Tuesday, June 25, 2013
Installing Shrink Sleeves: How Important Is Pre-Heat?
Another common question: How Important is Pre-Heat?
Short answer: ABSOLUTELY CRUCIAL
Long answer: When installing a heat shrinkable sleeve as a field joint coating, you are doing so for one reason and one reason only. You are protecting that field joint (girth weld) from corrosion. Of course, the idea is, that if you can seal that field joint in a way that prevents water and oxygen from reaching the bare steel; then it will be impossible for corrosion to occur.
We do this by installing a heat shrinkable sleeve that is coated with a sealant or adhesive of some kind. If we can install that sleeve so that we get the best possible bond between sealant and bare steel (and factory applied coating) then we begin the life of that field joint from the very best possible position: properly coated and protected.
Of course the first step of installation is making sure we've achieved the proper cleanliness standard on the bare steel. Different adhesives will have different requirements there and I won't discuss them here (but I did touch on surface prep for WPCT here). The next step however is preheating the steel (and adjacent factory applied line coating) to the proper temperature. Why is this necessary?
Any hot melt adhesive or mastic coated shrink sleeve will have an optimum temperature at which the adhesive bonds to the substrate. When we look at it very closely, we must create an environment where the bond-line reaches a temperature sufficient for ideal bonding. The bond-line is that area where the hot melt adhesive of the mastic come into contact with the substrate (pipeline). That interface is critical.
Even though there is a significant amount of heat put into a heat shrink sleeve during the installation process; when you consider the heat insulation properties of the heat shrink sleeve and when you consider that the steel pipe will act as a heat sink (carrying temperature away from the girth weld as it acts to spread any elevated temperature through the steel in the immediate area) it isn't possible to actually achieve the necessary bond-line temperature exclusively using heat from the sleeve shrinking itself.
As a result, the steel must be preheated. This assures that the bond line temperature is met and that the adhesive/steel interface will be properly prepared for the bond to occur. All Covalence heat shrink sleeves will be supplied with an installation sheet in each box. No matter the product; there will be a 'minimum recommended preheat temperature' listed on that installation sheet. It will certainly cause no problem to heat higher than that; with two exceptions.
- When installing a shrink sleeve system that utilizes a liquid epoxy (DIRAX, HTLP60, HTLP80, etc) it is important to keep the preheat temperature down below 200F. Installing S1301M epoxy on a pipe that is heated above 200F will result in some smoke - and result in the epoxy more or less flash curing (which we want to avoid since we want the epoxy chemically bonding with the sleeve adhesive; and that can't happen in the epoxy cures "early")
- Most all shrink sleeves (all PE shrink sleeves anyway) shrink at ~267F. We would not want to preheat (most of the time) to a higher temperature than that, as it could result in the sleeve shrinking before the installer is ready (as an example, before the closure has been secured).