In four books about British locomotive mechanical engineering, I've seen brief information on a type of piston valve called the semi-plug valve, described as an American invention.

G.J. Churchward, while chief mechanical engineer of the Great Western Railway, read about it in the technical press in 1906, bought a license for the Great Western to make them in its Swindon Works and employed them on many GWR locomotives. It appears they were a success.

I'll endeavor to include a cross sectional drawing of the valve, scanned from Page 117 of H.A.V Bulleid, Master Builders of Steam, London, Ian Allen, 1963.

[img]http://i1059.photobucket.com/albums/t437/StevenSmith3/Semi-Plug%20Valve/543a3879-2e92-4dfb-9d72-de34139ad352_zpsfd47e713.jpg
[/img]

There are also two photos showing some components of what is labeled as a semi-plug piston valve to be seen at:
http://www.martynbane.co.uk/6024/Overha ... iveMar.htm They are the 7th and 8th photos from the top in a group taken on March 22/23, 2003.

Bulleid wrote that "the semi-plug piston valve rings are forced into and locked in their operating steam-tight position when steam enters the steam chest. With the regulator closed, however, the rings are no longer in contact with the walls of the steam chest so that when drifting there is practically no wear and, incidentally a very free-running engine."

Harold Holcroft, in his book An Outline of Great Western Locomotive Practice 1837-1947 included a similar brief description but added "The one small disadvantage over other types of piston valve was the rather heavier weight, and larger bush surface was necessary to carry it." (Although Holcroft didn't mention increased inertia force resulting from the added weight, that would seem to be another disadvantage.)

Judging from these descriptions, I suppose the ring that contacted the steam chest wall when there was steam pressure in the chest had to undergo a slight increase in diameter over what it would be in the drifting condition. Hence the circumference of the ring would increase slightly. Simply having a gap in the ring that would open up slightly upon diameter and circumference increase wouldn't seem to work, since a gap would let steam flow from the steam side to the exhaust side. So it would seem that the ring had to have some other means to provide circumference increase—maybe some interleaved sliding segments? It tweaks my curiosity.

Since the valve was an American invention, I wonder whether anyone who posts to this forum might have had experience with such a valve on a preservation project, and could explain how ring diameter expansion was accomplished.

The inventor would surely patent such a valve, but my attempts at patent searching to date have produced great frustration and zilch for results, so I close with the question: Does anyone happen to know the patent number?

Well I see that my image inserting skills rank right down there with my patent searching skills! I had it in Photobucket and thought it would appear here.

Here is the image J-1e5344 meant to post:




(You need to use the Photobucket "get link code" button, then copy the code they provide.)

ALL piston rings seal against the cylinder wall based on the steam pressure in the cylinder working in the groove. Few are specifically designed to use it as a design feature, however......I think the best feature of this design is the OD of the ring being in the form of many narrower rather than two wide surfaces which should provide much less blowby. I'd be interested in knowing how cylinder wear in this style of ring compares with the more common design, and how maintenance intensive the rings themselves are compared to the usual.

dave

J-1e5344, when will you share your amazing steam era stories
with us??? I especially love the one where you got to fire a NYC Hudson while wearing
your best suit... and some Rutland stories would be nice as well :)

Stephen

Thank you, FLO, for helping me out.

Well Stephen, I don't think Interchange has a department called "A Few Stories" such as on the WW&F Railway Museum forum. But I'll risk going Off Topic with a little about that incident, which happened in 1948.

To be honest--I didn't really fire the engine. The fireman had long since gotten the stoker jets tweaked just right, so I just followed his instructions to regulate the main stoker steam valve to keep a light grey haze at the stack. Maintaining water level was simple, as the engine had a Coffin feedwater heater with centrifugal feedwater pump, and the steam valve for that within easy reach. I knew from many cab rides on commuter engines to get the blower on as soon as the engineer called he was going to shut off.

