I downloaded a while ago some very cool simulations of valve movements from a link that was here in the forum. The simulations all have one thing in common: the inside admission simulations are depicted with piston type valves, and the outside admission simulations are depicted with slide type valves.
So, my question is are all piston type valves inside admission? And, are all slide type valves outside admission?
They don’t have to be – but look at what goes on between the superheater header and the exhaust nozzle. Inside admission ought to keep more heat in the steam down to the ports (and snifting is facilitated a bit, on piston-valve engines at least, if there’s outside exhaust). Meanwhile, balancing slide valves is critical – piston valves of course don’t have that problem – and that may make outside admission a bit more logical. It’s rare to find superheat and slide valves together – lubrication, for example, is traditionally more difficult with high superheat.
I suspect that the type and arrangement of valve gear would have something to do with the choice – Stephenson, inside the frames, is the ‘classical’ gear for later slide-valve engines, and outside gear like Baker or Walschaert is typical on piston-valve engines. The orientation of the gear normally determines the axis of the valve rods…
I look forward to hearing more on this from people with experience. Don’t remember what Angus Sinclair might have said on this subject, but someone with a copy of Development of the Locomotive Engine might look in there. I’d be interested to see where the admission was preferred for rider-valve engines, for instance.
jruppert -
First question - are all piston type valves inside admission? No.
Second question - are all slide type valves outside admission? Yes.
Second question first: For slide valves, the live steam from the throttle comes into the valve chest on top of the valve, and the exhaust steam comes from the cylinder into the cavity underneath the valve. The difference in the pressures is what keeps the valve tight against its seat. If the opposite was true, and steam from the throttle was admitted to the cavity underneath the valve, the pressure would tend to lift the valve up away from its seat.
Piston valves were usually inside admission. Outside admission applications weren’t unknown, but required the valve rod packing to withstand full pressure from the throttle; with inside admission, exhaust steam is at the ends of the valve and the rod packing has a much easier job. It can be seen that as pressures increased, this could become critical. This wasn’t as big a problem with slide valves, because engines using them usually operated at lower boiler pressures.
I believe that Baldwin 60000, the water-tube boilered three cylinder compound 4-10-2, was probably the last locomotive built in this country with outside admission cylinders - the two outside low-pressure cylinders were both built that way, evidently to allow direct porting in compound operation from the center high-pressure cylinder, which was inside admission.
Hope this is some help.
Old Timer
jruppert -
First question - are all piston type valves inside admission? No.
Second question - are all slide type valves outside admission? Yes.
Second question first: For slide valves, the live steam from the throttle comes into the valve chest on top of the valve, and the exhaust steam comes from the cylinder into the cavity underneath the valve. The difference in the pressures is what keeps the valve tight against its seat. If the opposite was true, and steam from the throttle was admitted to the cavity underneath the valve, the pressure would tend to lift the valve up away from its seat.
Piston valves were usually inside admission. Outside admission applications weren’t unknown, but required the valve rod packing to withstand full pressure from the throttle; with inside admission, exhaust steam is at the ends of the valve and the rod packing has a much easier job. It can be seen that as pressures increased, this could become critical. This wasn’t as big a problem with slide valves, because engines using them usually operated at lower boiler pressures.
I believe that Baldwin 60000, the water-tube boilered three cylinder compound 4-10-2, was probably the last locomotive built in this country with outside admission cylinders - the two outside low-pressure cylinders were both built that way, evidently to allow direct porting in compound operation from the center high-pressure cylinder, which was inside admission.
Hope this is some help.
Old Timer
The quick answer is NO!
The real consideration was to avoid high pressure superheated steam on the glands of the piston valves.
The Western Australian Government railways developed designs with outside admission piston valves, with large inlet ducts on the ends of the piston valves. The New South Wales Government Railways built a number of TF class consolidations from 1912 with outside admission piston valves and smokebox superheaters based on Baldwin designs. When these were converted to Schmidt superheaters from 1918, the outside admission piston valves were retained until the cylinder castings required replacement, when larger diameter inside admission valves were substituted.
Outside admission is the simplest arrangement with slide valves with the live steam assisting the sealing of the slide valve, and inside admission is the simplest arrangement for superheated steam to minimise the stress on the outer valve glands.
Peter
Wow! I’m surprised how quickly I got an answer. And I guess what you guys are saying makes a lot of sense. I guess if I had thought a little more, I would have seen the answer myself, but hearing it from somebody else is reassuring.
Thanks!
Of course, there are such things as direct and indirect valve gear . . .
There was an article in TRAINS back in the ought-eighties that explained it pretty well.
Old Timer