US State Boiler Code

[Steamchick]

K A Olson (I guess that is your name?), thanks for that. The ASME regulation I refer to is the design regulation. The ASME limit of 100psi for silver soldered copper boilers is based on material properties which do not change by nation or state, so maybe it is good for us all to follow that limit (e.g. In the UK or anywhere else outside of USA).
The hydraulic test I found from ASME, based again on permissable stresses in annealed copper at elevated temperature.
To quote: UG-99: "The hydrostatic test pressure shall be at least 1.3 times the Maximum allowable working pressure multiplied by the lowest ratio of the stress value St for the test temperature for the design temperature." - In the case of a boiler for Max. 100psi (Max limit as required by ASME for Copper boilers) the ratio of St/Sd = 6700/3142 = 2.132: So the test pressure (Hydraulic at 70degrees F.: designed for 100psi = 338degrees F.) shall be 1.3 x 100 x 2.132 = 277psi.
My information source is a design document written by Kozo Hiraoka published in 2006, so I may be out-of-date by now?
The logic I expressed is based on sound engineering principles, of stress calculations and compensation for copper at elevated temperature, but if there is a Service test for in-service annual tests (as the UK test at 1.5 times the NWP) then that is accepted for that certification. But for new boilers there is usually a design hydraulic test - at least done by the designer or agent to prove the strength of the design is good for the simulated stress for 8 x NWP and factored for the material strength at the temperature of the NWP.
My recommendation to Wazrus was that if he had a boiler deform due to pressure, then that pressure should have been at least 8 times the NWP. So there was something wrong with the original design.
If you think being able to withstand 8 times the NWP is excessive for your boiler, then you misunderstand the purpose of the Regulations, in my opinion. It is the regulating authorities' decision that this is the level of safety required to prevent injury to people. So I have re-rated some of my boilers that were "inadequate" when I applied the ASME principles to the designs of those boilers. But I am just an engineer (following Kozo Hiraoki's explanation paper on ASME Boiler and Pressure vessel code and general engineering calculations for stress), with my opinion expressed on this site, so please check with your local test authority if you need your calculations or testing to be confirmed/ratified.
My prime interest is the safe operation of steam boilers by safe design.
Happy ( and safe!) steaming!
K2

 

[Steamchick]

Hi Wazrus,

1. I am fascinated by the boiler design: Do you have any general arrangement drawings you can show so I can better understand the firebox and fire tubes planned? I have had a notion that "conventional flat-sided and stayed" fireboxes are appropriate only because they copy full size Locos. Cylindrical fireboxes are much easier to manufacture and "naturally stronger" than flat plate constructed fireboxes. (I have a "scrap" super Simplex boiler with possibilities of repair - but the stayed flat sides limit the pressure it can take - and leak madly!).
2. I am curious about your idea of the fire-tube burner. Have you lit it yet? What does it do in a fire-tube? Where does the "venturi" fit?

I have attached a copy of a USA Government design of venturi. The jet goes at the left-hand end. The air inlet (between jet and venturi - needs to have a cross-sectional area of 120% of the area of the opening into the venturi. The area of the opening into the venturi is critical: this needs to be at least 250 x the jet cross-sectional area. - then all other dimensions are scaled from that dimension. The drawing attached is suitable for a propane jet about 0.55mm. dia., mounted about 11mm. or 7/16" from the end of the hole, by my reckoning.... but the US government paper suggests "0.5 x D" which would be 0.7" or about 18mm from the end of the venturi. So some degree of adjustment should be made so you can tune the gap between the jet and the venturi to get the optimum flame in your fire tubes.
Scale up or down as appropriate for your planned jets?
Of course, it is intended that the venturi blows the gas-air mixture into a burner - such as the burner that Susan Parker has = a lot of slots in a long tube! But I reckon you will do better with a blow-lamp type burner anyway.
Have you estimated how much heat you need for the boiler to generate the steam you want for the Pea to run happily? - That is the key so you can decide how much gas you need to burn - and then that dictates the jet sizes, etc.

