Residual Life Calculation For Brittle Carbon Steel

To be on the safe side, I would predict a residual life of zero, and replace the vessel ASAP, before the need arises to replace other equipment and personnel.

In addition to Milo for metallurgy, Abdel Halim Galala and blackpanic73 are highly knowledgeable on pressure vessels. If they reply to this thread, please take note.

Without pretending to be an expert, but with some understanding; my the first thought is that the cracks you see could be "brittle", "fatigue", or "stress" cracks. Brittle would relate more to something arising out of a welding process (hydrogen embrittlement, precipitation, shrinking stresses) and could well have been present from day one (or soon after). Stress corrosion is usually more chemical in nature and I'm not immediately alarmed by iron/steel in a caustic (i.e basic or high pH) environment.

Many if not the majority of mechanical failures are fatigue failures rather than straight overload yield type failures, and the usual design criteria for iron/steel is to use 10,000,000 cycles as the endurance fatigue (i.e. if the equipment lasts this long it will not fail).

In aircraft, where aluminium is often used, high stresses are the norm and the material does not have an endurance limit, it is common practice to "fly" the aeroplane in a test rig and watch the cracks develop. One just has to make sure that the laboratory plane has more hours on its airframe than those in the field! I cite this as a precedent for "watching the crack grow".

So in your case you need to do an assessment of what form of crack you have, and if no critical near yield stresses are evident you might consider watching the crack closely to see if it grows. Any evidence of substantial growth would pretty much demand an immediate replacement of the pressure vessel or, as one post says, you might be replacing more than just equipment - and looking forward to a seat in a court room somewhere.

Even if the crack is monitored regularly, catastrphic failure could happen without warning is in this case.

The reason I say this is because of material considerations.....A516 Gr 70 is also known as Cor-Ten and is a carbon steel with copper added. That makes it susceptible to "hot short cracking" when welding. Unless proper pre-heat and cooldown procedures are followed when welding, A516 is subject to sudden cracking at welds in the heat affected zone. It's hard to do with a large workpiece like a fabricated vessel and is therefore a potentially very dangerous condition for a pressure vessel.

I would have to agree with other posters....for safety and liability reasons the predictable life of the repaired vessel should be taken as zero.

i agree. i would'nt think anyone would design a tank using anything but mild steel. presure changes should,nt harden the steel. it may have been over stressed from excess air pressure. your only alterative is replacing the tank.

It is not possible to predict life time of the vesssel in a brittle crack/fracture case.

I think your crack is related with stress corrosion crack because high stress carbon steel as 516 G70. Please see if there is any stress concentration, welding or something which brings in stress around the original crack.

If you have already repaired the crack by welding, please make sure PWHD is properly performed and the whole area is NDE checked. Then the vessel could be still used. But care has to be taken to monitor for further cracking.

yes kevin, the cracks are SCC due to the nature of application- caustic service at high temperature. I will re-frame the question for you: After taking all the care ie. proper preheat, proper welding techniques, proper PWHT, can the brittleness in the plate (though localized) be removed and the life be predicted? My ans:NO. Your take! All others thanks for putting in your valuable responses.

Hi Maheshi, the life time of equipment by any type of brittle crack would not be predictable (though in theory it is possible to do so but in engineering I would say no).

By proper preheat/post weld heat treatment, as well as welding material/procedure, you can remove some SCC sensitivity of the material so reduce the possibility of cracking/or delay the formation of cracking for sure. But, I would sugget you to take design issues into consideration if possible, to reduce the stress level especially at the cracking place.

SA516 Gr 70 plate, Working pr: 32 kg/cm2, working temp: 200 deg cent, service: hydrocarbon+caustic.
Replace it.
Ron

Here is an article on "hydrogen embrittlement" that may apply to your situation.

If it forms part of a pressure system that is covered under burst indemnity insurance, the thing that will predict the remaining operating life is the Engineer/Surveyor from the company that supplies burst indemnity insurance cover.

At this temperature is the SA516 correct material? What was tha age of the vessel?

You may check this link