Steam Dome Throttle

[little giant]

Allen Models sells casting for dome throttle valve with the flat plate design. Little Engines sells a rotary type throttle.

[John_S]

Hmmm, that has possibilities. I'll look into similar ball valve installation into my steam dome.

Regarding the ball valve itself: How do you find the operating characteristics? Can you get a smooth launch without a jump forward or instant speed or easy wheel slip? How about low speed regulation? Can you accelerate gradually and easily modulate low speed?

Dave

Dave, I've never experienced any "jumping" from a start with the ball valves. I get a nice smooth startup without any slipping or lurching. Most of my running is with 1/2 throttle or less with the exception being up ruling grades pulling a heavy load. I have almost never run full throttle with the exception of when we've run "mega train" (50+ cars with multiple locos at the head and at least one at the rear) at the CSPPRY where I have run wide-open. My little mogul barks so loud and so nice under those conditions!

[dnevil]

More information on Throttle valves can be found here:

ibls.org/mediawiki/index.php?title=Throttle_valve

Regards,
Daris

[Fender]

Daris,
That link didn’t work.

[Short Line Tom]

That link didn’t work.

Here, try this one:
http://ibls.org/mediawiki/index.php?tit ... ttle_valve

Cheers!
Tom

[BillF]

I'm sorry to be so late to this thread, but my locomotive is located at a secondary residence.
I want to show Tom Artzberger's design for the steam dome throttle in the RGS 20,
The view is looking down into the steam dome, and the throttle is about halfway open. The white throttle disc is PTFE, machined with a slight recess that keeps it aligned over the throttle body as it slides fore and aft.
The bulk of the throttle body is a rectangular piece of steel welded to and supported by the dry pipe. This is Marty Knox's work and I'm sure he can provide further details. The visible throttle bearing surface is a separate piece about 1/2" thick, with a polished top surface and the slot-shaped steam passage milled into it. I'm sure that the design of this slot is where the fluid-dynamics knowledge has to be brought to bear.
The linkage consists of a piece of 1/8" thick bar stock bent into an L, with the lower end held by nuts on the threaded end of the throttle rod, and the upper end attached to the disc by a long screw threaded into the PTFE disc with a loose fit through the L bar.
The throttle rod is routed through a pipe near the base of the steam dome and into the cab. The packing gland is located in the cab.

I know there are concerns about the PTFE softening at high temperature, but note that it is well supported, and the net force can only be the boiler pressure times the slot area. The fact that the slot is relatively narrow means that the PTFE disc only has to support a bending moment over that short distance. Also note that this design provides a very linear throttle response as the open area varies linearly with rod position.

I had reported earlier that this throttle woks so smoothly that, on my first steam-up, the force of the boiler pressure on the throttle rod area was enough to cause the throttle to creep open, until I tightened up on the packing gland. On the whole, I think this is a very elegant design.

- Bill F

[jkimberln]

I agree that Artzberger's throttle works well. However, we had to put a ball valve to shut off the delivery pipe during boiler tests. Apparently the teflon block leaks badly when the boiler is completely filled with water as it would be during a boiler hydrotest.

[Fender]

About Artzberger’s throttle valve: a friend had to put a spring on top of the sliding teflon valve for positive downward pressure on the valve. Otherwise back pressure could pop it off the seat.
But I have to say, that the “feel” of this throttle is very prototypical!

[BillF]

Thanks for the tips about the problems with lightweight teflon. After considering the potential problems of working with very small parts like springs inside the steam dome (and losing said parts into the boiler) I decided to just try putting a weight onto the stud, above the L-bar. I cut off a 1/2" length of 3/4" bronze (the heaviest bar stock I had at hand) and threaded a hole through the center. The weight has about 1/16" clearance to the top plate, so this ought to at least limit the upward travel of the teflon block.

Unfortunately it will be some time next summer before I can steam up again and see how any of my winter modifications work.

[daves1459]

The reason I've been so concerned about low speed operation is I'm modeling the NYC 999 in 1.6" scale. The drivers end up being about 11 3/8" diameter. A scale 100 mph is an actual 13 mph and the drivers would rotate only 390 rpm. At an actual 5 mph, which is pretty fast for most live steam tracks, the drivers rotate at 150 rpm. At a more reasonable 3 mph the drivers rotate at only 90 rpm. Then there is the issue of acceleration from stand still. Being a 4-4-0 it will be "slippery" to begin with. A good set of cylinder cocks will be very handy during starting and early acceleration.

I have done a couple of installation studies to fit a ball valve into my steam dome. While the steam dome itself has enough room the location of the turret pipe make getting linkage to the side of the valve a real problem. I also looked into what is called "V-port" ball valves. These valve have a "V" shape in the valve ball instead of the usual round hole and offer very good low port opening flow characteristic if graphed have a concave curve. Unfortunately the smallest V-port valve I can find is 1/2" pipe.

Then there is Kozo's tapered plug throttle explained in this months Live Steam magazine. Leave it to Kozo to come up with something clever. He claims his tapered plug gives a concave flow curve at incremental openings. I'll have to study that as I don't see how he gets a curve flow with a straight sided constant tapered plug. The design does, however, offer lots of flow possibilities with a variable tapered plug. Also, being a plug it is has no side thrust due to steam pressure when opened and so is balanced. While any kind of flat slide valve, even with a Teflon wear disc, has to deal with the thrust and friction of steam pressure.

Regarding the flat slide valve throttle with Teflon disc as described above is the spring to keep the valve on the port a leaf spring? If so that is a common attachment to flat slide type throttles I've seen to keep the valve on the port during low or zero pressure conditions.

Dave

[Bill Shields]

I use a tapered plug 'pull' throttle in several of my locos and find them very easy to operate...

You can also consider having a small orifice I the throttle to keep response under control.

My 1-1/2 camelback has a 3/16" hole from the throttle to the 2" diameter cylinders and 9" drivers...the tip of the throttle is a 1/8" nose that clears the orifice after you have the throttle about 1/2 way open.

It is very easy to start and you REALLY HAVE TO TRY to get the wheels to slip....as in whack pull the throttle COMPLETELY OPEN and be on wet rail.

You could start with a 1/4" orifice and put a tapered plug with a 3/16" (or so) nose on the end.

The advantage to this type of setup is that you can easily pull the throttle plug out of the back of the boiler and change the dimensions without ripping anything apart to play with dimensions.

I can send pictures / drawings if you like, but it doesn't take a rocket scientist to figure it out.

Of course, I designed everything around this setup...and if you already have a boiler without the back head fitting(s) to allow this, you may not be able to travel this road.

[Fender]

There wasn’t a problem with the teflon slide lifting off with pressue in the boiler. But when moving the loco with a cold boiler, it could get out of position. I think he used a coil spring, but a leaf spring would be better.