Comparison · Valves

Safety valve vs. relief valve: what is the difference?

Name: Safety valves and relief/bypass valves: key values from 36 manufacturer data sheets
Creator: shukchok
License: https://shukchok.com/en-qa/policies/terms-of-service

What EN ISO 4126-1 and the ASME code mean by a safety valve, a relief valve and a safety relief valve, how to recognise a certified valve, and what the German word „Überdruckventil“ really sells. Based on 36 manufacturer data sheets and about 160 shopping offers in Germany and the US.

By Alexander Scharff, Responsible for contentUpdated 4 November 202613 min read12 sources · 1 dataset

Contents

In short Answer in one sentence

Use a certified safety valve (EN ISO 4126-1 component mark or ASME stamp, sealed) whenever pressure equipment must be protected. Use an adjustable relief or bypass valve only to limit or hold operating pressure.

ASME terms: safety valve = pop action (gas, steam); relief valve = proportional (liquids); safety relief valve = either. In Europe both count as safety valves under EN ISO 4126-1 ^{1, 7}.
Limits: set pressure ±3 %, full capacity within +10 % overpressure (EN ISO 4126-1; ASME VIII: 10 % or 3 psi) ^{1, 7}.
21 of 21 certified safety valves in our data-sheet sample cannot be adjusted by the user; 15 of 15 relief and bypass valves can, and none states a blowdown ^{4, 5}.

Key terms in 30 seconds

Safety valve (SV, PSV): Certified valve that protects pressure equipment from excess pressure; factory-set and not adjustable.
Relief / bypass valve: Adjustable valve that holds or limits operating pressure – not certified as a safety device.
„Überdruckventil“: German everyday word with no standard behind it; it can mean a safety valve or a pressure limiter.
Set pressure: The pressure at which the valve starts to open.
Overpressure: How many percent the pressure must rise above set pressure until the valve is fully open (full capacity). Smaller is better.
Blowdown: How far the pressure must fall again until the valve closes.
Component mark: Test mark on the valve, e.g. TÜV·SV·…·D/G in Europe or the ASME stamp in the US; the final letter names the medium.
Seal: Seal on the adjusting cap; removing it turns the safety valve into an uncertified part.

The differences at a glance

Feature	Safety valve	Relief / bypass valve
Purpose	Protect pressure equipment from exceeding its allowable pressure ^{1, 2}	Limit or hold operating pressure, bypass ⁵
Code	EN ISO 4126-1; Pressure Equipment Directive 2014/68/EU, category IV; in the US the ASME code ^{1, 2, 7}	no safety code; compressed-air relief valves PED category 0 or 1 ⁵
Marking	Component mark TÜV·SV·…·D/G, F, H or W, CE with notified body number, or ASME stamp ^{3, 4, 7} (advantage)	none ⁵
Set pressure	fixed, factory-set, sealed (12 of 21 explicitly) ⁴	adjustable by the user (15 of 15) ⁵
Set pressure tolerance	±3 % or ±0.1 bar ¹ (advantage)	not stated (0 of 15) ⁵
Overpressure to full capacity	EN ISO 4126-1: max. 10 % or 0.1 bar; ASME VIII: 10 % or 3 psi; data sheets 5–10 %, diaphragm SV 20 % ^{1, 4, 7} (advantage)	10–67 % (5 types with data) ⁵
Blowdown	gases 2–15 %, liquids 2.5–20 % ¹ (advantage)	not stated (0 of 15) ⁵
Characteristic	full lift (pop action), normal, proportional ^{3, 7}	proportional
Testing in service	Building services: at commissioning and at least every six months (manufacturers, citing EN 806-5) ^{8, 9}; German plants: with the periodic inspections under the Industrial Safety Regulation ¹⁰	no inspection duty as a safety device
Price ½″median, Google Shopping	$12 (diaphragm) to $92 (ASME in title, US); €7.49 to €31.90 (Germany) ⁶	German shopping: 1 of 78 offers ⁶

✓ = advantage, measured against the task “protection against excess pressure”. Prices: medians of our Google Shopping analysis for ½″, German prices mostly incl. VAT, US prices excl. sales tax, 2026-11-02 ⁶.

