Hi....someone could say me any information about type of grid when, I want to take thickness readings, doing fitness for services in bottom plates of AST´s? The API PR579 don´t talk so much about that.
07:29 Aug-04-2004 John O'Brien Consultant, - Chevron ETC , USA, Joined Jan 2000 278
Re: API PR579 API 653 and API RP 579 both deliberately avoid proscribing the amount of data and grid increments to be used. Its up to you to consider the type and physical attributes of the damage mechanism you expect. Pitting versus General metal Loss etc all have different attributes. therefore you need to select a grid increment that provides adequete data points to substantiate your calculations. Some operators take small samples on localised areas and then extrapolate using statistical methods.
If you have the tank out of service make a large coarse scan with MFL then say a 2 x 2 mm grid on the worst areas identified.
04:26 Aug-10-2004 Mike Trinidad Consultant, API 510 570 & 653 Marine Inspection Service Pty Ltd (MIS), Australia, Joined Jan 2003 138
Re: API PR579 To add some extra personal comments in:
1. UT spot readings on tank floors is a total waste of time as the amount of area actually being inspected would be miniscule.
2. Given that metal loss on floors is most commonly underfloor pitting caused by a variety of reasons and the chances of locating with UT spot readings is highly unlikely.
3. Tanks for whatever reason are delegated as simple fixed plant yet inspection and detection of problems can be more difficult than say a pressure vessel. Mr O'Brien's comments are critical for this type of inspection in that given the operating conditions you must base your inspections on expected corrosion mechanisms, not guesses.
4. The next step is to have an inspector with a lot of AST experience to look for the not so common ie dip plates not seal welded, improper material used for previous lap patches, columns with no doubler plates, anodic sumps, etc etc....
Interestingly enough I was just showing a presentation on tanks showing a coke can versus a AST. The coke can is proportionally stronger as its thickness far exceeds the requirement for the given weight of liquid, for packaging reasons. Also a coke can is built for pressure which an AST is not.
----------- Start Original Message ----------- : Hi....someone could say me any information about type of grid when, I want to take thickness readings, doing fitness for services in bottom plates of AST´s? The API PR579 don´t talk so much about that. : Thanks. ------------ End Original Message ------------