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Featured Engineering Project:

Steam pressure safety valve (pop valve):

We've got to recondition the seats and faces of a pair of safety pop valves from Tornado, these are fitted to the boiler to set the maximum boiler pressure that can be achieved - once the pressure is exceeded the valve will open with a loud 'pop' to release the pressure - hence the name.

Once the valves are stripped and cleaned, the erosion to the castings from the rush of steam can clearly be seen, this has had the effect of creating a small recess underneath the valve head - in this case the actual

valve seat looks in good condition, but there is an excessive clearance between the underside of the valve and the body casting. We need to machine away both the seat, the land about the underside of the valve head and remove material around the area around the seat area.

Although the valve seat looks fine on the valve, we skim a tiny amount from the sealing face to ensure that we have a flat surface to work with when we later come to lapping the parts together. The valves are placed in the centre lathe, carefully clocked and material removed until a flat even face is observed.

Next we mount the valve body into the lathe, we always take great care in clocking the parts to achieve perfect concentricity and minimal run-out. We machine away quite a lot of material from the seat (the thin band near the centre of the part) and then take back the wider band that will sit beneath the valve head. We need to achive a clearence of 0.008" to 0.012" between the valve head and the body when the valve is completely closed. The area of steam erosion can be machined away as far as necessary to produce a suitable step between the valve area and the casting main body.

The valve is checked on machine numerous times to ensure that the seat faces are machined to achive the correct clearances.

Once we're happy with the clearences we apply engineers blue to the components to check the contact area of the sealing faces. We then lap the parts together in pairs using the lathe to drive the body about the valve with an application of extremely fine lapping compound on the sealing faces. One must take great care at this stage as because the components are manufactured from fairly soft materials (phosphor bronze) we take care not to apply too course a compound or too much pressure

as we perform the lapping asit would be easy to damage to newly machined seats.

We check all gasket faces and skim where necessary to ensure that a good seal can be achived. All the threads are cleaned out with a tap and any scale removed.

 

 

 
Component was featured on the website home page (April 2013).

 

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