Check Valves, Relief Valves and Pressure Regulators for Aerospace and Ground Systems
Check valves let flow pass one way, relief valves open to protect a system from overpressure, and regulators hold a set downstream pressure. Flight hydraulic relief valves are bought to MIL-DTL-5523, ground pressure systems at NASA sites follow NASA-STD-8719.17, and code vessels need relief devices marked under ASME rules.
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Valve types and where they are used
Choose the valve by its job in the system first, then by fluid and pressure. A check valve, a relief valve and a regulator can look alike on a panel, but they protect against different failures and are tested differently.
| Valve | What it does | Where it is used |
|---|---|---|
| Check valve | Opens at a set differential pressure and blocks reverse flow | Pump discharge lines, pressurant feeds, purge lines, preventing backflow between parallel sources |
| Pressure relief valve | Opens on rising inlet pressure and recloses when pressure falls | Hydraulic systems, gas panels, pressure vessels, test stands and GSE |
| Thermal expansion relief valve | Relieves pressure built up by trapped fluid as it warms | Isolated hydraulic lines that can be blocked in and heated |
| Rupture disk | Bursts at a rated pressure and does not reclose | Vessels and lines where a one-time vent is acceptable, alone or under a relief valve |
| Pressure regulator and back pressure regulator | Holds a set downstream or upstream pressure during operation | Gas supply from cylinders and tube trailers, test stand supply legs, purge control |
The table matters for one reason buyers often miss: a regulator is a control device, not a safety device. NASA-STD-8719.17 states that pressure regulators or back pressure regulators shall not be used to provide overpressure protection to a pressure system. If the drawing shows a regulator, look for the relief device downstream of it, and buy both.
The specifications that govern these valves
Flight hardware and ground hardware are governed by different documents. Flight hydraulic relief valves come from the military detail specification family, while ground pressure systems are governed by consensus codes and, at NASA sites, by NASA-STD-8719.17. The status column shows what we could verify on EverySpec copies of DLA records, NASA and standards publisher pages.
| Document | What it covers | Status as verified |
|---|---|---|
| MIL-DTL-5523E | Detail specification: valve, relief, hydraulic pressure | Dated 23 November 2011, superseding MIL-V-5523D; listed as inactive on EverySpec, so check whether your program allows it for new design |
| MIL-V-5523D | Hydraulic pressure relief valves (1991) | Superseded by MIL-DTL-5523E |
| AN6279 | Detail sheet for a two port hydraulic pressure relief valve | Acquired together with MIL-DTL-5523; the sheet states that both make up the procurement requirement |
| MIL-V-5527A | Aircraft hydraulic thermal expansion relief valves (1951, with a 2001 amendment) | Confirm the current revision on ASSIST or SAE |
| MIL-V-5027 | High pressure oxygen check valves; the 1949 Monthly Catalog of government publications lists it as superseding AN-V-13a | Historical; confirm the current revision on ASSIST or SAE before citing it |
| NASA-STD-8719.17 Revision D | Ground-based pressure vessels and pressurized systems, including pressure relief devices, regulators and their retest intervals | Approved 26 April 2022 |
| ASME BPVC Section XIII | Rules for overpressure protection: pressure relief valves, rupture disk devices, pin devices and their certification and marking | Introduced as a standalone section in the 2021 edition; 2025 edition published |
| ASME PTC 25 | Performance test code for pressure relief devices | NASA-STD-8719.17 uses the 2014 edition's definition of a pressure relief valve |
MIL-V-5027 is a useful reminder that oxygen check valves have had their own procurement documents since the 1940s. If an old drawing calls a valve by that number, treat the call as a requirement to be resolved with engineering, not a part number to be matched on a surplus listing.
Have a part like this?Send it in under a minute. A person replies within 1 business day.Cracking pressure, reseat pressure and set pressure
Every check or relief valve purchase should state the opening and closing points and how they are measured. These numbers drift with spring wear, seat condition and temperature, so they belong on both the drawing and the test record.
