At Hydrotest Pressure how close are we typically to structural failure

[plantprowler]

While witnessing a Pressure Vessel Hydrotest in progress I got thinking: Just out of curiosity how close could we be at that point to structural failure of the Pressure Vessel, given the typical factors of safety in the design codes?

i.e. The test was being performed at 1.3 times the MAWP & the vessel was a SS316 Hydrogenator approx. 8 kL capacity.

So typically at what further pressure would the vessel fail? I suppose there will be some statistical variation but just wondering how much margin is left at the 1.3xMAWP point of test.

Or is there no good general answer to this question & it would depend hugely on the specific design?

 

[zdas04]

The stresses in a vessel under pressure are easy enough to calculate. Generally the hoop stress is an order of magnitude higher than longitudinal stress and can often be ignored (although programs like Compress calculate longitudinal stress and display it, it is rarely a big number in pressure vessels).

Nominal yield point for a metal is published (the published value is usually 1 standard deviation below the experimental mean). The code tends to further reduce the yield point by another 25% or so when they set the multipliers for a static test. When I've checked, I generally find my maximum test pressure around 40% of yield stress.

One time I was building an ASME B16.5 Class 150 vessel (MAWP 280 psig, test 150% or 420 psig) and the QA/QC guy was new and knew for a fact that all tests were 900 psig (i.e., 150% of Class 300). He didn't last long in that job. Anyway, he tested my vessel to 900 psig for 3 hours. Nothing broke. Nothing showed any external signs that the vessel had yielded except the studs on an 8-inch blind flange lengthened about 0.5%. We replaced the studs and re-tested. I lost track of the vessel at 9 years in service and it was still fine. In that case the margin for overpressure was something over 200%. The only static test failures that we ever see are due to material defects.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[TGS4]

A couple of corrections to zdas04's post.

First, the hoop stress in a pressure vessel, away from any structural discontinuity, is exactly twice that of the longitudinal stress.

Second, the "published" values in the ASME Code are actually minimum-specified values. In order to meet the material specifications (in your case, likely SA-240), the material test must achieve at least the minimum-specified value. I do agree with zdas04 that actual values are typically higher by about 25% (typical yield values are often 25% higher, typical ultimate values are typically 15% higher or less), although that will depend on the material.

Now, to answer plantprowler's question... It is useful to understand the various design margins in play for pressure vessels. The allowable stress that is published in ASME Section II, Part D for ASME Section VIII, Division 1 pressure vessels (I am assuming Division 1 because your multiplier is 1.3 - other Codes use different factors) is based on the lesser of yield divided by 1.5 or ultimate divided by 3.5. SA-240 316 has a room temperature allowable stress of 20ksi, a room temperature yield of 30ksi, and a room temperature ultimate of 75ksi. So this material is yield-governed.

Your minimum hydrostatic test pressure of 1.3*MAWP will likely increase the hoop stress from the calculated value of 20ksi to 26ksi. Many owner/users or EPCs would allow the calculated hoop stress to get to 27ksi or even 28.5ksi. Even still, for a vessel to burst, you are going to have to increase the hoop stress to about the ultimate stress, so you have a factor of 75/26=2.88 against the thing bursting - based on minimum-specified material properties. Based on actual material properties, your factor will be even higher.

Of course, this all assumes that the material behaves in a ductile manner. For carbon steels and other materials that display a ductile-to-brittle transition at lower temperatures, it is critical to stay on the ductile side of the behavior - by at least 30°F (17°C) above the MDMT. Many (most) hydrotest failures that I am aware of have occurred because the material is being tested at a temperature at which it is not ductile.

 

[MJCronin]

Plantprowler, Its important to note that, under overpressure conditions flanged joints will commonly leak ....

MJCronin
Sr. Process Engineer

 

[plantprowler]

Plantprowler, Its important to note that, under overpressure conditions flanged joints will commonly leak ....

Thanks @MJCronin!

So is the flange joints starting to leak a telltale sign that you have tested too far? Is that because of just too much elongation?

Are there other obvious warning signs in a hydrotest that one has gone too far? Is exceeding the yield point physically obvious visually etc.?

 

[TGS4]

If the deformation is obvious from a distance, and results in permanent deformation after the pressure is removed, then your test pressure was excessive.

Leaking from flanges from overpressure is typically due to excessive flange rotation.

Is exceeding the yield point physically obvious visually etc?

By a little bit - no, not really. It's got to get really bad before deformation odds visible, in my opinion.

 

[r6155]

Any defect in welds can cause failure during pressure test, though calculations were well.

Regards
r6155

 

[TomBarsh]

Being a structural engineer involved with pressure vessels the term "Structural Failure" has some connotations for me.

Are we talking about a catastrophic structural failure? A tiny pin-prick sized leak under high pressure?

Others have made good remarks above. I would only add that "in general" the vessel is not in danger of catastrophic structural failure at test. However, there can be related issues that can cause substantial failures under pressure. One being material failure due to brittle fracture if the test temperature is cold enough that this may be an issue (ASME Code recommends maintaining test at least 30° F warmer than the MDMT of the vessel).

A vessel "failing" under hydrostatic test would likely not be "too catastrophic"...in the sense of danger to life, limb, and property (other than the vessel itself). But a vessel under pneumatic test is much more efficient at storing energy and a failure under such a test could in the very most critical sense of the term be a catastrophic structural failure. It could very possibly blow up like a bomb. Even flange bolts rupturing and sending off shrapnel is a very real danger. Most shops maintain a healthy distance for personnel during pneumatic tests.

 

[berkshire]

Tom Barsh,
As an apprentice I was too close to a square tank we were testing by the pneumatic method when a bad weld showed up in one seam, the tank opened up and blew me about 6-0" across the shop floor. This test was being conducted at under 1 bar.
B.E.

You are judged not by what you know, but by what you can do.

 

[EdStainless]

When we hydro test tube samples to failure I will stand right next to them, but for air test I am on the other side of a blast shield. We commonly leak test tubing with 250psi air.
When you hydro, if you actually get all of the air out of the system there is usually no such thing as catastrophic failures. The only stored energy is in the stretch of the metal and the compression of the water, which are both fairly low. When a failure starts a small amount of water leaks and the pressure drops instantly. But that doesn't mean that you haven't caused some distortion.
The exception is when you have brittle failure, those cracks will propagate with almost no additional energy input.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[weldstan]

Generally the applied stress is less than 90% of the specified minimum yield stress and most often much less. Weld defects, principally cracks, can easily lead to rupture during hydros where the applied stress is greater than approximately 70% of YS and the toughness is low at the test temperature.

 

[blacksmith37]

I agree with Barsh reply; Hydro test failures I have seen (other than pin-holes or gaskets ) were caused by lack of toughness , not stress/yield problems. This opens a discussion of flaw size and shape,etc. and linear elastic fracture mechanics, - too complicated for me. However , one of the most notable failures I saw was a carbon steel vessel being gas pressure tested ( at 80 F ) to a pressure Of 70% of what it had been operating at at about 500 F a few days before. A very impressive brittle fracture ( 8 ft diameter ).