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Boiler leaks - Any ideas about this one
Posted by: Ian Vinton (
Date: November 14, 2006 01:55PM

Last year I scorched my wire wound boiler that does not have stay bars pretty badly, how I did it is embarrassing so I am not telling you.
I brought a tube expander and spent several weekend under the car expanding the tubes using air and soapy water to initialy track and fix the leaks and then a full hydraulic test. I steamed the car on four occasions with no problems and thought I had solved the leaky boiler, passed my boiler inspection and everything was set to head off to a show.
The day of the show I fired up and drove it out of the workshop, everything was great. Turned the burner off, got out of the car to lock up the workshop and there was an whoosh noise, the car enveloped in steam and water started pooring out of the burner. Quickly drove it back into the workshop and there it has stayed until last weekend.
I went through the same fix routine as previously used, pumped it up with air and only a few minor leaks showed, fixed these and the hydraulicly tested the boiler. Not a single wet patch has shown at 500 psi.
My next thoughts are warm it all up and then pump it to the 600 psi and keep it there for some minutes.

Does anybody have any theories about why the boiler failed yet did not show any serious leaks when cold ?

Re: Boiler leaks - Any ideas about this one
Posted by: Jeff Theobald (Moderator)
Date: November 17, 2006 04:45PM

Hi Ian,
I too have a boiler that gives this sort of trouble, it has never been scorched, but has given trouble from new, at about six months old it suddenly dropped water through the fire plate, when the auto shut down the fire, after first firing from cold, then it would do a week or two without trouble. In the end the boiler was removed and returned to the builder who fitted ferrules top and bottom, the boiler was filled with water and leak compound and hydraulic tested to 1000psi, and was totally dry.

From that day on it was tight for about five years, being tested each year, and then out of the blue it did it again. I then took note of what started the leak, and noticed that it would occur when first steamed, if allowed to come up on the automatic, and then be left standing while getting passengers in and preparing to move off, it would also do it on a long downhill run when still pumping water but with the burner shut down on full pressure. As you say water would pour from the ventures, but I found that if you could get the fire back on, it would instantly seal, and not show the symptoms again for some time.

I spent a long time flushing and cleaning the boiler, I ran an ultrasonic over the shell and the results showed no wasting, again the boiler was filled with water and leak compound, pumped up too 1000psi and again all dry. Again the boiler was put back into service doing about 2000 miles a year for another 12 years, when again it did the same thing during the tour in Devon. Careful driving for the week got us through, on returning home, again a complete clean inside and out, the same treatment as before, and it has been tight since.

Why does it do it? Well I think oil gets back to the boiler getting between the tubes and boiler plate, if the boiler has not gained an even temperature or the fire plate is chilled, movement takes place between copper tubes and steel plate, promoting the leaking. This same boiler is now over 20 years old, still no sign of wasting, and still passing a 1000psi test.

Hope this helps, all the best, Jeff.

Re: Boiler leaks - Any ideas about this one
Posted by: Ian Vinton (
Date: November 18, 2006 04:39AM


Thanks its nice to know that others have had gremlins as well.
What leak compound did you use ?

Jeff so you use boiler treatment ?

Ian Vinton

Re: Boiler leaks - Any ideas about this one
Posted by: Mike Clark (
Date: November 18, 2006 05:33PM

I did a few sums on the effect of the relative expansion of copper tubes in a steel boiler after seeing steam suddenly in the smoke hood after running a strong pilot under a cold boiler for perhaps 40 minutes without turning on the main burner.

These sums suggested that although the copper tubes do expand more lengthwise than the steel boiler casing the expansion in diameter of the tubes is enough to compensate for this and makes a tighter seal the hotter the whole issue gets. The lengthwise expansion force on the tubes relative to the casing is however always there when the boiler is hot.

This explains why a boiler which will not hydraulic when cold may be perfectly tight when hot. However when you pump cold water in it drops to the bottom and shrinks the bottom end of the tubes thus showing up any weakness in the tube/bottom plate seal. The rest of the tube, being mostly hot still has a lengthwise expansion force on it which possibly causes the bottom of the tube to move a little relative to the bottom plate, hence the rather dramatic leak. In my case when testing the pilot I must have got the tubes over the pilot hot enough to expand lengthwise and loosen the top ferrule as the leak was only on the tubes over the pilot and at the top end. The boiler had, in the 40 minutes, actually got some steam pressure on from the exertions of the pilot alone - it was a bit strong!

I suppose after firing up as Jeff and Ian described there may be a period before the casing has got truly heated up when the bottom tube seal is vulnerable when the fire is turned off and the coldest water sinks down there. I would expect it to take a few minutes for the bottom plate to cool down enough after turning the burner off for such a leak to occur.

I re-expanded all my tube both ends after this experience and have seen no further signs of leakage.


Edited 2 times. Last edit at 11/18/06 05:43PM by Mike Clark.

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