Cheers!
K2

 

[wazrus]

G'day, Steamchick
Fascinated by the boiler design? As far as I'm aware, the design is almost public domain and I certainly agree with your remarks about the circular firebox. What I have altered, though, is the placement of the boiler 'cylinders' relative to each other. That doesn't sound clear, does it? The boiler, for coal firing, had the barrels of the firebox and the main barrel almost in line at the top, to accommodate the firegrate, below which was, of course, the ashpan. The ash space was, in my opinion, far too small and certainly too small for the high ash coal I had on hand. The steam space above the firebox was reduced. My version has pretty much 'inverted' the boiler, so the firebox 'outer' (such as it now is) centreline is considerably above the barrel centre line. So all I've done is simply 'roll the boiler over' and dispensed with the firebox altogether. Other than the staying, the increased material thicknesses and the gas firing arrangements (flues), nothing else has changed. It's still pretty much a Sweet Pea ((Bagnall) style boiler, with dome and my larger flue tubes running from front to backhead, through what was the coal-type firebox area. So the thing is just two cylinders, silver brazed at the circular throat joint and the 38mmx3mm flues go straight through. Why not make it a simple cylinder? Good point, but I had the barrel(s) and some flanged plates, so why change at this stage? I've also gained much needed steam space and, with the addition of a dome on the larger diameter part, a drier steam collection point. Priming was a problem with the original boiler.
I like your inverted commas used with my 'venturi'. Venturis had sort of been confined, in my private pantheon, to a mysterious black art, as had burners, and no I haven't had the burner 'stick' in a flue, so I'm still in the black art area. I had very little room for a venturi of any sort and looking at the pipe reducer types, it seemed that they might be made shorter, say along the lines of a carburettor intake, with the bell shape. The outlet is probably woefully undersized. You could say that it's all not much more than guesswork and you'd be right!
But I do really appreciate your input which has thrown a little light into the black places.
Note also that my larger flue tubes in the small Pea boiler, have been reduced to two tubes. I plan to use three in the larger boiler.

 

[Steamchick]

Thanks Wazrus. I'll do a bit go digging and find the tube sizes of a regular pea boiler. Are your tubes bigger? And what wall thickness? I'll do a few sums on your fire tubes. I really think a blowlamp burner for each tube is probably best. You get very good control of each burner with screw-down taps, so you can "idle" the boiler by turning down the fires, then "Whack 'em open" when you want to pull away with a load. Blow-lamp makers spend a lot of time and money making their burners "the best" so why try and re-invent the technology? You can buy nozzles for the cost of the brass! (I made a steel burner, but much easier to buy a scrap blowlamp for the burner!).
Cheers!
K2

 

[wazrus]