Which one do you need?

Work through the questions from top to bottom. The first one that applies decides.

Do you have to protect pressure equipment and hold operating pressure, e.g. on a hydraulic power unit or with a pump bypass?→ Both: relief valve in addition to the safety valve
Can the pressure exceed the allowable pressure PS (MAWP) of a vessel, boiler, storage tank or line? Then a protective device is mandatory ².→ Safety valve
You only want to hold or limit operating pressure, and no equipment can exceed its allowable pressure?→ Relief or bypass valve

Then check on the safety valve

1. Medium and code letter (Europe): steam, gas or air → D/G; liquid → F; heating water up to 3 bar → H; water heaters → W ⁴.

2. Set pressure: no higher than the allowable pressure PS (MAWP) of the protected equipment. Under ASME VIII, where several valves are fitted, the additional ones may be set up to 105 % of MAWP, UG-134(a) ¹¹.

3. Relieving capacity: is the capacity the data sheet states at 10 % overpressure enough? If not, choose the next larger size.

ASME: safety, relief and safety relief valves

US readers meet three terms. A safety valve opens with pop action and is used for gas and steam; a relief valve opens in proportion to the pressure rise and is used for liquids; a safety relief valve can do either ⁷. PSV usually stands for pressure safety valve, PRV for pressure relief valve.

The ASME code sets the overpressure at full capacity at 10 % or 3 psi for pressure vessels (Section VIII) and 3 % for boilers (Section I). Certified valves carry the ASME stamp with the designator „UV“ (Section VIII, UG-129) or „V“ (Section I) ⁷. Accumulation above MAWP may reach 10 % with one valve, 16 % with several and 21 % in a fire case, UG-125(c) ¹¹.

API 526 (flanged dimensions) and API 527 (seat tightness) are the related API standards; we have not reviewed their values and quote none here.

Buying in Europe

If you buy in Europe, look for the TÜV·SV component mark or CE with the number of the notified body ^{3, 4}. In Europe safety valves and relief valves in the ASME sense are both „safety valves“ under EN ISO 4126-1 ¹.

Which term means what?

Safety, relief, overflow and pressure-limiting valve are often used interchangeably. We assigned 40 terms and key values to the codes they come from: EN ISO 4126-1, the European Pressure Equipment Directive, the former German boiler rules (TRD) and the ASME code.

Sample: 40 terms and key values
Selection: EN ISO 4126-1 (previews), Pressure Equipment Directive 2014/68/EU, withdrawn TRD 421 and 721, ASME/API glossary of a manufacturer manual, manufacturer terms from hydraulics, pneumatics and heating
Updated: 2 November 2026
Limits: AD 2000 sheet A 2 (2020-01), VdTÜV sheet SV 100, ASME VIII and API 520 not reviewed in full text; their values only from withdrawn TRD, manufacturer documents and technical articles

