Pressure Limits of Tapered Pipe Thread Connections
An experimental study was conducted by Wright et al* to determine the maximum allowable pressures for tapered pipe thread connections used in the waterblast industry. Its main points are reviewed here.
The two popular equations for calculating burst pressure of a cylinder are as follows:
Barlow equation for thin walled cylinders (ID/wall > 10)
Lame’ equation for thick walled cylinders (ID/wall < 10)
Where P = burst pressure, S = tensile strength of material, t = wall thickness, D = outer diameter, w = wall ratio (OD/ID). All the test samples in this study had a wall ratio less than 10. Using a safety factor of 2.5, specified by Waterjet Technology Association, the maximum allowable operating pressure can be calculated with P/2.5.
Tests were done to verify these two equations applied to tapered pipe thread connections. Test samples (including 6 solid plugs, 24 male pipes, 16 pipe couplings) were made from 303, 304A bar, 304L seamless schedule 160 pipe and 17-4 H900 stainless steel. Tests with different thread engagement and multiple assemblies were also done. A single stroke intensifier pump (80 kpsi maximum operating pressure) was used for most of the tests. In some cases leakage of test fitting exceeded the capacity of the intensifier pump and then a triplex plunger pump (40 kpsi maximum) was used instead.
Reported test results include failure pressure and failure mode of each test, photos of tested samples showing the failure modes, as well as comparison with calculated values. To the left is a graph that illustrated the ratio of calculated values to actual failure pressures. It appears that Lame’ equation most accurately predicts the failure pressure.
*Wright, D., Wolgamott, J., and
Zink, G. (2003) Pipe threads --- what is the limit? Proceedings of the 2003
WJTA American Waterjet Conference,
Beyong High Pressure
· Courtesy to Prof. O.V. Krishnaiah Chetty, an electronic copy of his paper “Studies on Recharging of Abrasives in Abrasive Water Jet Machining” is now available at http://www.qualjet.com/Newsletter/RechargingAbrabsive.pdf. Professor Chetty can be reach at email@example.com.
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