Steel boilers

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Aug 29, 2007
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The design/construction book for the steam roller I'm building calls for a welded steel vertical boiler...

4.5" OD DOM tubing, 0.25" wall thickness boiler shell
8 copper firetubes (L grade, 5/8" OD, 0.037" wall)
2 steel tube sheets (0.25" thick)
1 CRS 5/16" boiler stay

Working pressure = 100 psi
Hydro test pressure = 300 psi

Construction calls for (TIG or MIG) welding the tube sheets to the stay, inserting the structure into the shell and welding the tube sheets to the shell. (Since I don't weld, I would have that work done by a professional welding shop.) The firetubes are then inserted into the tube sheets and expanded in place. Beyond this expansion, no sealing of the firetubes is specified. The author claims that this will provide a satisfactory seal. I have my reservations but I'll give it a try.

I'm interested in building the boiler as designed (hear that, Zee?). Steel is cheaper than copper and the 1/4" wall thickness means that bushes are not required. Welding is strong and the massiveness of the boiler should make it very safe at the relatively low working pressure.

I have some concerns though and I'm hoping that this post will elicit responses from the boiler experts on those points.

My first concern is, of course, corrosion. On my copper boilers, after use, I drain the boiler and open the steam pipe to the atmosphere and continue firing at low level until no steam exits. I would plan to do the same thing with this boiler. Is that sufficient to minimize corrosion on the steel boiler or are there other procedures I should follow?

If I ever get this thing built (a very big if), it would probably be run only very occasionally - perhaps once a month. Given that, how often should the boiler be hydro-tested?

Barring a laparoscope, how do you assess the internal corrosion?

Having never worked with steel boilers, what important questions have I NOT asked? Please offer whatever wisdom you think I should consider. I (and perhaps others) will welcome any steel boiler information you have.

Finally, may I request that we limit this thread to a discussion of STEEL boilers. I'm well aware of the advantages of copper and, frankly, would feel more comfortable if this boiler were copper. Nevertheless, I'd like to understand the pros and cons of steel before I commit to building this thing. If your comments scare me enough, I may yet elect to build a copper boiler.

expanding the tubes is standard practice in 12 inch to the foot boilers and condensers for that matter, make sure that the holes are the correct size you use a three roller expander with a tapered drift wound in to expand they look like this

to asses the corrosion you would use a ultrasonic thickness gauge such as this

one final point make sure the welder is coded for pressure vessel work and will provide the proof , the steel should be the correct spec for boiler work and have a paper trail to source
use a water treatment for the feed water
"Boiler water treatment - Heatreat 502
Helps remove oxygen from water, stops build up of solids around foundation ring and boiler tubes. Supplied with instructions.
A43 1 litre £16.91 "

look at this site may be useful

have a meaningful chat with your boiler inspector or the insurance company the will carry the risk

As Stuart mentioned expanding tubes with a roller is a common practice on boilers and all types of heat exchangers.

This boiler falls in the scope of AMBSC part 2 for steel boilers. I opened my copy to check the requirements for tube sheets as 1/4" seams a bit thin to get a good tube seal to me. The minimum thickness for tube plates for boiler shells up to 300mm is 12mm.

I know that the AMBSC is not a legal requirement at your location but I consider it the best source model boiler design information for steel boilers. It is well worth the price if you are considering a steel boiler.

As for layup of steel boilers there are 2 acceptable ways to lay one up. Dry layup is common and you might consider a low watt buib to keep the boiler warm if tools rust in the storage location. The other method is completly full of water or wet layup.

mklotz said:
I'm interested in building the boiler as designed (hear that, Zee?).

I did indeed. I'm looking forward to seeing pics...regardless of outcome (i.e. the big 'if').

mklotz said:
Given that, how often should the boiler be hydro-tested?

