US State Boiler Code

Home Model Engine Machinist Forum

Help Support Home Model Engine Machinist Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
I live in Murfreesboro TN. There is a business in town called, Boiler Supply Co. they, service, sell, repair boilers. I talked to Dave the owner he gave me a copy of TN State Law. I can build my own boiler if it is smaller than 5 gallons. It does not have to be, inspected, tested, licensed, or anything. I built a 4.9 gallon boiler for my 4"x4" steam engine it ran the engine good.
Awesome, you know what you are doing. Have you looked into the ASME Boiler and Pressure Vessel Code for additional reference? Just a thought.
Kind regards, Al
I have uploaded all of the boiler info for the State of Washington on our Kitsap Live Steamer's website at this link:
Our state is also more relaxed on hobby boilers than it used to be, but all boilers must be certified, but once this has been done to their satisfaction, they no longer require the annual inspection as in the past.

However, be careful if you are in the market for a live steam locomotive. I have helped others in the state learn how to do the calculations and prepare the documents for boiler inspections, but I have found three hobby boilers that were a disaster! A 1 1/2 gauge pacific with crown sheet stays that had deteriorated so badly that they no longer connected to each other, another 1 1/2" pacific that the boiler was built with no stays, and a third that had girder stays that were not large enough to support the crown sheet and when we cut into this boiler, the welds were not full penetration.

So just because because our hobby boilers are truly "overbuilt" for the allowable working pressure they were designed for, if they are not properly taken care of, they can become a potential problem.

Ken Olsen
A boiler, incorrectly built and maintained, is a ticking bomb. I worked 38 years on oil platforms in the Gulf of Mexico. Boilers and pressure vessels were my livelihood. Our pressure vessels had to undergo periodic non-destructive testing to monitor the wall thickness of the vessel's shell. Boilers have to be removed from service and sent ashore for complete inspection and evaluation by a certified facility. Ken, I agree that you can't stress enough the potential dangers that boilers can pose.
Kind regards, Al
As Dan Rowe said, it appears that Australia is the only country which has a miniature boiler safety code. As a model builder, I have built three boilers using the AMBSC (Australian miniature Boiler Safety Committee) code for copper boilers and as Dan's thread suggested, the codes are available through the magazine Australian Model Engineering. The codes are well researched and documented and I do use them as a construction manual. Without looking at them, I think their scope covered boilers up to 12" diameter and I think that was in steel. I think the copper maximum was about 7". I'm a little surprised to see that no Australians have joined the discussion???
The UK also has its own codes for miniature boilers.
The Boiler Test Code 2018VOLUME 1-Boilers 3 bar litres to 1100 bar litres
The Boiler Test Code 2018VOLUME 2 -Boilers under 3 bar litres
I'm retired now, but worked with boilers and steam power systems my entire career - everything from low pressure to super-critical utility boilers.

Any boiler failure can be extremely dangerous - even a small hobby boiler. In the absence of any other mandated safety regulations, there are several things that are a must for a hobby boiler.
1. It should be periodically hydrostatic tested to 1.5 times the maximum operating pressure
2. It should have a tested relief valve
3. There needs to be a means to quickly remove the heat source

Something that most hobby boiler operators rarely take into account is boiler water chemistry. As steam is generated, the dissolved solids in the water (typically calcium & sodium) get left behind, and if allowed to concentrate can form scaling that can promote corrosion and lead to potential failure over time. These deposits also reduce the heat transfer and thermal efficiency of the boiler. Commercial and industrial boilers use a combination of water treatment to inhibit scaling & corrosion, and blowdown to limit the cycles of concentration. For a small hobby boiler, the easiest solution is to flush the boiler with clean water after each use.

Just my $0.02.........
The US ASME boiler code does recognize what they refer to as "Miniature Boilers" under "Part PMB, Requirements for miniature boilers".
PMB-2 under Scope notes the following:
PMB-2.1 The classification miniature boilers applies
to boilers that do not exceed the following limits:
(a) 16 in. (400 mm) inside diameter of shell
(b) 20 ft2 (1.9 m2
) heating surface (not applicable to
electric boilers)
(c) 5 ft' (0.14 m3
) gross volume,' exclusive of casing
and insulation
(d) 100 psig (700 kPa) maximum allowable working
PMB-2.2 If a boiler meets the miniature classification,
the rules in this Part shall supplement the rules for power
boilers and take precedence over them when there is conflict.
Where any of the limits in PMB-2.1 are exceeded,
the rules for power boilers shall apply.

However, each state can modify the code. For example note that the ASME code limits for ASME miniature boilers to 100 psi, the State of Washington has set the limit at 150 psi. However, the boiler calculations must show that the weakest component will withstand this pressure. Otherwise, the weakest component will prevail. The exception is that copper boilers are limited to 100 psi.
....The codes are well researched and documented and I do use them as a construction manual. Without looking at them, I think their scope covered boilers up to 12" diameter and I think that was in steel. I think the copper maximum was about 7". I'm a little surprised to see that no Australians have joined the discussion???
Ok Wazrus, I'll take the bait! If you are near Sydney, are you a member of a local club? Anyway, as you know, our boiler codes are well researched and documented. They are also used in many parts of the world where live steam builders don't have a suitable local code.