Term	Code	Safety function?	Marking	Key values
Safety valve (EU)	EN ISO 4126-1; PED: safety accessory, category IV ^{1, 2}	yes	Component mark TÜV·SV·…·letter or CE with notified body number	set pressure ±3 % or ±0.1 bar; overpressure ≤ 10 % or 0.1 bar; blowdown gases 2–15 % (≤ 0.3 bar), liquids 2.5–20 % (≤ 0.6 bar) ¹
Full-lift, normal, proportional SV	AD 2000-A2 (formerly TRD 421/721) ³	yes	D/G, F, H, W – the code letter names the medium or application	full lift: opens with pop action within 5 % pressure rise. Term from TRD 721 (1982); manufacturers still use it, e.g. ARI-Armaturen to EN ISO 4126-1/AD 2000-A2 ^{3, 4}
Diaphragm safety valve	heating (H), potable water (W), EN 1489 (water heaters)	yes	TÜV·SV·…·H or ·W	heating in Germany max. 3 bar; opening/closing difference 20 % (Caleffi) or 0.5 bar (IMI) ⁴
„Überdruckventil“	not a standard term	only if certified as a SV	–	–
Pressure relief valve (hydraulics, compressed air)	technical term in hydraulics and pneumatics (manufacturer data sheets); no safety code ⁵	no (standard series); a special type-examined series is a SV	none, or PED category 0/1	adjustable, pressure–flow characteristic
Bypass / overflow valve (process, heating)	manufacturer term ⁵	no	none	proportional; heating 0.05–0.5 bar
safety valve / relief valve / safety relief valve (US)	ASME/API ⁷	yes, certified; nameplate with ASME stamp and „UV“ (Section VIII) or „V“ (Section I) ⁷	ASME stamp	overpressure 10 % or 3 psi (Section VIII), 3 % (Section I) ⁷; accumulation above MAWP 10 % (one valve), 16 % (several), 21 % (fire) ¹¹

SV = safety valve, PED = Pressure Equipment Directive 2014/68/EU. Own assignment, 2026-11-02.

What does this mean for you? A valve is a safety valve because it carries a component mark or ASME stamp and is sealed – not because of its name. An adjustable “relief valve” without a mark must not protect your equipment against excess pressure; it only limits operating pressure.

Mini study: how to tell them apart on the data sheet

Comparison pages usually explain the difference in words. We counted instead: what do the data sheets of safety valves say, and what do those of relief and bypass valves say?

Sample: 36 manufacturer data sheets: 21 certified safety valves, 15 relief and bypass valves
Selection: Safety valves from Goetze, ARI-Armaturen, LESER, SYR, Caleffi, IMI Pneumatex, Bosch Rexroth, Alfa Laval and one private label; relief and bypass valves from Bosch Rexroth, Mankenberg, Danfoss, Oventrop, Afriso, Norgren, SMC and one compressed-air catalogue. Counted: component mark, seal, adjustability, overpressure, blowdown, set pressure tolerance
Updated: 2 November 2026
Limits: Selected by availability of public data sheets, not a random sample; “not stated” does not mean “not present”. Percentages of the compressed-air relief valves calculated by us from pressure rise and setting

Feature	Safety valves (n = 21)	Relief/bypass valves (n = 15)
Component mark or type examination in data sheet	21	1 (PED category 0/1, no safety function)
adjustable by the user	0 (19 explicitly no)	15
sealed (explicitly)	12	0 (14 explicitly no)
overpressure to full capacity stated	17 (5–20 %)	5 (10–67 %, partly own calculation)
blowdown stated	8 (10–20 % or 0.5 bar)	0
set pressure tolerance stated	1	0

Manufacturer data sheets, date per data sheet; counted on 2026-11-02 ^{4, 5}.

How to read the chart

Each row is a valve type. To the right: by how many percent the pressure must rise above set pressure until the valve is fully open. Further left is better: the valve reaches full capacity sooner.
Orange are component-tested safety valves (upper group), aubergine pressure limiting and bypass valves that are not tested (lower group). A bar instead of a dot means the data sheet gives a range.
The dashed line at 10 % is the EN ISO 4126-1 limit.
The diaphragm safety valve (marked with an asterisk) sits at 20 %. According to the manufacturer it is component-tested to TRD 721 or SR sheet 2 (code letter H for heating, W for potable water); its data sheet states 20 %.
Example: the full-lift SV LESER 459 is fully open at just 5 %. The differential bypass valve for heating needs 67 %, the 2 bar air relief valve 50–65 %.

