What is the purpose of a hydrostatic test conducted at 1.3 to 2.0 times the maximum allowable working pressure? Is this primarily a leak test? Does this pressure validate the structure integrity of the component? Your opinions, comments and intrpretations are most welcome.
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Hydrostatic Testing 6
- Thread starter JB23
- Start date
It's not just to see how good your welds & pipelines are.
There's also the autofrettage effect to produce favorable stresses.
At hydrotest, overpressurization produces yielding stresses in highly stressed areas [@ P> operating pressure].
When you take the load off, these go into compression.
Then when you run @ the normal pressure, they're < yield.
There's also the autofrettage effect to produce favorable stresses.
At hydrotest, overpressurization produces yielding stresses in highly stressed areas [@ P> operating pressure].
When you take the load off, these go into compression.
Then when you run @ the normal pressure, they're < yield.
Well said, arto.
I would add that this autofrettage (as you call it) has importance only in a fatigue environment: the fatigue resistance of the vessel may be improved, certainly not the resistance to bare pressure.
Of course, JB23, it's not only a leak test, but also a proof test: it ensures the structural integrity of the vessel to the design pressure with a margin of safety added to it. prex
Online tools for structural design
I would add that this autofrettage (as you call it) has importance only in a fatigue environment: the fatigue resistance of the vessel may be improved, certainly not the resistance to bare pressure.
Of course, JB23, it's not only a leak test, but also a proof test: it ensures the structural integrity of the vessel to the design pressure with a margin of safety added to it. prex
Online tools for structural design
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At times, pressure vessels are required by CODE to be stress relieved and yet hydrostatic testing is still preformed.
I agree with prex; that by far it is a Proof Test. During every survey conducted at work; we routinely conduct hydrostatic testing on all pressure vessels.
I would caution you about the test pressure and temperature in view of the following:
ASME Sect VIII, Div 1; bases its allowable stress on tensile/3.5(used to be 4.0 prior to 1999 addenda) or 2/3's of the yield which ever is lower; tabulations are outlined in Sect. II , Part D. It is Dangerous not to reflect your hydrostatic pressure against the minimum yield value of the material in question especially at any factor above 1.8 X the design pressure.
Special attension must be in place for the minimum metal design temperature. The interesting question would be why did ASME move from X1.5 --> X1.3 factor for hydrostatic pressure against the design pressure and yet did not relax the same testing condition for pipelines designed against B31.3/4/8
I agree with prex; that by far it is a Proof Test. During every survey conducted at work; we routinely conduct hydrostatic testing on all pressure vessels.
I would caution you about the test pressure and temperature in view of the following:
ASME Sect VIII, Div 1; bases its allowable stress on tensile/3.5(used to be 4.0 prior to 1999 addenda) or 2/3's of the yield which ever is lower; tabulations are outlined in Sect. II , Part D. It is Dangerous not to reflect your hydrostatic pressure against the minimum yield value of the material in question especially at any factor above 1.8 X the design pressure.
Special attension must be in place for the minimum metal design temperature. The interesting question would be why did ASME move from X1.5 --> X1.3 factor for hydrostatic pressure against the design pressure and yet did not relax the same testing condition for pipelines designed against B31.3/4/8
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- Thread starter
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PVRV, Please elaborate on the caution regarding reflecting the hydrostatic pressure against the minimum yield value of the material, especially at 1.8xdesign pressure. How was the 1.8 factor derived? Are you suggesting that the proof pressure should be selected to ensure that the allowable stress is not exceeded?
Is this applicable to cast iron material? ASME Code requires a safety factor of 6.67 for hydrostatic testing of cast iron vessels and specifies a standard hydrostatic test pressure of 2.0 times the maximum allowable working pressure?
Is this applicable to cast iron material? ASME Code requires a safety factor of 6.67 for hydrostatic testing of cast iron vessels and specifies a standard hydrostatic test pressure of 2.0 times the maximum allowable working pressure?
JB23,
1. Ductile_General (CS)
It often that the allowable stresses are drived from the minimum tensile strenght. When hydrostatic testing factors are exceeding 1.5 the design pressure one is getting dangerously close to the minimum yield strenght value.
