Cleaning a boiler - limescale deposits?

Apologies for the extremely long post which has got bigger as I think of other things to ask. Also, I'll try and take some photos to add here next time I get a chance.

A friend was recently given a very nice Stuart 10V engine mounted on a baseboard, complete with a gas-fired boiler, displacement oiler and gear-driven feed-pump. Almost sounds like I know what I'm talking about, doesn't it? He apparently thinks I do as I've been asked to get it working...

It has apparently been stored in an attic under a cloth for at least twenty years, but the donor did recall it running under steam, although it was not made by him. Not sure where he got it from.

After some cleaning and oiling, I got it running on compressed air fed directly into the intake (boiler disconnected), which it does nicely, down to very low revs. We then hooked up some tubing to the feed pump and established that that worked, and that it would also, with the correct adjustment of valves, fill the boiler. A few leaks needed fixing by gently tightening unions, but generally, all good.

To cut a long story (slightly) shorter, there are a lot of white deposits in the lower part of the boiler. What would be the best way to clean these out? I've set him off filling it with white vinegar and leaving it to stand, but I suspect this may be a long process. Is it likely to damage the boiler?

More info on the boiler - horizontal, no lagging, with four integral legs. It looks very well made, possibly professionally made but with no apparent markings. It's not brass (I know that's a no-no for boilers from what I've picked up here) but does not appear to be what I would immediately recognise as copper although it may be. It's more of a grey-brown colour. Gun-metal or bronze maybe? It's what I would imagine bronze to look like, but I've not heard of bronze boilers - however, I have little experience with steam engines of any sort other than very small Mamod ones.

Oddly, the end plate where the sight glass, pressure gauge and pipe from the feed pump are mounted looks to be steel, but no evidence of rust, nor can I see how it's been fixed to the rest of the boiler. It may just be a cover plate, but not convinced as the fittings screw directly into it as far as I can tell.

Filling (other than by the pump) is via a screw-in safety valve at the top - this leaks under the slightest pressure at the moment. It's a bit like a Mamod, but much sturdier - searching shows similar valves being sold by Polly Engineering and others, it may end up just being replaced if I can't get it apart to clean it, or if ultrasonic cleaning doesn't help.

The only way of draining it down is via a valve at the lower end of the sight glass, which is how we found the white deposits - nothing was coming up the sight glass, only down from the top, so I removed the valve stem and poked through with a bit of wire. I would have thought a proper drain plug fittred low down to be essential. Virtually impossible to see inside the boiler due to this omission.

The burner looks a bit like a blowlamp, and is inserted into a large-bore tube which is about a third of the way up the boiler, two fire-tubes (if that's the correct term) then run back and exhaust out of the chimney. There appears to be a rudimentary superheater formed by running the steam pipe from the top of the boiler (firedome?) into the burner tube and back out again. Pressure gauge reads 0 to 100 psi, but I've not seen it move yet due to the safety valve leaking under rudimentary hydraulic testing using the feed pump to pressurise the (full) boiler.

I'm not even going to start looking at the firing of it until we are satisfied everything else is working as it should.

Supplementary question for those who have stuck with it so far... once we do get it sorted and in a position to do a proper hydraulic test, what would be a suitable maximum working or running pressure for a Stuart 10V? I'm guessing 30 to 50 psi?

Thanks for any advice, either answering my specific questions re cleaning or just generally about running small stationary engines.

Rob

Edited By Grindstone Cowboy on 12/07/2023 16:09:45

White vinegar should work (it will work faster if you heat it slightly) , but you can buy kettle descale, which is designed for the job. Don't be tempted to use sulphuric acid, calcium sulphate isn't soluble in water

Thanks Duncan, wasn't sure about the kettle descaler's effect on what may, or may not, be copper. Although I guess they have copper kettles. Probably plated these days, though.

Rob

You can buy a stronger White Vinegar from hardware stores now that they sell for home cleaning. I use that and its works very well on removing scale.

Near 100% ethanoic acid is often sold as glacial acetic acid. Other concentrations are available. PPE required for these concentrations and preferably some chemistry knowledge helps a lot. Glacial is used by soap makers and 80% by bee-keepers, for instance.

Kilrock-K(?) for descaling kettles is Formic acid. Citric acid will also work.

Some deposits can be very difficult to remove. I bought a water still that had really thick deposits and I needed nitric acid to effectively clean that - it was stainless steel - but nitric (even if one can easily get it these days) will attack copper very quickly, so not an option.