Vacuum testing in tanks

Published 7 June, 2015 by admin

By Carlos Molina.

I am writing this post because of this video that a good friend  sent to me about vacuum box testing, that I want to show you. Besides, I will give you instructions for the construction of your own flat vacuum box, but first a little background.

MY FIRST CONTACT WITH STORAGE TANKS

My first contact with aboveground storage tanks was in 2011. I had been working before as a penetrant testing inspector for a large pipe project in Rubiales (my record was 64 penetrant tests in one day), and a friend of mine named Oscar, which you are going to see in the video, called me with an offer to work in tank repair.  Of course I accepted, switched cities and later became API 653 certified.

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One of the earliest tasks I had was to have some welded-on bottom patch plates vacuum tested. But i didn´t know how to do it and the company had no vacuum box. We only had the vacuum pump.  The bottom of the first tank in maintenance was finished and ready to be blasted for paint. But there was no vacuum box the day scheduled for blasting. Luckily I met someone that had one and I took some pictures of it, then made my own for more or less $35 dollars using plexyglass, “sieve rubber”, a discarded foam retired from the peripheral seal of an IFR of another tank the company repaired earlier, wood glue and a tap. It worked fine the first time and was the best vacuum box I´ve ever had. I didn´t need anyone´s help to use it because the foam was so soft. It was so quick. I fell in love with it.

vacuum-box-tanks

My first self-made vacuum box

That first contact made quite an impression in me, much more later when I found out how much they charge for a vacuum box.

WHERE TO USE A VACUUM BOX

Vacuum boxes are widely used troughout the industry for leak testing. They can be used in tank bottoms, tank roofs, containment liners, pressure vessels like heat exchangers and condensor, and pipes for the detection of leaks, in parts during fabrication or for completed parts. The primary objective of vacuum box testing is, of course, detect any leaks in the surface or joint being examined.

According ASME BPVC V article 10, “the objective of the vacuum box technique of bubble leak testing is to locate leaks in a pressure boundary that cannot be directly pressurized”. That is exactly what happens with tanks, that because of its large volume cannot be overpressurized to test weld seams, so a vacuum box has to make the job. If we apply this logic, then vacuum boxes are more likely to be used in low pressure, large equipment than in high pressure, small equipment. However, it cannot replace hydrostatic testing, because of the higher head pressure you can achieve with water.

In the following video, that was sent to me by my good friend Oscar Andrade, you can see the basics of vacuum box testing in tank bottoms.

WHEN IT IS USED 

Vacuum Box testing can be used on the following instances of construction a welded storage tank.

In the first pass of the internal shell-to-bottom weld if approved instead of MT, PT, or Pen. Oil. 7.2.4.1e
In the bottom welds. 7.3.4a
In the welds of roofs designed to be gas-tight if not air tested. 7.3.7.1
In all seams of internal floating roofs exposed to liquid or vapors if not tested by penetrating oil. H.6.2
In seams of flexible membrane liners for leak protection. I.6.2
In welded shell joints above the hydrostatic test water level unless tested with penetrating oil. 7.3.6.1
In the shell-to-bottom weld joints. 7.2.4.3c.

WHO TO MAKE A FLAT VACUUM BOX

The following diagrams will help you make your own flat vacuum box. I hope the diagrams are completely explanatory.

diy-vacuum-box

You will need rounded corners plexyglass, a rubber called “sieve rubber”, foamy rubber, a 1/4″ NPT tap, and a knife. I used wood glue to make the parts together, but there are better options.

diy-vacuum-box-details

With this instructions you can make a flat vacuum box for $35 dollars. Make sure to use a foamy rubber at least 1 inch thick and don´t let the connections pass trough under the plexyglass level. Otherwise when the vacuum is made, the soap film solution will enter your vacuum hose. You can use any soap for the test solution.

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WARNING

Off course, I don´t take responsibility for the wrong use of this vacuum box. I am only giving away what has worked to me in the past. Vacuum box testing can be dangerous and should be conducted by qualified personnel.

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Latest comments

Thanks. Good idea !!!

- Daehan Koo

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