Gents,

A group I've been doing some engineering drawing work for has asked me to investigate making some modifications to a locomotive they're considering restoring, a mid-size mikado.

Basically, the locomotive has your standard backhead steam dome throttle on it, but at some point in it's languishing the throttle assembly has disappeared (along with some other, not-as-crucial, parts.) They're interested, since a new throttle assembly will need fabricated anyway, in installing a "grapevine" type throttle linked to the steam dome with a linkage that runs above, and outside of, the boiler. The reasoning behind this is that they're concerned that a packing failure at the point where the throttle rod enters the backhead would result in high-pressure steam being released back into the cab. (Yes, I know there's plenty of backhead-entry locomotives running with no problem. I presume the reasoning is "if we've got to make a new one anyway...")

I know the NKP performed this type of mod on some of their locomotives, particuraly their mikes. I'm working on getting out to see #539 in Bloomington, IL to have a look at how they set their's up. My question is whether anyone's attempted this in the past, and does this sound like it's even worth the time and expense of doing. In my not-all-that-experienced mind, a new throttle assembly would need made, modifying or replacing the valve and linkage in the dome to recieve the new assembly, and patch the old hole in the backhead. That doesn't seem like major surgery in the grand scheme of things, but again, I'm no expert.

Historical accuracy and correctness put aside for a second, whaddya think guys, should I tell these folks no way, or can/should this be done? One note, obviously if this modification were to proceed it would be done with a locomotive boiler contractor taking the lead, not a novice such as myself. So don't worry about that one!

Please feel free to hit me back channel if you'd like more info.

Best,

TB

Careful, Tim. Alterations are not to be entered into lightly.

That said, I've never known of any failure of a throttle rod packing that was more than annoying since anybody with half a brain would replace it before it got worse. Worst I had was when an engineer used an electric drill to remove hard old packing, making a nice groove down the rod in the process. We had a sprinkler in the cab when the throttle was open. It ran that way until the next washout two weeks later, when we could braze up and polish the rod.

I'm thinking you don't need to do this - at least not for the reason you reference here.

Now, if you want to add superheaters and a front end throttle - then we have something worth a good discussion.

dave

Tim-

I am not at all familiar with the modification you are speaking of, but I would be interested in knowing more about it.

As far as the packing failing, I believe they might experience a water glass failure first. You can't relocate those outside the cab. My first inclination is that they are looking at a $50 solution to a $10 problem.

I'm with Dave on this one. I don't think the alterations are worth the hassle. Doesn't hurt to ask though.

Ryan

I once worked on two 190lb engines equipped with flange mount forged steel full port opening ball valve throttles, they tended to last 10 years or so seeing some 120+ service days per year. No leaks at all, and once worn out it only needs to be sent off and rebuilt. Off the shelf and only a fraction of the cost of a new "traditional" throttle body. The amount of control one gets out of these is absolutely incredible!
There is nothing wrong with new technology, especially when it has been proven in service.

I appreciate the feedback guys. I was sort of under the same thoughts, why mess with something if it isn't exactly broken, but I'm glad to hear it from others with the experience record. I will pass this input on to the power that be...

Dave - the locomotive is superheated, but it uses a somewhat funky arrangement, Schmidt I think? I know I'd never seen them until this one. I'd love to see what would be required to install a front end throttle, and what the pros/cons of such a mod would be, but I'd wager that would be getting rather expensive.

Jason - I'd be very interested in hearing more about this ball valve throttle engine. My "day job" is designing custom ball valves for manufacturing processes. Was this in a dome or front-end application?

Again, thanks.

TB

Jason and I also worked on a locomotive with a butterfly valve in that application - his balls work better and more reliably (that didn't come out right - you know what I mean).

Front End Throttle advantages are superheater elements are always "wet" with steam from the boiler, and aren't part of the steam circuit volume between throttle and valves, and superheated steam is available for the appliances. This means applicances run more efficiently, and elements last longer without fear of burning out. For a locomotive seeing only occasional service, neither is an issue, so again probably not worthwhile.

Our friend Jack Siffert not so far from Pittsburgh near Cleveland can show you a front end throttle if you want to see one, partially torn down.

dave

Dave,

I have indeed seen front-end throttles torn down before, and as you said such a modification wouldn't be practical or particuraly needed anyway.

I've heard of small live steam locomotives using ball and butterfly valves for throttles, but never on something full size. Most interesting.

Thanks again for your input folks.

TB

This was an in dome application. While I cannot remember the manufacturer or what type of seat material it had I can tell you that they were of forged stainless construction, 6 0r 8 bolt flange on abpt a 3" pipe. Doubt the stainless was required but is what they ended up with.

