Construction: A high-lift (high-lift relieving) safety valve is a spring-loaded safety valve with a large lift. The body houses a seating (valve seat) and a disc/valve which seats on the seat. The valve is held closed by a powerful spring (compressed by a loading screw) acting through a spindle. A high-lift valve is so designed that once it lifts a small amount, the effective area over which the steam pressure acts increases (the valve disc is shaped with a skirt/hot face and guide collar) so the valve "pops" fully open to a high lift (over 1/8 of the seat diameter), giving a rapid discharge. It has a guide/athwart arrangement to centre the disc, a lifting/escape arrangement, and a pressure-adjusting mechanism (spring loading screw, sometimes with a locking nut and a pawl/lever).

Working: When the boiler steam pressure reaches the set pressure, the upward force of the steam on the valve disc exceeds the downward spring force; the valve lifts. In a high-lift valve the design (the pop action from the steam acting on the enlarged area of a guide collar/lifting ring) makes it open rapidly to full lift, venting steam to reduce pressure. When the pressure falls (blowdown), the increased closing force over-rides and the valve slams shut. The valve discharges steam to atmosphere via the escape piping.

Adjustment: The set (relieving) pressure is adjusted by turning the spring loading screw/nut, compressing or releasing the spring so the set point is at the required pressure (e.g. that stamped on the Name plate, usually the working pressure + a margin; set just above the normal working pressure). Adjustment is done by slackening/changing the spring loading, and the release (blowdown/warning) may be adjusted by an adjusting ring/gland on the seat (the "lifting ring") which alters the amount the steam has to lift before full flow. Always adjust so it opens at exactly the set pressure and reseats with a small blowdown. Lock the adjusting nut to prevent tampering, and re-certify/date the seal. The valve must be tested to lift and reseat correctly before the boiler is put back in service.

Purpose: The accumulation of pressure (accumulation) test verifies that, on full boiler firing, the total relief capacity of the safety valves is enough to prevent the steam pressure rising more than 10% above the boiler's maximum allowable working pressure (MAWP), when the steam is not being taken off. It proves the safety valves can handle the total generation capacity.

Procedure: 1. Confirm certification/maintenance of the safety valves; the boiler is at normal water level; the main engine/auxiliary steam demand is shut off (connect to atmosphere/bypass) so all steam goes to the valves. 2. Note that the test must be done at the correct load; bring the boiler up to working pressure and to full firing (all burners). 3. With all steam outlets closed, the safety valves should lift and keep pressure from rising more than 10% above MAWP. 4. Record the highest pressure reached (accumulation) and confirm it does not exceed 1.1 x MAWP; check all valves lift and reseat. 5. Record results in the log; ensure the valves are correctly set (set/relieving pressure) before the test.

Safety precautions:

• Only certified/competent engineers conduct it; ensure the escape piping is correctly led to a safe discharge point and nothing/nobody is in the discharge path (blow loads).
• Do not over-fire; have the boiler isolated, the firing supervised; ensure water level correct and that the boiler cannot run dry.
• Use appropriate PPE and post/no-go around the steam outlet; watch pressure rise; have the ability to shut the firing down quickly.
• Ensure the test is timed and recorded; never exceed safe limits; if accumulation exceeds 10%, do not operate until the causes are corrected and re-tested. The boiler owner's surveyor/maker's requirement may be involved.