A stock phrase you might remember from old infomercials promised, “And it works at home just like on TV.” Someone adapting that thought to industrial users might say, “And it works in your plant just like in the lab.” But beware of such claims for corrosion monitoring systems, because while they might work well in a test lab environment, they may fail when applied in the real world of a refinery or chemical plant.
Jake Davies examines this point in an article he wrote in the February 2018 issue of Flow Control: How to Specify a Corrosion Monitoring System. It’s subtitled, “Make sure it works in the field, not just in a lab.”
He begins by citing some astonishing statistics compiled by the National Association of Corrosion Engineers on the effects of corrosion. Long story short, the costs are huge, equivalent to 3.4% of global GDP. The organization suggests this figure could be reduced by up to one-third if companies implemented basic corrosion control practices.
Some corrosion monitoring programs try to infer metal loss by monitoring the corrosiveness of the liquids involved and then estimating how deeply they have likely attacked the equipment, but in most situations, this ends up being largely guesswork. The other way is to try to measure actual metal loss.
Two main types of corrosion monitoring systems are available: traditional inline probes that place a sacrificial element that deteriorates from corrosion inside the process fluid, and ultrasonic systems that measure wall thickness. Inline probes require a hole to be made in the side of the piping/vessel so the probe can be placed in contact with the fluid. This access fitting is often specified when the plant is built. Inline probes measure the damage to a sacrificial element of the device, from which the damage occurring to the actual equipment wall is inferred.
Ultrasonic methods have been in use for many years, but historically have been primarily portable devices used to measure metal thickness at a given point. This is useful in many applications, but not practical for compiling continuous data in a strategic area such as a pipe elbow. Whatever method is used, it must be evaluated in light of what is practical in real-world situations.
For monitoring purposes, the ability to continuously detect and measure wall thickness loss with a high degree of repeatability is generally more important than the absolute accuracy of a single measurement. Some vendors quote resolution and accuracy without supporting data from the field. Other vendors quote resolution as the number of decimal places the thickness measurement is capable of under ideal conditions. And some vendors quote results from data collected under controlled laboratory conditions, which is generally not achievable under real field conditions.
Getting meaningful data over time requires installing permanent sensors in strategic locations, capable of monitoring metal thickness continuously. As Davies suggests, repeatability is the more critical element of the measurement, and the devices have to stand up over the long term. Anyone considering a monitoring system needs to ask some difficult questions.
Ask to see real field data where wall loss has been detected to determine how small a metal loss is measurable, and how quickly that loss can be recognized from the measurement data. Also ask to see real field experience that shows measurement quality will not degrade over time; for example, piezoelectric transducers in permanent contact with hot metalwork age and introduce distortion into signals. Transducers that require a coupler or are glued onto the metalwork will also likely degrade over time in industrial applications.
This is only the beginning of the discussion. The article does a deep dive into a whole list of elements of a monitoring program including hazardous area considerations, temperature limits, installation methods, communication, data handling and more. Success depends on having a system able to work well in the extreme environments often found in chemical and other plants and facilities for years at a time. The Permasense product family of corrosion sensors can take this type of the punishment, documented through year of operating experience, while delivering accurate and repeatable measurements.
You can find more information like this and meet with other people looking at the same kinds of situations in the Emerson Exchange365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the WirelessHART and IIoT Groups and other specialty areas for suggestions and answers.