Direct answer · August 2026
How do you size a pressure safety valve to API 526?
Size a PSV per API 520 Part I (gas/vapour) or Part II (liquid) for required relieving rate at set pressure plus allowed overpressure. Select an API 526 orifice letter (D through T) with rated capacity equal to or greater than the calculated requirement. Choose spring-loaded conventional PSV for general duty; pilot-operated PSV when near-set-point seat tightness is required. Body material (WCB, SS316, WC6) follows service temperature and corrosion.
API 526 PSV Sizing and Selection Guide
Technical guide · Supreme Valves India · August 2026
Correct PSV sizing is non-negotiable on pressure vessels, boilers, and hydrocarbon equipment. Undersized valves do not relieve fast enough; oversized valves chatter and leak. This guide walks through API 520 sizing and API 526 valve selection for procurement teams sourcing PSVs from India.
Standards chain
| Standard | Scope |
|---|---|
| API 520 Part I | Sizing and selection — vapour/gas relief |
| API 520 Part II | Sizing and selection — liquid relief |
| API 526 | Design, materials, and marking of flanged steel PSVs |
| API 527 | Seat tightness testing of PSVs |
| ASME Section VIII | When a PSV is mandatory on pressure vessels |
Sizing steps (summary)
- Identify relieving scenario — blocked outlet, fire case, thermal expansion, control valve failure
- Calculate required relieving rate (mass or volumetric) at relieving pressure and temperature
- Apply API 520 equations for critical vs subcritical flow (gas) or liquid sizing equation
- Select API 526 orifice with certified capacity ≥ required rate
- Confirm inlet/outlet size — outlet typically one size larger than inlet on conventional PSV
Spring-loaded vs pilot-operated PSV
| Type | Best for | Seat tightness | Typical size |
|---|---|---|---|
| Spring-loaded conventional | General steam, gas, liquid | Metal seat per API 527 | DN25 to DN150+ |
| Pilot-operated | Gas compression, near MAWP operation | Bubble-tight to ~95% set pressure | DN25 to DN300 |
| Balanced bellows | Variable back pressure | Same as conventional with bellows | When built-up back pressure > 10% |
Body material selection
| Material | ASTM | Service |
|---|---|---|
| WCB | A216 | Steam, oil, gas, general process — lowest cost |
| SS316 / CF8M | A351 | Corrosive, offshore, chemical |
| WC6 | A217 | High-temperature steam to ~538°C |
| LCB | A352 | Low-temperature and cryogenic relief |
What to send on an RFQ
Include fluid name and phase, set pressure, operating pressure range, relieving temperature, calculated or estimated relieving rate, back pressure if any, flange size and class, material, and IBR/API monogram requirements. Supreme Valves India lists PSVs by size and material at psv-manufacturer-india.html with 252 dedicated product pages for export RFQ reference.
Frequently asked questions
What are API orifice letters?
Letters D through T designate standard orifice areas per API 526. Each letter has a certified relieving capacity at stated overpressure.
What overpressure is used for sizing?
Typically 10% above set pressure for unfired pressure vessels; boiler codes may differ — confirm with applicable code.
Is IBR required for boiler PSV in India?
Yes for steam boilers under Indian Boiler Regulations. PSV must be IBR certified with Form III-C and authorized inspector acceptance.
Can I reuse an existing PSV on a new vessel?
Only after engineering review — set pressure, capacity, and material must match new service per API 520.
Request a quotation
Send valve type, size, material grade, class, quantity, and destination. Engineering response within 24 hours.
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