Dot and bar chart: pressure rise to full capacity in percent of set pressure for five safety valves and four relief and bypass valves, with the 10 % limit of EN ISO 4126-1 — **Fig. 1** Source: Manufacturer data sheets (9 types with data), EN ISO 4126-1; own analysis, 2026-11-02 ^{1, 4, 5}

The diaphragm safety valves for heating and potable water sit at 20 %, right of the line; IMI states 0.5 bar, which at 2.5–3 bar set pressure works out at 17–20 %. Their manufacturers name the former TRD 721 or VdTÜV sheet SV 100 as the test basis, not EN ISO 4126-1 ⁴. So the 10 % line is not the stated yardstick for these valves.

Hydraulics is a telling case: Bosch Rexroth builds the same design (DBD) as a freely adjustable pressure relief valve and as a type-examined, sealed version DBD..K. Removing the seal voids the approval ^{4, 5}. Testing and the seal make the part a safety valve, not its design.

What does this mean for you? Check three things when buying: a component mark (or ASME stamp) with the code letter for your medium, a fixed and sealed set pressure, and a set pressure no higher than the allowable pressure of the protected equipment. If one is missing, you have a control device, not a safety device. How to read the rest of a data sheet is explained in Reading data sheets correctly.

Dataset: Safety valves and relief/bypass valves: key values from 36 manufacturer data sheets36 rows, 2 groups, 14 fields · Public manufacturer data sheets, counted on 2026-11-02; wording shortened, “n/a” = not stated in the data sheet© shukchok, all rights reserved

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Market study: what shops actually sell

In Germany many buyers search for „Überdruckventil“ (literally “overpressure valve”). We sorted the Google Shopping offers for two German and two US queries by their titles.

Sample: 158 offers: „Überdruckventil ½ Zoll“ (38), „Sicherheitsventil ½ Zoll“ (40), “1/2 inch safety valve” (40), “1/2 inch pressure relief valve” (40)
Selection: Google Shopping Germany and US, retrieved via DataForSEO; classified by title, duplicates removed
Updated: 2 November 2026
Limits: Classified by title: an offer without a mark in its title may still be certified

How to read the chart

Two bars, two German queries: top „Überdruckventil ½ Zoll“ (relief valve), bottom „Sicherheitsventil ½ Zoll“ (safety valve). Each bar is all of about 40 offers, i.e. 100 %.
Colours show what is really sold: orange component-tested safety valve (TÜV/CE in title), aubergine diaphragm safety valve for heating/potable water, dark grey safety valve with no further detail, hatched (thin strip) pressure limiting/overflow valve, light grey other product.
Example „Überdruckventil“: 40 % + 50 % = 90 % are safety valves. A real pressure limiter is only the thin hatched strip (2.6 %) – a single offer out of 38.

Stacked bars: shares of product classes among Google Shopping offers for the German queries „Überdruckventil ½ Zoll“ and „Sicherheitsventil ½ Zoll“ — **Fig. 2** Source: Own analysis of about 40 Google Shopping offers per query, 2026-11-02 ⁶

Class (US)	“1/2 inch safety valve” (n = 40)	“1/2 inch pressure relief valve” (n = 40)
ASME-marked (title says ASME or “certified”)	17.5 % (median $92)	–
Diaphragm safety valve, heating/potable water	25 % ($12)	–
Relief/safety valve with no marking in title	57.5 % ($63)	–
Water/heating relief valve (ASME/CSA, calibrated)	–	32.5 % ($105)
Relief/safety valve for air, gas, industry	–	40 % ($44)
Not a safety device at all (ball valves, pressure reducers)	–	27.5 % ($70)

Share of offers, median price of the class in brackets, US prices excl. sales tax. Retrieved 2026-11-02 ⁶.

