What Is a Pressure Relief Valve?
A pressure relief valve (PRV) is a safety device fitted to a pressurised system — a vessel, a hydraulic circuit, a lubrication line, a storage tank — that automatically opens to release excess pressure before it damages equipment, ruptures a line, or injures someone nearby. It's a mechanical, self-acting device: no power supply, no electronics, no operator action required. Once system pressure climbs to the valve's set pressure, it opens ("relieves"), vents the excess, and then closes again once pressure drops back to a safe level.
"Pressure relief valve" and "safety valve" are used interchangeably in most day-to-day industry conversation, and that's the plain-English usage this guide follows.
How a Pressure Relief Valve Works
Most relief valves hold a disc closed against the inlet with a spring, resisting the pressure trying to push it open. As pressure rises toward the valve's set pressure, that spring force is gradually overcome and the valve lifts.
- Set pressure: the pressure at which the valve is designed to start opening.
- Opening behaviour: the two most common fluid types behave differently. On gas and other compressible fluids, the valve "pops" open rapidly once flow hits the deflection ring around the disc, giving a fast, full-lift response. On liquids, which don't expand the same way, the valve tends to open more gradually — sometimes described as "gushing" rather than popping — which is one reason spring and disc selection differs between gas and liquid service.
- Blowdown and reseating: once pressure drops, the spring reasserts itself and the valve closes again. The gap between the opening pressure and the lower closing pressure is blowdown, and it's an engineered part of the design. Codes typically call for reseating within about 7% of set pressure under lab test conditions, though real-world blowdown on an in-service valve can run wider than that — one reason periodic testing matters more than a one-off factory setting.
- Orifice size: relief valves are also classified by standardised orifice letter designations (roughly D through T, smallest to largest). A bigger orifice gives more disc area for pressure to act on, and therefore more relieving capacity for the same set pressure.
Types of Pressure Relief Valves
Spring-loaded (direct-acting)
The simplest and most common type. System pressure acts directly against a spring-loaded disc, with no intermediate mechanism. Reliable and mechanically simple — the default choice across most industrial, lubrication and hydraulic applications, including the grease-system and hydraulic-circuit relief valves in AIMS's own range (see the Relief Valves section below).
Pilot-operated
Uses a small auxiliary "pilot" valve to control a larger main valve, rather than relying on a single spring to do all the work. The pilot senses system pressure and controls a dome of pressure sitting on top of the main valve's disc; when system pressure reaches set point, the pilot bleeds off that dome pressure and the main valve lifts.
- Pop-action pilots snap the main valve fully open, the same way a spring-loaded valve pops.
- Modulating pilots open the main valve in proportion to how far over pressure the system is, reducing product loss during minor excursions.
The main advantage is seat tightness much closer to set pressure — little to no leakage right up near the trigger point — which matters most in high-pressure systems where a spring-loaded valve would start weeping well before it needs to open.
The Australian Standard for Pressure Relief Valves
Two standards work together here, and it's worth being precise about which does what:
- AS 1271 — covers the valve itself: design, construction, testing, capacity rating and certification. This is the standard a relief valve is actually certified against.
- AS 1210 — Australia's pressure vessel code. It sets out what relieving capacity the protected equipment needs, and requires that the relief device's capacity be certified in accordance with AS 1271.
AS/NZS 1200 ("Pressure equipment") is the parent code covering the full lifecycle of boilers, pressure vessels and pressure piping, but it references AS 1210 and AS 1271 for the specifics rather than detailing relief-device requirements itself.
Set Pressure, Overpressure and Accumulation
A relief valve isn't set to open at the system's normal operating pressure — there's a deliberate margin, and the codes this space is built on (API 520, ASME Section VIII) define how big that margin is allowed to be:
| Scenario | Allowable accumulation above MAWP |
|---|---|
| Single valve, normal (non-fire) service | Up to 10% |
| Multiple valves protecting the same equipment | Up to 16% combined |
| Fire exposure scenario | Up to 21% |
This is the practical answer behind a question that gets searched informally as the "3 rule" for pressure relief valves — there's no single formally-named "rule of thirds" in the standards themselves, but the three accumulation thresholds above (10% / 16% / 21%) are the figures people are usually trying to recall.
Testing and Inspection Frequency
There's no single blanket interval — valves should be inspected and overhauled as often as needed to keep them in satisfactory operating condition, and that varies with service:
- Corrosive or fouling service, vibration, pulsating loads, or a tight margin between set pressure and normal operating pressure all push testing more frequent.
- A valve with a solid track record of passing tests can move to a longer interval over time; a new installation should get an early first inspection to establish a safe, appropriate interval from real data rather than a guess.