1. Ductile_General (CS)
It often that the allowable stresses are drived from the minimum tensile strenght. When hydrostatic testing factors are exceeding 1.5 the design pressure one is getting dangerously close to the minimum yield strenght value.
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- #9
"The interesting question would be why did ASME move from X1.5 --> X1.3 factor for hydrostatic pressure against the design pressure and yet did not relax the same testing condition for pipelines designed against B31.3/4/8"
The basis of the revised test factor has to do with the change in design margin (Allowable stress increase) from Specified Minimum Tensile Strength (SMTS) x 1/4 ---> SMTS x 1/3.5 with the 1999 Addenda of ASME SC VIII Div 1.
Under the old SMTS x 1/4 criteria, the resulting hydrotest stress was roughly 0.375 x SMTS (SMTS x 1/4 x 1.5 = 0.375 x SMTS).
With the new SMTS x 1/3.5 criteria, what factor would maintain the same hydrotest stress of 0.375 x SMTS? Or...fill in the blank for the following...
SMTS x 1/3.5 x ___ = 0.375 x SMTS
If you solve this, you get 1.3125, which rounded to two significant figures gives you 1.3.
The allowable stress bases in the B31 standards are not the same, so there was no reason for them to change.
The basis of the revised test factor has to do with the change in design margin (Allowable stress increase) from Specified Minimum Tensile Strength (SMTS) x 1/4 ---> SMTS x 1/3.5 with the 1999 Addenda of ASME SC VIII Div 1.
Under the old SMTS x 1/4 criteria, the resulting hydrotest stress was roughly 0.375 x SMTS (SMTS x 1/4 x 1.5 = 0.375 x SMTS).
With the new SMTS x 1/3.5 criteria, what factor would maintain the same hydrotest stress of 0.375 x SMTS? Or...fill in the blank for the following...
SMTS x 1/3.5 x ___ = 0.375 x SMTS
If you solve this, you get 1.3125, which rounded to two significant figures gives you 1.3.
The allowable stress bases in the B31 standards are not the same, so there was no reason for them to change.
ananda2610
Mechanical
for large pressure vessels, especially mounded lpg storage vessels that are placed on a sandbed and covered by a large sand/earth mound, hydrotest is a key evaluator for foundation integrity and the quality of sandbed compaction.
At a practical level, very often, its the foundation that really gets tested. Particularly if its a piled foundation that can tend to containmany more variables.
At a practical level, very often, its the foundation that really gets tested. Particularly if its a piled foundation that can tend to containmany more variables.
Can anyone enlighten me on the following:
a vendor of a sectional cast iron boiler (Weil Mclain 88, just told me that he could not meet the spec of 100 psi pressure rating because cast iron sectional boiler could only be rated up to 80 psi "by code".
He did not know which code this requirement came from.
We're pumping out of the boiler (per O&M), not into it (as spec'd).
Thanks
Reinhard
a vendor of a sectional cast iron boiler (Weil Mclain 88, just told me that he could not meet the spec of 100 psi pressure rating because cast iron sectional boiler could only be rated up to 80 psi "by code".
He did not know which code this requirement came from.
We're pumping out of the boiler (per O&M), not into it (as spec'd).
Thanks
Reinhard
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The subject of fatigue has been mentioned. The hydrotest is one cycle in the expected fatigue life of the vessel. I have been in inspection and fabrication for a number of years, and vessel DO leak during the hydrotest. Fatigue failure occurs because no one yet has figured out hwo to pour steel than on cooling is crack free. During the first pressure cycle any cracks (usually on the microscopic scale) grow, thus reducing the life of the vessel. The 1.3 limit has been carefully considered. Parts of any vessel that is subject to the hydrostatic proof test will experience yielding. Have a care. The codes give very good advice.
Good software such as CodeCalc and PVElite which I use safeguard the user in this regard.
Good software such as CodeCalc and PVElite which I use safeguard the user in this regard.
I am rather interested in discussing real or imagined benefits from initial shop hydrotest on an ASME Sect III or Section VIII Div 2 vessel. With the quality and testing of material, the level of design and fabrication scrutiny, it surely cannot be a “proof test”. Is anyone aware of a Section II or VIII Div 2 (or for that matter Div 1) vessel failure at initial (shop) hydro? I can understand the periodic hydrotests – but not the initial hydrotest.
