Are we too comfortable with existing standards?
I recently read about Robert Frith in an article by Michael Abrams, an independent writer. Frith is a mechanical engineer and Founding Director of FE Consultants in Brisbane, Australia. He reviewed the process for evaluating the integrity of pressure vessels at the time of fabrication. He determined that when pressure vessel fabricators make a new pressure vessel they have a few options available to them.
To take full advantage of the strength of the vessel, they need it to be inspected for an additional cost. Alternatively, they can go in for limited inspection, add some material and claim 85% weld efficiency. Or they can choose not to have any inspection at all, make the vessel even thicker and claim 70% weld efficiency.
Frith tried to find out where these numbers came from and looked into ASME history. He couldn’t find anything and decided that somewhere along the line history has lost it. The most he came up with was that sometime in the 1920s a number of pressure vessels were pumped up until they leaked and, based upon that data, the subsequent standards have been in effect ever since.
But whatever the level of inspection, a pressure vessel is also subject to a hydrostatic test, which pumps it up beyond the pressure the vessel was intended to maintain. Frith asked the question: “Why don’t we just push it to the limit? Why don’t we go backwards and say: ‘How much pressure do we have to apply before it’s genuinely distressed?’”
The hydrostatic test would seem to test weld efficiency as well as any inspection when pushed to such limits. “I’m saying, why can’t we put the control back to the fabricator, let him make the call and we’ll prove him right or wrong with a test?” asked Frith.
Another issue is weld technology. “All the tools are from the 1920s. You’d think in 90 years we might have more control of our welding product,” said Frith. “Submerged weld techniques basically give you an all but defect-free weld. Why can’t we start to leverage off that?”
Frith laid out such arguments in a paper published in the International Journal of Pressure Vessels and Piping, called: ‘A proposed new pressure vessel design class’.
There was an institutional resistance to his proposal and discomfort with the idea of foregoing inspections. Nobody seemed to want a vessel like that without inspections!
Currently, fabricators have to balance the cost of inspection against the cost of additional material. If they want to have a non-inspection, they’re penalised and have to buy more steel. That increases the cost for fabricators and ultimately for pressure vessel consumers. It’s no surprise then that pressure vessel producers have shown enthusiasm for Frith’s idea.
Frith does not suggest a sudden abandonment of all inspection, of course. “But it needs further thought,” he said. “We need engineers to sit around the table and hammer it out. They have to allow themselves to think outside the square.”
Frith intends to submit a questionnaire to fabricators with questions such as: ‘How often do you get weld failures in your inspection?’ The answers to such questions would prove essential in determining the value of inspections as they stand.
“I don’t deny for a minute that it’s early days,” said Frith of his proposal. “It’s out there to challenge the industry. Can we work to get a more modern solution to what we had in the 1920s?”
The article raises some questions about the origins of standards. Have the basis of the standards, the requirements for inspection and acceptance standards been adequately scrutinised at regular intervals and appropriate adjustments made for advances in fabrication processes and non-destructive testing capabilities?
Robert Frith’s article may not result in changes to existing standards but they will hopefully stimulate discussion, review and scrupulous examination about the origins of these standards.