SUPERLOK® NORTH AMERICA · VALVE KNOWLEDGE
Relief Valve Set Pressure: How It Is Chosen and Tested
Where a setting sits between normal operation and the weakest part of the circuit, how it is changed, and what a bench proves.
Relief valve set pressure is the one number an engineer picks on purpose, and the rest of the device’s behaviour follows from it. This post covers how the number is chosen, how it is changed and how it is checked on a bench. Superlok® relief valves are instrumentation devices with an adjustable opening point, which the catalog calls the cracking pressure. According to Control Global, a process-control trade publication and not a manufacturer, set pressure is the pressure at which the valve is set to start to open, and a reader answer in its column adds that the setting can equal but not exceed the maximum allowable working pressure. For vocabulary, see relief valve vs safety valve and thermal relief valves.
What Relief Valve Set Pressure Fixes
Set pressure is the inlet pressure at which the valve begins to open. It is not the pressure at which the valve passes full flow. A glossary on the Control and Instrumentation reference site, which names no author, states it as inlet gauge pressure under service conditions, and treats popping pressure as a separate point where the disk moves rapidly to full open. A 2025 paper from the Budapest University of Technology and Economics reads it the same way.
Superlok® catalog pages say cracking pressure for the adjustable opening point. The SRVL listing gives 10.0 to 250 psig and the SRVH listing gives 225 to 6000 psig. Neither page defines the term further, so we read it as the opening point and ask that the intended setting be confirmed per order.
Choosing Relief Valve Set Pressure on a Real Circuit
Work from both ends. At the bottom sits the highest pressure the line normally sees. At the top sits the weakest component. Chip Eskridge, writing in Chemical Processing in 2006, puts the test plainly: compare the highest developed pressure to the weakest component in the system. One of his examples is a temperature switch with a small dead-band at about 100 psig. On instrument circuits the limiting part is often a switch, gauge or transmitter and not the tubing. The pressure ratings page covers the tube and fitting side.
Leave a margin below the setting. Crowl and Tipler, in Chemical Engineering Progress in October 2013, report that spring-operated valves begin to leak at 92 to 95 percent of set pressure. By simple arithmetic, a line running at 90 psig sits at 90 percent of a 100 psig setting and at 75 percent of a 120 psig setting. The Superlok® pages state no leak onset, so treat the range as general guidance for spring designs.
Leave room above the setting too. The valve does not reach rated flow at set pressure. Spirax Sarco, a steam-equipment maker whose pages are manufacturer publications, gives overpressure for the safety valves it describes as normally 3 to 10 percent for compressible fluids and 10 to 25 percent for liquids. Treat that as a planning allowance, not a Superlok® figure. The ranges leave different gaps. SRVL tops out at 250 psig against a 300 psig maximum operating pressure. SRVH lists 6000 psig for both, so a setting at the very top leaves no gap.
| Check | What to establish | Basis |
|---|---|---|
| Operating pressure | Highest steady pressure at service conditions | Your process data |
| Weakest component | Lowest rated part, after temperature derating | Eskridge, 2006 |
| Margin below setting | Spring valves leak from 92 to 95 percent | Crowl and Tipler, 2013 |
| Room above setting | Setting plus overpressure under the weakest rating | Spirax Sarco |
| Seal temperature | Buna (NBR) listed at -40 to 121 °C | Superlok® relief valves page |
Items summarised from the sources listed below, not from a Superlok® test. The seal range is a compound range, not a rating for any one model.
How the Setting Is Adjusted and Locked
The SRVL listing describes adjustable set points and lock wire features. The SRVH listing describes an adjustable cracking pressure feature, and one variant, SRVH-S-6-N, is listed as lock wire capable. Neither page describes the adjustment procedure, so this post does not either; ask for it when you order.
What follows the adjustment matters as much. Arizona’s rule on safety and safety relief valves, published through Cornell Law School, allows external adjustment only by trained employees of the owner and requires resealing with a metal tag naming the organization that made the change. An instrument circuit is not governed by that rule, but the lesson carries: record the final value and lock it.
The boundary needs stating once and plainly: instrumentation relief valves are not a substitute where an ASME code-stamped safety valve is required.
