Combination Injectors

Combination injectors (those fitted to firebox backheads and incorporating the steam and water valves and clackbox in the same casting as the injector) are notorious for leaking clack valves as they get older and it seems very difficult to cure them.. I have my own theory as to why but before I say anything, I wondered if anyone else had any thoughts on the subject, both why the problem exists and how to solve the problem?

 

Hi Steve,

I'd be interested in hearing your theory, but mine is that they are simply worn out. I recently rebuilt the backhead injectors for a Quarry Hunslet. This involved replacing the clack seat, valve, and cap to bring everything back to original drawing dimensions. I also made a "Grinding Plug" that is needed to properly lap the clacks on the Gresham and Craven design. The clacks are now working just fine.

Jonathan

 

Far to easy to give them a clout with a spanner/hammer should they stick open leading to distortion of the clack box! I've seen plenty of them with evidence of abuse in the past.

 

I've a pair of Davies & Metcalfe 7mm injectors which I've been doing battle with for some time. They were originally sent to the successors of D&M for an expensive overhaul and came back no better. They were then sent to a well-known south coast establishment for another expensive overhaul and came back a lot better. But not for long. The clacks soon started leaking again. I've spent hours re-cutting seats and grinding valves, all to no avail. They are perfect until you come to use them when they soon start to leak. My theory is that the valve is virtually the same diameter as the internal diameter of the screw thread into which the cap screws. Because of this the largest seat cutter that you can use is the same as the valve itself, so there is no overlap. This means that if there is any slight lateral movement with the valve it won't seat properly, allowing steam to pass. I'm toying with the idea of slightly reducing the diameter of the valve to allow for this but I wondered if anyone else had any ideas before I do it.

 

Not sure of the detailed arrangement for the Davies and Metcalfe clack, but I would normally expect the valve to overlap the seat on both the OD and ID. The Gresham and Craven design has a flat inverted "T" valve, a removable seat with a hex in the center hole, and presents a fairly narrow raised face to the valve. Does the Davies & Metcalfe design seat directly on the body casting? Could you let in a replaceable seat ring to create some overlap? On the rebuild I did I also had to replace the cap and the valve to restore the fit around the valve spindle and prevent it wandering around during operation.