vertical steam boilers

[david johnParticipant @davidjohn66754]

I have a vertical steam copper boiler which I constructed about 20yrs ago. It was built to standard drawings and is silver soldered throughout. It was tested to 180psi which was twice the working pressure. On overhauling it recently I noticed that there were two small bulges in the inner ring around where the firehole is situated. I have used the boiler on several occasions to power a Stuart 10 vertical steam engine without any problem. I havn't noticed any leaks at all and am satisfied that it's still intact. I am wondering if I inadvertently over pressurised it when using it and if so , have I weakened it so as to be unsafe?

[david johnParticipant @davidjohn66754]

[david johnParticipant @davidjohn66754]

The test pressure should have read 160psi not 180psi

[Jeff DaymanParticipant @jeffdayman43397]

If in doubt, fill completely with water and hydrostatically test to twice working pressure. If it holds steady for half hour or so at twice working pressure under hydrostatic test, it's likely OK.

If pressure won't hold, and there's no leaks visible, something structural may be moving, and therefore unit is less than safe or predictable.

Good luck, JD

[John OlsenParticipant @johnolsen79199]

Do you really need 90psi to run a Stuart 10V? I would have thought 40psi or so would be plenty. My own 10V will run when blown hard by mouth.

Copper is very forgiving material, you are not very likely to have corrosion problems even after this time. (But make sure it is not limed up inside.) The inner firebox is a tube under external pressure, so is not a stable configuration. So if it was slightly out of round or dented at the start, the pressure will tend to make it worse. This is why fireboxes are usually stayed, or sometimes in full size, corrugated. For a model the usual approach is to make the material a bit thicker, but there is no easy way to calculate how much thicker it needs to be, and it is a bit late for that anyway unless you want to commit major surgery.

I would suggest that if you can, see if you can ease the bulges back to round, and then retest, possibly for a lower working pressure unless you are desperate for as much power as possible. If you read some of the articles about loco boilers, you will see that they sometimes develop a pincushion effect on the flat stayed surfaces on test…however if these are pushed flat again and retested they usually stay flat, since the copper has been work hardened. The same may apply here…the area around the firedoor will have been well annealed when the boiler was built

John

[DMBParticipant @dmb]

John,
Not sure what you mean by pincushion effect but can guess. Many years ago I saw a large 5″G loco boiler made by an experienced loco building club member. The flat sides of the firebox had bulged so much that it looked more like an old style sofa with buttons rather than a pin cushion!
He said he had test gauge checked after the bulging and gauge found to be wrong and the estimated pressure was 750PSI!
John.
Said boiler used for quite a while but probably wont be steamed again as I believe it was put in a museum.

[DMBParticipant @dmb]

Woops, above is what happens when an extra paragraph is added and whole draft posting not proof read before final posting.

[John OlsenParticipant @johnolsen79199]

This is one of the nice things about copper….it will usually distort quite a lot before it lets go, letting you know something is amiss. Maybe we should check our gauges before the test rather than after.

Actually I am going through the throes of getting a larger boiler through the full size certification, for that I am expected to provide a calibrated gauge, eg the gauge has to be checked before each inspection. Gets expensive.

John

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