Boiler stays and heat expansion

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Boiler stays and heat expansion

Post by rkcarguy » Mon Jan 15, 2018 7:42 pm

I've been looking at a lot of boiler designs and drawings just out of curiousity, and one thing keeps bothering me so I'd figure I'd ask why.
Most designs have stays, bolts or rods, welded from the inner to the outer firebox. My knowledge of metals and fabrication says because these parts will see different heat levels they will expand and contract differently and crack/break these stays. Sure enough, looking at many pictures of boilers/fireboxes the are coming apart for rebuilds...lots of broken stays. However, I'm not afraid to say that my knowledge of boilers sucks, beyond the basic concept.
Why have the stays? Why not have the inner firebox welded or even bolted with a slotted connection to the bottom of the outer firebox only, so it "floats" and has room to expand and contract?

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Re: Boiler stays and heat expansion

Post by rkcarguy » Mon Jan 15, 2018 7:47 pm

To clarify, this is for our modeling purposes. I get it, that full size boilers need it because the metal is thinner in terms of scale....for example, most of your boilers, scaled up to full size, would be ~2" thick, which I'm pretty sure they aren't.

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Re: Boiler stays and heat expansion

Post by Harold_V » Mon Jan 15, 2018 7:51 pm

I should leave this response to those who have expert knowledge, but the idea of having opposing flat surfaces not supported by stays makes no sense. Keep in mind, the pressure inside a boiler accumulates. It's measured as pounds per square inch--so if you have ten square inches and the boiler pressure is 100 psi, the pressure on the unsupported surface (ten inches) is 1,000 pounds. What must it be when the area of unsupported surfaces is huge? Stays are not a luxury---they are a necessity.

By using stays, the pressure is balanced, reducing the amount of movement that can be achieved by the flat surfaces.

Now to hear from those who know!

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Re: Boiler stays and heat expansion

Post by Kimball McGinley » Mon Jan 15, 2018 8:09 pm

Prototype boilers also feature flexible stays in addition to rigid stays. They account for the differential expansion, which may be transient; the problem is mostly in warm up and cool down, I think.
I suppose they could all be flexible, but rigid stays are far less complex and therefore considerably less expensive. Rigid stays are also easier to check. A small hole drilled down the center of a stay will spit water if the stay separates from tension.

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Re: Boiler stays and heat expansion

Post by pat1027 » Mon Jan 15, 2018 8:39 pm

The difference in expansion between the firebox and the wrapper is small. Most model boiler failures are due to corrosion.

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Re: Boiler stays and heat expansion

Post by Marty_Knox » Mon Jan 15, 2018 8:54 pm

100 - 110 years ago when the railroads started going with wide firebox boilers they began to have trouble with staybolts breaking. The older designs with keyhole fireboxes allowed the sheets to flex. The new wide firebox boilers had large flat areas, with the staybolts breaking in the corners and along the top where the expansion is greatest. This was the impetus to develop flexible staybolts, which held the sheets the correct distance apart but could move laterally without breaking. If you look at the side of a wide firebox boiler you can see the pattern of the flexible staybolts.

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Re: Boiler stays and heat expansion

Post by RET » Mon Jan 15, 2018 9:10 pm


I don't consider myself to be a "real" expert, but I can answer some of your questions. If you look at the Big Boy boiler we just tested, on the firebox, the stays are at 1" spacing in both directions. On the flat side of the firebox there are 5 stays vertically and 11 horizontally. Because each stay is in the center of its own 1" square, this gives an area of 6" x 12" which is supported by the stays. Since we pressure tested to 200 psi. this gives 72 sq. in. x 200psi. or 14,400 pounds (7.2 tons). And this is on a model! You can see that even in a model the forces get big in a hurry.

Full size steam locomotives like Big Boy ran at a working pressure somewhere between 225 and 275 psi with some even higher. Now going by our test pressure of 2 times working pressure, you are looking at test pressures of more than 500 psi. In practice, they tested at something less, but you get the idea. Also in full size, boiler plates were MORE than 2" thick.

