How does a telltale hole fail in a pressure vessel? 4

[JustinME]

I am referring to section UG25(e) of the ASME BPVC Section VIII, where dimensional details of telltale holes are described.

I am investigating the value that a telltale hole can provide when integrated into a pressure vessel containing air below 200 psig, temperatures between -40 °F and 120 °F, and total volumes under 20 gallons (typically near 3 gallon volumes).

What is the failure mode of a telltale hole? Is it similar to the pressure relief valve opening up, with no dangerous phenomena occurring?

A failure could be caused by a cyclical stress that exceeded the fatigue life, or by a stress that exceeded the UTS. Are these two failure modes [of a telltale hole] practically different from one another? If so, how?

Is the purpose of the telltale hole to provide a safe failure mode, or is the purpose to provide a feature that deforms to indicate that the pressure vessel has corroded sufficiently and that the wall thickness is no longer sufficient to safely contain the internal pressure?


Thank you,
Justin

 

[LittleInch]

I'm not sure I understand your issue. A tell tale hole in a repad is there to show, amongst other things, that there is a leak in the shell or welds which the re-pad is covering.

http://www.eng-tips.com/viewthread.cfm?qid=25516

The hole itself does not "fail".

Unless you mean something else??

A picture or diagram might help



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

 

[JustinME]

Hi LitteInch,

I am referring to a telltale hole in the wall of a pressure vessel. I do not know what a repad is, but I do not think that repads exist in my application.

I am not sure if I am allowed to directly quote the ASME spec on this forum, due to copy write issues. However, as I read section UG25(e) of the ASME BPVC Section VIII, it seems that the purpose of a telltale hole in a pressure vessel is to give an indication that the thickness of the pressure vessel has corroded and is now thin enough to fail when internally pressurized.

Interestingly, the ASME code specifies that the telltale hole should be located opposite from where the corrosion occurs. Note: moisture accumulates in pneumatic pressure vessels, and then the water pools in the bottom of the pressure vessel.

So I am wondering if adding a telltale hole will provide a safe failure mode, as opposed to a dangerous failure mode that would occur if corrosion resulted in a pressure vessel wall thickness that is too thin.

 

[metengr]

You mean a pre-drilled hole of certain depth into the vessel wall from the OD surface that once internal wastage reaches this depth, a leak will occur and not rupture.

The depth of the hole would be perhaps minimum wall thickness depth so you have plenty of time to evaluate the ID surface from wastage, once a leak develops.

 

[david339933]

Hand Held UT thickness meters along with regular inspection has eliminated a lot of the need for tell tale holes drilled in the OD of vessels. That being said weeping from a 1/8" TTH would be better than a multi-inch rupture....but I have never seen one in an air receiver.

 

[JustinME]

Thanks everyone!

 

[NBrink]

Never seen one in an air receiver either.

Have seen a few fail on hot oil piping from uniform corrosion. In that case it presented as a faint whisp and a ball of coke forming. My opinion is that they're not really a great option IF you have good inspection programs, as they can force a shutdown at an inopportune time, whereas if you find it with UT, you can conduct an engineering assessment and shut the equipment down safely for repair.

Nathan Brink

 

[JustinME]

Thanks NBrink.

The application I am concerned with is the case where residential construction crews use an air receiver to increase the stored air capacity on their jobs sites. The stored air is primarily used to power a pneumatic tools. I would be surprised if any of these folks ever conduct inspections of the air receivers.

 

[NBrink]

I would be surprised too. Even on sites with very mature inspection programs, air equipment can sometimes slip to the wayside. Telltale holes probably are a decent thought for this application provided they are appropriately sized so as to not serve as a fatigue initiation site.

Nathan Brink

 

[JustinME]

Thanks again NBrink.

UG25(e) of the ASME BPVC Section VIII provides sizing and positional guidance for the TTH.

I imagine that if a fatigue failure did occur at the TTH, the significantly increased material thickness immediately around the TTH should still result in a safe failure mode. In other words, the hole depth should be large enough so that fatigue failures do not propagate outside of the TTH.

I find it acceptable for a fatigue failure to occur, so long as it occurs safely.

Does that train of thought make sense to you? Is it unreasonable to expect that the fault line of the fatigue failure would be fully contained within the diameter of the TTH?

FYI - if I do implement a TTH, I will test failure modes under corrosion and fatigue. I suppose their are two ends to the failure mode spectrum: one being corrosion with Ultimate Stress Failure, and the other being corrosion with Fatigue Stress Failure.

 

[JStephen]

I think the intent is that that kind of hole doesn't "fail", it just starts leaking with excessive corrosion.
The assumption made is that holes below a certain size don't require reinforcing, so they are assumed not to necessarily be a weak spot, either.
I would think on a small vessel, overall failure mode is more likely to be at the welds.
Generally, the use of tell-tale holes assumes also that the vessel has a specified corrosion allowance.