- A valve that has seen a fire event needs re-testing and a fresh seat gasket, no exceptions.
API Standard 576 and the National Board Inspection Code (NBIC) are the reference documents this guidance is drawn from, alongside whatever a site's jurisdictional inspector requires.
Signs a Pressure Relief Valve May Be Faulty
A few practical warning signs, worth checking before a valve is written off or a system is run past its comfort zone:
- Chattering: the valve opens and closes rapidly in quick succession rather than lifting and reseating cleanly, usually caused by dirt or debris in the seat area, or a valve sized larger than the system actually needs.
- Weeping or leaking at rest: slow leakage below set pressure, often from debris on the seat, a worn seat, a broken spring, or a plugged balance passage.
- Failing to reach set pressure, or not reseating: the system either exceeds its safe pressure without the valve opening, or the valve opens but won't fully reseat afterwards. Both point to incorrect calibration, wear, or contamination, and both are reasons to stop and investigate rather than keep running.
Root causes across all three are consistent: contamination (dirt, dust, corrosion) getting into the seat, wear and material degradation over time, incorrect calibration, or a valve that was undersized or oversized for the application in the first place. Regular testing — bench testing, in-line testing, or a manual pop test where the application allows it — catches most of this before it becomes a failure.
Relief Valves at AIMS Industrial
AIMS stocks spring-loaded, direct-acting relief valves covering grease lubrication systems, hydraulic circuits, and tank venting:
- Alemlube 1/8" BSP, 1 to 5 psi Relief Pressure Valve 47200B
- Alemlube 1/8" BSP, 0.25 to 1 psi Relief Valve 317400B
- Alemlube 1/8" NPT, 7.5 to 15 psi Relief Pressure Valve 47640
- Alemlube 1/8" NPT, 400 to 600 psi Relief Valve 50500
- Macnaught Pressure Relief Valve 2500 psi PRV-25
- Dixon 2" and 3" Air & Vacuum Relief Tank Vent Valve TVPC200-300
- Dixon Bronze Relief Valve With Side Outlet Male
- Dixon Anfo Loader 1/2" BSP Replacement Relief Valve Assembly 690 kPa
Browse AIMS Industrial's Relief Valves range →
Frequently Asked Questions
What does a pressure relief valve do?
It protects a pressurised system — a vessel, hydraulic circuit, lubrication line or tank — by opening automatically to release pressure once it climbs to a preset level, then closing again once pressure drops back to a safe range. It's a purely mechanical safety device with no power supply or operator input needed.
What is the "3 rule" for pressure relief valves?
There's no standard formally called the "3 rule," but the figure people are usually after is the set of allowable overpressure (accumulation) limits from API 520/ASME: up to 10% above MAWP for a single valve in normal service, up to 16% where multiple valves protect the same equipment, and up to 21% in a fire scenario.
What is the Australian standard for pressure relief valves?
AS 1271 covers the valve itself — design, construction, testing and capacity certification. AS 1210, Australia's pressure vessel code, sets out what relieving capacity the protected vessel needs and requires that capacity be certified to AS 1271.
How do you know if a pressure relief valve is faulty?
Three practical warning signs: chattering (rapid open-close cycling instead of a clean lift and reseat), weeping or leaking while the system is below set pressure, and failing to reach set pressure or fully reseat after it opens. Contamination, wear, incorrect calibration and incorrect sizing are the usual root causes — worth investigating rather than just replacing the valve outright.
Why does a pressure relief valve keep leaking after it's been replaced?
Usually the same root causes as any weeping valve — debris on the seat from the system itself (not the new valve), a set pressure too close to normal operating pressure so the valve never fully seats away from its trigger point, or an installation issue such as misalignment or over-tightening distorting the seat. Worth checking the system side, not just assuming the replacement part is faulty.
How often should a pressure relief valve be tested?
There's no fixed interval that applies everywhere — it depends on service conditions. Corrosive or fouling environments, vibration, pulsating loads and a tight margin between set and operating pressure all call for more frequent testing; a valve with a consistent pass record can move to a longer interval. API 576 and the National Board Inspection Code are the reference standards for setting that interval, and any valve that's seen a fire event needs re-testing regardless of its normal schedule.
What's the difference between a spring-loaded and a pilot-operated relief valve?
A spring-loaded valve uses a single spring acting directly on the disc — simple, reliable, and the standard choice for most lubrication and hydraulic applications. A pilot-operated valve uses a small auxiliary valve to control a larger main valve via a pressure dome, giving much tighter sealing near set pressure — an advantage mainly in higher-pressure process applications.
Have a question about a specific application, or need help choosing the right relief valve for your system? Get in touch with the AIMS Industrial team — we're happy to help you find the right fit.