Small Steel Boilers

Hi,

Once again a content warning! My brain is slowly but surely leaking away all the information it used to have, and as all this is based on stuff I did 20 years ago it cannot be considered reliable!

Next a couple of corrections, senior moments in my earlier post. I got my chromium and molybdenum the wrong way round, the correct figures for ASTM A335 Grade P22 is 2.25% chromium and 1 % molybdenum.

Finally, for some fairly gentle information on magnetite films in boilers, this is a good link.

I am sure there have been reports on this forum of brass boiler fittings failing due to de-zincification.

Neil

I think some folks are missing an important point regarding the differences between steel and copper for boilers. The need to make steel boiler shells thicker is not so much to do with the ultimate tensile strength but more to do with potential pitting and corrosion. Normally water treatment plants are required to ensure the water is of an acceptable standard and often chemicals are added to adjust the local water supply to that which will give the lowest corrosion.

The other point is the tensile strength of steel versus copper. Although for a given thickness of boiler shell wall thickness steel is better than copper the elastic limits are quite different. Steel has a lower ability to "swell" under pressure, which is good as it can withstand higher pressure than copper of the same wall thickness. But copper has a greater elastic limit and can balloon safely under high temperatures and to some extent return to its original form after an over pressure event. Steel on the other hand resists swelling better but once it reaches it ultimate limit tends to rupture.

Now there is also a huge difference between mild steel tube bought for structural and other uses and genuine steam pipe and boiler shell duty. Most steel pipes are rolled and seam welded, which is a potential weak point in the circumference when internal pressure is applied. Genuine boiler tubes and casings are seamless drawn and hence much more expensive. There are nowadays not many steel manufacturers who can supply genuine steam pipes and boiler shell material. I am only aware of one in Europe, in Germany, who can supply genuine steam boiler tubes etc. There may be more but I haven't come across them. I know for a fact that the locos down in the depot locally import their boiler tubes from Germany and they are ten times the cost of locally made welded seam tube.

Welding of pressure vessels is a highly skilled job and in this country (South Africa) may only be undertaken by government certified coded welders, who are few and far between with the necessary ticket. I have two friends who are coded welders, one has the top certification possible the other is one step below so he cannot tackle all the jobs and has to pass on them.

Steel when subjected to repetitive heating and cooling work hardens and stress fractures can appear, it needs to be a very special grade to withstand this arduous duty. Vickers Steel in the old days was the only British steel maker who could make the required plate and tubes. Riveted boiler construction obviates the need for welding but in some cases it cannot be avoided when repairs have to be made.

Lastly, as others have mentioned, copper will outlast steel in boiler applications by a large amount, so if you want to make it and forget it then copper is the way to go. Annual boiler tests will have in most cases to be done but a well constructed copper boiler should have no difficulty in lasting 30 years or more as long if it has been handled correctly and not allowed to run low on water.

My 2c worth!