Re. the suit: What was a feller supposed to do if wearing a suit when he suddenly got the chance of a lifetime to ride his very favorite class of engine?

Dave, I wish I knew how to get the information you mention about those valves, which would indeed be interesting. Apparently the semi-plug type was widely used on the Great Western Railway, and quite a few GWR engines are run by various preservation groups in the UK, but I don't have any contacts with them. Perhaps the Library at the National Rail Museum in York has information. It's a great storehouse of information, with a friendly staff.

Another question occurs: In the early 1900s, when the semi-plug valve was apparently invented, wasn't the usual approach to preventing drifting vacuum simply to leave the throttle slightly open during drifting? That would seem to make it a challenge to proportion the components of the semi-plug valve just right, so that the outer ring would contact the chamber or liner walls only when the throttle was well opened. And it might have been especially tricky to achieve consistent operation, as superheating was rather new and causing problems with lubricating oil.

ALL piston rings seal against the cylinder wall based on the steam pressure in the cylinder working in the groove.

Dave, please clarify this. Do you mean steam going into the groove and under the ring pushing it outward towards the cylinder wall? I'm not sure I'm understanding why it would do this. Sideways force from an area of high pressure to one of lower pressure but I'm not getting the idea of outward force. The steam pressure on the outside edge pushing inward would be the same, no?

Steve

Yes, the steam actually does sneak down the side of the ring on the high pressure side, and forces the ring outward against the cylinder wall. The moderate amount of springiness of the ring is simply to let the ring float and leave a gap for the steam to work in.

It looks as if this design has a beveled inner ring that steam forces against a beveled center ring to force the outer groovy sealing ring towards the cylinder wall. There's a diagional split across the outer ring to allow for expansion....so to my mind that's an invitation to blowby through that gap.

dave

I guess I'm just not understanding why the pressure would be greater under the ring than on the outer surface of the ring.

Steve

> I guess I'm just not understanding why the pressure would be greater
> under the ring than on the outer surface of the ring.

Think about Bernoulli's principle. If the ring is not completely in contact with the cylinder surface, and steam is blowing across the gap, the speed goes up and the pressure goes down. That will give you enough pressure differential to get the 'action' to work.

Once the ring is effectively 'sealed' against the cylinder, it should be fairly obvious where the pressure differential to keep the ring sealed will come from. If there is a water or hydrodynamic film present between ring and cylinder, only a very small amount of area is present to drive hydrostatic pressure, and again the pressure 'behind' the ring will expand it.

I'm tempted to add 'you can argue all you want, but it works that way, so you might want to amend your assumptions to fit the observed reality'... reminds me a little, again, of what Angus Sinclair said about 'fooling the steam' with respect to patent-valve drawings.

No one is arguing. That's very interesting as I would not have considered the velocity of the escaping steam. Thanks.

Steve

Looking through photos taken at the STEAM museum in Swindon in 2011, I came across two forgotten pictures I took of a semi-plug piston valve on Great Western Railway 2-8-0 No. 4248 without realizing that it was anything special.

The valve diameter looks to be about 8 – 9 inches, and from that I estimate that the diagonal gap is three eighths to half an inch wide.





The legend for No. 4248:


The museum also uses No. 4248 to tell visitors about the role women played in the Swindon works in WWII while the men were in the fighting forces. The legend suggests that the woman in the light-colored jacket is heating a rivet, but her partner in the smokebox certainly doesn't appear prepared to drive it. (Details, details.)



STEAM's featured artifact is GWR No. 4073, Caerphilly Castle, delivered in August, 1923, the first of 171 Castle class engines. It's over a well lighted pit open to visitors.




This shot of the driving gear and Walschaerts valve gear of the 2-cyl. inside engine (which drives the leading drive axle) was taken from a spot under the middle of the 4-wheel truck. Makes it easy to see why Brian Reed in his chapter on the GWR 4-cyl 4-6-0s refers to maintenance on the inside engine as being "irritating" as well as very time consuming.