Impact Exemption Comparison - API 650 to ASME

[michigander]

Can anyone shed any light as to why API's impact exemption curves are so different and more restrictive as compared to ASME? 3/8" thk SA-516-70 is good to a temp of -20F in ASME, but good only to about +8F in API 650.

I have an issue where a company1 detailed an API tank and used SA-516-70. The engineer of record (company2) used SA-516-60 (good to -20 in API). Engineer of record never reviewed the drawings. Drawings went to client and were approved. Tank is built, painted, and ready to ship and now, at this late date, the mdmt issue rears its ugly head.
Client is playing hardball. Not sure of what their ultimate goal is at this point. Maybe they want samples cut from each piece of material in each tank so they can be impact tested, but tanks will have a quilt work of patches. I doubt that's in their, or the tanks best interest.

Logically, it seems that given the ASME utilization of 3/8" in pressure vessels to -20 is proof that the materials are fine. If the tanks weren't built, corrective actions could be taken. But everything is complete and the client, for whatever reason, wants to play hardball.

Does anyone have a get out of jail card free suggestion?

Thanks in advance,

Michigander

 

[vesselfab]

i am not familiar with API so i can't comment about that

but in ASME the fabricator is soley responsible for the mechanical design of vessel, regardless of who does or does not approve the drawings.

in asme there are charts that allow plates to go lower than the exemption to -20

another choice is to get scrap from the same heat numbers (I guess API makes you track by heat numbers) and do impacts. OR call the plate supplier. often times there are additional MTR's or test results where another client has requested impacts done but not forwarded to the other shop since they did not ask.

any way, the fabricator, if he knew the MDMT (on data sheets or in specifications) for the API tank and said he was going to fabricate per API bears the burden of proof that the tank is good for the MDMT per the code fabricated to.

 

[JStephen]

If I remember correctly, if the impacts are required, the preferred method per API is to impact test the thickest plate from each heat. Contact the mill and see if maybe this information is available. An alternate approach is for the mill to show from previous experience that materials will meet the impact test. Or hunting up the scrap as mentioned above might help if it can be done at this point.

As to the reasoning- API doesn't say, but note that stresses are much higher in API than in ASME.