On this page12 sections
What does a pressure relief valve enquiry have to state?
A pressure relief valve enquiry, often written PSV on a requisition, has to state the duty before it states the valve. The orifice letter, the body size and the spring are outputs of a calculation, not inputs to one. A supplier who is sent a size and a set pressure can quote something; a supplier who is sent the relieving case, the fluid, the set pressure and the back pressure can quote the right thing.
That is a paperwork problem before it is a knowledge problem. The datasheet exists somewhere in the project, and what reaches the supplier is the line from the valve schedule. The two are not the same document and they do not carry the same information.
Flowsynergy Solutions Limited supplies industrial valves, pipeline fittings and flow-control equipment from Level 41, Emirates Towers, Sheikh Zayed Road, Dubai, and safety and relief valves are one of the categories we quote. This page sets out what the catalogue records say those valves are governed by, and what a datasheet has to carry before anyone can price one.
Which document covers sizing, and which covers the valve?
Two different documents do two different jobs, and a specification that names only one has left half the question open. API 520 Part I covers how big the device has to be; API 526 covers what the valve itself is. Confusing them is why an enquiry sometimes arrives with an orifice letter and no relieving case, which is an answer with no question attached.
The catalogue record for API STD 520 Part 1, tenth edition of October 2020 at 172 pages, gives its title as Sizing, Selection, and Installation of Pressure-relieving Devices, Part I: Sizing and Selection. It states that the standard covers equipment with "a maximum allowable working pressure (MAWP) of 15 psig (103 kPag) or greater" in refineries and chemical facilities, and that the devices it covers are "intended to protect unfired pressure vessels ... against overpressure from operating and fire contingencies". Its sizing procedures are described as "based on steady-state flow of Newtonian fluids".
The steady-state Newtonian basis is worth reading twice before a datasheet is filled in. A two-phase relieving case or a markedly non-Newtonian fluid is not what a steady-state Newtonian procedure assumes, so it needs a method written for it. If the case in front of you is one of those, say so at enquiry rather than at commissioning.
The catalogue record for API 526 describes a different kind of document altogether. Its abstract reads, in full: "Acts as a purchase specification for flanged steel pressure-relief valves." Its contents include a section titled "6 Orifice Areas and Designations" and one titled "11 Pressure-temperature Tables". In our wording rather than the record's: a purchase specification is what makes two makers' valves interchangeable on a datasheet, and it is not a sizing method.
Is API 526:2017 still the current edition?
The 2017 edition is not the current one, and a requisition that names it is naming a superseded document. The catalogue record for API 526:2017 states that it was superseded on 22 August 2023 by API STD 526:2023. That is a two-line fix on a requisition and an expensive argument at inspection.
What changed between the editions is in the documents themselves, and the procurement point does not depend on it: name the standard and the year. A valve bought to a 2017 purchase specification and inspected against a 2023 one is a dispute nobody needed to have.
What does ISO 4126-1 cover, and when does it appear?
ISO 4126-1 is the general safety valve standard that arrives with European equipment and European specifications, and in the Gulf it lands on the same site as the API suite. Its catalogue record states that ISO 4126-1:2013 "specifies general requirements for safety valves irrespective of the fluid for which they are designed". That scope is deliberately wide where API 520 Part I's is deliberately narrow.
The record for ISO 4126-1:2013, shown as the current revision published on 15 July 2013, also gives its limits of application: safety valves having a flow diameter of 4 mm and above, for use at set pressures of 0.1 bar gauge and above, which the record prints as 0,1, with no limitation on temperature. The title itself is Safety devices for protection against excessive pressure, Part 1: Safety valves.
The practical consequence for a buyer is the same one that runs through every two-system question on a Gulf project. A package built to one system and a requisition written in the other will not reconcile themselves, and the cheapest moment to notice is while the datasheet is being written.
Safety valve or relief valve: is it the same thing?
It depends on which naming system the words are being used in, and that is worth settling before the datasheet is written. ISO 4126-1 uses safety valve as its general term for the device; the API catalogue records use pressure-relief valve and pressure-relief device for the same family. Within the API vocabulary the two words also describe how the valve lifts: a safety valve pops open at set pressure, a relief valve opens in proportion to the overpressure, which is the distinction our guide to steam, pressure and protection sets out. Write the word the governing standard uses, so the requisition reads the same way as the document it cites.
| Document | What the record says it is | Scope limit the record states | Record shown |
|---|---|---|---|
| API STD 520 Part 1 | Sizing, selection and installation of pressure-relieving devices: Part I, sizing and selection | Equipment with a maximum allowable working pressure of 15 psig (103 kPag) or greater; unfired pressure vessels; steady-state flow of Newtonian fluids | Tenth edition, October 2020, 172 pages (NORMSERVIS) |
| API 526 | A purchase specification for flanged steel pressure-relief valves | Contents include orifice areas and designations, and pressure-temperature tables | 2017 edition, superseded 22 August 2023 by API STD 526:2023 (Standards Ireland) |
| ISO 4126-1 | Safety devices for protection against excessive pressure: Part 1, safety valves | General requirements irrespective of fluid; flow diameter 4 mm and above; set pressures 0.1 bar gauge and above; no temperature limitation | ISO 4126-1:2013, current revision, 15 July 2013 (Nimonik) |
What does a relief valve not protect against?