Bench-Testing a Set Pressure
A bench test answers one question: at what inlet pressure does this valve start to open? The Control Global column, published on 20 November 2020, notes that valves are normally tested on air or nitrogen, that a larger test volume gives a more accurate setting, that even a few inches of piping between chamber and valve can matter, and that pressure should be read on NIST-traceable instruments. It also cautions that a small bench often lacks the gas capacity to verify discharge, so a set pressure test is not a capacity test.
The glossary cited above defines cold differential test pressure as the pressure at which the valve is adjusted to open on the bench, including corrections for back pressure or temperature, and gives the leak test pressure under API 527 as 90 percent of set pressure. On timing, the Control Global answers place testing before installation, after any fire event and at a plant-set frequency.
The Superlok® listings state no factory test procedure, tolerance or leak criterion. Ask for the test record you need when you order. For installation, see the installation guide; for a cracking figure on a different valve type, see our note on check valve cracking pressure.
Questions
Is cracking pressure the same thing as set pressure?
On the Superlok® SRVL and SRVH pages, cracking pressure is the adjustable opening point, which is what the sources call set pressure. The pages give no separate definition, so confirm the intended setting per order.
Can the setting equal the maximum operating pressure?
Not sensibly. Spring valves are reported to leak from 92 to 95 percent of set pressure, so the setting belongs above the highest normal pressure with a margin, and below the weakest component.
Does a bench test prove the valve will pass enough flow?
No. Control Global notes that air or nitrogen on a small bench often lacks the capacity to verify discharge. Superlok® pages state no relieving capacity, so none is claimed here.
Need a setting confirmed?
Send the operating pressure, the lowest-rated component and the fluid, and we will check them against the listings.
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Sources
- Control Global, “Test options for pressure-relief valves,” reader Q&A moderated by Béla Lipták, 20 November 2020 (trade press). Supports the set pressure and MAWP statement, bench test features and the service-interval advice. controlglobal.com
- Control and Instrumentation, “Glossary of Pressure Relief Valve Terms,” undated, copyright 2006 (reference site, no named author). Supports the set, popping, cold differential test and API 527 leak test definitions. controlandinstrumentation.com
- Sáfrány and Hős, “A Detailed Fluid Mechanical Analysis on Pressure Relief Valve Blowdown,” Periodica Polytechnica Mechanical Engineering, 2025, Budapest University of Technology and Economics. Supports the definition of set pressure. pp.bme.hu
- Chip Eskridge, “Assessing Pressure Relief Needs,” Chemical Processing, 10 February 2006. Supports the weakest-component comparison and the example ratings. chemicalprocessing.com
- Daniel A. Crowl and Scott A. Tipler, “Sizing Pressure-Relief Devices,” Chemical Engineering Progress, October 2013 (AIChE). Supports the 92 to 95 percent leak onset for spring-operated valves. aiche.org
- Spirax Sarco (steam-equipment manufacturer; manufacturer publication), “Safety Valves,” Learn About Steam. Supports the overpressure ranges. spiraxsarco.com
- Arizona Administrative Code R20-5-413, via Cornell Law School Legal Information Institute. Supports the external adjustment and metal tag resealing requirement. law.cornell.edu
- Superlok® North America, SRVL and SRVH product listings and relief valves page (manufacturer of record). Supports the cracking pressure ranges, maximum operating pressures, adjustable set point and lock wire wording, and seal temperature range. SRVL and SRVH
Technical note: The sources establish what set pressure means, how a margin to the weakest part is reasoned, and what a bench test can and cannot show. The glossary is undated and attributes no author, and the Spirax Sarco overpressure ranges describe safety valves, so both are used as general guidance. Superlok® pages give cracking and maximum operating pressures, an adjustable set point and lock wire wording, but no adjustment procedure, test record, tolerance or relieving capacity. Body material is listed as 316 stainless steel on specific variants, a corrosion-resistant alloy; grade and rating confirmed per order.
Trademark disclaimer: Superlok® is a registered trademark. Control Global, Control and Instrumentation, Chemical Processing, AIChE, Spirax Sarco, ASME, API and NIST are named only to identify sources and standards and belong to their owners. Cross-references are made by form, fit, and function only.