Sentinel 7109's whistle valve and how does it work?

A peculiar looking valve had been hanging around Sentinel 7109's stores looking for a use. Eventually, I was advised that it was a whistle valve.

Grubby looking thing!

Alternative view

With mounting flange and whistle-mounting elbow

Whistle valve roof-mounted in Isebrook

Whistle valve roof-mounted in William

Unlike Isebrook and William, Sentinel 7109 has the whistle and valve mounted vertically on the cab front.

Front-mounted whistle (the valve is inside)

So how does a whistle valve work and how does it enable the 'pop' action needed for the whistle?

Functional Parts

The main valve body allows steam in through the top right connection where the air-hose adaptor is shown here. The steam is held back by a conical valve pressed against a valve seat by a spring and also by the steam pressure.

Conical valve (not to scale!)

Valve seat inside end of valve body

Spring and retainer which screws into the end of the valve body

A rod passes through the valve body and rests against the conical valve's base. To sound the whistle, the conical valve is pushed off its seat by the operating lever's movement which pushes the rod against the conical valve's base.

Rod (not much more to say on this!)

Operating lever

The steam can then pass out to the whistle through an aperture in the valve body's flange.
The steam pressure holds the conical valve in place until the lever is used to push it open. This is a sudden action which is also terminated abruptly by the steam forcing the conical valve shut again. Hence the whistle valve enables a quick 'pop' to be achieved as well as the long 'poop'! 

There is one more aperture in the valve body shown with a copper tube attached in the Isebrook valve picture (but, oddly, not on William).

Condensate drain

As a result of the whistle being sounded only occasionally (hopefully), the valve body will generally be cold when steam is admitted. Thus the copper drain tube is added to allow the resulting condensate to be drained away.

When it's all assembled, it looks like this.

Note the elegant MDF cab front substitute and supporting clothes' peg!

Finally, when I first took the valve apart originally for renovation, the conical valve seemed to have a leathery pad between it and the seat. As the pad was in an 'aged' condition, I discarded it wondering where to find a replacement. On subsequent closer examination, it looked as if the conical valve and seat were both in good condition and, after a small amount of regrinding, were clearly perfectly satisfactory. Thus I concluded that either the leathery pad was accumulated muck or was a misguided bodge from the past! Who knows? But it isn't there now!