Steamchick/K2
I must apologise for a bit of a bollocks in the 'explanation'. What isn't clear is that there are none of the usual firetubes at all in my 'revised' boiler. As in Kozo Hiraoka's Pennsy switcher, there will be just two or perhaps three larger (38mmx3mm copper tube) flues, in which I plan to place the 'burners'(!). The tubes (flues) themselves, seem to be OK as regards crushing. By the 'tubes' for a 'regular' Pea boiler do you mean the small fire tubes? If so, in the original, 20 are called up, 1/2" ODx20SWG. These have been eliminated. If you mean the outer barrel/tube, it is 5" ID. The firebox outer diameter isn't shown on the drawings (pages 102 and 103 of Jack Buckler's book)) , but mine is about 7" diameter
Note that I have not used tube, but have rolled the barrels myself and they have
been jointed using Alec Farmer's version of a castellated joint. This joint was debunked by some pundits here in Oz, so I doubled up and strapped it, to be sure. All platework/barrel etc is out of 4mm copper. During dismantling, the joint let go: not the castellated, but the strap, so a friend TIG welded all the legs of the castellations and I plan to strap over that, too. I used Jack Buckler's book 'Build Your Own Steam Locomotive', which is a blow-by-blow construction manual for a 5" Sweet Pea.
I plan to install some radial staying where possible. The large flue tubes will probably get in the way, so that's a work in progress. Longitudinal staying as per the words and music, referring to the AMBSC guidelines. I hope I've made my ideas a little clearer. The burners, as you say, shouldn't be a case of re-inventing the wheel, but I have reservations about whether a blowtorch flame will be long enough to reach the 'end' of my flues and what the characteristics of that flame might be. I know it's hot: I've been experimenting and have made a long lance-type burner, with air intake holes at the 'handle' end and a large head on the other, The result is a large, bushy roaring flame, which I use on household weeds! But the flame isn't very stable and I suspect there's not enough inducted air. For the head I copied a Bulgin burner, but located the air holes remotely. maybe the holes are simply too far away from the head for stability. it tends to be 'touchy' and inclined to self extinguish. I suppose the long 'wand' might be replicated in the boiler flues.....hmmmm.....
Again, many thanks for your input.
Wazrus

 

[Steamchick]

Thanks Wazrus, I have a better understanding of what you are doing. Using my ready-reckoner (spreadsheet for calculations) I deduce that the 38mm OD x 3 mm wall copper tubes can withstand 100psi external pressure with a factor of safety of 11. - Good in my opinion!
This means you have a 32mm hole to put a blow-lamp into. Looking at manufacturer's data, a Bullfinch blow-lamp - dia 32mm (ref 1240) generates about 8.5kW of heat. As these burners are designed to draw-in maximum air - balanced to the max. gas through the jet - they are much more stable than my home-made equivalents, and this is as big as you'll get a burner in that hole. This is a good as it gets in a flue tube. The flame will progress 2/3rds of the way down the tube if not all the way, so you should get 75% of the heat before the hot flame/exhaust goes up the lum! If you can put a simple triangular tube of Stainless wire mesh inside the tube you'll increase this by at least another 10% so you'll get 85~90% of the heat into the boiler - by my reckoning.
3 tubes like this at 85% efficiency will be ~21kW... which should pull the skin off a rice pudding... - But only if 2 tubes with burners you will keep the safety blowing-off when in the station, but run out of puff when you reach an incline! - Can you fit 4 or 5 fire-tubes and burners in the 5" boiler tube? - You'll need 19mm of metal between each tube and from tube to outer edge of the tube plate.
The biggest problem is lighting burners. This needs a clever and reliable arrangement of spark ignition, or a means of withdrawing burners for ignition.
Have fun!
K2

 

[wazrus]

Steamchick/K2
Many thanks for your efforts and I do like the idea of not making burners or jets, especially, as you suggest, the flame should blow about 2/3 of the way down the flues. That sounds pretty well OK by me! As you say, too, your efforts (and mine) building burners etc weren't too auspicious. I have ordered a couple of 'sets' of weed burners, which comprise handle and other bits and the only items of real interest are the burner heads themselves. The only issue is that the burner 'sets' come in 30mm, 40mm and 50 mm diameters. the 30mm is the one of most appeal, of course. Not quite the 32mm, but the set is priced at $A30, so I won't quibble too much: there shouldn't be too many issues adapting the 30mm job to the 32mm flue. like you, I think, I'd rather not mess around making the things, if it can be avoided!
Medical things have intervened yet again and it may be a while until I get sufficient time in the shed, especially now that the Southern hemisphere is entering summer and the stinking heat is on the way. I do hope, too that my Australian sense of humour hasn't been mistaken for insult, in calling Cds4byu a 'cheeky bugger'. That's Australian for a sort of endearment.
Wazrus

 

[Steamchick]

