Is it a…
Pressure Relief Valve?
Relief Valve?
Pressure Safety Valve?
Safety Relief Valve?
or simply what it is…
a Safety Valve?
Protection of life and property against undesirable overpressure events worldwide, is usually maintained by the application of Pressure Relief Devices (PRD).
‘Pressure Relief Device’ or ‘PRD’ in ASME/API parlance, is a broad term and covers a multitude of devices for protection against overpressure. The ‘device’ can be a reclosing or a non-reclosing type and includes, Bursting Discs, Rupture Pin Devices, Weight Loaded Valves, Pressure/Vacuum Valves, and the topic of this article…
Pressure-relief Valves or Safety Valves?
We often hear or read about, Safety/Relief Valves, Spring Operated Valves, Relief Valves, Safety Valves, Pilot Valves, SRV’s, Thermal Expansion Relief Valves, PRD’s, PRV’s TRV’s and even wrongly or rightly(?) Pressure Safety Valves (PSV’s) etc.
Many of these terms have come from years back from the evolved USA based ASME Codes and API Standards, which over time have established specific descriptions against the various valve designs used on various applications.
People reading this article may well already be familiar with the established definitions of Safety Valve, Relief Valve and Safety/Relief Valve as stated in codes such as ASME PTC-25, ASME I & VIII and API Standards 520 and 526 etc. These specifications are used world-wide, often as the only reference available for PRD’s.
In a nutshell, a Safety Valve is characterised by its rapid or pop-opening action and generally confined to compressible fluids, A Relief Valve opens in proportion to the increase of inlet pressure and is used on incompressible fluids (liquids), and a Safety Relief Valve is a design that can be used on both compressible and incompressible fluids dependant on the application (though some components may change). A prefixed qualification of design is often added such as Spring Operated or Pilot Operated Pressure-relief Valve.
The generic term in ASME/API, covering these valve types is given as…
Pressure-relief Valve (PRV)
Just to bring some clarity to the ASME World of Pressure-relief Valves and how the user standard API-526 fits in, the graphic below depicts what I call the ASME/API PRV Universe.
This illustration concentrating on process (pressure vessel) and power (steam boiler) applications, details only the ASME PRV sections and basic PRV designs in the ASME Codes covered in the scope of API-526 – Flanged Steel Pressure-relief Valves.
It must be appreciated that ASME & API PRV terms do not always translate into the numerous languages of the world.
For example, ‘Safety Valve’ in other languages is written as, Soupape de Sécurité (French), Sicherheitsventil (German), Zawór bezpiecze?stwa (Polish) and Válvula de Seguridad (Spanish). Of course, there are many more languages and translations. Likewise, acronyms like PRV and SRV are purely English worded abbreviations and will not always readily translate.
Hence, for the most part in non-English speaking countries, the local language term is used irrespective of the design or application and translates simply as to what the device is…
Safety Valve
After all, the device is there purely to protect the safety of life and property. ‘Safety’ is the most apt term for what the valve is there for.
Besides the ASME Code and API Standards, there are other similar regulations in use around the world. From back in May 2002, all EU countries Safety Valve requirements now come under one mandatory regulation namely,
The Pressure Equipment Directive – (now) 2014/68/EU - ‘PED’.
What is important to understand about the PED is that the qualifying directives must be met for pressurised equipment, including safety valves installed in the EU (Like ASME in the USA). However, the actual design standards used for PED compliance safety valves are voluntary and can even be ASME/API. The ideology of the PED was to collate all existing EU country safety valve standards into one governing regulation, harmonising the requirements throughout the EU and to ensure mandatory PED Essential Safety Requirements are met.
In a post-Brexit UK, the applicable regulation is UKCA – UK Conformity Assessed. Presently in transition and mandatory from 1st January 2022.
There are also ISO International Standards for safety valves, bursting discs and other overpressure protection devices under the ISO-4126 series. This is not a mandatory standard and is voluntary and can also be specified for PED applications.
ISO-4126 even references ASME and API standards for certain safety valve requirements such as API-527 for seat leakage testing.
‘Safety Valve’ has been the term adopted by the International Standard for Safety Devices - ISO-4126 which is published in different languages. Note that the generic term used in ISO-4126 is ‘Safety Devices’.
