Nice job Pat your right on the money with the .015.

If some one wants to play with the math they can check me on this. I took an expansion coefficient of 9.4 for copper and 7.3 for steel, at a temperature of 500 F
With a boiler of 14 inch’s tall, the total difference between the steel outer pipe and the copper tube will be .0268 Saturated steam or water temp at 600 PSI is around 486F
The outer shell is insulated and the tube’s may see a bit higher temp in the tube but the inside water temp should be more of the constant.
The bottom sheet will go down the same as the top sheet will go up so split the difference and you only have .0134 inch’s on each end. About three sheets of copy paper.
Not much movement.

If you do more maths Rolly you will also find that the tubes expand more tightly on their diameter into the tube plate holes as the boiler gets hot - which is why a cold hydraulic test can be misleading. Tubes which seep a bit when cold will tighten up when its hot.

Mike

So what is the effect of fitting 4 equally spaced stay bars to a 24" boiler ?

The added support to the bottom tube plate must be advantageuos but does the extra support cause a problem with the longitunidnal expansion of the copper tubes, in comparison to the now stiffer tube plates (taking into account Mike point of curcumfrencial expansion of the copper) ? Fretting of the tube over time ?

And if it makes a better boiler ? Why did Stanleys not adopt this approach ? Is it wrong or were they too set in their ways ?

Thoughts please ?

Ian V

Ian, As we have covered here, the 750 copper fire tubes act as stay bolts, all holding both the tube sheets together. There isn't a need for stay bolts in a Stanley fire tube boiler. All of the copper fire tubes work equally together at the same expansion rate. All of the copper fire tubes are swaged in their holes giving each one an equal amount of stress (tension) to keep them steam tight. Using four steel stay bolts in the mix to hold the tube sheets equally spaced, prevents the fire tubes from working in unison with the tube sheets. All because the stay bolts have a different expansion rate. Another factor too that I have experienced is what Jeff has discovered, and it was just mentioned by Mike. The copper tubes get tighter in the tube sheet with the higher heat. When they get hotter, the stay bolts do not get tighter at the same rate as copper. Therefore you have an unbalanced stress in the tube sheet that upsets the tight balance that an all copper fire tube boiler need to have. All of the copper fire tubes work together to make a steam tight tube sheet. If you have one loose tube, the other tubes will not prevent it from leaking. Instead the one loose tube will make four other adjacent tubes leak. I have tried to tighten just one fire tube by swaging it. It always ends up that I have to swage every tube across the whole tube sheet from one side to the other, just so that they all have the same tension. If four or any number of those tubes were steel stay bolts holding the tube sheets together, there would not be any chance at all of having everything in a tube sheet equally stressed (tensioned) because the steel stay bolts expansion rates are not equal to the copper fire tubes expansion rates.



Edited 1 times. Last edit at 05/17/07 08:55AM by SSsssteamer.