Safety Valves


What are they?

‘A valve which automatically, without the assistance of any energy other than that of the fluid concerned, discharges a quantity of the fluid so as to prevent a predetermined safe pressure being exceeded, and which is designed to reclose and prevent further flow of fluid after normal pressure conditions of service have been restored’

Safety valves, as the name implies, have a specific function

to protect equipment & personnel

EN ISO 4126 only refers to safety valves & covers –

• Safety Valves
• Relief Valves
• Pressure Relief valves
• Safety Relief Valves

Why we need one?

4 main reasons –

  • Blocked Discharge
  • External Heat
  • Thermal Expansion
  • Failure of pipeline component, i.e. control valve


Set pressure :

The pressure at which the valves starts to open measured at valve inlet
normally 1.1 x working pressure or working pressure + 0.7 bar for water whichever is the greater.

Overpressure :

Pressure at which valve has to achieve its full discharge capacity normally set pressure +10%

Accumulation :

Pressure increase over the maximum working pressure of the system during discharge through the safety valve.

Operating states

There are 3 operating states for safety valves –

1. Equilibrium
2. Fully open
3. Fully closed


Blowdown :

The pressure difference between set pressure and pressure at which the valve reseats expressed as a % of the set pressure.

Reseat Pressure :

Pressure at which valve is fully closed.

Working Pressure :

Pressure at which the system being protected normally operates.

Discharge Capacity ;

The amount of water / gas/ vapour the valve will pass at a given pressure.

Equilibrium :

Forces acting to close are in equilibrium (balance) with the forces acting to open and the seat & disc are just in contact

>defined in EN ISO 4126 as the Set Pressure

>this is the point when flow is about to start

>in practice flow commences as the equilibrium point is reached

Fully open :

The position the valve must achieve to pass its rated capacity at its specified over pressure
note – this is not the Set Pressure
different designs of valve have different relationship between flow & pressure
some designs have a rapid increase in flow for a small increase in pressure -others offer a gradual increase.

Fully closed

the position when the valve has re-seated i.e. fully closed – nil leakage

the difference between the re-seat pressure & set pressure is often referred to as the Blowdown Pressure.

the blowdown pressure will depend on valve design, the faster the flow increases on opening the lower the blowdown pressure is



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