Hydrostatic testing of pressure vessels 5

[CengMBA]

What is the real reason for the hydrostatic testing of pressure vessels used in industry. Is it:
1)To Validate the design calculations?
2) To ensure that there is sufficient safety margin in the design?
3)To redistribute the stresses induced into the vessel/structure during the manufacturing process?

Should industry continue with this practice and waste time, money and resource or are there better ways to ensure the integrity of vessels and structures.

 

[JStephen]

I understood the reason was to confirm the strength under conditions where the vessel was less of a bomb. If it fails, the energy release is much much less if filled with a relatively incompressible substance like water.

Normally, it wouldn't validate the design calculations unless the calculations were off by a factor of 3 or so. And it doesn't show the safety margin, exactly- it could be underdesigned and still pass the hydrotest. Redistribution of stresses, if any, I would consider an added benefit, rather than the reason for the test.

Would you go out to the airport, and get on a jet that had never been flown before, because someone's calculations showed that it ought to fly?

 

[deanc]

CengMBA...This does sound like a MBA question. Do you have any kids? How would you feel about about a new untested boiler or pressure vessel being installed in the building where they are?

Over the last 30 years I have probably witnessed pressure tests of 100,000+ vessels. 99.99% have been fine,but there have been several failures. These failures were unkown defects of material or workmanship.

If it saved ONE life it was all worth it!!!!!

 

[metengr]

This is a good question. I can attempt to answer based on my involvement with Code bodies. Hydrostatic testing was a test method conceived during the early development of the ASME Boiler and Pressure Vessel Code. The ASME Boiler and Pressure Vessel code uses a design by analysis approach.

As you listed above, hydrostatic testing was intended to verify the design of the component (in other words will wall thickness and geometry sustain actual pressure in service) and to look for gross defects or flaws in material used to fabricate the component.

Over the years, as hydrostatic testing evolved, other benefits were discovered like local yielding of material at stress risers or inherent fabrication flaws. The local stress redistribution during a hydrostatic test can result in decreased risk of crack initiation and propagation after the component is placed into service.

Should industry continue with this practice? For new boiler and pressure vessels this is a must, and will probably never be removed. Hydrostatic testing is a means of assuring structural integrity of materials used to fabricate a new pressure retaining item. At this point, we are not using hydrostatic testing to prove previous designs, it is used to assure no gross defects or workmanship. Safety comes first.

 

[MJCronin]

CengMBA,

Nope, Mr. MBA, your superior intelect has finally discovered our secret....us wacky mechanicals insist on hydrotesting pressure vessels and other equipment simply to waste time, materials and resources. All of this time, the worldwide engineering profession has been trying to fool people like you........but you figured us out !!!

You state:

"Should industry continue with this practice and waste time, money and resource or are there better ways to ensure the integrity of vessels and structures ?"

Sure, there is a much better way to ensure pressure vessel integrity.....through the use of lawyers and punative lawsuits. Have you seen these links ?

http://www.memagazine.org/backissues/february2000/features/setting/origin.html

http://www.casti.ca/books_ebooks/lite/B31_3_lite.pdf

(pages 1 to 4)

http://www.google.com/search?num=50&hl=en&lr=&safe=off&q=asme+explosion+Grover+massachusetts+

In fact, I believe that any PE who is arrogant enough to seal a pressure vessel calculation or drawing recommend hydrotesting should be flogged in public to within an inch of his life...

MJC

 

[vanstoja]

Uninstalled, uncoupled pressure vessels are hydrotested for reason 2, ie, to prove safety margins exist for the component only. To improve stress conditions (reason 3), resort is usually made to annealing thermal treatment rather than to hydroforming by hydrotests which can be damaging if repeated too often. Generally, initial hydrotests are run with a substantial margin, like 150% of vessel design pressure. Subsequent hydrotests, when needed are usually limited to a lower test margin like 110% of vessel or fluid system design pressure. For coupled pressure vessels used in fluid systems, hydrotesting is usually done to prove structural adequacy for transient loading conditions above system design pressure such as swing check valve reverse flow slamming pressure transients which may raise contained fluid pressures as high as 150% of system steady-state design pressure. Also, coupled PV system hydrotests prove the adequacy of coupling devices or procedures such as flanged joints and connection welds between PVs like piping, heat exchangers, pumps, valves, reactor vessels, etc. There is nothing frivolous about hydrotesting pressure vessels and pressurized fluid systems. To ignore hydrotesting such components and systems would be unprofessional, unwise and possibly criminal if design codes are involved.

 

[arto]

The shop guys think it's to check their welds for leaks,
the engineers think it's to test their design,
but the REAL reason is per Mr. METENGR:
to yield local highly stressed regions so that they go into compression & are preloaded under normal operating pressure.

That way they never cause a fatigue problem or crack in service.

 

[jte]

metengr-

Good points. I'll point out a minor issue, though. You say that "The ASME Boiler and Pressure Vessel code uses a design by analysis approach."

In fact, this is only partially true. For Section VIII, Division 1 is considered to be a "design by rule" code while Division 2 (and Section III) is considered to be a "design by analysis" code. The new Div 2 will apparently emphasize that even more.

jt