Northtown offers a
complete line of nylon pipe bushings in all thread sizes from 3/8 x
1/4 inch NPT to 2-1/2 x 2 inch NPT. Our nylon bushings are made with a
heavy hex for wrenching.
Northtown nylon pipe
bushings are designed specifically to insulate connections in water
systems between piping of dissimilar metals, such as iron or
galvanized connected to copper or to high-copper content
brass.
Preventing Electrolysis
& Galvanic Corrosion
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.
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All thread sizes (3/8 x ¼ to 2-1/2 x 2)
of Northtown 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.
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.
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