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Tuesday, September 30, 2014

Trapped Air Under a Shrink Sleeve

Air Pocket Under Shrink Sleeve

     Occasionally, when an untrained installer is shrinking a heat shrinkable sleeve on a pipeline, it is possible for his shrinking technique to inadvertently cause an air pocket to become trapped underneath the shrink sleeve resulting in a bubble that can be clearly seen by an inspector. 
     Does this mean it is time to panic?  Time to call in the cavalry?  Time to tear that darn specification up and begin the coating analysis programs anew?  Should we summon the sand blasting equipment and plan to strip that field joint over the course of the next few hours so we can start fresh with a new installation? (keep in mind - shrink sleeves are not designed to be removable, that's why they do such a good job as a coating!)
     Calm down.  Take a deep breath.  Go grab a cup of coffee (maybe a decaf).  We're going to be able to help you fix this.
     For starters, make sure you still have the torch and propane tank nearby.  Also, make sure you've got a couple of our silicone rollers on site.  These are going to be what saves the day.
     The first thing we've got to do is re-heat the area of the sleeve with the air entrapment.  You will also need to heat a "path" between that air bubble and the edge of the shrink sleeve.  It is important that you aren't overheating the sleeve here.  If you begin to see smoke, or if you begin to see the finish on the shrink sleeve begin to show a rainbow, you are heating too much.  This will require patience and a tender touch, but it shouldn't take more than a minute or two at most.  You will want to be sure it is the yellow portion of the flame that is in contact with the sleeve and you will want to make sure that your torch isn't open full bore.
     Now, you will know you've heated the sleeve enough when you touch the sleeve backing with a gloved hand and see that the adhesive sealant underneath the sleeve is soft.  It will be obvious.  Then you will take your silicone roller and slowly push that air pocket toward the edge of the sleeve.  You must be a bit careful, we do not want to displace all of the mastic that is under there.  It will be apparent how much pressure is enough to move the air without displacing the adhesive.
     Then you will slowly work that air pocket until it is released back into the atmosphere.  This too will be obvious as you will hear the air popping its way out.  You aren't finished yet though.  We want to go back and roll over all of the area that the air travelled.  We do not want to create any new air pockets and we want to make sure that the adhesive is well bonded to both the bare steel underneath and the polyethylene jacket.  It is possible - if you have a long way for the air pocket to travel (we do make sleeves as wide as 36") that you could have to reheat the sleeve along the way.
     In any case, this is actually quite a rare occurrence.  If the installer is moving his torch vertically, and the installer is shrinking the sleeve circumferentially, and the installer is working from the middle toward the edge, something like this should never happen to you.

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