The AMBSC boiler codes are in 4 parts. They are recognised by authorities in all Australian States.

Part 1 Copper boilers 50-203mm dia and 1-25 litres, 100psi, 68pages
Part 2 Steel Boilers (Certified Mild Steel) to 356mm dia and 50 litres, 100psi incl full loco and Briggs types, 66 pages.
Part 3 Sub-Miniature Copper Boilers to 50mm dia and 1 litre, 35psi
Part 4 Duplex Steel Boilers 325mm dia and 50 litres, 100psi, 44 pages

The codes are fully detailed with drawings, charts, plate thickness, flange depth, weld preps, stay dia and spacing, bushes, safety valve design etc etc. No trouble to design and build a complying (safe) boiler or air receiver using these codes, they are very comprehensive.

See Australian Model Engineering Magazine and go to the Shop.
Last edited:
Alberta, Canada code does not require a boiler to be registered if it is <152mm I.D. and <42.5 litres volume, maximum working pressure <1725kPa which translates to approximately 6" I.D. x 23" long and 250 psi.
Using copper pipe and the maximum pressure is 100 psi.
I think the links to the Washington state boiler code are broken, both the link on the original page, and the link by kaolsen1728 on Feb 6, 2020. They both give me a page not found error.

I worked 40 odd years as an engineer, designing High voltage switchgear and stuff, car stuff, and was the Ex-spurt Regulation engineer a t the car factory for around 20 years.
I learned that "in the eyes of the Law" ( I.E. words passed by the Legislature that allow anyone to get money from any pot there is.... to pay lawyers, and possibly compensate "injured parties" as they put it!) you shall not intentionally nor innocently break the rules within the Regulations, in case something goes wrong, or someone can challenge your "safety"..... for fear of being caught Breaking the law".
Exemptions are just there so the lawyers can argue and charge (whoever has the money) for their time.
So I advocate that you comply with ASME regulations to the best of your ability..... e.g. join a club and use their experts to check what you are doing (it is comforting to have someone next to you when you stand in the dock) and if possible have the club's insurance cover your public activities.
I have spent a lot of time pondering what I know from my engineering expertise to be sensible calculations of the strength of boilers. But I have not yet found any official guide to the calculations required to prove a design. Also, by my cautions demonstrate the weakness of many designs - typically the "pressure of failure" determined is less than 8 times the NWP. (ASME requirement is "more than 8 times.). So I think there are a lot of boilers that "by design" do not have the required Factor of Safety of 8 times NWP. The hydraulic test at 1.5 or 2 x NWP does NOT prove a good design, just that there is some margin of safety.
I am now de-rating my boilers accordingly...... because to not do so would be "negligent" if it ever came to court.
I cannot responsibly suggest you do anything but try and achieve what the Regulations require....
That's the world we live in and enjoy.
- Incidentally: I can strongly recommend a cheap way to "get the Regulation info" - Buy a back copy of "Live Steam and Outdoor Railroading": Volume 40 No. 6. The article by Kozo Hiraoka is really the best text book I have seen on the subject: He is a genius teacher. - I use this in place of all the other books I have, as it has such clear and uncompromising details on what MUST be achieved as a minimum to be approved to ASME Regulations.
But I must add, that this does not cover any tubes experiencing COMPRESSION forces from the water/steam - such a flue tubes. In the case of COMPRESSIVE forces on COPPER tubes, you need to de-rate the Maximum Permissible Stress to 21% of the Max. Permissible Tensile Stress - according to the worst case I have read from internet sources....
And considering that many boilers:
  • with a single fire-tube (for a blow-lamp) have cross tubes,
  • Outer-shells have penetrations for Steam domes,
  • Inner firebox tubes on vertical boilers have penetrations for coaling,
there is another ASME regulation applying to pressure vessels - including boilers - that states the STRESS CONCENTRATION FACTOR shall be taken as 3.3 - Whatever else an individual may think, calculate or ignore.... - This has the major effect of reducing the NWP to 30% of that otherwise calculated (I.E. If you have missed/ignored the SCF).
Basically this has caused me to de-rate some of my boilers, because I had originally used other stress concentration factors (typically 2.2 ~2.7 depending on tube size, shape and hole size) because I designed boilers based on my general engineering knowledge, and some text book tables, in ignorance of the ASME requirement.
In summary, unless you feel you can fight a court case for "exemption", you have to do a hell of a lot of research to find the Regulation quirks that affect you designs... And I don't know where I can go to ask for confirmation or correction of what I am doing! Most "inspectors" can inspect to their rules, but can't confirm your calculations, because it isn't their job to know or be able to do so.
And as far as I can determine, outside of the USA, one has to "Do one's best" as the Regs are so hard to find and interpret...
Sorry to be so long-winded about this, but I have pondered and researched for many hours to try and get to this semi-sensible stage...
I've missed replying to Radial 1951, as HMEM seems a bit spasmodic with its contacts. Just today, i received an e-mail telling me that the thread seems to have been resurrected!
