What does API 526 standardize for flanged pressure-relief valves?

API 526 standardizes key purchase details including API letter orifice designation and area, inlet and outlet valve size, pressure rating, body and spring materials, pressure-temperature limits, center-to-face dimensions and nameplate marking requirements.

Does API 526 apply to both spring-loaded and pilot-operated pressure-relief valves?

Yes. API 526 includes purchase specification requirements for direct spring-loaded and pilot-operated pressure-relief valves within its scope. The correct design still depends on the medium, operating pressure margin, relieving capacity, back pressure, leakage requirements and project specification.

How does API 526 relate to API 520, API 521 and API 527?

API 521 helps establish the credible overpressure scenario, relieving load and discharge system conditions. API 520 is used for pressure-relief device sizing and selection. API 526 standardizes the flanged pressure-relief valve purchase configuration, while API 527 is used when seat-tightness testing and leakage acceptance criteria are specified.

API 526 Flanged Steel Pressure-Relief Valves: Orifices, Dimensions and RFQ Requirements

Q: Is API 526 a safety valve sizing standard?

No. API 526 does not establish the governing relief load or calculate the required relieving area. API 521 is commonly used to define the relief system basis, while API 520 is used more directly for sizing and selecting pressure-relieving devices. API 526 then standardizes the flanged valve configuration used for procurement.

Q: What information is needed to request an API 526 PSV quotation?

Provide the protected equipment, governing relief scenario, medium and phase, set pressure, required relieving capacity, relieving temperature, back pressure, selected API orifice if known, inlet and outlet size, flange class, facing type, body and trim materials, applicable standards and required test documents.

Specifying API 526 alone is not enough to select a pressure-relief valve. The standard provides a common basis for flanged steel valve configurations and dimensions, but the correct valve still depends on the required relieving capacity, set pressure, fluid properties, temperature, back pressure and applicable code requirements.

This guide connects the API 520 sizing result and API 521 relief-system conditions to the API 526 orifice, inlet and outlet size, flange class, materials, dimensions and documentation needed for a complete, reviewable PSV specification and RFQ.

API 526 Quick Links

API 526 is commonly used as a purchase specification for flanged steel pressure-relief valves in petroleum, petrochemical and natural gas processing projects.
API 526 standardizes key procurement details such as orifice designation and area, inlet and outlet size, pressure rating, materials, pressure-temperature limits and center-to-face dimensions.
API 526 does not replace API 520 sizing or API 521 relief scenario review; it helps convert the sizing result into a standardized flanged PSV configuration.
Final valve selection still requires protected equipment data, certified capacity, applicable code, back pressure review and manufacturer engineering confirmation.

››› API 526 Overview

API 526 turns a calculated relief requirement into a standardized flanged steel PSV purchase specification

After the required relieving capacity is defined, the buyer still needs to specify a valve that can be manufactured, inspected, connected and documented correctly. API 526 helps standardize the flanged steel pressure-relief valve configuration used in many refinery, petrochemical, gas plant and EPC projects.

For procurement teams, API 526 is most useful when the RFQ already includes set pressure, relieving capacity, fluid, temperature, back pressure and applicable code basis. The supplier can then confirm an API orifice, inlet and outlet size, flange rating, materials, center-to-face dimensions, nameplate data and test documents.

What this page helps you confirm

When API 526 should be used for flanged steel pressure-relief valves.
How API 526 connects with API 520 sizing and API 521 relief system review.
What data is normally needed before selecting an API orifice and flange rating.
Which dimensions, materials, pressure-temperature limits and nameplate details should be checked.
What documents should be requested for EPC, refinery, petrochemical and gas processing projects.

››› What API 526 Defines

API 526 focuses on standardized flanged PSV procurement details

API 526 is not a complete relief study. It is a practical purchase specification that helps make sure the selected flanged steel pressure-relief valve has standardized dimensions, connection ratings, materials and documentation expectations.

Orifice Designation and Effective Area

The API letter orifice is normally selected after required area and certified capacity have been checked. The orifice designation should match the sizing basis, not only the existing valve tag.

Inlet and Outlet Size / Pressure Rating

API 526 helps standardize common inlet and outlet combinations and flange pressure classes. The connection must also match the piping specification and ASME B16.5 pressure-temperature rating.

Center-to-Face Dimensions

Standardized dimensions help EPC engineers, skid builders and maintenance teams replace or compare valves without redesigning piping layouts.

Body and Spring Materials

Material selection should be checked against medium, corrosion, temperature, sour service, low temperature and project specifications.

Pressure-Temperature Limits

Valve pressure rating, body material, flange rating, set pressure and relieving temperature must be reviewed together. A high flange class alone does not automatically confirm service suitability.

Nameplate and Marking Requirements

Nameplate data should support inspection and traceability, including set pressure, size, orifice, capacity, material, tag and applicable standard information required by the project.

››› Sizing to Procurement Workflow

How API 520, API 521 and API 526 work together

Many RFQ mistakes happen when API 526 is used before the real relief case and sizing basis are confirmed. Use this workflow to avoid selecting a valve by table size only.

Step 1

Define the relief case

Use process data and relief study assumptions to define fire, blocked outlet, compressor, regulator failure, tube rupture or thermal expansion cases.

Step 2

Size the safety valve

Use the required relieving capacity, set pressure, medium, temperature and back pressure to calculate the required area and select the orifice.