Hi Wazrus,
I have no problem making jets down to 0.25mm drill. But without the boiler for tuning against back pressure, and cooling (which all changes the combustion pressures and air fuel ratio) the burner is cheaper and easier if a proprietary item. I have bought a few second-hand blowlamps for burners, gas- taps & gas connections. Then fitted them in my own applications. But I have made my own blowlamp style burners, with the trials and tribulations of the various boiler influences on the combustion. From experience, I have even re-worked old early attempts and made better burners. So it isn't quite as easy as one hopes!
Maybe your 30mm burner in a 32mm ID tube will leave 2mm for a wire to the spark for an igniter.? Extracted from an old cigarette lighter? For the wire, I would use flame-proof wire from a scrap electric cooker. But then making the sparky end is a problem I have not yet tackled. There may be room inside the 30mm burner - where there is more space? - to insert a piece of ceramic tube, such as from a domestic fuse, with steel or copper wire up the middle to where a spark will light the burner? This wire takes the spark from the wire off the piezzo igniter. Bit of design thought needed.... Over too you?
K2

 

[Steamchick]

I could use some of your Stinking Heat. 22 deg. C in summer - MAX. now 5 deg. C but feeling sub-zero. We may even get down to -2deg.C this winter if it is very cold... We have had snow for 2 days already this winter, but the roses and chrysanthemums are till in flower. The cold killed the dahlias though... Some spring plants are starting through... - they will die if we do get a week of freezing weather.. No wonder we Brits complain about the weather all the time!
K2

 

[HMEL]

The states have jurisdictional authority of all codes. And it is in this respect one must be careful because what some do is reference the ASME code and put additional requirements in another section. These requirements can include special testing requirements, permits, fire protection and a host of other things they deem important. The ASME code is a recommendation not a law. However the state makes it a law. Military follows a different set of rules.

I now of one State that did not have any laws until they blew up a 30,000 lb hr boiler with a steam explosion. They now have more codes then you can shake a stick at.

Now for purposes of administration they sometimes limit the cubic volume of some systems because for instant, the pressure vessels of CO2 containers would just be to difficult to inspect and monitor.

Insurance companies do most of the inspections with certified inspectors. But the ASME code does provide a section on miniature boilers. If commercially built require an ASME stamp in most places.

The issue becomes more serious when you bring these model boilers out in to the public where they can hurt someone. Most of these are small enough not to present a hazard. There are exceptions.

And it is to that end you have a responsibility to determine just what you have built. Especially when you take it to a model steam show. We are not likely to build a boiler under a ASME stamp shop permit but I would hope you would use good engineering principles to construct your unit. The ASME provides good guidelines to follow.

 

[Steamchick]

Thankyou HMEL. I think this supports my statements expressed in post# 81. "The ASME regulation I refer to is the design regulation. The ASME limit of 100psi for silver soldered copper boilers is based on material properties which do not change by nation or state, so maybe it is good for us all to follow that limit (e.g. In the UK or anywhere else outside of USA).
The hydraulic test I found from ASME, based again on permissible stresses in annealed copper at elevated temperature." etc.
good to know someone else thinks following ASME is a "good idea", even when not actually law....I do notice a reticence in some contributors to follow the natural laws of the strength of materials, Engineering calculations limited by those laws, etc. on the basis of "the laws don't apply where I live". In some cases "Ignorance is not Bliss!". - Just risky.
K2

 

[Steamchick]

Hi again HMEL. Please can you advise the sections of ASME code covered here? - "the ASME code does provide a section on miniature boilers."
Possibly I have been using the wrong section/limits for my "miniature boiler" calculations?
Thanks,
K2

 

[wazrus]

