With regard to the safety valve pressure test that number is to insure the safety valve lifts within the pressure it is designed to protect.Thanks HMEL. The problem "in my head" - is simply a major discrepancy between the books on model boilers and my previous (all be it decades ago) design work including some pressure vessel work.
e.g. a book I have been using for 20 years or more (written in the 1960s) has a lot of good advice on design, yet nowhere does he consider compressive strength of tubes with "atmosphere" inside and "boiler pressure" outside. As is well known amongst Engineers, the performance of anything "real" (I.E. not the mathematician's perfect shape) has factors that de-rate the design performance of an object - although they may have been derived mathematically or empirically, they must be considered. E.g. ovality, dents or distortion, built-in "stress-raisers" such as holes for gauges, fire-doors, pipe-work, etc.
- Compressive forces on tubes - not covered by all the texts on boiler design that I have read.
- No consideration in Boiler design texts for stress concentration factors... that are a routine consideration for structural design engineers. (or so I was taught?).
A question: How should I consider a fire-door hole affects the strength of the firebox tube in a vertical boiler? - It is a gap in the "hoop" for hoop strength calculations, and as such raises stress in other parts of the tube that is under pressure from the water jacketing the firebox? The "shape distortion" - and subsequent non-uniform stress distribution - will have a significant effect of the collapse pressure of this tube. How can I factor this into the tube material stress determination?
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Many years ago I collected photocopies of pages from text books as a "quick guide" when I was doing calculations in work. I think these text books still hold truth. Yet in all the texts and Regulations I have seen relating to Boiler design, many things - such as stress raising "holes" in shells - are not considered in the design of steam boilers for modellers. - Yet the rules still apply of "good design". IMHO: Steam is natural, and has no brain to decide when to escape a boiler or not. If the boiler isn't "good enough" - by design or manufacture - or mis-used, then steam will escape, and being naughty, will hurt people. I am allergic to hurt, especially by steam. It makes me scream. So I want to correctly design boilers.
Additionally, where a design may have a factor of 8 (for safety) - or other value - built-in to the calculations to reduce the stress at NWP from "UTS" to "reasonable" in the calculations (or other methodology), the Regulations only require an Hydraulic test at up to 2 x the NWP. As the Safety valve shall inhibit pressures over 106% of the NWP (or other - depending on "Nationality" of the Regulations), why do we not test at twice the "106% of NWP"?
- The first Hydraulic test should first prove the Design is sound. Thereafter confirm the manufacture and materials are correctly applied. (Well, that was normal practice when I was a Designer.) Regulations seem to be "confused" about this?
In a technical article by Kozo Hiraoka, he explains (very well) how the ASME regulations require the consideration of temperature on the strength of Copper, such that a boiler to be worked at 100psi shall be designed using 3000psi as the maximum permissible stress for the silver soldered copper components and boiler construction. But when cold (room temperature The copper has an equivalent maximum permissible stress of 6700psi. Should we not then at ambient temperature be hydraulically testing the Design by taking a boiler to at least 2.23 times the NWP - to at least prove it can work safely at 100psi? (Even NASA can get calculations wrong... as proven by un-planned destructive testing! "I am not alone".). He mentions that the UK test is only 2 x the NWP, and the ASME test is to 1.5 times the NWP...
In my humble opinion, we are not proving the Design with these tests, only that there are no major defects in manufacture. That is OK where a Design is already proven. But where a design is not proven, as with an inherited boiler of unknown design or NWP, surely someone (like me?) should do the sums to determine the SAFE NWP when bringing it back to service after any repairs?
Finally, in their own homes people do a lot of things that are not legislated, or permissible in the public domain. Yet all the guys I know belong to clubs that do have competent Engineers as members for advice, and for testing and certifying boilers. The clubs also hold certified test equipment, procedures for testing, etc.. (Usually above the minimum required by their insurers, for use of members' models in public).
My lack of knowledge is the "standard good practice" for determining by calculation, the strength of various components of a boiler subjected to external pressure. - Internal pressure in tubes is easy to manage. A bit high and the tube pops-out any dents!. But in compression, tubes collapse catastrophically. So I want to do the sums before I destroy boilers. It is just a bit more than "Rocket science" - (They only apply internal pressure to tanks, etc. Which is why they land with some residual gases inside the tanks... and sometimes go "Bang").
Thanks for any advice.
With respect to the copper silver soldered joint it represents a dissimilar metal bond and credit should not be given to the full strength of a good copper alloy. I think the countries that have seen the most failures tend to look at recommendations differently.
Most of the publish recommendations are based on a long history of experienced failures. To achieve that safety margin we use math to get to those minimum standards.
And believe me I have seen well trained engineers argue over the correct way to analyze some of these problems. This also goes for recommendations as to what pressure should be used for hydro testing and the temperature at which it is to occur. You can actually seal off a leak if the water is hot enough during a test. Inspectors do not like that trick.
After looking at your model and if I were doing the calculation for the door I would treat it as a penetration of two cylinders. The external shell and the inner shell. Because one wall sits close the flame it will operate at a different temperature. Thermal expansion will be interesting but because the dimensions are short are probably not going to cause a lot of problems. The ring forming the door should be fairly thick and will act as a large stay bolt for this little unit. I suppose you could calculate the external pressure limitations but I dont think it would be necessary if its at least the same thickness as the shell walls.
Now personally if I was to build this little unit the door located there would go and I would use a refractory fire box to sit the boiler on and put the door in that refractory wall. Keep the heat transfer the same size but make it a little taller and save myself some issues with calculation. However it is just the way I would do it. doesn't necessarily make it better but sure would be easier to build.
Take Care and good luck with your project.