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February 14 - February 20, 2011

What is the maximum number of holidays that should be permitted for linings used in immersion service, and why?



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Selected Answers

From Marimuthu Govindarajoo of Freelance on April 5, 2011:
       True, no holidays, no exceptions. But then....when should the holiday tests be done. Most of the specs I have worked with call for holiday testing upon completion of the final coat. I, however, beg to differ. If the end user desires to see value for his money, then the testing should be done from the primer onwards.

From B Palanivel of VELOSI on February 27, 2011:
     Keeping zero holiday defects is SAFER.

From Mr.Chaiyan Dejtisak of Blasting Group Service Ltd.,Part. on February 23, 2011:
     I agree with those who previously answered;  otherwise, what is the reason to test for pinholes. The repairs are also very important.  The surface has to be clean and roughened  for adhesion of touch up paint.

From Gunnar Ackx of SCICON worldwide bvba on February 15, 2011:

     Frankly speaking, NO holidays should be allowed on linings in immersion service. First of all, the lining is often installed because the medium stored is corrosive to the substrate. So, every pinhole left over is likely to lead to local corrosion, which, depending on the medium, may lead to rapid perforation of the substrate and take the storage tank out of service prematurely.

     For example, a rubber-lined sulfuric acid storage tank was perforated within 3 months after the refitting of a new rubber lining that was not 100 percent holiday-free. Second, tank linings are often installed in order to prevent contamination of the medium (or cargo) with corrosion from the substrate. Again, with any pinholes in the coating, this is likely to react with the medium or cargo, leading to a loss of product quality. For example, six TDI storage tanks had to be taken out of service and relined because recent coating repairs (on an inorganic zinc tank lining) were not done properly, leading to pinpoint corrosion, which reacted with the TDI cargo and degraded the quality of the high purity TDI. And on a final note, leaving even small amounts of small holidays or pinholes in a tank lining has a HUGE potential for setting up a galvanic corrosion cell (if any other dissimilar materials are used inside the storage tank, which is often the case). At that point, the surface ratio between the anode (pinholes/holidays) and the cathode (other dissimilar metal) will promote rapid corrosion of the substrate, which again is likely to lead to perforation in a matter of weeks or months. For example, when the carbon-steel outer shells of a condensor were lined, perforation of the condenser housing was observed within 18 months. Investigation revealed a couple of 'weak spots' in the lining (where the scaffolding had been placed), exposing small bare steel surfaces that had now become a very small anode versus the HUGE Cathode that was also present inside the condensor, being the Titanium tube-plate & heat-exchanger-tubes. In that respect, EVERY tank lining should be holiday-tested over 100 percent of its surface, e.g., as per NACE SP188-2006. If any holidays are found, they should be repaired as per the lining manufacturer’s instructions, and the repairs themselves need to be re-tested as well (as a repair is not always a guarantee for a holiday-free surface). In the event of premature corrosion or coating failure, the coating repairs themselves can already be quite expensive (e.g., if scaffolding has to be re-installed), but most often the collateral damage (cleaningcosts, downtime, loss of income and other costs) are going to be  much greater than the costs of the coating repairs themselves. And 'holiday-testing' is not just rubbing a copper-wire brush over the coating-surface; it takes a well-trained and experienced coating inspector to properly execute this type of quality inspection.

From Lee Edelman of CW Technical Service on February 14, 2011:
There should be no holidays. A uniform lining applied within the range of minimum and maximum dry film thickness will perform better than a lining that is not in spec. I have never seen a specification that allows holidays for a lining used in immersion. Most specifications for linings require a holiday test be done. If holidays are allowed, the test will fail and the lining will fail. Maybe the question should be, how many jeeps should be allowed by a holiday detector before the lining fails.

From Joe Isbell of SAWS on February 14, 2011:
Zero, to stop corrosion.

From kamal el sayed of adco on February 14, 2011:
The maximum number is zero, bacause a holiday  will be a corrosion riser point .

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Tagged categories: Immersion service; Linings


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