Threaded plastic connections are often the weakest component of an irrigation system.
If installed incorrectly, they can easily lead to leaks or complete failure of the connection
due to improper application of sealant products and more commonly, over-tightening.
Male and female pipe threads are tapered. Male threads get wider with each thread while
female threads get narrower. This allows them to wedge together to form a better seal. However,
sealant is also required to fill in voids between the root and crest of the threads
that otherwise would be an opportunity for fluids to escape.
As I mentioned, over-tightening is commonly the reason for the failure of threaded PVC
connections. If the male end of a threaded fitting is forced down to far into the female
side it creates stress that the plastic cannot withstand. Using too much thread tape will
increase this stress and eventually cause the fitting to crack or split.
Manufacturers of PVC fittings and other experts in the field seem to agree that the best method
for making threaded connections is to use pipe thread sealant vs. PTFE thread tape,
commonly referred to as Teflon tape. It’s said that the lubricating characteristic of
thread tape is intended to prevent galling on metal fittings. Using it on PVC can give
the installer the false sensation of the fitting not being tight enough, so they end up over-tightening
the connection.
On the other hand, the Plastic Pipe and Fittings Association recommends using PTFE thread tape.
They state that there are many types of oil-based thread sealants, designed for metal, that
are chemically incompatible with PVC. Using the wrong type of sealant can also damage
spray heads and valves. This is why you will often see “use no pipe dope” printed on
some of these products. If you choose to use a thread sealant, it should be non-hardening
and approved for use on PVC.
Whether you prefer to use PTFE tape or thread sealant, it’s important to know how much
to use, how to apply it, and how far down you should tighten the fitting. If you decide
to go with thread tape, use white PTFE tape that’s at least 3-mil in thickness. The
tape width you’ll want to use will depend on the size of the fitting you are working
with.
The tape should be wrapped in the same direction as the thread spiral. This will prevent the
tape from bunching up or unwrapping as you make the connection. Making sure the threads
are clean, hold the fitting in your left hand and align the edge of the tape over the threads
so that it’s parallel to the end of the fitting. You want to cover the first thread
but don’t allow the tape to extend past the edge of the fitting.
Press down on the end of the tape with your thumb to hold it in place and begin wrapping
in a clockwise direction. As you wrap, pull with enough force to stretch the tape slightly,
forcing it down into the threads without slicing thru the tape. Each successive wrap should
cover at least ½ of the width of the wrap below it. Once you’ve made no more than
2 to 3 rotations, press down on the tape where you want to make the break and pull. Finally,
smooth the loose end of the tape down into the threads.
When you make the connection, it should be
finger-tight plus a maximum of 2 full rotations. Tightening more that 2 rotations will apply
unwanted stress to the female side of the connection. It’s also important not to back
the threads out after you’ve made the connection. Using serrated gripping tools is not recommended.
Use a strap wrench or crescent wrench depending on the fitting you are working with.
If you prefer to use a thread sealant instead of tape, apply the paste in an even coat to
the male threads only, mostly filling in the crevices. Be careful not to apply too much
or the excess can squeeze out and possibly clog valves, spray heads or emitters. As with
the thread tape, finger-tighten the fitting as you make the connection then complete no
more than 2 additional full rotations.
One last thing to note: if you are connecting nipples or male adapters to an irrigation
valve, refer to the valve manufacturers installation instructions. Most only require a half to
1 full turn past finger tight when tightening the connection.
Regardless of your personal sealant preference, hopefully this information will help you make
better leak-free threaded connections in the future. Both PTFE thread tape and thread sealant
products can be found at your local Ewing branch. To find a location near you or to
shop online, visit us at ewingirrigation.com.