I'd never thought about retesting over time. Is this recommended for copper non-steel boilers as well?
Sounds like a pretty good thing to do.

here in the UK with boilers that come into the regulations ( I think yours is out side the regs but not sure ( size it the factor ))
copper boilers 1.5 times WP every 4 years with a steam test every year ( accumulation test for safety valve )
Steel boiler pressure test , ultra sonic thickness test and steam test ever year


Steel boilers are great.

Why have I not made one................simple.............Like you I am not a certified pressure welder and the costs of employing one are beyond my means.

Our code is an initial hydro of 2 x WP combined with an accumulation test which means at maximum firing rate the boiler pressure should not exceed WP + 10% and the time taken from full to empty gauge glass is measured. Subsequent hydros are carried out every 2 years at 1.25 x WP.

IMHO dry storage is better than wet. This can be closed or open. If closed trays of lime are placed inside a nominally dry boiler before closing or burning charcoal is left inside to eat up the oxygen present when it is sealed. If open air circulation must be provided and the light bulb idea is great to maintain the shell above the dew point.

To store wet the boiler must be completely full to the air cock(s). An oxygen scavenger is normally added to the water which brings the boiler from W to WW.

The most important things for the boiler water are:

A pH between 10 and 11 with the ideal at 10.5 this can be achieved quite cheaply using Caustic Soda.
A TDS of < 3000ppm. Controlled by blowing down.

I hope this helps.

Best Regards


Corrosion seems to hit the tubes first and one advantage of expanded tubes is that they can be replaced if corroded, a guy at our club replaced the tubes in his traction engine and gave a talk on the procedure.

Some very interesting information in this thread.

Hope this helps



I have just joined this nice forum in order to get some advice.
I'm considering building an iron boiler. The boiler shell measures 16cm diameter and 37cm in length/ a bit over six inches by 14.5 inches long. The material is 3mm thick. Main shell gives 7,6l of displacement.
The material is iron.

I have 6 flues to use, but probably I'll use a lot less of them, mostly because of the limited welding room(also, the boiler design is quite prehistoric). My brother has experience with welding, he can weld it together for me. This boiler will not go under any regulation or rule - here in Estonia I don't think any rules even exist! :-[ I'll keep it under 3 bar pressure.
Most likely it's going to be a "pot" boiler, without any flues or water tubes. Wood/charcoal-fired. Weighted safety valve, pressure gauge, water gauge. Also a steam blower and multiple steam takeoffs if needed.

What I'm worried about is the corrosion. How should I take care of the boiler, so it doesn't corrode through and cause harm? I know steel boilers are a delicate topic, especially homebuilt ones, so I'll ask for help before I try to do anything.
Also, I'll ask this. I know silicone tubing can be used as a cheap sight glass. For a temporary solution I might go for that but I feel like I should ask - what pressure is safe for silicone tubing?

I hope I've written down everything now. Please share your thoughts on this - if it's too risky, then say so. This is mainly an excuse for searching for bigger copper pipes - I have very limited resources.

Good night,
Timo :)
You can use a water treatment, most are tannin based, this will help restrict corrosion. Also when not in use for short periods keep the boiler completely full, if you are not going to use it for some time then completly empty it and use some form of heat to dry it.

Having said that 3mm is thinner than I would want to go for that dia even with the low pressures you intend as there is really no allowance for wasting (corrosion) I'd be looking for double that thickness. And for your own peace of mind base your calculations on a recognised code even if its just for your own use and no one else will be anywhere near it.

Is the tube you have actually iron or do you mean steel ? I would imagine iron pipe to have less strength than steel and its also a lot harder to weld

The pipes are raw iron.
Uhh, I can't imagine adding water treatment to a boiler. Looks like I'm a candidate for a riveted copper non-steel boiler.