- Cracking pressure. The minimum differential pressure across a check valve at which the first indication of flow appears. Swagelok's technical guidance notes that this is not necessarily the point at which the valve is fully open.
- Reseal or reseat pressure. The differential at which the valve closes again without visible leakage. A check valve usually needs some back pressure to seal well, and Swagelok notes that the poppet and seal design and the number of check valves in series both affect the reseal point.
- Set pressure. For a relief valve, the inlet pressure at which it is adjusted to open. Ask for the set point tolerance and the reseat or blowdown behavior, since a valve that reseats too low can vent a system well below its working pressure.
- Test medium and temperature. Cracking and set pressures measured on nitrogen at room temperature can differ from service in liquid or at cryogenic temperature. Say which medium the acceptance test must use.
NASA-STD-8719.17 recognizes how these numbers are used on low pressure systems. Code stamped relief devices are generally not available below 15 psig, so the standard allows check valves with known cracking pressures, or lift disks that open on their own weight, as relief devices on those systems, provided they are retested like any other relief device.
Relief devices on ground pressure systems
For hardware that will operate on a NASA site, the relief device is part of the system certification, and its records have to be kept. NASA-STD-8719.17 Revision D sets out what a buyer should expect to document.
- The location, design, operating parameters, last test date and due date of each reclosable relief device must be recorded in the system's configuration management records.
- Relief valve set points are retested or the valves replaced at set intervals consistent with National Board NB-23: steam systems annually, gas systems above 200 psi maximum allowable working pressure every 3 years, relief valves combined with rupture disks every 5 years, corrosive, flammable or toxic fluid systems every 2 years, and all others under center procedures but no more than 5 years.
- The standard points to NB-23, Part 4, Supplement 3 for the acceptable shelf life of relief valves held in storage.
- Downstream of a pressure regulator, the system must either be rated for the source pressure or carry a relief device sized for a full open regulator failure. A regulator certified to CGA E-4 can stand in only with Pressure Systems Manager concurrence and only where no vessel sits downstream.
- Hydraulic pump discharge relief valves can count as the relief device if they are sized for full pump flow at maximum allowable working pressure, verified for function and retested at no more than 5 year intervals.
The practical buying point: order relief valves with a bench test record that shows the set point and the date, because that date starts the retest clock. A valve that sat on a shelf for years before installation may be outside its storage limit on arrival.
ASME code relief valves and UV marking
Relief devices on ASME code pressure vessels must be devices the code permits, marked with the ASME Certification Mark and the right designator. Since the 2021 edition, the device rules sit in ASME BPVC Section XIII, while the referencing code, such as Section VIII for pressure vessels, decides which devices may be used. The UV designator is the one associated with pressure vessel relief valves, and UD with rupture disk devices.
NASA-STD-8719.17 states that pressure safety relief valves shall be used in accordance with the applicable ASME code of construction, and that adjustments and repairs to code stamped valves must follow the applicable code. It also allows one exception buyers see on vacuum chambers: where relief is needed for internal pressurization sources, code relief capacity rules still apply, but the device need not be UV stamped if its set pressure is below 15 psig.
What this means on a purchase order: when the vessel is code stamped, require a code marked relief valve with its nameplate data (set pressure, capacity, fluid basis) and the manufacturer's documentation. Do not accept a valve that has been reset by anyone other than an organization authorized to do so, because the reset breaks the certification.
How the part number is built
Valve part numbers come from a detail sheet such as AN6279 or from the manufacturer's catalog, and the coding of size, port, set pressure and material differs between sources. We did not find a coding scheme we could verify across these families, so we do not decode one here.
Instead, get the detail sheet or catalog page the number comes from and confirm the set pressure or cracking pressure, port type, materials and seals directly from it. For AN6279 relief valves, the sheet itself says the requirement is the sheet plus MIL-DTL-5523, so put both on the PO. Never substitute a valve because the body and ports match: two valves with the same envelope can have different springs, seat materials and set points.