Steamchick

Around 1993, I built my first steam creation, a Bolton No.7 mill engine. This sat around for years, slowly deteriorating. It had done some steaming using the bulging vertical boiler. After the bulges, steaming stopped, but the engine lives on and has been the subject of a full restoration and 'prettying up'. It came up looking pretty good, I reckon! So that's been my time-consumer of late and the work can be carried out in (relative!) comfort downstairs in my shed. In our stinking Summer heat (some days at +40C., Sydney), the mezzanine of the shed is uninhabitable and I do move from one end of the shed (downstairs) to the other, 'following' the sun.
Medical issues seem to be on the back burner, but more heart issues loom for the not-too-distant future.
The vertical boiler also lives on and I have bashed out the bulges during dismantling. I'd rather not try to add stays, as from some experience, this is a sure way to have a whole lot of other stuff let go. So I'll de-rate it to 80 PSI, light the kettle and run.
The other two boilers are waiting for Winter. You'll agree that a near red-hot boiler on a 'nice' Summers' day isn't for an old wannabe engineeer.
I do note another contributor's post where he suggests K.N.Harris's book 'Model Boilers and Boilermaking'. My experiences of that book leave something to be desired. But the AMBSC codes, copper and steel(s) are excellent.
The 'in flue' burners I'm contemplating are still current and to this end, i bought a 'pipe burner' from the internet and this thing makes my efforts look like rocket science. This is basic stuff with a capital 'B'! Not much more than a pipe tee, a reducer and a 400mm length of pipe. All in stainless steel though and the jetting arrangements via the 'venturi' (the reducer) look reasonably capable.
Rgards,
Wazrus

 

[HMEL]

Hi again HMEL. Please can you advise the sections of ASME code covered here? - "the ASME code does provide a section on miniature boilers."
Possibly I have been using the wrong section/limits for my "miniature boiler" calculations?
Thanks,
K2

k2:

Without doing research to see if these specs have been updated I will share with you what I have. To be considered a miniature boiler the following criteria are used.

1. Does not exceed 16 inches in shell diameter
2. Less then 20 sq feet of heating surface
3. Less then 5 cubic feet of volume
4. 100 psig of Maximum Allowable Working Pressure

They must be tested at three times the Maximum Allowable Working Pressure.
Note that if you exceed 100 psig you are not a miniature boiler. Regular boilers can be tested one and half times allowable working pressure.

All other stress calculations and material constraints still apply although not necessary to heat treat welds or do radiography.

So in short you are doing the right things.

Not sure this helps but that is how I interpret this section.

 

[GreenTwin]

I would be a little cautious about testing to three times pressure.
In the electrical industry, there are high-pot tests that can actually damage the conductors.
For a welded steel boiler, or a correctly silver soldered boiler with sufficient copper thickness, I would think the 3 times pressure may be ok, assuming it is a good design without upsupported flat surfaces, but some model boilers will probably not withstand 3X pressure without damage.

You have to watch the rated pressure for fittings, and things like that, so you don't exceed their rating.
It is not necessarily just the boiler to be tested, but the boiler assembly with all fittings and devices.
.

 

[comstock-friend]

HMEL: "Without doing research to see if these specs have been updated I will share with you what I have. To be considered a miniature boiler the following criteria are used.

1. Does not exceed 16 inches in shell diameter
2. Less then 20 sq feet of heating surface
3. Less then 5 cubic feet of volume
4. 100 psig of Maximum Allowable Working Pressure"

Exactly what my July 1, 1974 copy says!

Concern about damage at 3x pressure is slight as the minimum wall thicknesses specified should take care of this. Last time I delved into the Boiler and Pressure Vessel Code was December 1984 so it's been a while. But I was pretty good at it until then!

