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Today, there are millions of miles of plastic piping with threaded fittings, providing reliable, leak-free service. However, a tiny percentage of those threaded plastic fittings may leak or break. The reason for this is improper assembly of threaded joints. Here are some of the Do's and Don'ts of PVC joint assembly. Don't over-tighten joints by giving them "one more turn to be sure." Do finger tighten plus one or two turns- No More. Don't wrap Teflon tape or Teflon paste or pipe dope to add bulk to or to lubricate the joint. Do use a sealant for threaded joints. Don't use "stronger" Schedule 80 threaded fittings on the assumption that they may solve the problem of splitting through over-tightening. Do use only Schedule 40 threaded fittings with Schedule 40 pipe and fittings.

Don't over-tighten. Do finger tighten plus one or two turns.

On threaded male PVC fittings each successive thread is slightly larger in diameter than the one before it. Female threads get successively smaller. This is called taper and the amount of taper is specified (13⁄4 degrees) in the American National Standard B2.1. All pipe manufacturers voluntarily follow these standards to assure their customers they are receiving quality materials. Because 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. The amount of strain increases as the size of the pipe decreases. Therefore it is easier to split smaller diameter threaded joints than larger ones. It is also easier to over-torque smaller diameter fittings because their resistance to torque is less. Table 1 gives Strain and Tensile Stress levels according to pipe diameter. "Stress" (tensile stress) is the force exerted by the strain of the male thread multiplied by the resistance of the PVC. The resistance of PVC is 400,000 pounds per square inch (psi). The strain per turn past finger tight for one-inch PVC pipe is .00447, so the stress per turn is 1,788 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. 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).

Table 1

Strain and Tensile Stress levels of PVC Threaded joints (Schedule 40 and 80)

Finger-tight+2 turns Size Strain/turn Stress/turn Max allowable Hydrostatic (IPS) (in/in) (psi) stress (psi) 1⁄2 .00588 2352 6704 3⁄4 .00461 1844 5688 1 .00447 1788 5576 11⁄4 .00349 1396 4792 11⁄2 .00302 1208 4416 2 .00239 956 3912 21⁄2 .00287 1148 4296 3 .00234 936 3872 4 .00180 720 3440

The right way to assemble a threaded PVC joint-Schedule 40 or 80 is finger tight plus one to two turns-no more. Two turns past finger tight plus the stress of the system pressure is within the tensile strength of one-inch PVC. ([1,788 psi x 2] + 2,000 psi = 5,576 psi). Don't use Teflon tape, Teflon paste or pipe dope. Do use a sealant. Teflon tape, Teflon paste and pipe dope is intended for metal pipe and fittings. 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. Plastic fittings do not need this lubrication. When Teflon tape is wrapped around plastic male threads it adds to the strain and tensile stress. The tendency of most installers is to incorrectly wrap several thickness of tape around the male threads, increasing stain and stress further. Teflon paste and pipe dope, just like Teflon tape, make threaded joints slippery. Their use on PVC fittings can be an invitation to over-torque. When working with threaded plastic fittings do use a proper sealant. The right sealant for threaded joints is non-hardening, compatible with plastic and doesn't add slipperiness. A non-hardening compound is forced by water pressure into potential points of leakage, thereby performing a true sealing function. Tapes and hardening pastes permit a leak path to develop when a joint is backed off, mechanically flexed, or expands with rising temperatures.

A sealing compound must be compatible to plastics. Many brands of pipe sealant contain oils, solvents or carriers that can damage plastic. A proper sealant must be certified by the manufacturer to be harmless to the fitting material and to not contaminate fluid in the pipe. Finally, a sealing compound must not lubricate the joint to the point that over-tightening is encouraged. Several sealants on the market meet all these requirements. Don't use Schedule 80 threaded fittings in a Schedule 40 System. Do use the same Schedule threaded fittings with the same Schedule pipe and fittings. Many plastic piping system installers who encounter problems with splitting assume Schedule 40 fittings are weak. They conclude that the problem can be solved by switching to "stronger" Schedule 80 fittings. There are several fallacies in this reasoning. First, all the problems inherent in over-tightening apply as much to Schedule 80 systems as they do Schedule 40. While the walls of female Schedule 80 threaded fittings are thicker, wall thickness does not change stress and strain levels. See Table 1. Second, installers believe Schedule 80 systems are stronger because they have higher pressure ratings than Schedule 40 systems. This is true only when comparing systems with components that have been cemented together with solvent. See Table 2. Introduce even one PVC threaded pipe or nipple, and the rating of the entire system must be reduced by 50 percent.

Table 2

Maximum Static Pressure Rating* of Type 1120 PVC at 73° F

Size Schedule 40 Schedule 80   (IPS) Solvent Weld Solvent Weld Threaded Joint 1⁄2 600 850 425 3⁄4 480 690 345 1 450 630 315 11⁄4 370 520 260 11⁄2 330 470 235 2 280 400 200 21⁄2 300 420 210 3 260 270 185 4 220 320 160

This decrease in rating is due to a reduction of the fitting's wall thickness caused by threads. In addition, most plastics, including PVC, are "notch sensitive." When the smooth wall of a plastic part is notched, the part loses a significant portion of its original strength, just as a thick sheet of glass will break along a scribed line on its surface. This is why the presence of even one threaded fitting in a system requires a 50% reduction. With these Do's and Don'ts in mind, many of the unnecessary headaches and costs of improperly installed systems can be avoided.