Valves
Pressure relief and safety valves (PSV / PRV)
CRS PLUS sources pressure-relieving devices for vessels, piping and storage tanks — flanged spring-loaded safety relief valves to API 526, balanced bellows and pilot-operated designs, small threaded thermal relief valves, and pressure-vacuum relief vents for atmospheric tanks. Each valve is quoted against its set pressure, service and orifice, and supplied set and tested.
Relief devices we quote
Sourced through the qualified supplier base. Sizes, ratings and grades are shown as ranges; send your bill of materials and every line is quoted against its exact specification.
| Product | Standard | Sizes (inlet × outlet) | Rating | Materials |
|---|---|---|---|---|
| Spring-loaded safety relief valve, conventional | API 526 / ASME VIII | 1 × 2 in to 8 × 10 in; orifice D to T | Inlet class 150 to 2500; outlet class 150 to 300 | A216 WCB body, SS316 nozzle and disc |
| Balanced bellows relief valve | API 526 | 1 × 2 in to 8 × 10 in | Inlet class 150 to 2500 | A216 WCB, A351 CF8M; SS316L or Inconel bellows |
| Pilot-operated relief valve | API 526 / ASME VIII | 1 × 2 in to 8 × 10 in; larger on request | Inlet class 150 to 2500 | A216 WCB, A351 CF8M, A352 LCC |
| Thermal relief valve, threaded | ASME VIII / manufacturer | ½ × ¾ in to 1 × 1 in | Up to 6000 psi set | Carbon steel, SS316 |
| Pressure-vacuum relief valve (tank breather) | API 2000 / ISO 28300 | NPS 2 to 12 | Set in mbar / inches water column | Aluminium, carbon steel, SS316 |
How a relief valve is specified
A relief valve is identified by its inlet and outlet size and rating, its orifice letter, set pressure, back-pressure and service fluid — for example "1½ in × 2 in, inlet class 600 RF, outlet class 150 RF, orifice F, set 45 barg, conventional, gas service". The orifice and set pressure come from the relief sizing calculation (API 520) on the process datasheet, which is the most reliable basis for an exact quotation.
The outlet is larger and of a lower class than the inlet because, once open, the valve discharges into a lower-pressure system or to atmosphere. Conventional valves suit constant low back-pressure; balanced bellows valves compensate for variable back-pressure; pilot-operated valves seal tightly close to set pressure and suit large orifices.
Testing and set pressure
Every valve is set to its specified pressure and tested for seat tightness to API 527 before shipment, and the set pressure is recorded on a tag and on the test certificate. Valves for pressure vessels are supplied with ASME Section VIII certification where the specification calls for it.
Documentation and supply terms
- Mill test certificate
- EN 10204 3.1 certificate for every line, traceable to the heat number marked on the item. EN 10204 3.2 with third-party witness on request.
- Marking
- Material grade, size, rating and heat number marked on each item to the governing standard (MSS SP-25 for valves, flanges and fittings).
- Inspection
- Positive material identification, dimensional checks and third-party inspection arranged where the project specifies them.
- Sour and low-temperature service
- NACE MR0175 / ISO 15156 hardness limits and impact testing applied where the piping class calls for them, and stated on the certificate.
- Quotation
- Each line states the standard, grade, origin, lead time and certification before you commit. Listing does not indicate stock held by CRS PLUS.
Frequently asked questions
What is the difference between a safety valve, a relief valve and a PSV?
A safety valve opens with a rapid "pop" action and is used for gas and steam; a relief valve opens progressively and is used for liquids; a safety relief valve can do either. PSV (pressure safety valve) and PRV (pressure relief valve) are the instrument tags used on P&IDs for these devices.
What is API 526?
API 526 standardises flanged steel pressure relief valves: the orifice letters D to T with their effective areas, the inlet and outlet sizes and flange ratings for each orifice, and the centre-to-face dimensions, so valves from different makers are interchangeable.
Why is the outlet a different size and class from the inlet?
The inlet sees full system pressure, so it is rated for it. Once the valve opens, the fluid expands into the discharge system at much lower pressure, so the outlet is larger to limit back-pressure and is rated lower, typically class 150 or 300.
What information is needed to quote a relief valve?
Ideally the valve datasheet: set pressure, relieving capacity, fluid and phase, temperature, back-pressure, inlet and outlet connections, orifice and materials. Where only size and class are known, we confirm the remaining data before quoting.
What is a pressure-vacuum relief valve?
A pressure-vacuum relief valve, or breather valve, protects an atmospheric storage tank from overpressure during filling and vacuum during emptying or cooling. It is sized to API 2000 / ISO 28300 and is often combined with a flame arrestor.
Request a relief valve quotation
A structured offer follows within 72 hours.
Manufacturer and brand names, where mentioned, are used for identification only. CRS PLUS sources these products through its qualified supplier base and holds no distributor, reseller or agency appointment from any manufacturer.