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Hello again, The present configuration of my boiler uses 20 .625 o.d. tubes. This would give me a heating surface of about 500 Sq.In. I think this is more than adequate for the demand. I do have 5 tube patterns ranging from 14 - .625 tubes up to 31 - .500 tubes in Excell so It's easy to fiddle with. Most of the tube patterns seem to produce a heating surface in the 475 to 540 Sq. In. area, so 500 was chosen. The 20 - .625" pattern has a web spacing of between .450" and .475" spacing.

As I've mentioned I want to stay with commonly available sizes of Pipe. Looking at the attached drawing the water leg would have a spacing of 1.625. Going to a 7" pipe would reduce that space to about .750" which would increase the heating surface by about 50 sq. in.,but I think that spacing would be too narrow.

The upper tube sheet extention would use the same 6" pipe and thus the same spacing as the water leg. I'm wondering if that is going to leave enough steam space? I also wonder if the bottom tricock which is .5" above the bottom of the upper tube sheet and would be the absolute minimum water level should be raised?

Thanks for your input, John

Donkey Boiler 5 - New.JPG
Prospect said:
I want to stay with commonly available sizes of Pipe. . . . Going to a 7" pipe would reduce that space to about .750" [snip] I'm wondering if that is going to leave enough steam space?
I don't see a 7" steel pipe listed in schedules very often, probably because like 5" and 7" copper those sizes have been discontinued as stock sizes. Engineers and designers went to a 4"-6"-8" standard years ago. If you need enough of it the mills will draw it for you, but otherwise you won't find it in warehouses except as old old stock. If you haven't done so already, I have always found it handy to keep a listing of standard pipe sizes in my notebook. Here's a good one for steel: I keep them for copper also.

If it was me I would want a larger steam space, but this isn't a deal-breaker depending upon demand. If it's a "working" boiler, one which will carry a load, then as steam space is reduced you can expect the firing rate (fuel & water) to need to be increased, that is, more fuel and more water per time period, to the point where firing might be continuous. A flash boiler, which typically has essentially no steam space, carries this to its logical extreme where heat and water must be continuously force-fed into the system. A demonstration or test boiler usually has less demand, so the steam is drawn off at a slower rate, so it doesn't require continuous firing although the basic thermodynamics, energy-in for energy-out, are about the same.
GWR Thanks for the link. The site is a lot simpler and clearer than the one I was using.

The steam space does concern me. I think I'll lower the upper tube plate and see how it looks.

I don't think it will effect the heating surface that much.

I had though of maybe adding a - call it a steam dryer - into the well at the top of the boiler since it

is such a large availabe space, but if it was likely to prime and mounted after throttle it can cause

some very erratic engine operation. I'm not sure if I would want to mount it ahead. Nice space maybe

for a feedwater heater but I'm getting ahead of myself. John
DOM tubing comes in a 7" ID. It may have a heavier wall than you want, but that's easily remedied. If you know any hydraulic repair shops you might check and see if they have any old cylinder barrels you could whack a length off. It's kind of miserable stuff to machine, stringy chips and all, but plenty of strength for a boiler shell.

Just a thought
John did mention in the very first post that he was building to AMBSC rules. If that is the case not just any old chunk of tube from the scrap pile will be suitable. The types of steel allowed in the AMBSC steel code are listed in the front. Some of them are AS specs which are not easy to find on this side of the pond.

The thing to search for is Pressure Vessel Quality or PVQ steel.

Here is a post by a steel boiler maker in the US who has been building quality steel and a few copper boilers for a while now that states the types of steel suitable for boiler construction.

Anyone in the US looking for State boiler code information can check this link:

I do think it will be hard to find 7" PVQ tube but I have not searched. I had a similar problem as I needed 5" copper for the Saturated Steam boiler. There is a guy on ebay selling 5" copper at this time so I bought 16" so I am now redrawing the whole plan set because it used true 6" and 5" tube and the US standard is 1/8" larger.

I will start a thread on that boiler when it makes it to the workbench stage.

I'm by no means a boiler expert, but I routinely use DOM tubing in various sizes for hydraulic cylinder designs with working pressures exceeding 3500 psi with a 3x minimum burst pressure. Typical yield is around 70 ksi for A513/1020 DOM with good weldability. Obviously establishing the pedigree for a chunk of cylinder would be difficult if a specific certification with heat and melt number is required, but this material is so widely used in the cylinder business by all the OEM's I would be very surprised if it was anything else but. I have never seen the PVQ designation on a steel spec sheet, but would assume it applies to this. I took a look at the links you referenced, and in Marty Knox's first post he mentions

"Up to about 5 1/2 - 6" diameter and 100 PSI copper has many advantages,above that,low carbon steel is the material of choice. I worked as a boilermaker for DuPont. They have a whole department that works on material specifications. They never hesitate to buy the best material for the application. We worked with a wide variety of materials - Inconel, Hastelloy, and Carpenter 20 to name a few - yet all our boilers were low carbon steel!"

I think most people would consider Marty one of the leading experts in boiler construction, I know I have certainly benefited from his contributions to various forums over the years. It seems like we are definitely in agreement here with this material.