There is some benefit to reducing residual tensile stresses in welds and at stress risers but residual stress considerations is part of the design.
Is anyone doing NDE after hydro? Maybe that is a benefit (i.e. subsurface defects could open up to surface)?
Maybe a leaking gasket or a leaking seal weld could be found. But that is a leak test, not a proof test.
Historically, I believe hydrotests must have served vale at a time when prevention of brittle and ductile fracture were not well understood. Then it could have been considered a “proof test”.
There is some benefit to reducing residual tensile stresses in welds and at stress risers but residual stress considerations is part of the design.
Is anyone doing NDE after hydro? Maybe that is a benefit (i.e. subsurface defects could open up to surface)?
Maybe a leaking gasket or a leaking seal weld could be found. But that is a leak test, not a proof test.
Historically, I believe hydrotests must have served vale at a time when prevention of brittle and ductile fracture were not well understood. Then it could have been considered a “proof test”.
Cheops
Mechanical
- Jan 19, 2003
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As a reply to sect3: I would not stand next to a pressure vessel operating at its Max Allowable Working Pressure unless that vessel has already been hydro-tested at 1.3 times the MAWP. The hydro test covers for all uncertainties, whether it is a faulty design, a defective material, a defective weld, etc .... And believe me, these things do happen !!!! It is too dangerous to operate a vessel under pressure without having it previously tested with water or other non-compressible fluid.
cheops is right on.
The hydrotest in the shop makes any failure a direct responsibility of the fabricator- it is right up close and personal in his shop- and he has to fix it within the original schedule and cost. If it is discovered in a field hydro, there are endless arguments over who is at fault and it is not always a default judgement that it is a shop fault- it could be argued that it was caused by shipment, installation, testing ,etc.
The discussion about QC related to sect III or sect VII div 2 places a lot of faith in paperwork and glossy words- the rubber hits the road when you hydrotest.
The hydrotest in the shop makes any failure a direct responsibility of the fabricator- it is right up close and personal in his shop- and he has to fix it within the original schedule and cost. If it is discovered in a field hydro, there are endless arguments over who is at fault and it is not always a default judgement that it is a shop fault- it could be argued that it was caused by shipment, installation, testing ,etc.
The discussion about QC related to sect III or sect VII div 2 places a lot of faith in paperwork and glossy words- the rubber hits the road when you hydrotest.
I agree with Cheops & davefitz, a hydrotest is your last line of defense against problems due to any number of errors. Anything from an undetected flaw in a casting, a miscalculation in the design, last-minute revisions not being included in calculations, I've even seen one 300# vessel where the wall thickness on the heads on the approved fabrication drawing was thinner than the head thickness used for the design calcs, enough to reduce the vessel MAWP on recalculation to less than operating - resulting in a 300# boat anchor. A lot of things happen to a vessel between the time someone designs it and someone actually builds it, a lot of cracks to slip through. There's a reason you don't stand next to a vessel or piping system during hydrotest, they can (and have) kill people when components blow.
Are there any (ASME, EN, etc.) guidelines for failure testing of a pressure vessel? We have a couple small (1 & 5 liter), cast aluminum tanks with a bolt-on head for which I am currently writing a test spec. I'm struggling with what contitutes failure. The max operating pressure of each tank is 100psi and we pressure test every tank to 2X that.
But now we want to see where they fail. What would be the minimum design failure limit for a such a vessel? Is there a hard requirement or rule of thumb? Also, what constitutes failure? From what I understand, past testing suggests that the head deforms enough to cause the o-ring seal to start leaking. Almost like a built in relief valve.
Hopefully this isn't considered too far off the thread.
Thanks,
-Mike
But now we want to see where they fail. What would be the minimum design failure limit for a such a vessel? Is there a hard requirement or rule of thumb? Also, what constitutes failure? From what I understand, past testing suggests that the head deforms enough to cause the o-ring seal to start leaking. Almost like a built in relief valve.
Hopefully this isn't considered too far off the thread.
Thanks,
-Mike
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