Why use insulating bushings?

Even a low mineral-content water performs as an electrolyte would in a battery. No current will be produced if two pieces of the same metal were immersed in an electrolyte and connected together, but if two different metals are used, a current will flow in a circuit through the electrolyte and any metallic connection outside the electrolyte (water). The dielectric bushing interrupts the metallic circuit and the electric current.

If the electric circuit is complete, as in metal-to-metal contact of the different pipes, the current through the water-electrolyte produces ions at the water-to-metal interfaces that are destructive of one or both metals. In the connection of copper to iron or galvanized, destruction or corrosion of the iron occurs with a heavy buildup of rust (iron-oxide) inside the pipe. The pipe wall may become corroded entirely through, and the inside passage will become severely restricted.

If the outside of the copper-iron pipe connection becomes wet due to condensation, or if the joint is immersed in water, corrosion of the iron will also occur on the outside.

The mineral content and acidity of the water supply affect the rate of corrosion, and both the mineral and acidity vary in domestic water supplies. Hot water is much more active than cold water, and many times a hot water pipe may be choked to a trickle of water, yet the cold water pipe next to it may be only moderately corroded.

The corrosion typically occurs in pipes within only 3 or 4 inches of the joint in pipes of ¾ size or smaller. If copper tubing is connected directly to a galvanized hot water tank, the entire inside of the tank is likely to corrode, with the zinc galvanizing disappearing first, then corrosion proceeding into the iron. Many such progressions have been observed. Any pinholes or bare spots in a “glass-lined” tank will corrode rapidly if copper tubing is connected directly to the tank.

Since the current developed in this corrosion process has a potential of only about 1¼ volts, any insulating interruption of the circuit will stop this corrosion. Nylon in a tough material and in the formulation used in IVP Bushings is resistant to water and the minerals formed in domestic water supplies with intermittent use up to 180ºF. If your application approaches 180ºF. we recommend individual testing of the bushing for your application.

As the water temperature is increased above 180ºF, long-term deterioration occurs, with higher temperatures accelerating the deterioration. Even a few months of exposure to 250ºF. water will seriously deteriorate the nylon.

Special Applications

Electrical ground connections for telephone and house wiring systems are commonly made by straps or clamps to water pipes. The electrical codes specify that these connections be made between the water service entrance and the first shutoff valve in the building. Since the code specification is sometimes disregarded and grounding connections are made to some other point in the plumbing system, installation of dielectric bushings may interrupt the electrical ground. A quick check should be made of the electrical ground connections whenever a portion of a pipe system is insulated with dielectric bushings.

Water softening and treating equipment produces greater chemical activity in the nearby systems. Even small differences in steel alloys used for tanks and piping can cause electrolytic corrosion in these areas. Protection with dielectric connections is desirable in all connections to the equipment.

Installation Instructions

All thread sizes (3/8 x ¼ to 2-1/2 x 2) of IVP Bushings are made with a heavy hex for wrenching. The nylon bushings are somewhat pliable and must be tightened securely. Also, the thread tapers are designed so that some flow of the nylon into inaccurate metal threads will occur if the thread size is within the specifications of the American Standards Institute.

The use of pipe dope for assembly is recommended even though the nylon will normally flow sufficiently to fill poorly cut threads in the metal. The dope lubricates during assembly as well as filling minor voids. None of the common paste or stick dopes have been found to affect the nylon. A stick dope that is wiped onto the threads should not be crumbly. Varnishes are sometimes used in production assemblies and are satisfactory if allowed to dry before use. Shellac should not be used in any case, since it is slightly soluble in cold water and will wash out completely in warm water.

Hemp or other fiber, or Teflon tape should not be used as a joint filler with nylon. The nylon will normally fill any properly sized threads if tightened securely as instructed above.

To obtain proper performance, the bushing is first screwed into the female fitting until the hex end is not more than one full thread from bottoming on the fitting: screw the hex all the way against the fitting if possible unless the fitting has a deep chamfer. If the bushing does not require a wrench for the last three threads at this stage, the thread in the fitting should be checked for oversize dimension.

The male pipe end or fitting should then be entered into the bushing and screwed in as tightly as possible with an appropriate wrench. Not more than one thread should be visible above the bushing hex, less if it can be screwed in farther. TIGHTEN THEM SOLID!

If the pipe end has been cut to an extra long thread, the small end of the thread should be either cut off if the female fitting has a shallow hole, or the small end should be screwed completely through the bushing until the large end of the tapered thread is close to or inside the hex of the bushing.

How to Avoid Joint Failures

Almost 100% of reported leaky joints have been due to failure to tighten the joints as far as it will go. Do not fail to tighten securely!

The balance of joint failures have been traced to improper threads in metal fittings or failure to allow relief for movement of the pipe system due to heating and cooling. If a section of a pipe system containing a nylon bushing is anchored securely at both ends and the pipe is heated and cooled, the resultant change in length is likely to distort or tear the nylon so that a leak can occur after one or more cycles of heating and cooling.

The remedy is to follow good mechanical practice and allow sufficient movement of one end or the other of the section that includes the nylon bushing, or to use the expansion bends to relieve the pressure. One case was found where a 1-1/2” pipe was connected to a heavy hot water boiler and run straight out for 12 feet, then at a right angle inside a brick wall. The calculated expansion of this pipe was over ¼” and the nylon bushing at the boiler end was simply torn apart.