Because of the conductivity of the steam/water combination, the differential expansion between the inner firebox and the outer boiler shell wasn't a lot unless the water level was allowed to get too low but you still had to bring the boiler up to operating temperature and pressure fairly slowly.

As for me, I wouldn't want to be anywhere in the neighbourhood if one of those things ever let go.

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Re: Boiler stays and heat expansion

Post by Crosshead4 » Mon Jan 15, 2018 9:35 pm

No full size boiler had 2” plates. The Big Boy’s firebox sheets were probably 1/2”-5/8” thick. The barrel may be as thick as 1.25”. They ran at 300psi and would have been hydro tested at 375psi. Our scale boilers are quite out of scale in these ways.

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Re: Boiler stays and heat expansion

Post by rkcarguy » Mon Jan 15, 2018 11:04 pm

"Since we pressure tested to 200 psi. this gives 72 sq. in. x 200psi. or 14,400 pounds (7.2 tons)."

Standard 6" schedule 40 pipe has a max working pressure of 2,070 PSI(and that is calculated at 70%), so there is plenty margin of safety there.
I guess what I'm talking about, is I've seen fireboxes where the box is pretty much inserted into the bottom of the boiler pipe, and there is no stays. The bottom of the firebox isn't surrounded by water, and a good portion of the boiler pipe continues clear back to the end of the boiler inside the cab.

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Re: Boiler stays and heat expansion

Post by steamingdon » Tue Jan 16, 2018 7:58 am

Sounds like a Briggs style boiler that you are talking about to me . All model boilers have stays to keep the flat sheets from distorting from the internal pressures. Some boilers, like copper need plenty of stays because the copper is very ductile. I have seen some boilers that have no water surrounding the crown or side sheets. The builders in OZ often use this method.

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Re: Boiler stays and heat expansion

Post by michaellynn2 » Tue Jan 16, 2018 9:46 am

Boiler stays are serious business. The only welding on a full scale boiler is only allowed on a stayed area or flue sheet. The stays carry any weakness in the side sheets or crown sheet. Harold was correct in his calculations of the forces on the firebox side sheets. These forces are spread out by the stays. As a side note, I have on my desk a piece of side sheet from the 611. It is 5/8 inch thick.
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Re: Boiler stays and heat expansion

Post by Pontiacguy1 » Tue Jan 16, 2018 10:54 am

A briggs type boiler doesn't need any stays on the side sheets of the firebox because there is no pressure there. A true locomotive-type boiler with water surrounding the entire firebox will need stays to keep the outside firebox side sheets and the inner firebox side sheets from being pushed apart by the pressure of the boiler. The stays are put in typically at right angles to these sheets and are under tensile load while there is pressure inside the boiler. These will keep the boiler from distortion and splitting/cracking while under pressure. You also need stays on your crown sheet to keep it in the proper location and keep it from sagging.

Why don't we have a problem with huge swings in length of our boilers from temperature differentials between inner and outer sheets? One reason is that the water in the boiler acts a lot like a buffer to keep the inside of the inner sheets and the inside of the outer sheets closer together in temperature.

Thermal expansion rate of mild steel is given as roughly 9.5x10^(-6) (in/in*degF), where the 9.5 is the coeff. of thermal expanison and that is then multiplied by the length of the member in inches, and the change in temperature in degrees F. So basically, changing mild steel by 350 degrees only changes it's length by about .0000095" per degree per inch of length. So a boiler that is 42" long is only going to change by about 1/8" or so with a 350 degree change on the shell temp. This is the reason you DO need some amount of expansion allowance built into your boiler mounting method. Expansion by a boiler of 1/8", if it is bolted down tight at each end, will definitely warp up your frame.

The firebox, being only about 12" length or less on most models, is only going to change about 0.039" total with a 350 degree change, and that would be spread across the entire length meaning that none of the stays is going to be deflected more than about 0.020" with a 350 degree difference in temp, assuming that's what would happen when running a wide open fire like pulling a grade or something. So, with that small amount of deflection, in our model boilers, fixed stays work just fine and can easily absorb that amount of movement.

Hope my explanation works. I haven't done those calculations in a long while, so might have made some mistake. Had to use a general coeff for mild steel, and I used the higher end for that one

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