steam tables.
- Liquid prompts assume single-phase nonflashing flow with local specific gravity and viscosity prompts.
Limitations
- Does not determine governing relief scenario, fire case, blocked outlet, utility failure, runaway, depressuring, or flare capacity.
- Does not model two-phase, flashing, cavitating, viscous-table, reactive, toxic, corrosive, fouling, or polymerizing service.
- Does not select conventional, balanced-bellows, pilot-operated, rupture-disk, or multiple-device arrangements.
- Does not calculate inlet pressure loss, outlet pressure loss, discharge header pressure, reaction force, drainage, or safe discharge location.
- Does not determine inspection, testing, repair, NBIC/National Board, OSHA PSM, RMP, insurer, employer, or jurisdictional requirements.
References
- API 520 Part I source pointer for sizing and selection context.
- API 520 Part II source pointer for installation context.
- API 521 source pointer for relief-load, depressuring, and flare-system context.
- API 526 source pointer for flanged steel pressure-relief valve row/product context.
- ASME BPVC Section VIII Division 1 source pointer for pressure-vessel and relief-device code context.
- OSHA PSM and National Board source pointers for relief-system design-basis and PRD certification context.