When to use which: typical applications

Application	Medium	Pressure	Temperature	Operation	Right choice	Price ½″ (median, DE)
Closed heating system	heating water	2.5/3 bar	≤ 120 °C	rarely (fault)	diaphragm SV “H”	€7–17 ⁶
Storage water heater	potable water	6/8/10 bar	≤ 95 °C	may drip while heating up	SV “W” in the safety group	€7–17 ⁶
Compressor air receiver	air	8–16 bar	≤ 60 °C	rarely	SV D/G, TÜV-certified	€26–32 ⁶
Compressed-air network after the regulator	air	0.5–10 bar	ambient	often	relief valve (in addition)	–
Steam generator, steam line	steam	–	–	rarely	full-lift SV D/G	–
Hydraulic power unit	oil	60–500 bar	≤ 80 °C	regulating constantly	relief valve, plus a type-examined SV where required	–
Heating pump bypass	heating water	0.05–0.5 bar differential	≤ 120 °C	constantly	differential bypass valve	–

Typical applications. The data sheet and approval of the specific valve are what count.

What happens if a safety valve drips or fails?

A dripping safety valve usually has one of two causes: the operating pressure is too close to the set pressure, so the valve does not close cleanly because of the blowdown (up to 20 %) ¹, or there is dirt on the seat. SYR puts it plainly: if the valve drips constantly, it is usually dirt ⁹.

Never adjust a safety valve. It is factory-set and sealed; removing the seal turns it into an uncertified part ⁴.

How often must a safety valve be tested?

Diaphragm safety valve on a water heater: at commissioning and then at least twice a year by a professional. The valve is lifted until it clicks; afterwards it must be tight. That is what the instructions of Caleffi (series 531) and SYR (2115) say, both citing EN 806-5 ^{8, 9}.

Pressure systems in German plants: the safety valve is part of the periodic inspection under the Industrial Safety Regulation (BetrSichV), Annex 2 Section 4; the intervals depend on the part of the plant (No. 5.8, Table 1) ¹⁰.

Good to know: myth and reality

Common assumption~~A safety valve starts opening before its set pressure.~~

In factUnder EN ISO 4126-1 the set pressure is exactly the pressure at which the valve begins to open. The standard allows ±3 % or ±0.1 bar tolerance and requires full capacity at no more than +10 % ¹.

Getting this wrong leads to the wrong set pressure, or to adjusting a valve that drips “too early”.

Common assumption~~„Überdruckventil“ and safety valve are two different products.~~

In fact„Überdruckventil“ is not a standard term; 90 % of the ½″ valves sold under that name in Germany are safety valves ⁶. What separates them is certification, seal and characteristic ^{1, 2}.

Buyers think a relief valve is less strict – or take an adjustable pressure limiter for a safety valve.

Common assumption~~An adjustable relief valve protects just as well as a safety valve.~~

In factNone of the 15 data sheets states a set pressure tolerance or blowdown. Compressed-air relief valves fall under PED category 0 or 1, and one catalogue says explicitly “does not replace a safety valve” ^{2, 5}. A 2 bar relief valve needs 50–65 % pressure rise to reach full capacity ⁵.

The vessel is left unprotected, and the inspection body objects at the next inspection.

Common assumption~~If the safety valve blows off too early or drips, just turn it up a little.~~

In factCertified safety valves are factory-set and sealed (12 of 21 data sheets say so explicitly); on the Rexroth DBD..K the approval ends with the seal ⁴. Dripping usually means the operating pressure is too close to the set pressure (blowdown up to 20 % ¹) or there is dirt on the seat ⁹.

Do not readjust: have the valve checked by a professional and run the system with a margin below the set pressure.

Common assumption~~Safety valves may be used up to 3 % above MAWP; that is what separates them from relief valves.~~

In fact3 % is the permitted overpressure for boilers under ASME Section I and – under ASME VIII Appendix M as under AD 2000-A2 – the recommended or permitted pressure loss in the inlet line. It is not a rule that separates safety and relief valves. For pressure vessels ASME VIII allows 10 % or 3 psi ^{7, 12}.

Take the percentage from the code section that applies to your equipment, not from a comparison chart.

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Frequently asked questions

What is the difference between a safety valve and a relief valve?