John

 

[Steamchick]

Hi Green Twin. Your idea that a boiler may fail at 3 x NWP is contradictory to the design practices I used in industry - unless I mis-interpret what you mean?
I understand the codes (like ASME) declare a "limiting Tensile strength for copper of 210MPa (or whatever, depending on Code) for the Annealed Copper, then de-rate that so at 100 psi you have a Permitted Max stress for calculations of 3000psi at 400deg.F. (Sorry to mix units - 210MPa is 30457psi.) Effectively a FACTOR OF SAFETY of around 10....
Ergo, if the boiler is designed to such a code, then a 3 x NWP should not cause any permanent damage to a properly built boiler?
Facts are - in my garage - that I repair/de-rate/strengthen and re-rate/Scrap old boilers that people "give" me so they meet my interpretation of ASME (I use Kozo Hiraoka's well written magazine article on building the Shay Boiler).
But I am in the UK... So why use ASME? - Because I like the Engineering that is presented there. It is written in much the same way as UK Regulations that applied to all manner of features of Electrical equipment I design for High Voltage Electrical equipment. The Code simply tells you what to do, step-by-step, to determine what size everything should be... based on fundamentals of the material being used (Copper or various grades of steel). Certainly in industry, the various applicable Regulations, codes, etc. made the design Engineer's job simple - and if you didn't follow the rules you had to explain "why" to a board of professionals who were going to buy your equipment - or "Not"! - A 3 day design audit of my new design of 400kV switchgear - in front of 4 Fellows and Chartered Engineers from the Central Electricity Board Design department was the easiest exam I have undertaken - they just audited that all my calculations met the rules... and where the rules were not binding or didn't exist they approved all my calculations, developments and confirmation tests. Some of which were for strength, some durability, some destructive and some non-destructive. A clean sheet is my only commendation/reference. I just did my job as prescribed.
So from that experience, to worry about a failure at 3 x NWP seems contrary to all the codes etc. that I have read on model steam boilers, that has FOS of 6, 8 or more (even the old/obsolete treatise on the subject).
To that I add that all the "failed" boilers that I have experienced have not been due to poor workmanship but a lack of good design to any codes or texts. To calculate the stresses does not task many Engineers - many codes have look-up tables to by-pass "heavy" complex calculations.
Wazrus: Yes, due to development of computers and codes - based on real world experience - K.N.H. and others may use "old, out-dated", and "simple" limits that do not suit "today's" thinking. But his basic methodology is a lot better than nothing. I should like to share the parts of his book that you don't like, as we may have the same ideas.
Green twin: On the "High Pot Test" experience to which you refer of Electrical test that have failed seemingly good equipment, I am unsure what the jargon means in my English, so cannot comment. (I spent a few years in Busbar design for power installations 3kV to 33kV, including witnessed HV, thermal and Short circuit tests, etc. But I don't recognise the "High Pot Test" - unless that was a test following a session celebrating the success of a test series?- Smokin!).
I agree the rating of ancillary components is important, and should not be exceeded in Normal use, but the high factors of safety are applied to the "bare boiler" - and I read of a manufacturer who proof tested their designs at 8 x NWP "to be sure"... even though the UK Standard is a proof test of 2 x NWP for initial manufacture to an "approved design". A real issue may be that some boiler inspectors do not really appreciate how to check calculations used in a boiler design, and cannot spot omissions....
And there are many "old" heritage designs that will not withstand "modern" stress limits at test pressures> 2 x NWP!
That is before the can of worms when you consider modern thinking of fire-box temperatures, stresses, etc. and "rated" pressures for tubes in compression, etc. that are simply not covered in most text books, but are considered in ASME, etc.
Most "failed" boilers that I have seen have had thinner than needed copper flat plates, and poor joint design at stress-raising positions.
Wazrus: do a slow and careful hydraulic test while measuring deflection of the surfaces you know had previously bulged. I make a simple gauge to fit across the boiler at strategic points - with a few 'thou gap - and then determine the hydraulic pressure that takes up (say) 0.003" on the gap with a smaller feeler gauge inserted. Check after releasing the pressure that the copper return to the original gap. Call that the ultimate pressure limit, divide by 6 (or whatever Factor of Safety your local codes require) and you have a good NWP! So you can then do a safe Hydraulic test at 2 x NWP - set the safety relief valve accordingly to 1.1 x NWP, and run up to Safety lift without damaging or overstressing the boiler - or yourself!
I derated one boiler (that had regularly cracked a joint at ~90psi) to 15 psi - as that was the limit predicted by my calculations - and it is now certified and runs a suitable engine. Another regular failure at ~60psi was de-rated to 25psi, etc. The sums work for me! People "without sums" who select material that is generally too weak make boilers that bulge, crack joints, have repeated leaks, etc. and do not have a happy life.
Safe steaming! (Apologies for such a long contribution!).
K2

