• A couple of technical questions...

  • Discussion of steam locomotives from all manufacturers and railroads
Discussion of steam locomotives from all manufacturers and railroads

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  by Allen Hazen
Thinking abut some late steam locomotive designs, a couple of questions occurred to me:
(i) About the sides of the firebox. There is water between the inner firebox (the metal compartment containing the fire) and the outer shell of the boiler. With wide fireboxes (various late big steam locomotives had grates nine feet wide), there is a limit to HOW MUCH water space there can be along the sides of the firebox (I think in some of these same late designs, the lower firebox was at least tied for being the widest part of the locomotive). I remember reading (so: since I don't remember WHERE I read it, any references would be welcome!) that some of the Norfolk & Western's late steam locomotives (A? J? Y-6? all of the above?) had unusually thick water spaces here: this was praised as a design feature. I assumee the idea is that you getter water circulation around the firebox if the space the water circulates in isn't too narrow, and that this improves heat transfer from fire to water, and so boiler efficiency. Does anybody here know more abut this issue?
(ii) About boiler pressure. There are losses in the steam pipes. Does anybody know (a) a general estimate of how the boiler pressure compares to the pressure of steam at the inlet valves of the cylinders and/or (b) how different designs compare in this respect? (The importance of the issue was recognized: late steam designers payed careful attention to "internal streamlining," to prevent "wire-drawing" of the steam in its passage from boiler through engine.)

Any information, bibliographical or otherwise, would be appreciated. Even if a bit tangential to the questions as I have asked them.
  by Pneudyne
Hi Allen:

Re your item (ii), I think that this excerpt from Bruce (*) provides at least a broad-brush answer.

Bruce pp.390,391.png

On item (i), I don’t recall seeing anything substantive on the size of the water space around the firebox. We could use mudring width as a proxy for water space width, although the latter usually steadily increases above the mudring. The width of the mudring width seems to be a rarely quoted parameter in specification tables. It is sometimes shown in boiler diagrams, but not regularly so. A cursory look through the data on hand found the following numbers:

5 inches Pennsy 4-4-4-4
6 inches DM&IR 2-8-8-4, UP 4-8-8-4, NYC 4-8-4
7 inches N&W 4-8-4

Based upon a single example in a very small sample, one might say that the N&W opted for a slightly wider mudring than was common practice.

Steam locomotive boiler parameters seem to be variable over quite wide bands, no doubt with much interdependence, within which satisfactory performance can be obtained. Really, they have to be, as the boiler has to fit the chassis on to which it is mounted, whose dimensions in part are determined by the chosen wheel arrangements and driving wheel diameters. One might say of boilers that in unconstrained applications, “short and thick” is better than “long and thin”, but locomotive boilers have often, of necessity, veered towards the “long and thin” end of the spectrum.

In respect of mudring width, I suspect that there is a point where the gains from further increases become incremental, and do not offset the debits, such as the need for longer staybolts. I am not surprised that the N&W used what appeared to be a wider-than-customary mudring width. In some ways it [the N&W] operated towards the boundaries of parameter bands, and (heresy alert!) whilst that was fine in situ , I don’t think that its practices would always have been a prototype for wider application. For example, I doubt that the very stiff lateral characteristics of its 4-8-4, necessary to gain stable riding at very high speeds given its relatively small (for a fast passenger locomotive) drivers, would have been acceptable on roads where this type of locomotive had to be “jack of all trades”. Nor might the very high piston speeds have been liked by roads whose passenger locomotives habitually operated for extended periods at around 90 mile/h. The N&W also pushed the limits on factor of adhesion, and for example I expect that its 2-6-6-4 might have been rather slippery in for example typical Wyoming winter conditions.

(*) A.W. Bruce; The Steam Locomotive in America: Its Development in the Twentieth Century; Norton, 1952.

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  by Allen Hazen
Thank you! (Again!)
Thinking about it: will respond in due course.
The 7 inch mud ring from the Norfolk & Western is what I remember (from wherever I read it). It's remarkable, since these locomotives (I'm pretty sure) also had nine-foot-wide grates. (The J boiler was similar in size to the ATSF 3776 boiler, with a 108 sq ft, 9 foot by 12 foot, grate.) Add in the thickness of the firebox wall and of the outer boiler shell and the lower firebox is getting about as wide as the U.S. loading gauge permits!
What got me thinking about it was wondering about the Pennsylvania Railroad S-2, the experimental 6-8-6 steam turbine locomotive. It had a 120 foot grate area, but with the six-wheel trailing truck the grate was long rather than wide: 15 feet long by 8 feet wide (so: same grate width as, say, a New York Central Niagara). And it occurred to me that, IF there is an advantage to having a wide water space beside the firebox, the S-2's configuration (or the 4-8-6 configuration Lima proposed for their "Double Belpaire boiler" design) would have allowed full advantage to be taken of it.