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I Try to be Flexible...

TUESDAY, DECEMBER 3, 2013

By Warren Brand


More items for Quality Control

We are in the process of getting a water tank—85 feet in diameter and roughly 80 feet tall—back in service.

From start to finish, we have about four weeks.

Four weeks to:

1. Identify an optimal coating solution.

2. Write a specification.

3. Identify a contractor.

4. Get all parties through various classes so they can work on site.

5. Stage all vendors (need to rent lifts, DH, heating, etc.).

6. Schedule inspectors.

7. Order supplies.

And about 150 other things. Not to mention that we’re here in Chicago, where it’s not getting any warmer.

No problem. I actually thrive under pressure. This is due to one of the three following reasons:

1. I used to be a newspaper reporter and had a daily deadline.

2. I’m a martial artist, and we constantly train under pressure.

3. I’ve been married for 22 years and have three daughters.

TankInterior
Images: Warren Brand

Time is of the essence. But so is a successful coating. And there are hundreds of linear feet of seams to consider.

In any case, there it is.  The process is underway as I write.

Fixating and Flexing

We are responsible for all of the technical aspects of the job: coating identification, specifications and inspection services. We are designing a system to be maintenance-free for 20-plus years and are also providing associated consulting services as required.

Time is of the essence. And yet, I became almost maniacally fixated—and inflexible—on a question of flexing.

The contractor, coating manufacturer and my co-workers, were all doing their best to politely tell me I was off my rocker.

So I thought I would share my fixation and open myself to broader abuse.  

Eyeing the Angles

The issue is flexing, or oil-canning of the tank bottom.

Diagram1

Diagram 1: Most coating systems designed for these environments can accommodate flexing. My big concern? That tiny right angle between the plates.

For those unfamiliar with the term, when you are working on a large-diameter tank (say, more than 20 feet), the bottom will frequently flex—move and up and down, predominantly in the center.

I’ve seen flexing of roughly a foot in a 100-foot diameter tank, but have heard of flexing upwards of several feet in a tank bottom 200 feet in diameter.

Now, most coating systems designed for this environment (even coatings that appear brittle) can handle the flexing with no adverse effects.

However, I was concerned about the area where the welds overlapped each other. In particular, I was concerned about the tiny right angle (Diagram 1, above).

Now, of course, there’s not a clean right angle in our tank. There is a lap fillet weld. See the photo above or the sketch below.

LapFilletWeld

But the tank is large, there are hundreds of liner feet of seams; time is of the essence; we’re shooting for a 20-year, no-maintenance service life; and my reputation is on the line.

Seal and Deliver?

As the tank bottom flexed, and the plates moved up and down together, I worried: Would there be any bending in the teeny, tiny angle between the bottom plate and beginning of the weld?

Or, if there were areas where the weld was not continuous and the angle was steeper, could the movement cause the coating to crack (when the tank flexed upward) or be crushed (when it flexed down)?

I was fixated on applying a seam sealer (flexible or otherwise) over the welds and seams to flatten out that angle and make a smoother transition over the joint.

I spoke with a number of folks, including my good friend Ken Jacobi over at CB&I, and was assured that if the weld was done properly, applying a coating without a seam sealer (and, of course, stripe coating) would be fine.

Still, I’m uneasy. (And did I mention that it’s not getting any warmer?)

Tank experts, what do you say?

ABOUT THE BLOGGER

Warren Brand

Warren Brand’s coatings career has ranged from entry-level field painting to the presidency of two successful companies. Over nearly three decades, he has project-managed thousands of coating installations and developed specs for thousands of paint and coating applications. NACE Level 3 and SSPC PCS 2 certified, Brand, an MBA and martial-arts instructor, now heads Chicago Corrosion Group, a leading coatings consultancy. Contact Warren.

SEE ALL CONTENT FROM THIS CONTRIUBTOR

   

Tagged categories: Chicago Coatings Group; Coating failure; Tank interiors; Tanks; Warren Brand

Comment from Mike DesPres, (12/4/2013, 8:25 AM)

Werren, I have coated hunders if not thousand of tank interiors floors some for CB&I. I agree with your friend that if the welding and finish grinding is done properly flexing of the coating in the weld areas will be fine. A stripe is mandatory but do not exceed the manufacture recommendation for the DFT. I have found that appling caulking to the weld areas can create more problems then it fixs. I have coated tank floors as large as 320' Di and after the warranty inspection found no problems at the weld joints with thin mil coating ( 18.0 - 23.0 DFT). Follow the manufacture data sheet and good paint practice and a 20 year coating system is not out of the question.