 

[GreenTwin]

I think you summarize it pretty well in that there is a wide variety of boiler types and designs, and some are good, and some are bad.

And building a boiler requires some pretty rigorous skills sometimes, such as meticulous silver soldering, or certification-grade welding and fittings.

So any good boiler design built by a competent boiler builder should be able to withstand 3X test pressure.
But what I often see is poor boiler designs, and skill sets that are not what they should be for boiler construction.

So my concern would be that a 3X test may stress some of the parts, but perhaps not cause a failure. So then the stressed/damaged parts may fail during normal operation.

The 3X test seems to assume that the boiler is built correctly, and that the boiler will not be damaged by a 3X test, but I have my doubts, give some of the hobby-built boilers I have seen.

There is really no control over what some hobby person (like myself) assembles in the shop. Some folks are not aware that they must avoid flat unsupported surfaces on a boiler, and so these joints can and will crack.

All I can say is to perhaps follow a proven boiler design; avoid flat unsupported surfaces, and assemble it using a method that you have expertise in.

And the wall thickness of everything is of course important from both a strength and sometimes corrosion standpoint.

It is hard to make more than general statements without referring to some particular boiler design/build.

The "hi-pot" test is "high-potential dielectric strength test", where an elevated voltage is applied for some short period of time, and the leakage current detected. This to some extent mimics a voltage transient that the device or cable may see when in service.

If you get too aggressive with high pot voltages and times, you can actually punch tiny holes in the cable insulation. Medium voltage cables have multiple layers of insulating material, and they can be damaged with high pot tests in ways that only become apparent over time.

Sort of like pushing the gas pedal all the way down in your car with the car in neutral, and the engine rev's above redline, but does not fail.
So you assume that you did not damage the engine, when in fact you may have caused some hidden damage, ie: deformed part shapes or surface damage.

.

 

[Steamchick]

For an 18kV busbar system in a power station we were confirming at 27kV AC rms.... with a test transformer to be sure all clearances and sharp points were within design requirements. Before the main Generators and transformers were energised with 125MW. at 18kV that would blow big holes in the connections if a spanner was left in the works!
Working in a cable manufacturer every drum of cable was fully tested with AC and spikes at design voltages. And samples were tested at random to pierce voltage (under water). So I suspect your damaging test was beyond designed limits for the dielectric?
Fun!
K2

 

[Bentwings]

k2:

Without doing research to see if these specs have been updated I will share with you what I have. To be considered a miniature boiler the following criteria are used.

1. Does not exceed 16 inches in shell diameter
2. Less then 20 sq feet of heating surface
3. Less then 5 cubic feet of volume
4. 100 psig of Maximum Allowable Working Pressure

They must be tested at three times the Maximum Allowable Working Pressure.
Note that if you exceed 100 psig you are not a miniature boiler. Regular boilers can be tested one and half times allowable working pressure.

All other stress calculations and material constraints still apply although not necessary to heat treat welds or do radiography.

So in short you are doing the right things.

Not sure this helps but that is how I interpret this section.

Very interesting I had called a local heating company and was referred over several transfers to some boiler inspection department. The guy tol me that as long as I was going electric heat I did not need inspection . It was too small for them to bother with . They said I could not use a fuel boiler indoors so even some of the model boilers would have been illegal in doors where I lived. I’ve temporarily shelved the boiler as my steamers run fine on compressed air . I have a near silent air compressor hidden in a closet .