Hi Charles.
In my defence... I shall explain where I get my conclusions.
I follow the "latest" ASME information that I have - an article written by Kozo Hiraoka and published in Live Steam and Outdoor Railroading magazine in December 2006... Possibly this is out of date, or "not applicable" to where you are...?
But I have been a professional engineer for over 40 years, including some time with higher pressure air systems (NWP 28 Barg - 10in dia piston motor). During my time on that design work I experienced British standards for pressure vessels and Company standards, which didn't always match. The Company standard needed updating from its 1950s source, as the British standards were tighter limits for some odd bits. (I can't remember the details - just information in the making of pressure vessels...). So it doesn't surprise me that there are thousands of old boilers working satisfactorily to the "standards" of their time, yet do not comply with latest Standards and Regulations.
So with this background, and having collapsed a flue tube on a boiler that "shouldn't have collapsed", I set about checking the compressive strength of Copper, and confirming the hoop stress calculations (from the interweb...). I found that typically, at the temperature of steam at 100psi, the compressive strength of copper is only 21% of the tensile strength at that temperature (~200deg.C.). (A university paper - graph of strengths with temperature). There are also many empirical methods of calculating the deterioration of compressive strengths of tubes due to the distortion of the tube (Elliptical, distortion of round, etc.). And there is a Stress Concentration Factor (I was advised by someone who's job it was to know... but don't know which ASME clause, etc.) which must be considered when there are penetrations in the shell (both in tension and compression). The standard SCF for ASME is 3.3, to be used to determine the stress in materials in the design of pressure vessels. (I had been using SCF of 2.5, up until then, based on my "job standards" from 1980s).
I have been able to compile a spreadsheet which takes into consideration:
The Hoop stress based on:
* the calculation of hoop stress given by:
Shoop = σc = [(pi ri2 – po ro2) / (ro2 - ri2)] – [ri2 ro2 (po - pi) / (r2 (ro2 - ri2))]:
* A SCF = 1 - if without any penetrations, or = 3.3 if with penetrations (Irrespective of how they are reinforced).
* NO consideration for non-uniformity of the roundness of tubes in compression, as "straight and true and undamaged" tubes will be good, and any damaged or distorted tubes will fail by collapsing - probably during the hydraulic test (as did mine!). (Studies and calculations of deformation effects are complex! Straight and true is good, anything else isn't.).
The calculated max stress shall be less than the limits as defined:
* The permissible Tensile stress (based on ASME limits - Kozo's paper) based on temperature of the steam in the boiler.
* A reduction from the Tensile stress limit to 21% of that limit to account for compressive stresses instead of tensile stresses, as appropriate.
Now I appreciate that British standards are different to ASME, Australian etc. but I understood that ASME was effectively the same standard as that applied in Australia? But you would know, and I can only assume such.
Notwithstanding: as an ex-professional engineer, I cannot in all good faith, suggest anything less safe than the safest to the best of my knowledge and experience, and in accordance with suitable Regulations... What the standards were when the Simplex boiler was designed is immaterial to my calculations for helping someone make a safe design of boiler for someone to make. I suggest that with a FOS of 2, the Simplex boiler would be safe in almost all conditions of normal safe working. But that would not meet current regulations for the design of copper boilers for the USA. (ASME). I guess the ASME is based on a FOS of 8 or thereabouts?
I'll be glad if anyone corrects my assumptions and applications of the calculations, or simply corrects any errors I have made. (I am prone to dyslexic typing! - and thinking!). We are all here to learn from each other, so please ask questions, from which we may get better answers.
I hope this and the attached paper I wrote will help any understanding of where I am coming from... so you can correct me where I am wrong and teach me the right thing to do.
Steam safely.
K2
Copper:
Tensile strength = 210MPa. (Deteriorates with temperature)
Compressive strength MIN 45 MPa, (Deteriorates with temperature)
Extract from Kozo's article on Copper Boiler design: