PVC Threaded Plumbing... Use Plumbers tape?

I believe what you are saying is you screwed a union to the non tapered threads on the outside of the bulkhead? If so this is not a good idea as it will leak. The outside threads on a bulkhead are more similar to bolt threads so you can screw the jamb or locking nut on to hold the bulkhead in place. Pipe threads are tapered so the more you screw a fitting (union) on, the tighter and more drip tight the fitting gets. While a 1.25" union might catch a starting thread or two on the bulkhead it will not seal well since you are only sealing at one tiny point and not the entire length of penetration.

Personally I would buy a 1" thread x thread bulkhead and screw a 1" threaded PVC nipple or a 1"x 1.25" male adapter so you can connect to your existing 1.25" PVC. Make sure you spend the necessary time to properly install the bulkhead and to clean out the threads and sealing surfaces of the bulkhead so you only have to do this once without fear of leaks. If the bulkhead is supporting the weight of the plumbing try to build some sort of additional support bracket to take the weight off of the bulkhead so it seals well.

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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 an 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 use the same Schedule threaded fittings with the same Schedule pipe and fittings.

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 (1½ 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 number 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 ., 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).

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. 

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 too many thicknesses 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.

For more information, please visit Hebei Top Tape.

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 with plastics. Many brands of pipe sealant contain oils, solvents or carriers that can damage the 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.

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 joined together with the solvent cement.

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.

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.

For additional information visit:

www.Expert4PVC.com and PVC Piping Systems - Helpful Tips for Avoiding Problems.

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