Whenever I visit my cardiologist, I insist why doesn't he carry out a CT scan of my heart ? He laughs & tells it's just because he is my friend.
If he could find me visibly sick,he could do without asking.

Isn't the case so for our stress analyst's pre start-up walkdown too ? Unless some items like welds, flanges, supports look wrong,no hydotest leaks or post-start-up vibrations are observed, isn't deep check by stress analyst before start up a conservative idea ?

regards,

sam

Why no response at all from our forum mentors & commoners!

Have I really spelt out the unplesant truth that routine site supervision of all process pipng stress critical lines a costly proposition as it may call for many unnecessary as-built piping analysis and subsequent requirement of piping and support hardware changes ?

Aren't the usual mechanical completion punchlists & pressure tests enough for routine stress critical piping ? For very strain-sensitive rotating machines, analyst's supervision may be necessary.

Way back in June 1997 of ChE, Tony Paulin once wrote in a cautionary checklist for sources of pipe failures that 'the majority of catastropic failures are due primarily to operational failures and have little to do with pipe stress analysis'.

In today's overload of piping stress, what can be called 'necessary' site supervision by stress analysts ? Can't we weed out the unimportant & concentrate on the real issues properly!

regards,

sam

With all due respect to Tony Paulin, who has forgotten more about pipe stress analysis than I care to know, the article you refer to omits one critical concept. This can't be emphasized too strongly.

If we didn't do stress analysis to support our design drawings, the vast majority of catastrophic failures would be fatigue failures of overstressed piping.

This is a human failing - proactive procedures designed to avert catastrophic failures are expensive, and if followed consacientiously usually reduce the incidence of catastrophic failure to a level where people no longer choose to pay for the cost of the proactive procedures. In the case of CAT scans, the people who object to paying for them are the health insurers (or your government agency if you live in a country where that system prevails). In the case of process plants, it's the owners wanting to reduce "unnecessary" costs, supported by ill-considered statements from respected people like Tony.

Field walkdowns and hydrotests do a great job of verifying that piping systems have been installed and fabricated effectively. They can catch a vast majority of the possible types of fabrication errors before a catastrophic failure occurs. But they can't check the design for the things that a good stress analysis will catch. I could design a line to operate at 1000 F and 2500 psig to run 3000 feet in a straight line between anchors. Any competent contractor could install it so that it would pass hydro and any sort of visual inspection that ignored the deficiencies in the line routing and reastraint system I have chosen.

As to what's "necessary," that depends on many factors. Regulations, Codes and Standards, site practices, operating and maintenance practices, and the general competence of the installation forces that can be expected all play a part. One could write several volumes in order to give a proper answer to your one-line question; it's no surprise to me that you have found nobody here besides me who is nuts enough to try to answer it more briefly.

Thank You, Sir, for this reply. But, you missed one important point of the query.

Code & procedures govern which pipings are to be stress analysed.

But, as a routine as-built piping stress analysis resulting from field walk-down by stress analysts on not-so-sensitive stress critical piping can be costly.

Regarding CT scans or treadmill stress tests, similar argument works. What simple dieting & brisk walk can do, can't be done by these tests at great cost on normal active population.

Aren't we biased towards one side because of self interest?

regards,

sam

I believe the great amount of piping analysis going on now is a product of two things - easy access to analysis and reduced experience of the analyst/engineer.

This leads to the owner's or manager's question to change from "Is this system safe/reliable?" to "Did you analyze this system?".

The Codes I know still base their evaluation on the use of a sliderule. It's the industry that's fueling this analysis inflation. With Dynaflex thirty years ago, the typical job had about 20 pipes with one (thermal) load case. Now we have people running 1000's of elements complaining that 99 load cases is not enough.

More analysis may not be the final answer. Much is said about the value of a thoughtful walkdown in the American Lifelines Alliance document on seismic design of piping systems. A good set of experienced eyes can confirm adequate flexibility and ensure survival through a seismic event.

Yes, Sam, as I stated in my earlier post, one could write several volumes in response to the brief question you asked. I make no apologies for what I chose to omit.

As to self-interest; in my case my right to practice this profession is subject to revocation by the general populace as a whole if I should be involved in a project where one of my perceived omissions or errors leads to a serious accident. Of course I have a self-interest in doing an adequate analysis.

Thanks to both of you, Sirs, for these interesting replies.

I only asked for moderation in volume - both in design & walkdown based on available time & experienced analysts.

One set of experienced eyes can keep a watch on 10 confirmed suspects adequately. But, if the self-interest of industry raises the volume to millions of commoners as probable suspects, will the walkdown be thoughtful enough ? Let us eat what we need, not what we want or can afford!

sam