Hydrostatic Testing
Hydrostatic Testing
(OP)
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.





RE: Hydrostatic Testing
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.
RE: Hydrostatic Testing
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
http://www.xcalcs.com
Online tools for structural design
RE: Hydrostatic Testing
Pressure testing as arto and prex state is to redistribute fabrication stresses which can be of yeild magnitude. It's like performing a stress relief by monotomic overload.
RE: Hydrostatic Testing
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
RE: Hydrostatic Testing
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?
RE: Hydrostatic Testing
RE: Hydrostatic Testing
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.
RE: Hydrostatic Testing
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.
RE: Hydrostatic Testing
The allowable stress basis in the B31 are not the same. You are missing the point. Consider, the test stress as percentage against the minimum yield for guidance.
RE: Hydrostatic Testing
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.
RE: Hydrostatic Testing
a vendor of a sectional cast iron boiler (Weil Mclain 88, http://www.weil-mclain.com/literature/literature.htm)
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
RE: Hydrostatic Testing
RE: Hydrostatic Testing
Good software such as CodeCalc and PVElite which I use safeguard the user in this regard.
RE: Hydrostatic Testing
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”.
RE: Hydrostatic Testing
RE: Hydrostatic Testing
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.
RE: Hydrostatic Testing
RE: Hydrostatic Testing
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