In ASME terms a safety valve opens with pop action and is used for gas and steam, a relief valve opens in proportion to the pressure rise and is used for liquids ⁷. In Europe both are safety valves under EN ISO 4126-1 if they are certified ¹. An adjustable relief or bypass valve without a component mark only limits operating pressure and does not replace a safety valve ⁵.

What are the two types of pressure relief valves?

The ASME code distinguishes the safety valve with pop action (gas, steam) from the relief valve with proportional opening (liquids); a safety relief valve can work either way ⁷. European codes classify safety valves by characteristic as full-lift, normal and proportional ³.

PSV vs PRV: what do the abbreviations mean?

PSV usually stands for pressure safety valve, PRV for pressure relief valve. The difference is the opening behaviour: pop action for the safety valve, proportional opening for the relief valve ⁷. Both are certified safety devices; an uncertified, adjustable relief valve is not ⁵.

What happens if a relief valve is faulty?

A safety valve that drips constantly usually has dirt on its seat, or the operating pressure is too close to the set pressure ^{1, 9}. Do not adjust it, because it is factory-set and sealed ⁴. A professional checks it by lifting it; afterwards it must be tight ^{8, 9}.

Sources

EN ISO 4126-1:2013 + A1:2016 (DIN EN ISO 4126-1:2016-12), Safety devices for protection against excessive pressure – Part 1: Safety valves. Terms per ISO 4126-1:2013, clause 3; limits per ISO 4126-1:2013/Amd 1:2016, clause 7.2.1 (previews). Amendment A2:2019 only changes Annex ZA.
Directive 2014/68/EU (Pressure Equipment Directive), Art. 2 No. 4, Annex I Nos. 2.10, 2.11 and 7.3, Annex II.
TRD 421 and TRD 721, edition May 1982 (withdrawn), TRD 721 Nos. 3.1 and 4.9.3 – origin of the terms of today’s AD 2000 sheet A 2 (edition 2020-01, full text not reviewed).
Manufacturer data sheets, safety valves: Goetze, ARI-Armaturen (900005, edition 12/13), LESER, SYR, Caleffi, IMI Pneumatex, Bosch Rexroth (RD 25710), Alfa Laval and one private label; date per data sheet in the data set.
Manufacturer data sheets, relief and bypass valves: Bosch Rexroth (RD 25710), Mankenberg, Danfoss (VD55K803), Oventrop, Afriso, Norgren (8.240.500), SMC and one compressed-air catalogue; date per data sheet in the data set.
shukchok: Google Shopping analysis Germany and USA, 2026-11-02 – about 40 offers each for four queries, classified by title.
Dresser Utility Solutions (Flow Safe): Pressure Relief Valve Technical Manual, Bulletin TECK 596 Rev C – glossary (overpressure), ch. 5.1 (UG-129, UV designator), ch. 5.7, Fig. 5.2.
Caleffi: diaphragm safety valves series 531, instructions UW003886 (© 2025), commissioning section; Flamco: Securfix NG 4807, installation instructions, section 1 and maintenance list.
SYR: diaphragm safety valve 2115, application catalogue, pp. 273–274 (application range, maintenance).
German Industrial Safety Regulation (BetrSichV 2015), Annex 2 Section 4, Nos. 2.2, 5.3 and 5.8 Table 1.
S. P. Agarwal, Tata Consulting Engineers: Pressure Relief Valve – ASME Section VIII Unfired Pressure Vessel Code Requirements, 2023-03-02 (UG-125(c), UG-134(a)); ASME text not reviewed.
E. Stork, ARI-Armaturen: Gegendruckbeaufschlagte Sicherheitsventile (back-pressure loaded safety valves), technical article, 2004-04-29 – inlet pressure loss max. 3 % under AD 2000 A2, edition of that time.

How to cite this page

Scharff, A. (2026): Safety valve vs. relief valve: what is the difference?. shukchok.com, Updated 4 November 2026. https://shukchok.com/en-qa/wissen/sicherheitsventil-ueberdruckventil-vergleich