Tank Thickness Readings

Comparing the variation of the readings to the standard deviation will give some indication of a value that is outside the measurement error.

Take just 10 readings and see if they fit into +/-sqrt(N) deviation. Next best result will come from 100 readings. Often 10 sample readings are OK.

Questions A:

Is it a storage tank ?

material ? diameter ?

how old ?

what is your original corrosion allowance?

Was it designed according to API 650 ?

Question B: Which type of deteriorartion / corrosion do you have ? is it local ? is it general ? is it on the shells ? is it on the bottom ?

A + B above plus the use of API-653 and API 579 would allow you to:

- establish the correct grid for measuriing thickness and define exactly the overage on a certain location and also the minimum.

-establish if a repair is needed or not ?

-Establish the re remaining life of the tank

API 653 is enough , API 579 is much more refined and has 3 levels of analysis

I am just wondering why he stated the average + SD and not the average - SD which in my view would be more critical. Anyway I would guess that the average +/- SD would have to be within a thickness range according to some kind of spec (depending on application?). It might also be used as a continues quality control check where trends for moving average (+/- SD?) are checked against a minimum and maximum allowable thickness.

Not sure why the third calc is µ (average) + SD instead of µ +/- SD but it sounds like the intent is to show the distribution. Typically, you show sets of data that fall under the bell curve below and describe their distribution with µ +/- SD. If you want more of your data points to fall in your distribution then use µ +/- 2SD or µ +/- 3SD.

Thanks for that guys. I am comfortable with the concept - but the addition of the SD to the average and taking that as an important value still has me worried. Like Dutchy says, I would have thought the critical value would have been the average minus SD.

I think I am now pointed in the right direction, but will dust off my Engineering Statistics text. Many thanks.

The tanks are (mostly) standard carbon steel tanks as found on any pulp and paper site. They are built to a variety of standards, usually API653 and inspected regularly. Some tanks are more complex - esp the chip digesters which have the bottom cone made from a composite of carbon steel overlaid with stainless steel by explosive bonding. Exciting stuff.