The Injector Clack Valves

As mentioned in the problems pages the injector clack valves fitted to the Stafford work completely satisfactorily under normal circumstances (although I did see two other Stafford's exhibit the same failure at a Stafford Owners Rally).  This page explains how I made an alternative to the conventional ball bearing clack valve as an experiment to see if I could overcome my problems.
Stafford Steam Locomotive -O Ring Injector Clack ValvesAfter almost 18 months of operating my Stafford with injector clack valves that were prone to sticking open a fellow club member suggested that I should try using an O Ring clack valve to see if they would behave any differently.  Unfortunately we could not find any commercially available O Ring clack valves of a suitable size for the Stafford so I had to design my own.  The main component of the O Ring clack valve is obviously the O Ring which has to withstand the boiler water temperature of 172 degrees Centigrade at 120 psi.   Small Viton O Rings are quite easily available and can operate at temperatures up to 200 degrees Centigrade so they became my preferred option.  
The photo shows a completed O Ring clack valve with an original Stafford ball bearing clack valve inset for size comparison.  The component parts of the O Ring clack valve are also shown in the photo with a "shuttle" shown in its "as machined" state and after fitting with its O Ring.
O Ring clack valves require the shuttle that carries the O Ring to be guided as it moves up and down (open / closed) so that the O Ring will seat around the hole in the inlet, unlike a ball bearing valve where the loose ball simply drops into the hole.  This can be accomplished by having a fluted stem on the shuttle which slides up and down in the inlet port bore but that requires the inlet bore to be increased beyond the feed pipe bore so that the area available for the water to flow is maintained.  This will obviously increase the dimensions of the clack valve.  To overcome this size increase I decided to put the shuttle stem on the top of the shuttle and guide it by using a tubular extension of the valves top cap.  The length and tapered end of the shuttle stem were made so that the stem could be engaged in the caps guide hole before the cap is screwed down to ease maintenance of the valve, and obviously the guide hole in the cap has an opening into the valve chamber at its upper end to avoid any possibility of the shuttle stem becoming hydraulically locked as it moves up into the guide hole.  The valve seat is machined on to the removable lower section of the valve assembly which is both easier to machine and also easily removed for cleaning / servicing.  Full details of the O Ring clack valve can be obtained by downloading a JPEG copy of the CAD drawings by clicking here (use a "right click" and "save as" to download the image to your computer).

Ideally the clack valves should be manufactured using gunmetal or phosphor bronze and stainless steel for the shuttle, but to initially test the valves I used the cheaper option of brass for the main components.  The loco has now covered over 150 miles with these valves fitted and they have performed perfectly, always closing completely no matter what has happened with the injectors.  Why my original ball valves would not operate correctly is a mystery, but these valves have resolved my problem and have definitely become a permanent feature of my Stafford.  Their only disadvantage is that the O Ring has to be replaced after about 30 hours "in steam" as it starts to leak when there is no boiler pressure (e.g. before steam is raised).