Vaccum Testing of Tank's lapjoints

By omerjaved Date 07-25-2007 09:12

Hi,

We had an issue of demineralized water tank's bottom plate leakage. To rectify this we have changed complete bottom of the tank and now we are in phase of doing NDT's of the welding. As per the code API 653, vaccum box testing is to be done. We know that for vaccum box testing soap solution is applied on the weld, but unfortunately we are unable to apply soap solution on the lapjoints as it slips away. Can we use foam solution for the same test?? is it against the code ??We have contacted few manufacturers of tanks and they have given us a go ahead with the foam made by soap and shampooo.

early response will be appreciated.

Omer Javed
Inspection Engineer
Fauji Fertilizer Company Pakistan

By RANDER

Date 07-25-2007 13:34

API 650 10th edition ADD.4 2005
6.6 VACUUM TESTING
6.6.1
Vacuum testing is performed using a testing box
approximately 150 mm (6 in.) wide by 750 mm (30 in.) long
with a clear window in the top, which provides proper visibility
to view the area under inspection. During testing, illumination
shall be adequate for proper evaluation and
interpretation of the test. The open bottom shall be sealed
against the tank surface by a suitable gasket. Connections,
valves, lighting and gauges, shall be provided as required. A
soap film solution or commercial leak detection solution,
applicable to the conditions, shall be used.

Looks like they give you some leeway regarding the solution used. Is the foam solotion you want to use going to cause many small bubbles which will interfere with the interpretation of the test?

By dbigkahunna (****)

Date 07-27-2007 03:24

I use dish washing liquid. Be careful and not put too much in. Please do not ask for Cl per ltr. I use two squirts per gallon. That would be 1/3 squirt per liter. Be sure you have good lighting and the tester is not moving too fast. Also KEEP THE DURN PRESSURE ON THE VACUUM BOX DOWN!!!!!!!!. You want only enough pressure to pull 9-11 inches of vacuum. Too much vacuum and you can miss a crack! The bubble comes up, and you dont see it and you move on. Result, leak. Been there and done that. Before you do the vacuum box test, visually inspect the welds. It is called mooching!. If you see ANY pinholes in the weld on the product side, weld em up. When you mooch the corner watch for gas pockets at the toe of the weld espically if 7024 was used for the corner. Remember it has to be 2 pass.
Go slow and keep rags to clean off the vacuum box cover. I make my own with a single gasket over 3/4 inch Lexan. I make a lexan plate out of 1/8 inch and attach it to the 3/4 inch and put 1/8 inch squares where the vacuum attachement goes and a vacuum gauge at the other end. Dont use plexiglass as it scraches too much
BABRT's

By G.S.Crisi (****)

Date 07-27-2007 18:03

Next time you have to design and build a water tank, take into account that water tanks follow AWWA standard D100, not API 650.
The difference is that API 650 addresses oil and oil products storage tanks, which are highly flammable, whereas water is not.
So, API 650 is more stringent (and therefore tanks built to it are more expensive) than AWWA D100.
AWWA is the American Water Works Association.
Giovanni S. Crisi
Sao Paulo - Brazil