Pardon my ignorance, but I was just wondering - what are the advantages and disadvantages of outside vs. inside admission valves? In some of the old literature I've read on the subject, it's usually just mentioned that there is a difference and how it works, rather than why one was chosen over the other.

Maybe I'm not reading the right books, but it's kind of got me puzzled. I can see how it would at least make a difference mechanically in a slide valve - pressure pushing up on the valve vs. out on the steam chest - but:

1. What was/were the deciding factor(s)?
2. Which was preferred for slide vs. piston valves?
3. What difference did it make on piston valves?

Hello,
From what I've read on this subject, slide valves fell out of favor when locomotives started running higher boiler presures and using superheated steam. Both of these factors tended to wear the valve surfaces a lot quicker than a saturated steam locomotive. With a spool valve(piston valve) the pressure is equal inside the steam chest, whereas on a slide valve, it's all on one side of the sealing surface. Some locomotives were converted to spool valves later on if they got superheaters added. A good example I can think of is a surviving Boston and Maine mogul that was built as a saturated steam locomotive, but was superheated later on. If you look at a picture of it, you'll see that it had been converted to spool valves using what was called a "Franklin Economy Steam Chest". It was a bolt on adaptor kit basically that allowed the original cylinder saddle to be used.
L.F.

As I understand it, the main problem there was the problem of lubricating the slide valve surface. The superheated steam tended to make it very difficult to keep it lubricated properly, causing, as you mentioned, greater wear.



That's quite interesting - I did not know that they made conversion kits. I'll have to check that out and see if I can find some pictures - I'd like to see that. Thanks!

However, maybe I worded them wrong, but I feel I need to clarify my questions a bit. They were intended to be not so much about the merits of slide valves vs. piston/spool valves, but more about why you would hook them up as outside admission vs. inside admission, especially with piston valves, where I don't see an obvious difference.

As I understand it (please correct me if I am wrong) outside admission piston valves (steam is admitted to the cylinder past the outside edges of the valve) can allow for a more direct exhaust path design, but they have the disadvantage of having the stem packing subjected to incoming steam pressure. With the much more common inside admission valves (steam admitted past the inside edges of the valve) the exhaust path is usually longer, but the packing is exposed only to exhaust steam.

PC

Slide valves are almost always outside admission, wioth the steam helping hold the valve against its seating surface. The steam chest is filed with steam from the drypipe, and the exhaust runs out through the bottom central cavity in the valve. This worked well for lower pressure wetter saturated steam with tallow based lubricants. Hotter superheated steam at higher pressures not so much - lubes broke down and the pressure of the steam holding the valve down on to the steam chest became a problem. The increasing size of locomotives had let to large valves which required balancing to deal with the downward pressure towards the valve face even in saturated application by then. Stephenson gear was made heavier and bulkier to deal with the load - but the room for it between the frames was growing tighter as frames grew heavier at the same time. Increased pressures also affected the steam chest seals to the cylinder saddles and covers, and the junk packings used to contain the steam around the valve stems.

Piston valve conversions made with bolt-on stuff commonly accompanied conversions to superheated steam and outside, usually Walschaert gear, conversions as part of a general modernizing of power. They were of course outside admission since the cylinder saddles they bolted on to were set up that way. The addition of mechanical force feed lubrication and higher temperature lubricants along with the other improvements made things much better. Metallic packings replaced junk about this time.

Inside admission was built that way from the cylinders out through the valves - one casting (or later fabrication) held both. Steam was held within the central spool area, and admitted from the inside into the cylinders, then the exhaust went out around the outer ends of the valves. this allowed the packings to only have to hold in the lower exhaust pressure only during the exhaust stroke on the back end of the cylinder. Valve ring wear was more easily dealt with by replacing worn rings - no need to try to resurface the valve port area and the valve and then lap them to each other. More area for larger cored steam passages helped locomotives breathe better.

You need to look at the evolution of valves as a part of the bigger picture evolution of steam technology in general - slide valves were produced for little industrial and tank engines right up through WWII but were very uncommon on anything larger after WWI. It is interesting to compare the B&O Museum's 0-4-0T with New Hope Valley's, for example, although they were both built right up towards the end of steam.

Short answer - size plus temperature plus pressure all getting bigger created the need for a lot of technological changes of which valve design was one of an organic combination of several.

dave

Very informative - thank you all!

I've been fascinated with that period in time where piston valves were growing in popularity but the locomotives still had Stephenson valve gear as delivered. The ex Michigan Central atlantic inside the DT&M roundhouse is a good example as is C&NW 1385. Some visitors have mistaken the slant of the cylinders as Vauclain compound- now that's something that dates a locomotive!