I agree that Marty Knox is one of the most leading boiler experts around. He has several very well written articles in hobby and historic preservation magazines.

The important bit he had to say about the steel to use and why is this:

"The first quality of a boiler material that comes to mind is ductility - you want a material that will withstand many,many heating and cooling cycles, with out fatiguing and cracking. Strength is almost secondary - the strength of a boiler is in its structure."

I have not had much luck with PVQ in searches but others have mentioned it with steel boilers on this forum. Here is a link to a firm that carries the types of steel Marty mentioned in his post.

A53 was not mentioned by Marty I got that from the AMBSC Part 2 1995 Steel Boilers.

1020 is a low carbon steel, can certainly take lots of pressure cycles, based on my hydraulic experience.My only question would be the heating/cooling. It has a similar carbon content to the A53 spec you referenced, so potentially about the same ductility. The pipe spec didn't give a % elongation which would help equate the two materials better.

I'm a mechanical engineer by education and experience, but not a boiler maker. I might mention as a side note, I helped restore a full size Willamette logging donkey and ran the calculations necessary to get the boiler certified here in Washington by the state boiler board, but that had more to do with the existing material thicknesses and riveted joints.

I'm curious about the Australian materials, I should get a copy of their code. My company has a plant in Brisbane, with a full engineering staff. I could probably get some of the Australian material specs crossed into US or ISO material equivalents with my colleagues assistance down under if there was interest.

I could tell you were an engineer by your second post. I am a retired Marine Engineer also by education and experience.

The weak point of the AMBSC for some one in the US is the specs do not match what I can find.

I think that this issue is important enough to have its own thread. I will start with Marty Knox's post on the specs for an ASME steel boiler and add the AMBSC steel specs so maybe home builders can source the proper steel to make boilers with.


Edit: For boiler steel specs see:

If it was me I would want a larger steam space, but this isn't a deal-breaker depending upon demand.

GWR, I think you'll appreciate this. I was out in the shop today and out of curiousity I tacked together some pipe which gave me the approximate spacing of the upper drum to boiler shell which is about 1,625". Filled with non pressurised water and with a venturi mounted on the outlet location using air and filled to 2" from the top it sucked water like a darn. 3" down there was just a mist. It would be very interesting to make something up out of lexan so you could actually see what's happening. My current attached drawing is 3" to high water and 4.5" to middle trycock. This has dropped the heat surface from 500 to 450 sq in. and the operating water level to 3.5 Imp Gals. I'm really thinking that the maximum load I'm basing my engine demand calcs. on is probably excessive, but been wrong before - mostly. I was pleasantly surprised to find out that the Boiler Inspector for my area is a fellow I know who is very involved with local museums and "Model Engineers". I'm going to try and get a hold of him after the new year in case He does the snowbird thing. Take care, John

Donkey Boiler 5 - New.JPG
Sorry for not responding sooner . . . I've been busy building a boiler . . .
Prospect said:
I was pleasantly surprised to find out that the Boiler Inspector for my area is a fellow I know who is very involved with local museums and "Model Engineers".
I would be pleasantly surprised and very thankful to discover I had an ME-friendly boiler inspector too. When I started out our State (TN) had a chief inspector whose career began in the 1930s and he was rigid and inflexible and would not allow that amateurs could build or operate a boiler safely. Thankfully his preoccupation was with backwoods "skonk works" mine and sawmill boilers and we ran under the radar until he retired. Keep posting on your progress. And I would do the snowbird thing also, if I could.
Things got busy between farm work and Med Appoints since the new year so finally got back to my drawings. I talked to a Boiler Inspector ( not the one I want Yet ) and He gave some general info which I was already aware of, but talk is good.

I currently doing some drawings for weld preps. The lady who cleans house for me decided quite rightly that my Library needed cleaning. Unfortunately I can't find the books I need now - yet. Could someone enlighten me in regards to the asme code and butt welds, if I'm using a 60 degree combined angle for two plates, what is the required unbevelled root size and separation?

Thanks for your help. John
Prospect said:
what is the required unbevelled root size and separation?

Thanks for your help. John

I think that has something to do with the thickness of the plate.

Hope this helps

Best Regards
Sorry. This should be clearer. All other joints are similar but may be bevelled and welded from both sides depending on access. John

Found My Australian code book so far at least. The recomendation in it is for a maximum dimension of 1.5 mm, so that's what I'm going to draw. Also talked to a B-Pressure welder welding repair sleeves on a live 36" oil pipeline close by and He said 1/16".

Donkey Boiler 5 - Welds.JPG

Donkey Boiler 5 - Welds 2.JPG
Prospect said:
Hi, further to this boiler, I've been working on a spreadsheet where I can change dimensions and not make me wish I paid more attention in math class. While a work in progress it's functional.

<other stuff clipped>

Thanks for any replys, John

Have you done any more on this spreadsheet? Willing to share it? I was thinking of doing something similar but didn't want to reinvent the wheel, so to speak...

Prospect said:
Hi The spreadsheet is kind of a hodgepodge mess specific to where my mind was at the moment. I just haven't had time since the beginning of the year to play much. I'll post it if only to give you an idea of what not to do. John

Thanks, I'll take a look and see if I can make it work or adapt it. I'm trying to size safeties.


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