This article has been updated with more details on 2015-04-19
What are you designing right now?
I am designing, building and testing Laboratory pressure vessels and I have a limited experience with large, industrial pressure vessels of the size and class governed by ASME Section VIII, Div. 1 & 2
What do you think about the actual ASME code?
ASME Sect. 8, Div. 1 & 2 were written way back at the dawn of engineering time, long before digital computers were dreamt of. It is arcane, overly complex, badly written and organized. In recent years ASME has slowly woken up to the real world of digital computational engineering and made some small concessions, but really, it needs to be junked and re-written to reflect modern reality. For example, they now permit some level of FEA to be used in areas surrounding vessel supports, lugs and brackets – but it is quite constricted – and those areas are exactly where most of the interesting stress problems arise. The big problem, since much guesswork and empirical experimentation was used to develop that code with section after section being added-on, is that it is very conservative and therefore vessels come out rather heavy, and expensive. In my career, I have tried to avoid it altogether, but we have a more senior engineer in our office who has spent his career in large vessels and, when necessary, I ask him for advice on which section, sub-section, page, UG chart and Div. 2 table, etc., etc. to use, and even then it is painful. He uses FEA constantly as a check on what the ASME code numbers say, and if there is a conflict, I know he leans to the FEA results side.
What is the difference between lab equipment and much higher pressure vessel used in industry?
By comparison, lab equipment is generally small, generally much higher pressure, and is not governed by any code whatsoever – there is far too much diversity of design for that. The engineer is then left to his own devices and experience. In thirty years of working with FEA software and taking this approach to small vessel design I have yet to have one blow up or even crack out on me – the result of careful consideration of Margins of Safety and other factors. The same goes for (non pressure vessel) oilfield equipment generally. In that world there are fewer constraints outside of general directions for MAWP (Maximum Allowable Working Pressure) and test pressures, such as those defined by API (American Petroleum Institute and the SPE (Society of Petroleum Engineers) allowing us the freedom to pay attention to the details we know are important.
What do you think about the following statement?
Models are great for pointing out weaknesses that need to be beefed up prior to a physical test. They are really good for finding design bottlenecks that can be corrected prior to putting welder to metal. BUT, at the end of the day you have to build the vessel and break it. I’m seeing a lot of areas where people are replacing testing with FEA and other computer simulations. Sometimes with disastrous results. When you call modeling “proof” you get bad engineering, bad science, and horrible regulations.
I understand where that contributor is coming from – he is referring to the practice of Test Validation of every design – probably due to a fundamental lack of confidence in the software. Twenty five years ago, when we started using FEA (back when it was all 1’s and 0’s for turning the program flags on and off), yes, we did some validation testing because the technique was new and the entire engineering community, being a conservative bunch, tended to distrust their results and tested quite a bit. The last major validation test I ran on a model for the housing on a high-pressure downhole tool outer casing came in within 2% of predicted collapse values – not bad. The FEA design cost a couple thousand and the testing something like $25,000. Those days are gone. Today, for standard designs with known equipment, testing is the exception, not the norm, and I can tell you for sure that nobody in the pressure vessel industry is “building and breaking” multi-million dollar vessels. Why? Because the FEA codes are so good, so well understood and validated and produce results that are consistently accurate and predictable that few bother with expensive and time-consuming validation tests any more.
Do you think FE Analysis is a good tool for pressure vessel designers?
Of course, like any good tool, to get the most out of it, you have to know what you are doing. So, for example, model mesh quality is important. You have to pay attention to detail in areas of high stress concentration. And, you can’t just use a coarse mesh and assume that you are getting anything like realistic stress numbers.
So in your opinion, what would be the right approach to be able to obtain accurate results from FE Analysis?
The approach we use is to start with a relatively coarse mesh model, find the areas of interest and then focus on them by fining down the mesh in those areas until the changes in stress becomes a game of diminishing returns – but how far to go with that? If an engineer is interested in the accuracy of solution, then he would be well advised to follow the AB-520 standard set out by ABSA (Alberta Boiler Safety Authority), refer to the link below:
In there you will find the key to accuracy – minimising the difference between the Elemental and Nodal results (also called “Averaged/Un-Averaged” results). When that gets down to less than 5% you have an acceptably accurate stress result. It is the only standard for this I have seen, we use it here routinely, and I would recommend it to anyone seeking better results.” Also, you will be making decisions on the relevance of using the Von Mises (root/sum/squared) stress paradigm or Max Principal stresses – depending primarily on your judgement of the orientation and level of the Shear contribution to the stresses.
Any final advice for those in PV industry who want to go for FE Analysis?
One final point I would like to make: If it is properly understood and used by well trained engineers/technologists who are careful and prudent with their choices, FEA is a wonderful tool, freeing us to create new designs, the performance of which exceeds what was possible in the past thanks higher levels of knowledge of their limits and accuracy of results. It is why we can now confidently build drilling tools which break new ground at 35,000 feet beneath the ocean floor or probe the depths of the Marianas Trench, how Carbon fibre composites can simultaneously strengthen and lighten large aircraft, how the engines for those aircraft can be designed to burn very lean mixtures to improve their thermal efficiency beyond 50% – all of this is possible today thanks to the digital computer and advanced FEA software.
Thank you very much Gerry for those valuable insights!
Do you think that Gerry is Right?
Please state your own opinion in the comments to open to discussion