Comment from Warren Brand, (12/4/2013, 1:33 PM)

Hi Mike, Thanks much. That's very reassuring. Warren


Comment from Gordon Kuljian, (12/4/2013, 3:08 PM)

Warren- In order to achieve your 20 year service life, one needs to ensure the proper steps are taken: Pre job conference where everybody knows what we are trying to achieve, 24/7 operation of your heat/DH so you can work multiple shifts, proper air ventilation to evacuate solvents (if not using 100% solids), abrasive cleanliness checkpoint, profile checkpoint, substrate contamination checkpoint (conductivity I find is quick and easy), quality of prep (SP-10?), stripe coating, ensuring recoat windows are met, check mixing and thinning so that no unauthorzed thinners/ excessive thinner is used, holiday checks of the final system at the welds, and of course proper DFT's and proper touch-up after dismantling scaffolding. For the 20 year service life I would insist that the owner pay for full time inspection, not just at the checkpoints. I consulted on a similar job in the Chicago wintertime and yes it is a challenge - check out my article in the Nov issue of CoatingsPro.


Comment from David Cerchie, (12/5/2013, 7:37 AM)

Well I’m certainly not a tank expert and I can already see that other professionals have weighed in on the various elements required for successful tank coating. Thanks gentlemen. I’m always intrigued by the underlying scientific questions posed by Warren. Here I can’t help but ponder the points surrounding the issue of micro-cracking in the weld seams on the tank floor. One thing I keep coming back to is that if the lap weld maintains its integrity then the angle in the corner should remain the same in spite of the oil can flexing. Otherwise the floor plates would be breaking apart at the weld. It seems to me that the flexing stresses are being transferred to and absorbed throughout the field areas of the floor plates such that the angle at the weld remains constant. If this is the case then it would follow that very little additional stress would be transferred through the metal at the weld seam corner into a coating product. Therefore the coating would only need to be capable of withstanding its own flexing forces during the oil can movements. This of course would require that the coating product have sufficient modulus capabilities for the expected movements. Am I off base here?


Comment from Jesse Rigamonti, (12/5/2013, 8:30 AM)

I’m curious about what the paint system is? I usually call on a paint technician when it comes to warranty or specific technical questions.


Comment from Randy Gordon, (12/6/2013, 7:08 PM)

Thousands Mike? C'mon Wilt Chamberlin =)


Comment from Randy Gordon, (12/6/2013, 7:15 PM)

Guidelines for such details can be found within NACE SP0178. Really, there will be only one movement; during the initial filling; from a dry to a wet stage. Once the product is in the tank, the compressive force of the product is going to flatten your floor and its not going to move again until you drain the product completely. There will also be some thermal influences, but not enough to make a difference. All I do is paint tanks...you're safe, I promise.


Comment from robert conn, (12/13/2013, 8:28 AM)

A smooth transition in the weld that allows no puddle to form at the toe of the weld is critical in the welds at the bottom interior of railroad tank cars. If attention is paid to this aspect of coating the inside of the car, allowing consistant DFT including at the toe of the weld, our cracking problem has all but been eliminated. Whe a condition exists that allows coating to puddle at the toe of the weld, the high dft in that area caused the coating in that area to almost immediatley start failing.


Comment from Stephen Pinney, (9/17/2014, 1:26 AM)

You are correct that the throat of the weld provides the required strength for that stress circumstance of that tank and thet the throat should not be reduced by grinding. Obviously a pressure vessel requires more welding then a water tank. The weld design is therefore calculated by a design engineer who specifies the the required throat dimension and the length of the weld required. This is then accomplisher by skip welding or sealing welding. Fabrication Details, Surface Design Considerations for Tanks and Vessels to be Lined for Immersion Service, NACE Standard SP0178 then provides a method to select the required smoothness, for example C or D. A plastic mold is availably which illustration the various smoothness in available. If the joint is skip welded, the space between welds should then be sealed with an elastomeric sealer such as butyl or polyurethane which is compatible with the coating material. This should eliminate moisture permeation and allow for flexing.


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