Thursday, 25 August 2011

Trailing Engine's Boiler Feed Pump Valve Regrinding

The last episode left the inlet and outlet valves in need of a quick regrind. The process is simple and requires only a little fine grade grinding paste and a screwdriver to rotate the two halves together.

The rotation action is backwards and forwards in a two steps forward, one step back arrangement with the two rubbing surfaces never being allowed to stay in the same place for long. Otherwise the surfaces could stray from being flat and a good seal would only take place when the valve and seat had a particular position relative to each other. This would be clearly undesirable so hence the continual back and forth rotational movement.

Inlet Valve being reground.
After grinding, the remaining paste must be removed from all surfaces so that the paste does not get where it shouldn't during the pump's operation. I used Duck oil and a cloth to wipe away the residue. Paraffin is actually recommended but I didn't have any. However, there did not seem to be any residue remaining!

Inlet Valve

I did the inlet valve first and the result is below (apologies for slight lack of focus).
Inlet valve after the regrind
Deposits on the valve have gone and
the marks on the seat are lessened
At this point I spotted a problem which I had not previously noticed but was there all the time. The cap over the valve had clearly been over-tightened at some time sufficiently to break it apart.
Cap viewed from the side/below
Cap viewed from above
It had been tightened so much that it had pressed the centre section through so that the outer rim would seat but the whole piece would not seal. Whilst there would be little consequence to the pump operation because there is an outer cover to hold it in place, it has been remade to avoid any further trouble from it in future.
The original cap seems to have been made from gunmetal; however, a 1.5 inch diameter piece of brass rod has been machined for the new one.
Old and new
I decided to mill a groove across the top to enable the cap to be rotated. With valve grinding paste between the mating surfaces, it can now produce a good seal. At first I didn't think that water escaping into the cap cavity would be important; however, it could possibly act as an air bubble in the system and hence reduce the effectiveness of the pump (which was designed to work with an incompressible fluid). So I've convinced myself it will be better as a result!

Needless to say, the outer cover which holds the new cap in place should only be delicately 'nipped' tight to avoid further destruction!
New valve cap installed
Outlet Valve

The outlet valve was reground in a similar manner to the inlet valve.
Valve deposits removed compared to before
Seat deposits reduced
The outlet valve is now ready for action.

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