A relief valve relieves pressure. It does not keep a vessel standing. The fire case is one of the contingencies the device is sized for: the catalogue record for API 520 Part 1 describes devices "intended to protect unfired pressure vessels ... against overpressure from operating and fire contingencies". What the same record rules out is structural protection: "Pressure-relief devices protect a vessel against overpressure only; they do not protect against structural failure" when the vessel is exposed to extremely high temperatures such as during a fire.
The record refers readers to API 521 for ways to reduce pressure and limit heat input. The valve is sized for the fire contingency; it does not keep the vessel standing once the metal itself is hot enough. The fire case is a system question about depressuring and heat input, and the relief device is one part of the answer rather than the whole of it.
The API 520 Part I record is equally clear about what the standard does not reach. Atmospheric and low-pressure storage tanks are covered by API 2000 and are outside it. Pressure vessels for bulk product transport and shipping containers are excluded. Overpressure protection for fired vessels is handled by ASME BPVC Section I. A tank farm question and a fired vessel question are not API 520 Part I questions, and an enquiry that treats them as such will get an answer built on the wrong document.
What is the relieving case, and why does it decide the size?
The relieving case is the credible upset the valve has to handle: a blocked outlet, an external fire, thermal expansion in a shut-in line, a control valve failing open, or a tube rupture in an exchanger. Each case produces a different required capacity from the same vessel, so the case is what sets the size. A datasheet that does not state it cannot be checked by anyone.
An orifice letter on its own is the end of somebody else's calculation, and that calculation started with a case, a fluid and a flow. Send the case and the required capacity with the letter and the letter can be confirmed; send the letter alone and it can only be repeated back.
Where more than one case is credible, the governing case is the one needing the largest valve, and that is not always the one with the largest flow: the cases are compared at their own relieving pressures, temperatures and fluid states. State the governing case on the datasheet. Naming the others alongside it costs nothing and tells the supplier what the valve has to tolerate as well as what it has to pass.
Why does back pressure decide the valve type?
Back pressure is the pressure at the valve outlet, and it has two parts: superimposed back pressure, which is already in the discharge system before the valve opens, and built-up back pressure, which the valve's own flow creates in the outlet piping. Together they decide whether a duty suits a conventional, a balanced bellows or a pilot operated valve.
A valve discharging to atmosphere through its own tailpipe sees one situation. A valve tied into a flare or a closed header shared with other relief devices sees another, and that one can change while the valve is open. It is why the line is worth filling in even when the answer looks obvious from the P&ID.
| Type | The duty it suits | Why |
|---|---|---|
| Conventional | Superimposed back pressure low and steady and built-up back pressure low: outlet to atmosphere or a dedicated tailpipe | In the usual closed bonnet arrangement outlet pressure acts on the spring side, so a varying one moves the pressure at which the valve opens |
| Balanced bellows | Superimposed back pressure variable or significant, or built-up back pressure high | The bellows shields the disc area from outlet pressure, so the set pressure holds |
| Pilot operated | High back pressure, or an operating pressure sitting close to the set pressure | The pilot senses process pressure directly and controls the main valve |
Which of the three a duty needs is a sizing question, and the thresholds that separate them sit in the sizing standard rather than on this page. What the enquiry has to carry is the two back pressure figures and where the outlet goes, and the arrangement follows from them.
What goes on a pressure relief valve datasheet?
A datasheet that can be quoted without a phone call carries the relieving case first and the hardware last. Send the fluid and its phase, the relieving case and the required relieving capacity, the set pressure, the allowable overpressure and accumulation, the superimposed and built-up back pressure, and the inlet and outlet line sizes and ratings. Those are the inputs. The orifice designation, the body and bonnet material, the trim and the spring are what comes back.
- The relieving case. Blocked outlet, fire, thermal expansion, control valve failure, tube rupture. Each produces a different required capacity from the same vessel.
- The fluid, its phase and its condition at relieving. A sizing method assuming steady-state Newtonian flow needs to know whether that assumption holds.
- Set pressure, overpressure and accumulation. Set pressure is where the valve is meant to start opening, overpressure is the rise above the set pressure while the valve passes its rated flow, and accumulation is the rise above the equipment's own maximum allowable working pressure that the equipment is allowed to see. Two different reference points, which is why they are two numbers. Three different numbers, and easily sent as one.
- Back pressure, superimposed and built-up. What is in the discharge system before the valve opens, and what the valve's own flow adds. This is what decides conventional, balanced bellows or pilot operated, and it is an easy line to leave blank.
- Inlet and outlet size and rating. Including whether the outlet runs to a closed header or to atmosphere.
- The governing standard and its year. API 526 and its edition, or ISO 4126-1, or both where a package demands it.
- Materials and any service condition behind them. Corrosive service, low temperature or sour service each change the answer.
