Steel boiler

John, the steel tubes tend to rust through on miniature engines in about 10 years, this is due to them being exposed to the most heat, and yes most of us use boiler treatment. When I changed my tubes a few years ago they were pitted and wasted away to nothing in places, and I have seen several identical examples since then. However as far as could be seen through the manhole the much thicker boiler shell was in really good condition.

Chris Gunn

Boiler water treatment chemicals can be used, and are used in full size boilers. Frequent blowdowns under steam help keep rust and scale under control near the foundation ring. For full size boilers frequent water testing is often done to make sure pH, minerals and sediment are within limits. For full size steel boilers a slightly alkaline condition of the water is preferable. Many operators of full size traction engines here in Ontario store them when out of use empty of water , handholes/washouts open, and with a lamp or small heater in the firebox to keep them warm and to stop moist, condensation-prone air from being pulled into the water spaces.

For steel model boilers the use of distilled water with as little entrained air as possible helps. After use, one of two things can be done to minimize corrosion - either a) fill the boiler completely full of distilled water that has not been aerated/air entrained (completely full means absolutely NO air bubbles) or b) empty it completely, warming it to get it completely dry, then flow nitrogen or argon gas through it until cool, then plug all openings trapping an inert gas blanket inside, for storage.

Steel boilers are more heavily constructed than copper ones, to allow some metal loss due to corrosion over time. Always best though to try and minimize corrosion wherever possible by good maintenance. Most of my steel boiler shells for models are 1/4" or 5/16" thick wall steel pipe with rolled-in copper fire tubes. With the above precautions, I see almost zero corrosion inside and all boilers are hydrostatic tested yearly to twice working pressure before steaming.

I have noticed in full size boilers which have had water left in them with air above, closed or open, major pitting of the steel happens at the air/water line. If the boiler is fully wet or fully dry, there is far less corrosion. Oxygenated still water is the worst thing for any boiler, in my experience. JD

Not having a steel boiler but I believe a Tannin treatment helps,

Yes the commonly use Multitreat TS is a tannin base.

I suppose the odd old leaf and twig that got sucked up from the nearest ditch or pond by traction engines in the past had the same effect

The chemistry of this is "quite interesting".

Firstly, ordinary rusting is due to Oxygen and Carbon Dioxide dissolved in water. This doesn't have much effect when a boiler is operating because the two gases are taken away with the steam. But if the boiler is left to cool with water and air in it, then rusting will be aggressive. This is why it's important to either dry out an unused boiler or to completely fill it with boiled water and an additive.

Secondly, rust already in a boiler from the first cause provides a site at which an electro-chemical reaction will occur. This together with trace impurities will cause rapid corrosion and pitting.

Thirdly, water is a powerful solvent capable of directly attacking slag or other impurities in the steel. Additives reduce this effect.

Fourthly, the presence of Iron-Manganese Sulphide in boiler plate was once a serious problem as it promoted the formation of rust cones containing sulphates and sulphuric acid that were very liable to perforate the plate. Additives and quality control reduce this effect.

Fifthly, water is decomposed by Iron at about 350C to make Iron Oxide and Hydrogen. The necessary temperature is reached if a tube is coated with incrustation, which is prevented by only using pure water in a boiler.

Ordinary rusting due to air and water occurs on the outside of the boiler as does electro-chemical rusting around impurities. The later mechanism has a strong local perforating action. Consequently some steels are better for boiler making than others.

In a large steam plant like a power station very elaborate precautions are taken to use suitably treated water and materials. For example Copper Alloys are not used at all because Copper is soluble in high-pressure steam (2400psi at 500C). Silica, Chlorides, Oxides, Organics and Sulphates are all carefully removed from the water, not because they corrode the boiler but because they erode the turbine blades.

A carelessly operated boiler won't last long, but good management will extend its life considerably.

Dave

Hi John,

In my steel shell / copper tube boilers the shell is welded and the tubes rolled in. By rolled in I mean the copper is first annealed and then expanded with a special tool into the tubesheets. The tool is a steel mandrel with a stack of rubber washers mounted on the end with a screw and steel washer to tighten them. The screw is loosened, the tool fitted inside the tube end, and the screw tightened to expand the copper. The copper deforms/bulges outward on either side of the tubesheet to hold and seal the tubes. This process works best dry, no lube on the rubber, otherwise the tool tends to slip out.

I first heard of this method many years ago in reading articles by Bill Harris in Live Steam magazine, published in the USA. He used it in his Shay, steam roller, logging donkey engine, and Falk locomotive.

I should point out that this method may not be legal where you are, check with a local or club boiler inspector before building anything this way. I do not run my models in public, for insurance reasons. Never had any problem though. When I first used the boiler methods above I made a couple of test vessels to verify the joints and materials would hold, with tubes rolled in exactly as described. They both held 600 psi water pressure (hydrostatic test, vessels full of water) for six hours at a time, several times a few weeks apart. Never leaked a drop. I feel fine operating my boilers at 80 or 90 psi after these tests.

