Quality
Waterjet Newsletter – 11/13/2007
Testing of NPT and NPTF Connections NPT and NPTF connections are
commonly used in the water jet cleaning industry. As a higher and higher
working pressure is being used in this industry, a concern has been raised about
the use of NPT and NPTF connections. Lees and Crofton* did some testing in an
attempt to address this issue. Testing was done with two popular
sizes (1/4” and ½”) of NPT and NPTF connectors (with NPT and NPTF threads
mixed and matched). A “Threadmate” (a Parker Hannifin, Inc. product) sealant,
PTFE tape, and dry condition were used as another testing variable. Two
methods of applying torque were used. In the first method, the tightening
torque was progressively increased after each leak. In the second method, the
maximum torque (100 ft-lb for ¼” and 150 ft-lb for ½” connections) was
applied once and once only. The fittings for the tests were made from cold
drawn 316 stainless steel bar. Test fluid was hydraulic oil with a very low
viscosity. Connections in test were immersed in water to check leakage of
oil. Testing pressure was limited to 80,000 psi
maximum. Even though NPTF threads (also
known as “Dryseal”) are designed to form a better
seal (because no clearance is allowed between the rest and root of the
thread), the test results showed that no one combination (among NPT/NPT,
NPTF/NPTF, NPT/NPTF) consistently outperformed the others. Under dry condition (no sealant or
tape), the maximum torque tightening method had a higher leakage pressure
than the progressive tightening method probably because of plastic
deformation of threads. With the PTFE tape and the Threadmate
sealant, the leakage pressure was significantly higher than without (dry
condition) and the progressive tightening method performed as well as (if not
better) the maximum torque method. Therefore the fittings can be reused if
they are recoated with PTFE tape or sealant. There was no consistent
difference between the PTFE tape and the Threadmate
sealant in terms of leakage pressure. For ¼” thread it appears that there was
an optimum torque of around 50 ft-lb, beyond which the leakage pressure got
lower. There were evidences, when a proper tightening torque and a proper
sealant were applied, both the ¼” and ½” threads
were capable of withstanding 80,000 psi without
blowout or even leakage (a good safety margin for an operating pressure of
20,000 psi). Authors’ disclaimer: These results
and comments are not intended to be recommendations or approvals for the use
of any type of fitting at any pressure. * Lees,
William, A. and Crofton, P. Shaun (1999) Factors influencing the leakage
characteristics of NPT and NPTF threaded connectors, Proceedings of the 10th
American Waterjet Conference, l
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