A relief valve vs safety valve question usually surfaces on a requisition where one word was written and the other was meant. Under the codes these are different devices, and the difference bites hardest on instrument tubing, where people reach for whatever fits the thread. This post sets out how the terms are drawn, then places Superlok® relief valves inside that picture. According to Spirax Sarco, a steam-equipment manufacturer whose Learn About Steam pages are a manufacturer publication and not an independent standard, ASME/ANSI PTC 25.3 defines a safety valve as a device actuated by inlet static pressure and characterised by rapid opening or pop action, and a relief valve as one actuated by inlet static pressure with a gradual lift.

How the Codes Draw the Three Terms

The same page gives working definitions for each term. A safety valve is characterised by rapid opening, and the page says it is used primarily on compressible gases, especially steam and air. A relief valve lifts gradually and is commonly found on liquid systems, opening in step with the rise above set pressure. A safety relief valve may open rapidly or in proportion to the pressure rise, so on gas or steam it behaves as a safety valve and on liquid it behaves as a relief valve. The generic phrase pressure relief valve sits above all three.

Two cautions apply. Usage drifts between regions: the page notes that EN ISO 4126-1 words its safety valve definition differently, and that US and European usage do not line up exactly. A European data sheet and a US drawing can therefore use one word for two devices, so read which code the document cites. Second, we could not retrieve the API 520 or API 526 text for this post. The Spirax Sarco page lists API RP 520, RP 521, STD 526 and STD 527 among the relevant standards, but the definitions above are the ASME ones and nothing here is attributed to API.

Relief Valve vs Safety Valve on Instrument Tubing

On instrument tubing the question is rarely about a boiler. It is about a transmitter, gauge, sample line or short tubing run that must not see more than its rating if a regulator fails or liquid is trapped. That duty is the one Superlok® builds its relief range for, and we describe the SRVL and SRVH series as instrumentation relief valves.

The SRVL low pressure series is listed with a cracking pressure range of 10 to 250 psig and a maximum operating pressure of 300 psig, with adjustable set points and lock wire features. The SRVH high pressure series is listed to 6000 psig maximum operating pressure. Both list a 4.8 mm orifice, connections from 1/4 to 1/2 in. in tube OD and NPT patterns, and Buna N or FFKM O-ring options. Where a listing states the body material it is 316 stainless steel, a corrosion-resistant alloy; where a listing is silent, grade is confirmed per order.

When a Code-Stamped Safety Valve Is Required

Plainly: if the vessel, boiler or system you are protecting is built to ASME Section VIII, its overpressure device is a code-stamped valve, and an instrumentation relief valve is not a substitute. ASME describes Division 1 as covering the design, fabrication, inspection, testing and certification of pressure vessels operating at internal or external pressures above 15 psig, and says the Division includes the rules for use of the U, UM and UV certification marks.

The certificate system around those marks is real. The Arizona rule on safety and safety relief valves, published through Cornell Law School, refers to valve manufacturers holding V, HV and UV certificates, and allows resetting or repair work only where the shop also holds a VR certificate from the National Board of Boiler and Pressure Vessel Inspectors. Superlok® pages for the SRVL and SRVH series make no Section VIII claim and mention no such mark, and we do not offer them as stamped valves.

That boundary also decides what an engineer may do in the field. Do not substitute an instrument-grade valve because the connection matches or the set pressure looks close. If the drawing calls for a stamped device, order a stamped device from a manufacturer holding the certificate, and keep the instrumentation relief valve for the instrument circuit it was made for.

Choosing for an Instrument Circuit

Work through three questions in order. First, what governs the equipment: a vessel code, a piping code, or no code at all beyond the instrument rating. Second, which is the weakest component in the section being protected, because the set pressure has to sit below its rating; the pressure ratings page is the place to check tube and fitting limits. Third, can anything cut the relief device off from the section it protects? A closed valve between the two defeats the purpose, which is why the root valve vs isolation valve distinction matters when you place the device, and why block and bleed arrangements deserve a second look.

The same tubing carries the process fluid whichever device you choose, so the bore finish downstream of the valve counts as well. For what Superlok® publishes on the range, see the certifications page.

Questions

Technical note: The Spirax Sarco page is a manufacturer publication that quotes ASME/ANSI PTC 25.3; we did not retrieve the standard itself or the API 520 and 526 text. The ASME page confirms Section VIII scope and the U, UM and UV marks, not valve selection. The Arizona rule shows the certificate system but defines nothing. Superlok® SRVL and SRVH pages quote pressure and orifice figures but no relieving capacity and no code stamp. Grade and rating confirmed per order.

Trademark disclaimer: Superlok® is a registered trademark. Spirax Sarco, ASME, API, ISO and the National Board are named only to identify the sources and standards cited and belong to their owners. Cross-references are made by form, fit, and function only.