I have often wondered about the feasability of these different modifications as well. If one has all the original parts and nothing is awry of course you woudn't need to concern oneself. But while on the subject....does Juan have to be concerned about using say stainless bolts or other alloy bolts when using cast iron or cast steel in conjunction to this retrofit? I am wondering about expansion and contraction once again, interferring with these critical parts. Being that warm up is gradual it probably isn't an issue and this subject was asked about once before in regards to fabricated steam delivery pipes staying tight to joints of cyl and manifold coming off dry pipe. Having difficulty with origanal throttle valves leaking and or being lost or fubar'd does make this interesting to me too read about.

Jason, I hear your out and about, just make sure you come home and bring your swim suit when your done. Put the Copper Range combine back on her trucks yesterday. Now if she doesn't float away. Cheers, John.

Stainless - model live steamers have had very poor luck with stainless steel boilers, but I don't know if it is a specific grade of stainless, or the welding process doesn't like steam, or what - perhaps a metallurgist can tell us more. Based on that and my general cheapness and conservatism, I have never used stainless fittings in a boiler myself, but that doesn't mean it can't be done properly. Stainless ball valves built and certified for steam service would be an obvious start.

Not sure why fabricated piping would not hold in place as well as cast sections - since superheated elements are fabricated anyhow. I'd steer clear of other than ground-in standard joints in their connection, however, based on experience with some other stuff I have dealt with.

Perhaps somebody esle has had different experiences to report.

dave

Speaking of leaking throttle valves, the 1702 has a Chambers throttle. There is no lower seat, but there is a pilot valve in the valve slide. It was an amazinly tight throttle when closed. Cylinders were always stone cold in the morning.

Ryan

I'm no metallurgist either, but the company I work for makes many stainless steel ball valves for steam service, particuraly in the pharmaceutical industry. The main killer for ball valves and steam, particuraly superheated, is that the standard teflon seats start to get soft, and then turning the ball causes "teardropping," or pulling the softened teflon into the flowpath. Metal or Polyetheretherketone (PEEK) seats usually solve this problem.

We actually produce a throttling control valve, which is just a normal ball valve with a "v-ball" for better control.

(see: http://www.pbmvalve.com/productdetail.php?productid=121).

I'd like to try one of these in a live steam that already is built with a ball valve throttle, and see how the control given compares to a standard bore ball.

Dave - what's been the main issue the model live steam guys have been having with stainless steel?

I think it was stress corrosion cracking, but it has been several years back that I heard about it. Something in a welded stainless boiler didn't do well in service resulting in its being replaced with a standard carbon steel welded boiler. Again, I'm not a metallurgist and only recall the sketchy details but I think it had to do with a chemical reaction to the steam that made the stainless under heat and pressure that made it either brittler or more prone to cracking. Given I'm not a modeler and never expected to run across a full size stainless boiler shell, i didn't pay a lot of attention other than to generally decide to avoid stainless on general principle, certified valves and such obviously excepted.

I wonder if some bimetallic corrosive action could result from stainless fittings being screwed into a carbon steel boiler shell?

dave

Can someone find me the thermal coefficients for heat transfer for the type of stainless and low carbon steel in question?

Thanks
Robby

As quoted from http://www.sandmeyersteel.com
Found on Google where everyone researches info.

When properly annealed, Alloys 316 and 317L are primarily austenitic. Small quantities of ferrite may or may not be present. When slowly cooled or held in the temperature range 800 to 1500°F (427 to 816°C), carbides are precipitated and the structure consists of austenite plus carbides.

Melting Range: 2450 to 2630°F (1390 to 1440°C)

Density: 0.29 lb/in3 (8.027 g/cm3)

Modulus of Elasticity in Tension: 29 x 106 psi (200 Gpa)

Modulus of Shear: 11.9 x 106 psi (82 Gpa)

Coefficient of Linear Thermal Expansion

Temperature Range
Coefficients
°F °C in/in/°F cm/cm/°C
68 - 212 20 - 100 9.2 x 10-6 16.5 x 10-6
68 - 932 20 - 500 10.1 x 10-6 18.2 x 10-6
68 - 1832 20 - 1000 10.8 x 10-6 19.5 x 10-6

Thermal Conductivity

Temperature Range
Btu•in/hr•ft2•°F W/m•K
°F °C
68 - 212 20 - 100 100.8 14.6

The overall heat transfer coefficient of metals is determined by factors in addition to thermal conductivity of the metal. The ability of the 18-8 stainless grades to maintain clean surfaces often allows better heat transfer than other metals having higher thermal conductivity.

This is just the info for one type. There is a lot more if you have the interest. Feel free to look for yourselves.