Plastic Pipes Conference Association # 2021 Amsterdam
Chris Ampfer, Gene Palermo, Ernest & Oren Lever
This paper looks at the origin of the 10% allowable scratch depth rule for polyethylene (PE) pressure pipe developed in 1971 and research that has been performed over the years since its inception to determine the validity of this rule. The 10% rule has been used by natural gas pipeline operators since the 1970’s to determine if scratched pressure pipe is acceptable for service or should be removed from service. This rule has crept over into other pressure applications like water distribution. Polyethylene pipe can be scratched or gouged in many ways before, during, or after it is installed. The 10% scratch depth rule was developed on first generation PE pipe and was left unquestioned until after a test to predict slow crack growth resistance (SCGR) was developed in the late 1990’s. Since that time, research has shown that allowable scratch depth is dependent on the pipe’s slow crack growth resistance, operating temperature, and operating pressure. As the SCGR of pressure pipe resins have increased since the 1970’s, it has become the greatest factor in determining allowable scratch depth.
This paper concludes with recommended maximum scratch depths for polyethylene pipe of 35% for gas distribution and 10% for water distribution and non-regulated gas gathering dependent on average operating temperature over the pipe’s estimated life. Data from historic research was reviewed and re-analyzed to formulate a simplified equation for estimating allowable scratch depth based on failure time, PENT value, operating temperature, and hydrostatic design stress. Allowable scratch depth limits along with a depth measurement tool can be used by pipeline operators to determine if scratched pipe can remain in service or if it must be removed.
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