Continuous Thickness Monitoring: Precise Corrosion Rates in Less Time

Introduction

Continuous thickness monitoring is, in the opinion of the author, one of the more innovative technologies for corrosion control in the refining industry to come about in the last two decades. While this technology has already been successfully implemented by many in the industry, its unique ability to determine how process changes affect corrosion has not yet been fully realized and taken advantage of by most users.

The most common form of this technology is accomplished by attaching an ultrasonic thickness measurement device to a pipe, where it remains fixed in the exact same position, taking thickness measurements at a high frequency (i.e. daily or more) for a number of years or the time frame of interest. This allows the thickness measurement trend that comes from this probe to be very precise, but not necessarily accurate. 

The difference between accuracy and precision is important enough to be reviewed here. From study.com^[1]: “Precision and accuracy are often used interchangeably, but in science they have very different meanings. Measurements that are close to the known value are said to be accurate, whereas measurements that are close to each other are said to be precise.” Continuous thickness monitoring probes aren’t guaranteed to be accurate because they aren’t a calibrated instrument. They are precise because they take measurements in the exact same location, in the exact same way, every time they take a measurement. The author’s experience has been that the typical standard deviation for an ultrasonic thickness measurement probe has been demonstrated to be less than 2 mpy whereas the typical standard deviation of a condition monitoring location (CML) reading taken with a UT instrument can be as high as 30 mpy.