Pressure relief valve

01

Define set pressure, overpressure and blowdown under the code

Spirax Sarco defines set pressure as the gauge pressure at which a direct-loaded safety valve starts to lift, overpressure as the rise above set pressure needed for the lift that gives rated capacity, and blowdown as the difference between set pressure and reseating pressure. It summarises that an ASME Section VIII pressure-vessel valve (UV stamp) opens within 10% overpressure and closes within 7%, and an ASME Section I boiler valve (V stamp) opens within 3% and closes within 4%. [1] [2]

As an editorial calculation example only: a Section VIII valve set at 10 barg should reach rated lift by about 11 barg and reclose by about 9.3 barg. If normal operating pressure sits close to that, the valve may leak or reseat late; ask the supplier for the recommended margin between operating and set pressure for the valve type.

Spirax Sarco classifies valves by how they lift after opening, for example a full-lift valve that reaches the lift for rated flow within a pressure rise of not more than 10%, and describes pilot-operated valves whose main valve is controlled by a pilot. Choose the type for the fluid and service with the supplier. [2]

Give the supplier the protected equipment's maximum allowable working pressure, the code it is built to, the relieving scenario and required capacity, the fluid and its state at relief, and the relieving temperature.

02

Allow for superimposed and built-up back pressure

Spirax Sarco explains that total back pressure is superimposed back pressure, present at the outlet before the valve opens, plus built-up back pressure generated while it discharges. In a conventional valve, superimposed back pressure changes the opening point, while total back pressure affects blowdown and reseating; balanced valves, for example with bellows, minimise these effects. [2]

Where valves discharge into a shared (“common”) discharge header, state the superimposed back pressure and its variation, and ask whether a conventional, balanced-bellows or pilot-operated valve is needed. [2]

03

Keep inlet losses low and the discharge drained

Spirax Sarco notes that EN ISO 4126 recommends keeping the inlet pipe pressure drop below 3% of set pressure when discharging, with rounded inlet corners, the valve at least 8 to 10 pipe diameters downstream of bends or Y fittings, the bonnet vertical, and no take-off branches in the inlet. It notes EN ISO 4126-1 limits pressure drop in the discharge system to 10% of set pressure. [3]

For steam and gas, discharge pipework should rise, for liquids it should fall, and horizontal runs should slope at least 1 in 100 away from the valve so it drains; fluid collecting downstream impairs performance and corrodes the spring. On steam, mount the valve above the pipe so condensate cannot wet the seat. [3]

As an editorial calculation example only: at 10 barg set pressure, a 3% inlet limit is 0.3 bar and a 10% discharge limit is 1 bar; check both against the line sizes before ordering.

04

Check seat tightness and keep the valve undamaged

Spirax Sarco states that “The vast majority of safety valve seat leakage problems occur after initial manufacture and test”, from transit damage, misuse, contamination or poor installation, and that test procedures are “usually based on the API 527 standard”, for air holding the valve at 90% of set pressure. Systems should be flushed before installation, valves never lifted by the easing lever, and never gagged or loaded to stop leakage. [3]

Repairs should be made only by personnel approved by the manufacturer; for ASME valves the repairer must hold a National Board ‘VR’ stamp. Ask for the set-pressure test certificate and the seat tightness standard applied. [3]

05

Compare what suppliers offer

Suppliers linked to this page list safety and relief valves for pressure vessels and steam, sanitary relief valves, fire-protection and waterworks relief valves, high-pressure oilfield relief valves and instrumentation relief valves. [4] [6] [7]

J&O and China Sanitary Valve list tri-clamp sanitary relief and safety valves; Convista lists a 1319 ductile-iron pressure relief valve and spring full-bore safety valves; FITOK lists proportional, medium- and high-pressure relief valves for instrumentation. [4] [5] [6] [7]

06

Trace the relief valve components

This is a functional reference built from the sources cited; it is not one maker's parts list.

07

Fill the specification fields with values for your duty

A relief valve quotation needs duty-specific values. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

08

Send the protected equipment, relief case and piping data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Equipment: [MAWP; design code; protected item]
Relief case: [scenario; required capacity; fluid and state; relieving temperature]
Valve: [set pressure; overpressure; blowdown; type (conventional, balanced, pilot)]
Back pressure: [superimposed (constant or variable); built-up; header]
Piping: [inlet length and size; discharge route; drainage]
Construction: [body and trim materials; seat; connections; easing lever]
Documents: [code stamp; certified capacity; set-pressure test; seat tightness test]

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