In full size boilers steel tubes are welded in or rolled in much the same way as above but with an expander tool with wedges and balls to deform the tubes. The tool is usually driven by a heavy duty pneumatic drill. After rolling the tube ends are swaged over to a bead shape with a pneumatic hammer to help make a seal and to protect the tube to sheet joint from fire gases. JD

Edited By Jeff Dayman on 15/09/2016 12:55:20

Search boiler tube expanders, several examples have appeared in ME over eh years. I don't know if tehy are allowed by current boiler codes, they would best suit larger steel boilers.

Tubes are considered consumable items for full-size boilers.

Neil

In full sized boilers, the feedwater is usually either distilled or more often filtered, de-mineralized and de-aerated before even getting to the boiler, where the abovementioned chemicals are added to combat corrosion, scale, sediment and even corrosion in the steam lines and condensate return lines after the water leaves teh boiler. Yes, there is a whole science to it. It's a major part of operating a boiler whether a tiddler in the local laundry or a 10-storey high power-station boiler.

In industry we used to "press 'em up" full of treated water until it came out a vent valve at the very top of the boiler then close the valve before storing them out of use for even a few days over the weekend etc. Longer term, say taking out of service for months, they were drained, dried and ventilated, which is probably the best way for model sized boilers.

On start up, the water was drained (or filled) to quarter of a glass and the vent valve on top of the boiler left open while heating the boiler until all air had been expelled and steam was coming out. This gets that corrosion-friendly oxygen in the space above the waterline out of the boiler. Probably a good practice on models too. But I am sure your local club can tell you more about that.

John, there's no madness, only physics. Don't under estimate how much force is applied to a tube when it is expanded. Some tube plate holes may be plain while others may have a little groove in them, half way across the tube plate. For a 2 1/2" tube the groove may be about 3/32" deep. If you gas-axe one of these tubes out you see how the tube expander has deformed the steel tube to fill the groove. Even when you apply a pressure test of a few hundred psi that acts on the outside of the tube they (should) stay tight.

Reg

This is the type of tube expander which we use, both on full size boilers with 2" dia tubes and on my model with 1" diameter tubes. The expander consists of a number of conical rolls which are aligned inside a guide cage. The slots in the guide cage are angled slightly from the axis of tool (shown in the photo below). A tapered pin is inserted into a gap between the rolls and is rotated. The angular position of the slots in the cage causes the tapered pin to be drawn into the tool as it is being turned, causing the rolls to move outwards and expand tube. Normally by rotating the pin in a clockwise direction it is drawn in and by reversing the direction of rotation it is withdrawn. A massive force is exerted by the wedging action of the tapered pin. On full size boilers with 2" dia tubes, they can withstand 200 psi working pressure and 400 psi when being tested.

Below: Expanding the tubes on my model engine

Stationary boilers with condensers so that the condensate was pumped back into the boiler may well have used distilled water, locomotives used all sorts of rubbish. Some big depots had treatment plants which reduced hardness and any acidity, out in the sticks you got what was available. Some more enlightened railway companies used chemical bricks which were disensed into the tender. I've read somewhere that this was actually better than the big water traetment plants as it did all the water no matter where you got it from. Preserved steam on the mainline probably gets whatever comes out of the fire hydrant.

Edited By duncan webster on 21/09/2016 14:08:40

Hi John Lintorn,

Pics of full size expander and my model expander and a test piece done with it are below.

Not necessarily a good thing though!

Cheers,

Dave

Boiler water treatment can be a complex subject. One aim is to maintain a non corrosive ph level in the boiler. Another is to precipitate out any dissolved salts, which are then expelled from the boiler when you blow it down. If you don't blow down properly the mud can build up and cause the plates to overheat. The correct ph is alkaline. Because of this, you really don't want oil getting into the feed water...if it does it will produce a soapy substance when it reacts with the alkaline, and will cause the boiler to prime. If you look after a steel boiler properly it can last a lifetime.

The tubes for my watertube steam launch boiler are steel, and are expanded into the steam and mud drums. The tubes are about 5/8" diameter with about a 3/32" wall thickness. A tube expander of the same style as the others show above expands them with ease.

John

I didn't say it was, poor water quality undoubtedly resuts in shortened boiler life, probably due to overheating (scale is a good insulator, and if present in sufficient quantities will impede water circulation). This applies as much to models as full size. If you live in a hard water area you should descale your boiler, how often depends on how much it is used and how hard the water is. Clubs in such areas could batch treat water by filling 50 gallon drums and adding the right chemicals, but you'd need someone like SillyOldDuffer to advise what and how much. When our club tried distilled water (from local power station) we got lots of priming (before my time, so no detail)