Radial, what 'bait?'. And no, I'm not a member of any club and never have been. I have my own 5" track of about 300M. I built it around 2004, i think it was. However, some health issues have intervened and I haven't turned a wheel on the track for some years. Before the 'issues', I was in the process of re-aligning the track, as there had been some subsidence and the whole shebang was in the process of being re-sleepered (lots of white ants), re-aligned, re-graded and re-ballasted. The work is unfinished, particularly the sleepering and some of the banks need edges, for which I've started to use concrete blocks. Nor have I been doing much with live steam, although there are some projects in the pipeline. I have (or had) a complete Sweet Pea, which I've dismantled, as the original design of boiler had more than few limitations, which I was in the process of, let's say, 'correcting'.
I did let my AME subscription go some years ago, but I was a contributor since 1993 but, as I said, never a club member. Too much politicking for me.
But I'm still alive!
Current 'projects' ( ho ho ho) included LPG firing and burners for same, as good steaming coal is hard to get in what is one of the largest coal exporting countries. There's something very peculiar when I see Welsh steam coal being offered.. Now LPG burners are tricky little buggers and I'd like to place 'em in the flue(s) of my latest creations, as that Finnish gentleman has done, but without the 'glow cones'. Then, too, I've seen somebody else on HMEM remark about the paucity of data on the compressive stress of copper tube and this is a concern with larger boiler flues.
I'm in New South Wales, in Sydney. I'll look for your contact.
Thanks Russ, I am working through this tome, to try and find the relevant stuff for copper boilers up to 100psi. It seems all boilers in Texas below 160psi are called "hot water boilers". I must read more.
But as I am in the UK, I am not obliged to meet Texas , nor ASME, Regulations. However, the ASME Regs were written by engineers cleverer and more experienced than I, and as such I feel it is foolish to ignore them. "What price for Safety?".
Hi Wazrus. Good luck in finding burners for your loco project. Having been interested in this for a few years, the most successful gas powered locos seem to have dedicated designs suited to gas firing. For all designs though it seems most important to not just have a burner system that draws in adequate primary air for full and complete combustion, but the fire needs to have radiant elements to improve the heat flow from flame to copper.
Simply:a hot flame has the coldest part against the boiler wall, and the conduction of heat from hot gas to cold wall is poor. But if the flame heats some medium to red or orange heat, then the radiant heat is transmitted into the boiler walls very efficiently. Additionally, the volume of exhaust gas from flames is large, but the same burner with radiant element has a lower volume (due to the lower temperature) of exhaust gas. As most boilers are limited by exhaust gas versus flue size - so coal fired loco boilers force this with the exhaust steam blast - gas fired boilers need significantly larger flues to avoid back-pressure affecting the burners. The mathematics of all this are hugely complex (beyond my capabilities). So copy someone else's successful design for an easy life!
On tubes in compression. The tensile strength limit for copper is derated by temperature. When doing calcs at room temp. ASME permits 6700psi. Max. But at 100psi (the ASME limit for Silver soldered boilers) the Max. permissable Tensile stress is derated to 3142psi. Now from my research, the compressive stress is derated to 21% of the tensile stress, that means when you do the hoop stress calculations for the fire-tubes, you can only have a max. compressive stress of 660psi - relating back to ASME limiting tensile stress. Actually, for thick-walled copper flue tubes this is usually manageable. But for larger tubes for fire-tubes, etc. this means additional reinforcement of the tubes is necessary. Further, if you have cross-tubes, the stress concentration factor that must be used is 3.3 - defined by other ASME pressure vessel regulations for ANY side penetration of a tube in compression - means this is further derated to 200psi. MAX.
But please note, I am only a jobbing engineer (I did the job for 40 years in industry all to national standards and international regulations - not back-street mechanics "it's looks OK to me, Mister"), I'm not an expert in this field, so if any experts want to correct me I will listen closely!
Wazrus, I'll try and show some examples:
Tubal cain quotes an example of a 1" dia copper tube - 0.048" thick wall collapsing at 330psi: This means a factor of safety of 8 would give a NWP of 41.25psi: Except he did his test cold: (Max permissable tensile of 6700psi). But if we run a boiler at 41.25 psi the temperature is around 290F. So the max permissable tensile stress would be near 4157psi. I.E. 4157/6700 = 62% so if the test had been with steam at this pressure, we should expect a failure at ~205psi. - De-rate this by a factor fo 8 and we get a NWP of 25psi!
Martin Evans proposes 1" OD x 0.064" wall thickness should be OK for 100psi NWP... but I have calculated it is OK for 80psi NWP to ASME and other physical limits if you really apply all the factors and Factor of safety of 8. As the heaviest standard 1"OD tube listed is 0.080" wall, this is good for 95psi NWP for a FOS over 8 - meeting ASME as a fire tube with external pressure, but with NO cross-tubes. This also implies the tube is straight, and undeformed, as a non-straight tube, or oval or deformed tube must be derated a lot for external pressure.
For 100 psi NWP for a fire-tube - to meet all the requirements for safety from Regualtions, etc. I propose a 1" OD tube should be at least 0.085" wall thickness.

Latest posts