Materials, seats and the trade-offs
Body, spring and seat materials decide where a valve can go. The seat usually sets the temperature and fluid limits, and the spring sets how stable the opening point stays.
| Element | Choices | Watch for |
|---|---|---|
| Body | Aluminum, stainless steel, brass in some ground gas service | Galvanic coupling to the line, pressure and temperature rating, specialty metals rules on defense contracts |
| Soft seat or seal | Elastomer O-rings, PTFE, PCTFE, polyimide | Fluid compatibility, cold service stiffness, oxygen compatibility, shelf life of elastomers |
| Metal seat | Hardened or lapped metal seats | Higher allowable leakage than soft seats, sensitivity to particles |
| Spring | Stainless and nickel alloy springs | Relaxation at temperature changes the set point; cryogenic service needs material that keeps its properties cold |
Seat and seal materials for oxygen and cryogenic service are compared on our solenoid and ball valves page, and elastomer compounds and age control on the O-rings page.
Cleanliness for oxygen and propellant service
Valves for oxygen and propellant systems must arrive clean to a stated level and stay sealed until installation. Check and relief valves are hard to clean after assembly because the spring cavity and seat traps particles and lubricant.
- Name the cleaning specification and level on the PO. ASTM G93 covers cleanliness levels and cleaning methods for equipment used in oxygen-enriched environments, and ASTM G88 gives design guidance for oxygen systems; many programs use their own center or company specifications instead.
- Ask how the valve is cleaned: disassembled and cleaned by part, or flushed as an assembly. Assemblies that cannot be disassembled may not reach the required level.
- Specify any lubricant by name, and only one approved for oxygen service.
- Require the cracking or set pressure test to happen before final cleaning, or require recleaning after test.
- Require capped ports and double bagging with the cleanliness level and date on the label.
Our precision oxygen cleaning service and the cleaning reference explain levels and certificates.
Acceptance testing and the test record
Require a test record for every relief valve and every check valve in a safety or pressurant function. The record is what lets the receiving engineer and later the system owner trust the set point.
- Valve part number, serial number, and the specification or detail sheet and revision
- Cracking or set pressure measured, with the allowed tolerance
- Reseat pressure and seat leakage at a stated pressure, with the method and medium
- Proof or shell test of the body where the specification requires it
- Gauge or transducer identification and calibration date, test date and inspector
- For code valves, the nameplate data and certification documentation
Tube and line testing is covered on our tube and fluid systems guide. If you need an independent lab to bench test relief valves, see test lab time.
Ordering checklist for check and relief valves
- Full part number or detail sheet, plus the specification and revision, for example AN6279 with MIL-DTL-5523
- Fluid, temperature range, cracking or set pressure with tolerance, reseat requirement and allowable seat leakage
- Governing code: ASME code marking and designator for vessel relief, or the program flight specification
- QPL or QPD source where the specification has one
- Manufacturer and lot or serial traceability, material certs for wetted parts
- DFARS specialty metals clause flowed down for stainless and nickel alloy parts where the contract carries it; see our specialty metals page
- Elastomer cure date and shelf life limits for soft seated valves
- Cleaning specification, level and packaging for oxygen or propellant service
- Test record content, and a certificate of conformance; see our C of C guide
Counterfeit and quality risks, and how to receive valves
The main risks are reset or rebuilt valves sold as new, relabeled industrial valves offered against aerospace detail sheets, and surplus relief valves with no test history. A valve's set point cannot be seen from the outside, so source and records carry the proof.
- Buy from the manufacturer or its authorized and franchised distributors, and require chain of custody otherwise.
- Check nameplate, marking and seal wires against the documentation; a broken or missing seal on a relief valve means the setting is unverified.
- Bench test relief valves from any source other than the manufacturer before installation.
- Reject valves with damaged ports, missing caps on cleaned valves, or cure dates beyond your limit.
- Quarantine surplus valves until their history is established.
Our counterfeit parts avoidance process and receiving inspection guide cover the wider controls.