FYI, reinforcing plates put around nozzles on larger vessels normally have a tapped hole through them so the welds can be pressure-tested in initial construction and so that later leaks in the nozzle welds will be visible. Those holes are also referred to as "tell-tale holes", and about 99% of the time, when somebody mentions tell-tale holes, that's what they mean.

 

[zeusfaber]

Just looking at the application here - a pig on a building site.

Unless the telltale hole is big enough to vent the entire output of the compressor at something like half of working pressure, I should think there's a pretty good chance the users will just ignore it when it starts to leak.

A.

 

[NBrink]

That's much too pessimistic zeusfaber.

After all, you know full well at least half of them will oversize the hole, tap and throw a plug in there...

Nathan Brink

 

[rww88]

As background, from Chapter 3 of the yet-to-be published book "Design and Analysis of Steel Tank and Pressure Vessel Openings":
Telltale holes derive their name from the definition of a telltale, which means indicator. Telltale holes do not meet the strict definition of a tank or pressure vessel opening in that they do not completely penetrate the pressure restraining vessel element. They are included as a separate topic here because they are a requirement associated with certain opening connection reinforcing elements. Telltale holes serve multiple purposes and their API Standard and ASME Code specified (or unspecified) locations differ.
Briefly, with regard to the ASME Code, telltale holes have two purposes. A hole is placed in opening connection reinforcing elements to allow for the application of air pressure to sealed cavities that are formed by the intersection of the host shell, the opening connection shell and the reinforcing element during welding. This, in essence, results in a low-pressure pneumatic test for tightness of interior pressure boundary welds at the opening connection. In the case of a one piece reinforcing plate or saddle, a specific location for this hole is not given. Segmental reinforcing elements require one hole per segment, the location of each allowing a clear path to attachment welds intended for testing.
Optional telltale holes can also be placed in the pressure envelope of vessels that are subjected to heavy corrosion, erosion or mechanical abrasion. Leakage from these purposed holes provides a warning that the shell component has been reduced to predetermined allowable minimum thickness. Other than the fact that the blind telltale hole must be effected in the opposite side to the surface that will be subjected to the thickness reducing environment, no exact locations are suggested. Obviously the use of telltales for this purpose is not allowed where the vessel content is harmful or hazardous.
The stated purpose of telltale holes in the API Standards is for the pneumatic test previously mentioned. Like the ASME Code, API Standard 620 does not provide a specific, required location for the hole. API Standard 650 indicates that the hole must be located on the horizontal center line of the opening connection and that the openings requiring the hole are limited to the tank shell (wall). As with the Code, each segment of a reinforcing element that is fabricated from more than one piece must contain a telltale hole.

 

[JustinME]

Thanks rww88.

I am investigating the TTH that does not fully penetrate the tank, and is used as an early indicator of excessive corrosion. I am not referring to a TTH that is used for testing, which is later plugged.

Why does the ASME code state that the TTH should be placed opposite from where the corrosion is expected to be the most severe?

The air receiver in my application has a drain valve for water. The water comes from condensation after moist air is compressed into the receiver so corrosion should be expected everywhere inside the receiver, but is it possible that the corrosion at the bottom could get too thin before the TTH begins to leak?

 

[TGS4]

rww88 actually said the opposite

Other than the fact that the blind telltale hole must be effected in the opposite side to the surface that will be subjected to the thickness reducing environment, no exact locations are suggested.

The opposite in this context is that the corrosion is on the inside whereas the TTH is drilled from the outside. There is indeed no guidance provide as to the location. You, as the designer, needs to determine the optimal location for it.

 

[JustinME]

Thanks TGS4. Thank makes a lot of sense. I'm actually laughing at myself right now .

 

[LittleInch]

Or you could forget about it and just inspect the vessel with a UT probe every now and then?

The issue as you correctly determine is where do you put this hole? Corrosion has a significant tendency to pit as opposed to a uniform loss of metal and even 15 to 20mm away could mean failure from corrosion not picked up by the tell tale hole.

Deliberately weakening a pressure vessel never sounds like a great idea to me if there are alternative means of inspection. TTHs in pressure vessels started when being installed in locations where you couldn't see any corrosion.

The other issue is that it requires the remaining metal to fail when the thickness is reduced - this is not an easy thing to work out for such a small hole.

If you expect corrosion or erosion then deal with it by increased wall thickness, strengthening or lining.

I would be interested to see if this procedure is still routinely done now or is it just a hangover from a previous era when inspection techniques were less available?

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

 

[JustinME]

Thanks LittleInch.

This air receiver will be sold into an application where it will be used by folks without a high school education. I have no control over whether or not inspection occurs, and I have no confidence that the end user is competent to conduct inspection.

I have an expiration date on the air receiver, so the end user can know to discard the tank after the corrosion allowance has been consumed. However, this guidance may be ignored. My thought is that the TTH can constrain the failure mode to a low energy event.