The blowdown valve is used for clearing out (blowing down) accumulated scale, rust and assorted particles from the bottom of the boiler. 7109 was missing this item on arrival and it has taken a long time to find one capable of handling the high 275psi boiler pressure. John Goold has obtained a new old-stock type as in the picture with 4.25 inch diameter undrilled flanges.
New Old-stock Blowdown Valve
It has to be fitted on to a flanged "drain-pipe" at the bottom of the boiler (it has been cleaned up and the snow has melted since this December 2010 picture was taken).
Blowdown valve "drain-pipe" flange
And then there is the spanner needed to operate the valve from a suitable distance. And the angle of attack to avoid the spanner operator from being steam-cleaned!