Plastic fittings do not need this lubrication. Metal to metal fitting joints are more difficult to tighten than plastic the surfaces tend to gall without the aid of such lubricants as Teflon or pipe dope. Teflon tape, Teflon paste and pipe dope is intended for metal pipe and fittings. ( + 2,000 psi = 5,576 psi).ĭon't use Teflon tape, Teflon paste or pipe dope. Two turns past finger tight plus the stress of the system pressure is within the tensile strength of one-inch PVC. The right way to assemble a threaded PVC joint-Schedule 40 or 80 is finger tight plus one to two turns-no more. Strain and Tensile Stress levels of PVC Threaded joints The joint is bound to fail since the stress exceeds the 7,000 psi tensile strength of PVC, without even adding the tensile stress caused by the pressure inside the irrigation system (up to a maximum of 2,000 psi). Thus, a one-inch threaded PVC joint that is tightened four turns past finger tight will develop a tensile stress of 7,152 psi. 00447, so the stress per turn is 1,788 psi. The strain per turn past finger tight for one-inch PVC pipe is. The resistance of PVC is 400,000 pounds per square inch (psi). "Stress" (tensile stress) is the force exerted by the strain of the male thread multiplied by the resistance of the PVC. Table 1 gives Strain and Tensile Stress levels according to pipe diameter. It is also easier to over-torque smaller diameter fittings because their resistance to torque is less. Therefore it is easier to split smaller diameter threaded joints than larger ones. The amount of strain increases as the size of the pipe decreases. All pipe manufacturers voluntarily follow these standards to assure their customers they are receiving quality materials.īecause the threads are tapered, additional turns cause the female part to stretch or undergo "strain." This will split the female fitting just as a wedge, driven by a sledgehammer, will split a tree stump. This is called taper and the amount of taper is specified (1¾ degrees) in the American National Standard B2.1. On threaded male PVC fittings each successive thread is slightly larger in diameter than the one before it. Do finger tighten plus one or two turns.Do use only Schedule 40 threaded fittings with Schedule 40 pipe and fittings.Don't use "stronger" Schedule 80 threaded fittings on the assumption that they may solve the problem of splitting through over-tightening.Don't wrap Teflon tape or Teflon paste or pipe dope to add bulk to or to lubricate the joint.Don't over-tighten joints by giving them "one more turn to be sure."Do finger tighten plus one or two turns - No More.Here are some of the Do's and Don'ts of PVC joint assembly: The reason for this is improper assembly of threaded joints. However, a tiny percentage of those threaded plastic fittings may leak or break. Today, there are millions of miles of plastic piping with threaded fittings, providing reliable, leak-free service. The "Do's and Don'ts" of Assembling Threaded Plastic Fittings
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