Procedure:

1. Preparation: Isolate the boiler/ safety valve, allow it to cool and depressurise, and obtain the maker's manual and spares. Prepare a permit-to-work/ risk assessment.
2. Remove the safety valve: Remove the valve from the boiler (or the valve cover/ bonnet) and take it to the workshop.
3. Dismantle the valve: Disassemble the valve - remove the bonnet/ cover, the spring, the spindle, the disc/ valve head, and the seat. Note the arrangement and the setting.
4. Clean all parts: Clean the valve, seat, disc, spindle and spring using an appropriate solvent; remove scale, deposits and corrosion.
5. Inspect each part:

• Valve seat and disc (valve head): for wear, pitting, scoring, corrosion and damage to the sealing faces; check they are flat and smooth.
• Spindle/ stem: for wear, bending, corrosion and damage.
• Spring: for breakage, fatigue, loss of tension and corrosion; check the free length.
• Guide/ body: for cracks, wear and damage.
• Gaskets/ seals: for deterioration and damage.

1. Renew as necessary: Replace worn/ damaged parts (seat, disc, spring, spindle, gaskets) with genuine spares. The seat and disc are often re-faced/ re-ground or renewed together (matched).
2. Reassemble the valve: Fit the parts in the correct order, ensuring the disc seats correctly and the spindle/ spring are correctly fitted.
3. Refit the valve to the boiler with new gaskets, and set the valve (see part b).

Parts requiring close attention:

• The valve seat and disc sealing faces (must be flat, smooth and free of damage for a tight seal).
• The spring (correct tension, no breakage, correct setting).
• The spindle/ stem (straight, free, correctly guided).
• The gaskets/ seals (must be renewed to prevent leaks).
• The valve lift/ travel (correct clearance for the required capacity).

• The safety valve is set to lift at the correct pressure (the boiler's working/ design pressure, or as per Class requirements - typically the valve lifts at the working pressure, or at a set margin above it, e.g. the working pressure or a small percentage above, per Class).
• Procedure: With the boiler at operating pressure (or using a test/ gagging arrangement), the safety valve spring/ setting is adjusted so the valve lifts at the set pressure. The valve is tested by raising the boiler pressure (or by a test rig) and observing the lift and reseat.
• The blowdown (the difference between the lift and reseat pressure) is set to the required value (typically 2-4% of the set pressure, or as per the maker/ Class) to give a stable, clean reseat without chattering.
• The setting is verified and recorded, and the valve is sealed/ locked to prevent tampering.
• The safety valve must comply with the classification society requirements (e.g. lift at the working pressure, reseat within the set blowdown, and have adequate capacity to relieve the full boiler output).
• The setting is witnessed/ approved by the Class surveyor as required.