A Cable Condition Monitoring Strategy For Safe And Reliable Plant Operation

Abstract

Electrical cables provide essential functions such as delivery of power or instrumentation signals for most industrial monitoring systems. Most cables installed in plants use polymer insulation materials that can become brittle, crack, or degrade over time from exposure to harsh environmental conditions such as elevated temperature, moisture, vibration, mechanical shock and radiation. Wholesale replacement of cables can be expensive, time consuming and impractical. Therefore, implementing a condition monitoring (CM) strategy to identify and quantify degradation and estimate the remaining useful life (RUL) of the cables can be an effective way of managing aged cables. An overall CM strategy includes several steps to assess the health and manage the aging of cables during the operating life of an industrial facility. These steps include performing As-Found evaluations to determine the current condition of installed cables. These As-Found assessments are performed using a combination of destructive, semi-nondestructive, and/or nondestructive CM tests. Destructive and semi-nondestructive CM testing are performed by removing cable and jacket/insulation polymer samples from service and evaluating the mechanical, thermal, chemical, and electrical properties of the materials to determine their overall condition. Nondestructive CM tests are used to perform in-situ testing to identify and assess the condition of degraded sections of cable insulations as well as to identify potential issues in the electrical circuits including degraded terminations, splices and/or connections. Each of these CM methods provides unique and important information on the overall health and performance of cables and insulation polymers. Moreover, a combination of some or all of these methods can be used to assess the condition of installed plant cables, depending on the cable configuration, insulation materials, and the needs of the plant. Predicting RUL is accomplished by performing laboratory accelerated aging of samples for each representative cable polymer type. The accelerated aging methodology involves exposing the cables to elevated environmental conditions that cause the cable polymers (insulation materials) to age faster than the installed cables. During aging, CM tests are periodically performed to trend changes in the electrical, mechanical, thermal, and chemical properties of the cable and insulation material during aging. The Arrhenius method is then used to normalize the accelerated aging data to the cables' in-service temperatures, and this normalized data is then used to estimate the cable's RUL. The focus of this paper will be to describe an overall strategy for condition monitoring of cables installed in harsh environments using in-situ (i.e. nondestructive) and laboratory (destructive and semi-nondestructive) aging assessment techniques.