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Friday, February 7, 2014

Preheat Temperature of Steel in Overlap Area

Shrink Sleeve Installation Preheat Temperature

     I had this question today and it really was a great, great example of how sometimes in this world of pipeline coating....what seems perfectly logical actually turns out to be exactly wrong.  The question is:  when working with HTLP60 or HTLP80, what preheat temperature should I be achieving specifically at the overlap area (the area where the shrink sleeve wraps around onto itself)?

     The more I've thought about it - WHAT A GREAT QUESTION!  This is the type of question that a person who is really THINKING asks!  Why is it great?

     Well, the overlap area of a shrink sleeve (where it overlaps itself) is a complicated part of the coating.  For starters, you've got an area there that is doubly thick.  In addition, you've got the closure strip that; while thin; does block/absorb temperature from the torch.  Finally, that area where the sleeve is doubly thick is also the area where the sleeve goes from 2 layers to 1 layer - there is a natural step down (small - and the sleeve adhesive fills that area thanks to the shrink force of the backing....but I digress).  That is a pretty critical area as you absolutely do not want that step down to become some kind of leak path (which is one of the main reasons that you should be using a silicone roller to roll the overlap area of every single shrink sleeve that you install!). 

     So, this man's thinking process must be:  double thick shrink sleeve...closure strip...step down area....I know!  I will give extra preheat to that area and that will insure that my bond line temperature is properly achieved and then I will be able to sleep comfortably at night knowing that I installed that shrink sleeve as well as a shrink sleeve has ever been applied, and as a result - that sleeve will be a fantastic coating for one hundred years!

     Makes perfect sense on every level right?  Yes....except it is wrong.

     Look back at the first paragraph.  This man is using HTLP60 or HTLP80.  That is the Covalence three layer field joint coating.  The first of those three layers is a two part epoxy.  Covalence (formerly Raychem) specially formulated their epoxies and their adhesives to bond both physically and chemically as the epoxy cures and the adhesive cools. In order for that to happen, the sleeve must be installed over the still wet (not cured) epoxy layer.  See the problem yet?

     Epoxy cure time is directly related to temperature (and other factors, but ambient temperature has a tremendous impact on cure time).  If you put the S1301M Epoxy (one of the Covalence epoxies) onto a pipe operating at 50F, it will cure very slowly (likely take hours).  If you put it on a pipe that is 140 degrees F it will cure MUCH faster, but still give you time to shrink the sleeve in place.  If you put it on a pipe that is 220 degrees F the epoxy will likely 'flash cure'.  Flash cure - as in essentially cure instantly.  Why is that a problem?

     If the epoxy flash cures - then you will be installing your HTLP60 shrink sleeve on top of cured epoxy.  If you are installing on cured epoxy; the coating system will not achieve the chemical bond it was supposed to get (and whether it gets the proper physical bond or not is open to debate).  So, by OVER heating the overlap area (thinking that would improve our coating); we've actually drastically reduced our coatings performance.

     It seemed like we were doing something smart and good and productive....but we weren't.  So, was I right?  Was that a great question or what?? 

     Every box of Covalence shrink sleeves should include an instruction sheet to guide you step by step through the application process.  Please, please, please - follow it. 

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