Those seven lines are the enquiry. Send them to enquiry@flowsynergy.ae and an offer can be prepared against them. A pressure-reducing valve is a different device with a different sizing question, even though both get shortened to PRV on site. Our page on valve documentation and sourcing covers the certificate side of the same requisition, and our guide to steam, pressure and protection covers where these valves sit on a steam system. The wider selection question is in which valve a service actually needs.
What does naming API 526 still leave open?
Naming API 526 settles the orifice designations and the pressure-temperature tables its contents list names. It does not settle the size, because the size comes from a sizing calculation that API 520 Part I governs and that needs the relieving case as its input. Nor does it settle the material, the back pressure arrangement, the testing or the documentation.
- The required area. That is a calculation from the relieving case, not a catalogue lookup.
- The back pressure arrangement. Conventional, balanced bellows or pilot operated is a duty decision.
- The materials. Body, bonnet, trim, bellows and seat each follow the service, not the standard.
- The certificate and the test report. Separate documents, named separately, covered in valve documentation a buyer should demand, with the pressure test standard named for the test itself.
- The installation. Inlet pressure loss and outlet reaction are system design, and the standard's title covers installation as well, with Part I as its sizing and selection half.
Questions buyers ask about relief valve datasheets
A relief valve enquiry usually turns on a handful of questions, and all of them are answerable before a quotation is asked for. Each answer below says what belongs on the requisition, which catalogue record it rests on, and what gets decided for you at inspection if the line is left blank.
Can you quote from an orifice letter alone?
We can quote a valve, but not the right one with any confidence. An orifice designation is the output of a sizing calculation, and API 520 Part I is the document whose catalogue record describes it as covering sizing and selection, with procedures based on steady-state flow of Newtonian fluids. Without the relieving case behind the letter, nobody can tell whether the letter is right for the duty it is meant to cover.
Our specification says API 526. Is that enough?
Not without a year. The catalogue record for API 526:2017 states that it was superseded on 22 August 2023 by API STD 526:2023, so a line reading "to API 526" now points at two documents. Write the standard and the edition year on the requisition, and send the relieving case separately, because the purchase specification does not carry it. Leave the edition open and it gets settled at inspection, after the valve has been bought.
We have an ISO 4126 package and an API specification on the same project. Which governs?
Whichever the purchase order names, and it has to name one. ISO 4126-1:2013's catalogue record describes general requirements for safety valves irrespective of fluid, for flow diameters of 4 mm and above. API 520 Part I's record describes a narrower scope aimed at unfired pressure vessels above a stated working pressure. Cite both without saying which governs and the decision has been deferred to the inspector.
Does a relief valve cover us for the fire case?
It covers the fire contingency it was sized for, and nothing past that. If the fire case is yours, state it as the relieving case on the datasheet with the capacity your process engineer has calculated, because that is what sets the size of the valve we quote. What happens to the metal itself at fire temperatures is a separate question, and API 520 Part I's record points readers to API 521 for the depressuring and heat input side of it.
Does API 520 Part I cover our storage tank?
Say on the enquiry what the item being protected actually is, because the governing document changes with it. API 520 Part I's catalogue record places atmospheric and low-pressure storage tanks under API 2000 and outside its own scope, excludes bulk transport vessels and shipping containers, and leaves fired vessels to ASME BPVC Section I. Tell us the equipment, not only the connection size, and the quotation can cite the right document.
Send us the datasheet, not the part number
Flowsynergy supplies safety and relief valves alongside pressure-reducing valves, steam traps and strainers. Send the relieving case, the fluid, the set pressure, the back pressure and the line sizes and ratings, and we will tell you which valve answers the duty as stated and where the datasheet is still silent on something that changes the answer.
Call +971 4 313 2786 or email enquiry@flowsynergy.ae, Monday to Friday, 8:30 am to 5:00 pm Dubai time.
Sources: the catalogue records quoted on this page
Read 17 October 2026. Every figure, title, date, quotation and scope sentence on this page is read from a standards distributor's catalogue record of the document named, not from the document itself. The plain-language explanation around them is ours. No clause, table, equation, orifice area, pressure-temperature value or acceptance figure from any standard is reproduced here. Buy the current editions from API and ISO for the requirements themselves, confirm every line against the current edition, and your purchase order governs.
- API STD 520 Part 1, tenth edition, publisher catalogue record held by the standards distributor NORMSERVIS: the title, the October 2020 tenth edition and 172 pages, the 15 psig (103 kPag) scope statement, the unfired pressure vessel purpose, the steady-state Newtonian flow basis, the exclusions, and the statement that pressure-relief devices do not protect against structural failure.
- API 526:2017, publisher catalogue record held by Standards Ireland: the title, the 1 September 2017 publication date, the purchase specification abstract, the orifice areas and pressure-temperature table section titles, and the 22 August 2023 supersession by API STD 526:2023.
- ISO 4126-1:2013, publisher catalogue record held by the standards distributor Nimonik: the title, the current revision date of 15 July 2013, the general requirements scope, and the flow diameter and set pressure limits of application.
These designations are identifiers and publisher record facts only. No Flowsynergy stock position, lead time, accreditation, certification or distributor appointment attaches to any of them.