I can still use the shell as a water preheating boiler. That way it won't have any pressure.
mklotz said:
8 copper firetubes (L grade, 5/8" OD, 0.037" wall)
If you haven't already bought the material I would go with Type K. This will hold a seal a bit better.
Working pressure = 100 psi, Hydro test pressure = 300 psi
If some is good, then more must better, right? A lot of people seem to want to climb up this slippery slope but overpressuring on test is unecessary and can be harmful to copper. The current world standard is a 2X initial test follwed by 1.5X on subsequent retests on both steel and copper.
Steel is cheaper than copper and the 1/4" wall thickness means that bushes are not required.
I think bushings are a good idea even if not "required" by someone's code. My personal guideline is to have at least 2X the thread diameter in clear bushing thread depth, which means that the bushing OAL could be as much as 3X the thread diameter.
My first concern is, of course, corrosion. . . . . I would plan to do the same thing with this boiler. Is that sufficient to minimize corrosion on the steel boiler or are there other procedures I should follow?
Most steel boiler operators I know use a specific boiler treatment, the name of which I can't recall, but they now swear by it. Opinions vary (Imagine that!) on whether to leave a boiler full between steamings or blow it completely down. One technique I used to hear about was to pump a cup of kerosene into the warm boiler and then drain it, which left a protective coating of kerosene on the inside and didn't harm anything else.
operate . . . perhaps once a month. Given that, how often should the boiler be hydro-tested?
Most clubs require an annual retest but if you are operating privately every second year might be OK.

It is my opinion that many of the "Code" concerns and specifications (ASME specifically) which model builders cuss and discuss are or eventually become non-issues. Not because they don't add strength or safety to a boiler and should be followed, as often as not they do, but because it takes a lot more than just a "code" welder to make a "code" boiler and if all the requirements aren't met then it won't be "code" no matter who welds it. If one required opening is missing or undersized it won't matter if you used traceable code materials, that would be enough to nullify code compliance.
Bluecat said:
Uhh, I can't imagine adding water treatment to a boiler. Looks like I'm a candidate for a riveted copper boiler.
There will be NO discussion of copper the other kind of boilers in this thread! (Pssst . . . riveting isn't used much any more on copper boilers.) ;)
Steel Boiler Water Treatment

The 2 most important parameters are

pH - 10.5 to11.0 cheapest way is to buy Caustic Soda NaOH from a hardware store.

TDS - Total dissolved solids <3000 ppm. Controlled by blowdown. Make sure that the end of the blowdown pipe inside the shell is at the lowest point or you waste a hell of a lot of good water and don't get rid of enough solids.

To measure these you can buy small meters.

Hope this helps

Best Regards
The name of the boiler treatment that many US live steamers I know have gone wonkers over is "LSB8000." This is for steel boilers, not the other kind.
GWRdriver said:
There will be NO discussion of copper the other kind of boilers in this thread! (Pssst . . . riveting isn't used much any more on copper boilers.) ;)

My apologies! Thanks for the information though! :)
You do realize I was kidding don't you?

But at the risk of being tossed out of the thread, current conventional wisdom is that only as many fasteners (pins, rivets, screws, etc) as necessary to hold the boiler together for soldering are required. Of course one can use as many rivets as you want, but they won't add any appreciable strength to a boiler.
Yes, of course I do. I just like the looks of a riveted boiler, so I'll rivet the ends and the shell together.
I don't know if this is an open thread on steel boilers, but I'll post this here anyway.

I'm starting the Kozo A3 in 3/4" scale. It calls for a 3 1/2" boiler with three 3/4" tubes made of copper.
Would a boiler of this size be too small to fab from steel? Though I'm not a certified welder, I am very comfortable in my Mig welding skills. About 5 years of production welding Hyd. tanks. Way more comfortable welding than soldering.

If this is possible... What would the Materials consist of IE...pipe type and thickness?

I Know that the boiler is a long way off in my build, but just trying to get a game plan together.

Your two main problems with a small steel boiler are

1. you will have toi allow for wasting (corrosion) as the rate of wasting does not scale you would need to increase the plate thinknesses by about 1/8" this will seriously reduce the volume of a small boiler and also the grate area.

2. Steel is not as good at conduction heat as copper so you will not get as efficient heating.

I would stay with copper at that small size it should not be hard to make