How we source check and relief valves
We do not hold inventory or publish prices, and we are not an authorized distributor for any valve maker. We source valves for your order from the manufacturer or through authorized and franchised distributors, flow down the specification, set point, cleaning and test requirements, and check the paperwork before shipment.
Valves rarely ship alone. We can put them on one purchase order with the tube assemblies, fittings, filters and machined manifolds they mount to, arrange independent bench testing, and kit a panel's worth of parts together. For valves that are out of production, we search under the controls on our obsolete parts page. The rest of the family is on the fluid components page.
Request check, relief or regulating valves
Send the part numbers or detail sheets, fluid, set or cracking pressure, governing code and any cleaning requirement through the fluid systems quote form. Those facts decide which sources can supply and test the valve. A person replies within one business day, and rush requests are prioritized. For export controlled propulsion data, follow our controlled programs process.
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Part, quantity and date are enough to start. Drawings and the mutual NDA follow by reply, and controlled programs never send technical data through this site.
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Questions
Can a pressure regulator protect a system from overpressure?
Not on its own at a NASA site. NASA-STD-8719.17 Revision D states that pressure regulators and back pressure regulators shall not be used to provide overpressure protection. The downstream system must be rated for the source pressure or carry a relief device sized for a regulator failing full open. A CGA E-4 certified regulator can substitute only with the Pressure Systems Manager's concurrence and with no vessel downstream.
How often do relief valves need to be retested?
For ground systems at NASA sites, NASA-STD-8719.17 sets intervals consistent with NB-23: steam systems annually, gas systems above 200 psi maximum allowable working pressure every 3 years, relief valves with rupture disks every 5 years, corrosive, flammable or toxic systems every 2 years, and all others no more than 5 years. Programs and site rules can be stricter, so confirm with the system owner.
What is the difference between cracking pressure and reseal pressure?
Cracking pressure is the differential at which a check valve first shows flow. Reseal pressure is the differential at which it closes again without visible leakage, and it is usually lower. Swagelok notes that cracking pressure is not the point where the valve is fully open, and that seal design and valves in series affect reseal. Put both values, the test medium and the tolerance on the purchase order.
Is MIL-DTL-5523 still active for hydraulic relief valves?
EverySpec lists MIL-DTL-5523E, the detail specification for hydraulic pressure relief valves dated 23 November 2011, as inactive. It superseded MIL-V-5523D. Detail sheets such as AN6279 still reference it as part of the procurement requirement. Confirm the current status on ASSIST and ask your engineering authority whether the program accepts it for new design or only for existing parts.
Does a relief valve on a vacuum chamber need a UV stamp?
Not always. NASA-STD-8719.17 says ASME code relief capacity requirements must still be met for internal pressurization sources, but the relief device need not be UV stamped if its set pressure is below 15 psig. Code stamped devices are generally not available below 15 psig, so the standard allows non-code devices such as check valves with known cracking pressure, subject to retesting.
Can I buy a surplus relief valve for a test stand?
Only if you can establish its history and retest it. A relief valve's set point is invisible from the outside, and a broken seal wire or missing test record means the setting is unverified. Bench test any non-manufacturer valve before installation, record the result as the start of its retest interval, and check NB-23 shelf life guidance for valves that have been stored.
Related
Sources
- EverySpec: MIL-DTL-5523E Valve, Relief, Hydraulic Pressure
- EverySpec: MIL-V specification listing (MIL-V-5523D superseded, MIL-V-5527A)
- EverySpec: AN6279 two port hydraulic pressure relief valve
- Monthly Catalog of US Government Publications: MIL-V-5027 high pressure oxygen check valves
- NASA-STD-8719.17 Revision D, Ground-Based Pressure Vessels and Pressurized Systems
- ASME BPVC Section XIII, Rules for Overpressure Protection (2025 edition listing)
- Nimonik: ASME BPVC Section XIII scope summary
- Swagelok: Check valve basics, cracking and resealing pressure
- ASTM G93/G93M: Cleanliness Levels and Cleaning Methods for Oxygen-Enriched Environments
- ASTM G88: Standard Guide for Designing Systems for Oxygen Service
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