Anytime you see some outstanding claim of a high burst pressure that reaches some ‘amazing’ pressure value before bursting, you should question its validity.
A pressure test should have axial and hoop load to be legitimate, if the pipe is being constrained in the axial direction during a burst test the test is absolutely invalid in regards to the loads a piping system experiences during operation. A pipe in operation sees both hoop and axial load at the same time throughout much of the system. Therefore, a burst test with just hoop load is of limited value and is usually misleading in regards to qualifying a pipe for pressure service. Burst tests with both hoop and axial load can be as little as 50% of the value of a burst test with hoop load only.
It’s this simple, if the pipe in a burst test is free standing with viewable dome ends it’s loaded bi-axially. If you see any ‘apparatus’ restraining axial load in any way, the pipe is only bearing hoop loads.
In the end, tests with hoop load only can lead to fiberglass piping systems that have much lower factors of safety in service than owners and operators desire and in some cases can lead to premature failures during commissioning or at any point down the road.
So next time you see a burst test, don’t be fooled, question the results.
Hogni Jonsson on 06/14/2013 6:52 a.m. #
Let's clear up this common misconception.
There are many pipe installations where the pipe will not be subjected to axial thrust and no axial load is transferred through the joints. These can be both buried and aboveground installations. Thousands of kilometers of such pipelines are installed every year around the world. Go study the AWWA M45 Manual.
Chris Renoud on 06/18/2013 9:54 a.m. #
Axial loads are typically overlooked by engineers and burst test results are often misunderstood resulting in lower factors of safety of piping systems than expected. Since the 1970’s FSE has been involved in many pipe failure investigations and performed on-line testing of active piping systems to determine axial loads while the pipe is in service. We regularly see evidence of axial loads while performing these analyses and testing. We conclude that axial load in buried pipe is common and unavoidable regardless of the joining system that is employed.
You are correct in that axial loads cannot be transferred through unrestrained joints. Axial loads in underground pipe accumulate as the distance from unrestrained joint increases. We conclude this from full scale axial strain measurements on large diameter buried pipe with unrestrained joints. While the pipe is free to move at the ends near the unrestrained joints, frictional resistance quickly begins to develop axial load and approximately the center one third of each pipe “stick” can be nearly fully restrained and axial stresses exist. Of course, the magnitude of these stresses depends on the operating conditions and the specific properties of the pipe laminate.
The AWWA Manual M45 Second Edition, Section 5.8 addresses axial loads stating that, “Factors that contribute to the development of axial stresses in buried pipe are (1) hoop expansion due to internal pressure, which causes axial tensile stresses whenever the pipe is axially restrained; (2) restrained thermal expansion and contraction; and (3) pipe “beam” bending that may be induced by uneven bedding, differential soil settlement, or subsidence of soil.”
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