Boiler safety valves functions and design features

Safety valves are used to relieve the excess of accumulated pressure from a boiler in as short a time and as efficiently as possible. This ensured by designing the valve to:

• Achieve a lift as close as possible to the maximum effective lift of D/4.

• Prevent jamming under all extremes of temperature and pressure.

• Allow the waste steam to be carried away without impeding the action of the valve.

• Allow the valve to re-seat rapidly without excess loss of steam (blowdown)

• Maintain the seat profile to ensure that the valve seats properly without leakage.

• Protect the valve loading spring from excessive heat thus preventing distortion.

• Allow the valve to be manually lifted in an emergency.

• Allow accurate adjustment of the loading pressure.

• Secure the valve from unauthorized tampering.

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To allow the steam to be correctly diverted in order to act on the valve lip and increase the lift of the valve, it is important that the above relative dimensions are maintained (lift = D/4). If material is machined from the seat then equivalent amounts must be machined from surfaces F, G & H in order to keep the seat width constant and also to maintain gap E.

The maximum pressure is 3% above the designed working pressure of the boiler, as certified and stamped on the boiler drum by the classification society or other approved regulatory body. If the boiler is de-rated then the new maximum pressure is the certified de-rated pressure plus 3%.

Working clearances must be maintained in order to guard against any risk of valve misalignment or spindle jamming, which would thus impede the operation of the valve. Modern safety valves are designed to eliminate potential problem areas such as valve lid wings and fixed lifting pistons. Stem bushes are given adequate clearances and the valve lid is made a loose fit on the end of the spindle (secured with a pin) to allow it to seat cleanly.