A lot of spray systems and nurse tank systems are built or renovated each winter. And each spring we fight leaks that seem an inevitable part of plastic pipe fittings. Here are some tips I've picked up from plastic fitting manufacturers.
-Plastic pipe fittings have National Pipe Thread (NPT) threads. That means they seal against the sides of the threads, compared to National Pipe Taper Fuel (NPTF) fittings, which are manufactured to tighter tolerances and seal against the sides, peaks and valleys of their threads.
-Because of the "gaps" in their peaks and valleys, the threads on plastic NPT fittings require some sort of sealant. Teflon tape, Teflon pipe dope, traditional plumber's pipe dope or silicone sealant are options to seal plastic fittings.
-Teflon tape is quick and tidy, but doesn't like to be "adjusted" after it's been tightened. Teflon tape is such a good lubricant that there is risk of over-tightening plastic fittings and stripping threads.
-Teflon pipe dope and plumber's pipe dope allow a little adjustment after a fitting is installed. Yes, they're messy, but they seal well.
-Silicone sealant, the stuff used to seal gear cases and a hundred other jobs around the shop, makes good pipe sealant. The fittings must be dry, so silicone isn't good for in-field repairs, and there is less leeway for adjustment after the joint is cured. But once cured, silicone is about as leak-proof as you can get.
My personal preference? Teflon pipe dope, with silicone sealant a close second. I like the adjustability of Teflon dope, and the secure seal of silicone. As for the mess associated with any pipe dope or silicone---they don't pay me to stay clean.
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