Spring-loaded, balanced bellows and pilot-operated relief valves that open at a set pressure and protect vessels, pipes and pumps. Specified with Young & Cunningham, sized, supplied and supported in the GCC by Petromarsh.
Tap a part to see what it does. Blue is the system pressure at the inlet. When it reaches the set pressure the disc lifts and the red discharge leaves through the outlet. The scale on the right shows set pressure, overpressure and blowdown.
Cap. Covers the adjusting screw so the setting cannot be changed by accident. After testing it is sealed with a wire and lead seal. A lifting lever can replace it where a manual test lift is required.
Adjusting screw. Compresses the spring to set the pressure at which the valve opens. It is set on a test bench and locked with a lock nut.
Spring. Pushes the disc down onto the nozzle. The spring force divided by the nozzle seat area is the set pressure. Each spring covers only a limited range of set pressures.
Bonnet. Houses the spring. On a conventional valve it is open to the outlet, so back pressure adds to the spring force. A bellows or a pilot design removes that effect.
Stem. Carries the spring force down to the disc and keeps the disc guided and square on the nozzle as it lifts and reseats.
Disc. Sits on the nozzle and holds pressure until set pressure is reached. A shaped huddling chamber around it gives the fast, full lift (pop) of a safety valve on gas and steam.
Nozzle. The inlet passage and seat. Its bore is the orifice area that sets the relieving capacity, named by API 526 letters D to T.
Inlet. Connects to the vessel or line being protected. API 520 Part II limits the pressure loss in the inlet pipe to about 3% of set pressure, or the valve can chatter.
Outlet. Larger than the inlet, because the fluid expands as it leaves. It discharges to a flare header, closed system or safe place, as set out in API 521.
A pressure relief valve is the last line of defence against overpressure. It stays closed in normal running, opens on its own when pressure reaches a set value, releases enough fluid to stop the pressure rising, and closes again when pressure falls. Three terms describe how it behaves.
The inlet pressure at which the valve starts to open, set by the spring. It is usually the design pressure (MAWP) of the protected equipment. Keep normal operating pressure well below it, often 90% of set or lower for spring valves, so the valve does not simmer.
Pressure keeps rising a little after the valve opens, until it passes full flow. The rise above set pressure is the overpressure. ASME VIII allows 10% for a single valve, 16% for multiple valves and 21% for fire.
A relief valve does not close at set pressure. It reseats at a lower pressure, so it does not chatter. The difference is the blowdown, given as a percentage of set pressure, often around 5 to 10% on gas and steam valves.
The first three cards explain the names used on datasheets. The last three are the main designs. Tap a type to see where it fits.
Opens with a rapid full lift, called pop action, as soon as set pressure is reached. Used on compressible fluids where a large volume must be released fast.
Fitted on boilers, steam headers and gas systems.
Opens gradually as pressure rises above set, roughly in proportion to the overpressure. Suited to liquids, where a small release quickly drops the pressure.
Used on pump discharge, blocked-in liquid lines and heat exchanger thermal relief.
A valve built to act as a safety valve on gas and as a relief valve on liquid, depending on the fluid. Most process plant pressure relief valves are of this kind. Pressure relief valve (PRV) is the general name for all three.
The common choice on GCC process vessels and separators.
The simplest design. Back pressure in the outlet acts on the disc and adds to the spring, so the set pressure changes with back pressure. Best when it discharges to atmosphere or a low, steady back pressure.
Common on utilities, steam and atmospheric discharges.
A bellows seals the back of the disc from the outlet, so back pressure no longer changes the opening point. It also keeps corrosive or dirty discharge away from the spring and guide. The bonnet must be vented.
Typical on refinery and gas plant valves that discharge to a common flare header.
System pressure holds the main piston closed. At set pressure a small pilot vents the dome and the main valve opens. It stays tight close to set pressure and handles high back pressure, but needs clean fluid for the pilot.
Used on high-pressure gas, tank protection and where operating pressure is close to set.
A relief valve has two pressure zones: the inlet and nozzle at system pressure, and the body and bonnet at outlet pressure. Trim and spring must stay unchanged for years and still lift at the right pressure.
| Part | Carbon and low-temp steel | Stainless and duplex | Special alloys |
|---|---|---|---|
| Body and bonnet | Cast carbon steel, chrome-moly for high temperature, low-temp steel for cold reliefA216 WCB · A217 WC6 · A352 LCC | 316 stainless and duplexA351 CF8M · A995 4A | Alloy bodies for seawater and acidsMonel · alloy 625 · Hastelloy C |
| Nozzle and disc | 316 stainless as standard, Stellite faced for erosive or high-temperature duty316 SS · Stellite | 316 or duplex, soft seat inserts for tightness316 · F51 · PTFE / elastomer | Monel or alloy 625 for seawater and chloridesMonel 400 · alloy 625 |
| Spring and bellows | Carbon or alloy steel springs, coated or in stainless or Inconel for corrosive and hot service; bellows in 316L or alloy 625API 526 · ASME VIII | ||
A relief valve is a safety device, so the vessel code, the sizing and the certification all matter. Your datasheet names the codes, and each valve is supplied with its set pressure test record.
Flanged steel pressure relief valves: orifice letters D to T, sizes, ratings and centre-to-face dimensions.
Sizing and selection: the relief load, the required orifice area and the valve type.
Inlet and outlet piping, the 3% inlet loss rule and supports.
Overpressure causes such as fire, blocked outlet and tube rupture, and the design of flare and disposal systems.
Allowed leakage at 90% of set pressure, measured in bubbles per minute for gas.
Overpressure limits and certification. Valves for ASME vessels carry the ASME mark with the UV designator from a certified maker.
Safety devices for protection against excessive pressure, including safety valves and pilot-operated valves.
Safety accessories are category IV, with UKCA under the UK Pressure Equipment (Safety) Regulations 2016.
Inspection, testing and repair of pressure-relieving devices in operation.
Your engineers red-line our drawings, you approve the design twice before production starts, and you can witness the tests. Tap a step to see what happens and which documents you receive.
Young & Cunningham has made valves in Britain since the 1920s, with a long history of supplying the UK Ministry of Defence, and publishes practical guidance on specifying relief valves: operating and set pressure, media, temperature, materials and connections. Petromarsh, their exclusive representative in the GCC, adds valve selection and sizing. Where your project needs ASME UV or PED certified valves, we confirm the certification of the exact valve offered before you order.
A relief valve is sized for the worst credible case, not the pipe. Send the items on the right and we will check the orifice, select the design and reply with a proposal.