Irrespective of application, fluid, or design (spring, pilot etc.), in ISO-4126 ‘Safety Valve’ is the only term used. However, Spring Operated Safety Valve’ or ‘Pilot Operated Safety Valve’ etc., are again used as a qualifier.
Unlike ASME/API, ISO-4126 allows the use of other devices such as lift assisted safety valves and supplementary loaded safety valves and Controlled Safety Pressure Relief Systems (CSPRS), which utilise a different design concept.
ISO-4126 also does not have any distinction for safety valves used for fired or unfired vessels as does ASME I and VIII.
They are simply Safety Valves in ISO-4126.
Here is a depiction of the equivalent parallel ‘Universe of ISO-4126 Safety Valves’. I say parallel as this requirement coexists with ASME/API where specified (no wonder people get so confused!). Nuclear is covered differently in other standards.
(Other related ISO-4126 sections are purposely not represented for the clarity of the graphic).
There certainly has been a proliferation of terms, especially acronyms over the years relating to Pressure-relief and Safety Valves (there we go again!). Lets’ look at one that has in time been copied through the years and used erroneously since…’PSV’.
‘PSV’ - Deserves a special explanation. Contrary to popular use and belief, ‘PSV’ is an incorrectly used acronym. PSV is historically a prefix used extensively by engineers compiling PRV specification sheets for installations and projects for example, PSV-101, PSV-102 etc. Also known as the Tag Number that appears on the nameplate, certification, drawings and even the purchase order and invoice. Other acronyms such as RV-, SV-, SRV- etc., are also used as tags.
PSV is historically short for ‘Plant Safety Valve’.
‘PSV’ In this respect, is the customer identifier/tag number of a PRV instrument and location in a plants ‘P&ID’ (Piping & Instrumentation Diagram).
Hence ‘PSV’ in its original and true meaning, applies only to a PRV’s identification and position within a system and not its design type.
Interestingly, the only and minor mention of ‘PSV’ as Pressure Safety Valve, in ASME/API is in API Standard 521- Pressure-relieving and Depressuring Systems 7th Edition, 2020 - Paragraph 3.1.56 (copied in the header to this article above).
‘PSV’ Is not a recognised term in ASME, API or ISO-4126.
I can only assume that it appears as a secondary mentioned term in API-521 due to the misuse and proliferation of using ‘PSV’ in the valve industry, especially with aftermarket services. Even a major manufacturer has mysteriously used it in on recent occasions. PSV also means various other technical things unrelated to valves. Its use should be discouraged outside client P&ID and tag number use.
There. Done it. My contribution to eliminate ‘PSV’ and for people to hopefully use the correct and more qualified terms as written in established codes and standards.
What should be used?
There is no written rule only common sense, an attribute in decline for some time. Over the years use of the various valve terms have proliferated the original established terms and they are used interchangeably to a degree. Thankfully, users can count on the manufacturers understanding this, and specifying the appropriate product for the intended application and regulatory jurisdiction, whatever is specified.
Before the introduction of ISO-4126 (still rather new in use), ASME and API codes and standards dominated and continue to do so. Hence those terms are used globally in the technical English language using world.
I am by no means the authority to dictate what to use, but there is to my understanding, no global rule. As I stated, it comes down to common sense and an understanding of the topic. Companies may want to take note of this and mandate their own proper use of the applicable term.
Accordingly, Pressure-relief Valve or Safety Valve should be used in general discussion. The applicable term should ideally be driven by the design code or standard used.
NB: PRV Has been written in this article in its API styled format, Pressure-relief Valve.
ASME XIII - IMPORTANT INFORMATION
On 31st July 2021, ASME will be releasing a new section of the BPVC Code. This will be Section XIII – Rules for Overpressure Protection. This will cover the requirements of PRD’s presently in sections I, III, IV and VIII.
This first version will be technology neutral and will generally align requirements in the existing sections were there are some differences.
Going forward, it is the intention to incorporate improvements and adding criteria such as back pressure testing of PRV’s against flow laboratory capacity testing (for actual correction factors etc.).
Until this section is released and better understood, I have no more information to add at this stage.
I hope you have found this discussion helpful and informative. There are other related articles here on LinkedIn that I have authored such as Effective (API) Vs Actual (ASME) sizing, Using API Standard 526 and Bonnet Venting. These can be found on the featured part of my LinkedIn profile