Step 3

Confirm API 526 configuration

Convert the selected orifice into a standardized flanged steel PSV with correct inlet, outlet, pressure class, material and dimensions.

Step 4

Specify test and documents

Add seat tightness, calibration, pressure test, MTC, nameplate, drawing and inspection requirements to the purchase order.

››› Procurement Matrix

API 526 procurement data matrix

API 526 Item	Why It Matters	RFQ Data to Provide	Common Error
Orifice designation	Links sizing result to standard valve configuration.	Required area, selected API letter, certified capacity.	Choosing the same letter as an old valve without checking capacity.
Inlet size and rating	Controls mechanical connection and pressure-temperature suitability.	NPS / DN, Class rating, flange facing, piping class.	Assuming flange class alone confirms valve suitability.
Outlet size and rating	Affects back pressure, discharge piping and installation layout.	Outlet NPS / DN, Class, discharge route and back pressure.	Ignoring outlet piping or flare header pressure.
Materials	Controls corrosion, temperature and project acceptance.	Body, bonnet, trim, spring, seat, gasket and NACE or low-temperature needs.	Specifying only body material and missing trim or spring materials.
Center-to-face dimensions	Supports piping layout, replacement and skid fabrication.	Required standard, valve type, inlet/outlet orientation and GA drawing.	Ordering a valve that fits the process but not the piping envelope.
Nameplate and stamping	Supports inspection, traceability and maintenance.	Tag, set pressure, size, orifice, capacity, material, standard and certification.	Missing buyer tag, capacity basis or project-specific marking.

››› Where API 526 Is Used

Common applications for API 526 flanged pressure-relief valves

API 526 is most often requested when a project wants standardized flanged steel PSV dimensions and purchase data for process units and pressure equipment.

Refinery and Petrochemical Units

Pressure vessels, columns, separators, heat exchangers, reactors, flare-connected PSVs and process relief systems.

Oil & Gas and Gas Processing

Separators, compressors, dehydration units, gas plants, LNG / LPG systems and high-pressure hydrocarbon equipment.

Chemical Processing and Skids

Process vessels, reactors, package units, modular skids and EPC projects where standardized dimensions simplify engineering.

Steam, Gas and Vapor Service

Gas, vapor and steam relief valves where API sizing, ASME code basis and flanged steel valve construction are required.

High Back Pressure Systems

Balanced bellows or pilot operated PRVs connected to flare headers, scrubbers or closed relief systems.

Large Capacity Relief

Large vapor, gas or steam loads where API orifice selection, certified capacity and outlet forces must be checked.

››› Related Standards

Standards normally used with API 526

API 526 is normally used with sizing, relief scenario, pressure vessel, piping, flange and seat leakage standards. The exact combination depends on the project specification.

››› Document Package

Typical API 526 valve document package

Technical datasheet with tag, model, type, size, orifice, set pressure and connection standard.
General arrangement drawing with dimensions, weight, inlet/outlet orientation and center-to-face data.
Sizing calculation or certified capacity confirmation from API 520 basis.
Material certificates for body, bonnet, nozzle, disc, trim, spring and pressure-retaining parts.
Set pressure calibration certificate and pressure test report.
Seat tightness test report where API 527 or project leakage requirement is specified.
Nameplate data, tag list, inspection witness records and spare parts list where required.

››› RFQ Checklist

Information to send for an API 526 flanged PSV quote

A complete API 526 RFQ should include the process sizing basis and mechanical configuration. This allows the supplier to confirm the correct API orifice, flange rating, material, dimensions and document package without repeated clarification.

Protected equipment, tag number, MAWP / design pressure and applicable code.
Medium, phase, molecular weight, specific gravity, viscosity and special hazards.
Set pressure, operating pressure, relieving pressure, relieving temperature and required capacity.
API orifice designation if already calculated; otherwise provide API 520 sizing data.
Inlet and outlet size, flange class, facing, material and piping specification.
Back pressure, discharge route, outlet system and high back pressure requirement if applicable.
Body, bonnet, trim, spring, seat, gasket, sour service, cryogenic or corrosion requirements.
Required documents: GA drawing, sizing report, MTC, calibration, pressure test, seat tightness and inspection records.

››› Common Errors

Common API 526 procurement mistakes

API 526 is a purchase specification, but it cannot correct incomplete process data or incorrect sizing assumptions. These errors should be corrected before PO release.

Using API 526 before completing API 520 sizing

API 526 cannot tell you the required relieving area by itself. The valve must first be sized from the relief case, medium, set pressure and back pressure.

Selecting a valve only by connection size

An existing flange size does not confirm capacity. Orifice area and certified capacity must match the required relieving load.

Ignoring outlet size and back pressure

The outlet connection and discharge system can affect built-up back pressure, capacity and valve stability.

Missing material and service details

Sour gas, high temperature, low temperature, corrosive media, oxygen service or hydrogen service can change material and trim requirements.

Forgetting center-to-face dimensions

A valve can be technically correct but fail installation if dimensions do not match the skid, piping or replacement envelope.

Leaving out nameplate and test documents

EPC and end-user inspections often require complete nameplate data, calibration, pressure test, MTC and seat tightness records.

››› Related Standards & Engineering Pages

Continue your API 526 flanged safety valve review

Use these pages to complete the full workflow from relief case to sizing, flanged valve configuration, seat testing, flange selection and purchase documentation.