B31.8 fabrication hydrotest to obtain max MAWP but limited to test pressure of 1.5x flange rating

[alsy]

Hello All,

We have a B31.8 600# class fabrication (operating at over 30% hoop stress) that we will be hydro pressure testing with a customer supplied test range of 2160 psig-2225 psig. The customer wants a MAWP rating of 1480 psig. I have stated that the MAWP can only be rated at the highest test pressure/1.5, which if test drops to the lower end of range provided (2160) could result in a lowering of the system MAWP from the desired 1480 psig. I would like to increase the lower end of our test pressure range to 2220 psig to maintain a true MAWP of 1480 psig. Since we need to have some sort of working pressure test range, we would then need to increase the upper end. I typically like a 2% range to work with, which would take the upper range to 2265 psig. At this value, the stress in my piping system is under 80% of yield (so pipe design is not limiting). However, this value is slightly more than 1.5 x the flange rating. B16.5, 8.2.2 simply states that shell pressure test shall not be at a pressure LESS than 1.5 pressure rating of the flange. Are there any code provisions that would allow for this slight increase, or would a flange stress analysis need completed (such as appendix 2 in BPVC)?

Thanks,

"Success is not final, failure is not fatal: It is the courage to continue that counts."
Winston S. Churchill

 

[LittleInch]

The same section also says you can round off upwards to the nearest 1 bar OR 25 PSI. So that gets you part of the way and to be perfectly honest, this sort of minor "over pressure" is negligible and simply not worth bothering about. If necessary, just tighten up your test variance a bit, but a fabrication should only need an hours testing at most so I can't see where your pressure raise or rise is going to come from.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[alsy]

Thanks for the reply, however the majority of our customers require a 4 hour minimal, which is norm in the North East oil/gas producers. Our fabrications are large slug catchers,(and exposed to the weather). It is not uncommon to see a 40 psig pressure swing during the course of the test due to weather conditions. Rounding up only in 25 psig increments only gets me 5 psig gain (from 1480x1.5=2220 round up to 2225). It would not be practical and very difficult to be within a 5 psig variance.

"Success is not final, failure is not fatal: It is the courage to continue that counts."
Winston S. Churchill

 

[LittleInch]

Either which way, this level of variance from the 1.5 time level is negligible.

The actual section you need to use is 2.7 which states "Testing at any higher pressure is
the responsibility of the user, taking into account the
requirements of the applicable code or regulation."

Normally when people check B16.5 flanges to the ASME VIII code it fails at normal pressure, so it's not a get out of jail card.

So basically you can take responsibility and provide strict instructions not to exceed 2265psig on the basis that this very minor increase over 1.5 x rating pressure is negligible

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[SNORGY]

I believe you can test to 1.5 times *ceiling* pressures as tabulated in one of the appended tables in B16.5, which usually affords enough headroom that slight overages above 1.5 x *rating* are not of concern.

 

[TGS4]

ASME B16.5 8.2 (and subsequent sub-paragraphs) speaks to the pressure testing of flanged fittings, and only with regards to a shell pressure test. But note paragraph 8.1

Flanges are not required to be pressure tested

Don't worry about the flanges. They certainly won't break. And this so-called ceiling based on 1.5 multiplied by the rated pressure at the test condition is a fiction, akin to the 90% yield limit in VIII-1 construction hydrotest (which simply doesn't exist).

 

[alsy]

Thanks for the comments. We know flanges can take a higher test value than 1.5. How much higher is not something our customers typically want to engage/commit to. Most customers we deal with either want to see a code provision, or the engineering to back up deviations from code stated parameters. That was my original question: what is the max allowable on the flanges. After researching more into this, I cannot find any code provisions on flanges that give max value or testing parameters higher than 1.5, so any deviations for this particular customer (which is one of the larger natural gas midstream companies in the states), will need to be addressed through additional engineering/calculations on the flanges.

We did this particular test last Sunday. Per site conditions, we started the test later in the evening. Our 12,000 ft^3 (about 90,000 gallon) above ground fixture was still cooling down from being in sun and cooler night time temperatures. We had a significant pressure drop. We started test at 2225 psig, and 4 hours 15 minutes later (with no drips/leaks) we ended at 2162 psig (4 hour test is standard in this area, and some customers require an 8 hr test). Customer's acceptance criteria states no addition of medium, but allows bleed off if pressure is rising. Their acceptance criteria is not based off sound engineering as they acknowledge temperature/volume plays part in pressure change and allow you to bleed off when temperature climbing, but not to allow additional medium added if temperature dropping. Unlike indoor/controlled environments, we have to test outdoors at random times of the day, hence the desire to have a higher starting pressure so we can account for substantial pressure drop, while not dropping below a minimum amount which defines the MAWP. Fortunately in this case, their MAWP was 1440 psig, so we could drop to 2160 psig on our test, which if test would of been any longer, would of dropped below. We had proposed to start this particular test at higher amount, as we foresaw this big drop, but this particular customer does not allow testing over 1.5 of the flanges as stated above. Ultimately this test was accepted, and all good, but a few more degree pressure drop, or a slightly longer test and it would of failed per their criteria.





"Success is not final, failure is not fatal: It is the courage to continue that counts."
Winston S. Churchill