Safety Valve vs Relief Valve vs Bursting Disc: How to Choose

Safety valves and relief valves both open on rising pressure and reclose; they differ in how they open. In the ASME definitions, a safety valve pops open and is used mainly for steam, air and other gases; a relief valve opens gradually, broadly in proportion to the pressure rise, and is normally used for liquids [1, 3]. A bursting disc, or rupture disc, is a membrane that bursts at a set pressure: leak tight and cheap, but it does not reclose and must be replaced after every burst [4].

Relief sizing and selection must be done by a competent engineer, working from the design code and your plant's relief cases.

Why this matters now

In EN ISO 4126 Part 1, used in the UK and Europe, "safety valve" covers any valve opened only by the fluid's energy, which Americans call a "safety relief valve" or "pressure relief valve" [1]. So a UK enquiry for a "safety valve" can draw bids to different codes.

ISO confirmed ISO 4126 Part 3:2020, on valves and discs used together, as current in 2026 [7]. In Great Britain, a written scheme of examination under the Pressure Systems Safety Regulations 2000 (PSSR) must cover all protective devices, bursting discs included [8]; see our PSSR written scheme of examination guide.

The terms: PRV, safety relief valve and PSV

Pressure relief valve (PRV) is the general term for a valve that opens on inlet pressure and recloses once normal conditions return [1, 3]; on steam plant it can also mean pressure reducing valve, so spell it out. A safety relief valve pops on gas and opens in proportion on liquid [1]. PSV is a drawing tag, pressure safety valve; it says nothing about how the valve lifts.

Valve designs and bursting discs compared

A forward acting disc takes process pressure on its concave side, in tension; a reverse acting disc on its convex side, in compression [5]. ZOOK rates the operating ratio, the highest operating pressure as a percentage of minimum burst pressure, at 95% for its reverse acting series and 50% to 90% for forward acting [6]. Exceeding the ratio can shorten disc life or cause an early burst; ASME sets a burst tolerance of ±5% at 40 psig (2.8 barg) and above [5].

Fitting a bursting disc under a valve

A disc upstream isolates the valve from the process, preventing plugging, corrosion and leakage, allowing testing in place, and letting a standard material valve cut first cost and spares [4]. The rules:

• ISO 4126 Part 3 covers a disc within five pipe diameters upstream of the valve inlet, and the combination discharge factor used in sizing [7].
• Without a tested factor, the valve's rated capacity is cut by 10%, a factor of 0.9; under ISO 4126 Part 3, only if the disc petals stay in the holder [3, 4].
• The disc must not fragment, as loose pieces could block the valve [4].
• Both devices respond to differential pressure, so the space between them must be monitored or vented, typically with a gauge or indicator, try cock and free vent; a gauge alone on a closed space is not enough [4].
• Fike's summary of ISO 4126 Part 3, which predates the 2020 edition, puts disc burst at 90% to 110% of valve set pressure (or 0.1 bar, if greater, for the upper limit) [4].
• For ASME designs, Fike cites API 520: disc burst and valve set should share one nominal value; ASME combination tests need marked burst at 90% to 100% of set [4].

Set pressure, overpressure, accumulation and blowdown

Set pressure is where the device starts to open. Overpressure is the rise above set pressure, as a percentage of it; accumulation is the rise above the vessel's maximum allowable working pressure (MAWP). Blowdown is the gap between set and reseat pressure [3].

Emerson's handbook reflects the 2013 ASME code, so check the current edition [3]. Fike notes the European limit is 110% of design pressure where ASME allows 116% with several devices [4]. Keep operating pressure sufficiently below set pressure to avoid premature operation [8] and leakage, which rises as pressure nears set [4].

PSSR and PE(S)R duties in Great Britain

Under PSSR regulation 4, whoever designs, manufactures, imports or supplies a pressure system must fit the protective devices needed to prevent danger, with any release made safely, so far as is practicable; equipment supplied under PE(S)R is excepted [8]. HSE's Approved Code of Practice, L122, adds:

• fit at least one relief or limiting device as close as practicable to any item that could exceed its safe operating limits
• give devices enough capacity to reach full discharge within a set accumulation, discharging to a safe place
• examine devices at least as often as the plant they protect
• isolate devices only in very exceptional cases, and replace a vessel's only relief device immediately if removed
• investigate repeated discharge [8]

For new equipment, PE(S)R also requires the safety device set pressure in the marking, where relevant [9]. This is general information, not legal advice.

What to put in the datasheet and RFQ

• Fluid, phase, molecular weight or density, relieving temperature, solids and corrosives
• The governing relief case and required rate from the engineer's study, for example to API 520 (sizing and selection) and API 521 (relieving systems) [1]
• MAWP, set pressure, allowable overpressure, design code and conformity marking
• Superimposed and built up back pressure
• For a disc: forward or reverse acting, burst pressure, tolerance, operating ratio and vacuum support
• For a combination: a tested combination factor or the default 0.9, and the telltale
• Connections, materials, a seat tightness test such as API 527 [1], certificates, spares and lead time

Ask each bidder for certified capacity at the stated overpressure, so the technical bid evaluation runs on one basis.

Practical example

Take an illustrative reactor: MAWP 10.0 barg, operating up to 8.5 barg with corrosive vapour, needing 4,000 kg/h of relief, with no fire case; all rates and prices are illustrative.

One valve under ASME VIII is set at 10.0 barg (about 145 psig); at ±3% it may lift at 9.7 to 10.3 barg. Accumulation is limited to 10%, or 11.0 barg, matching the PE(S)R surge limit if PS is 10 bar. A reseat 7% to 10% below set, 9.0 to 9.3 barg, stays above operating pressure.

Option B is £1,700 cheaper, but only after going up a size for the 0.9 factor, and the engineer must recheck inlet losses and stability for the larger valve. After a burst, the disc must be replaced and the valve cleaned or refurbished [4].

Find suppliers for each option

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