How to Guide

102
Plastics Technical Manual
INSTALLATION PROCEDURES
Important Information on
Threaded Connections
Millions of PVC, ABS and CPVC threaded fittings have
been produced over the years. When properly installed
these fittings provide excellent service in both pressure and
drainage applications. Some of the most common installation
errors include over-tightening and the inappropriate use of
female adapters.
Tapered Threads
American National Standard B2.1 is the dominant standard
used for threaded fittings in piping applications. Adherence
to this standard ensures that mating parts will thread properly
and provide appropriate service. ANS B2.1 requires that
fittings be made with tapered threads. Fittings with tapered
threads work like a wedge; the wedge forming the water
seal like a cork in a bottle and the threads holding the two
parts together. However,
this wedge also exerts
tremendous force which
can crack female fittings
just as a small wedge
tapped into a hole can be
used to split giant boulders
in a quarry.
In piping applications the force generated when a tapered
fitting (wedge) is tightened is referred to as strain. If a
threaded fitting is over-tightened, the strength of the plastic
material can be exceeded, causing the material to yield and
the fitting to fail.
Strain increases as the pipe diameter decreases, making it
easier to split smaller-diameter threaded fittings than larger
fittings. At the same time, it is easier for an installer to over-
tighten small diameter fittings because less effort is required
to tighten them.
Threaded Fitting Applications
Threaded plastic pipe and fittings fall into two categories
of application. The first is when they are used in all-plastic
systems. The second is when they are used to transition from
metal to plastic. There are three possible combinations: 1)
plastic male to plastic female (recommended); 2) plastic
male to metal female (recommended); 3) metal male to
plastic female (not recommended). Threading metal male
thread into a plastic female thread produces very high stress
in the plastic fitting and is not recommended by Charlotte
Pipe. For reasons cited above, the Uniform Plumbing Code
expressly prohibits the use of CTS CPVC female adapters.
Why do metal male threads cause so much damage when
threaded into plastic female threaded fittings? Why doesn’t a
plastic male thread cause as much of a problem? The answer
is that when plastic-to-plastic threaded fittings are tightened,
the female fitting expands and the male fitting compresses.
The stress is shared equally between the two. However, when
a metal male thread is tightened into a plastic female thread,
stress is not shared equally. Since metal has a much greater
strength compared to plastic, it does not compress when
tightened. This places all the stress on the plastic female
fitting.
Female Adapters
An excellent example
of an application where
female plastic threads
can be a problem is the
use of Schedule 40 PVC
threaded caps to test a
domestic water system.
In this scenario a steel
pipe nipple is connected
to a newly constructed
domestic water system
and a PVC threaded cap
is used to seal the nipple as shown in the photograph.
There are several problems with this application. First, the
International and Uniform Plumbing Codes do not permit the
use of PVC 40 pipe and fittings to be used in domestic water
All pipe thread sealants must conform to the requirements
of IAPMO’s PS 36 and with the thread sealant manufacturer
to confirm that these sealants are chemically compatible
with ABS, PVC, and CPVC. Incompatible pipe thread
sealants may result in the degradation of plastic pipe or
fittings resulting in product failure and property damage.
• Verify that paints, thread sealants, lubricants, plasticized
PVC products, foam insulations, caulks, leak detectors,
insecticides, termiticides, antifreeze solutions, pipe
sleeves, firestop materials or other materials are
chemically compatible with ABS, PVC, or CPVC.
• Do not use edible oils such as Crisco
®
for lubricant.
• Confirm compatibility of pipe marking adhesive tape
with the manufacturer of the tape to ensure chemical
compatibility with CPVC pipe and fittings.
Exceeding recommended torque for threaded
connections may result in component damage, system
failure, and property damage.
Never use thread sealant when installing a P-Trap or a
Trap adapter with a plastic or metallic nut. Use of thread
sealants could cause seal separation or cause damage to
the fitting through over-tightening.
Maximum wrench-tightness is two turns past finger
tight. Plastic or metal nuts should be tightened with a
strap wrench only. Never use common wrenches or tools
designed for metallic pipe systems.