Corrosion monitoring is designed to mitigate several safety risks, but incorporating it into an existing maintenance program can be tricky.
“General corrosion leads to wall thinning,” explains James F. Dante, manager of the environmental performance of materials for Southwest Research Institute. “In a piping system, a reduction in wall thickness will eventually lead to a situation where the piping can no longer safely maintain the internal line pressure leading to burst pipes. Additionally, localized pitting corrosion can lead to conditions where stress is concentrated. For a deep enough pit, enough stress may be present at the bottom of the pit to induce the formation of a crack leading to catastrophic structural failure. Leaks often occur as a function of corrosion under insulation. Corrosion of electronic components could result in failure of automated control systems. To be clear, corrosion monitoring is performed using surrogate sensors, not the structure itself. However, information can be gained that defines the type of attack that the exposure environment causes. As an example, multi-electrode corrosion sensors can be used to determine whether corrosion is uniform or localized for a given environment. Other systems can be used to determine if water ingress occurs under insulation and whether or not the trapped moisture is corrosive.”
Corrosive liquids and gases are found in industrial processes — the chemicals flowing through the pipes and vessels of the process — and in the utilities for these processes — the cooling liquids for temperature control, waste water systems, or fire suppressant systems, says Mike McElroy, CorrTran account manager, Pepperl+Fuchs. “If the processes’ pipes and vessels fail due to corrosion, this would result in a chemical release, which could harm people and the environment,” he explains. “If the utilities’ pipes and vessels fail, then the cooling system may not be available for a runaway reaction and an explosion could result. Corrosion would contribute to the risks, and corrosion monitoring should be considered with other methods of mitigating risk. These other methods would consist of scheduled testing of safety systems, mechanical maintenance on valves and joints, and other common maintenance and safety practices done in a well-run plant.”
In plant settings, roughly 80% of catastrophic equipment failures are related to piping, says Travis Keener, technical advisor, industrial services — asset integrity management, SGS North America. “Depending on the damage mechanism occurring in the piping, loss of containment can result from pinholes, small cracks, and even sudden fishmouth openings,” he explains. “Corrosion monitoring not only covers tanks and vessels, but, per piping codes such as API 570, is intended to cover sensitive areas such as injection points, dead legs, CUI, and elbows. Several traditional and advanced nondestructive testing (NDT) methods are available to operators. Some of the NDT are point measurements, and others are able to look at large areas of piping at once. Without corrosion monitoring, facilities not only leave their risk profiles unnecessarily high, if their product storage capacities are above the thresholds set by OSHA’s PSM program, there’s a risk of fines and penalties.”
|Mike Bacidore is chief editor of Plant Services and has been an integral part of the Putman Media editorial team since 2007, when he was managing editor of Control Design magazine. Previously, he was editorial director at Hughes Communications and a portfolio manager of the human resources and labor law areas at Wolters Kluwer. Bacidore holds a BA from the University of Illinois and an MBA from Lake Forest Graduate School of Management. He is an award-winning columnist, earning a Gold Regional Award and a Silver National Award from the American Society of Business Publication Editors. He may be reached at 630-467-1300 ext. 444 or firstname.lastname@example.org or check out his Google+ profile.|
Despite the positive impact, monitoring corrosion isn’t typically a part of PdM. “In many cases, corrosion is not considered a major factor in predictive maintenance programs,” says Dante. “Focus is primarily given to functionality or structural reliability, and hence sensor systems are designed specifically to measure these factors. The role corrosion plays in each of these primary factors, however, is often underappreciated. That said, corrosion sensors play an important supplemental role in predictive maintenance. Components and systems experience variable environmental exposure spectra. Specific environmental factors affect corrosion processes and eventually equipment reliability. As an example, it’s known that cracking of components exposed to coastal environments are more likely to undergo stress corrosion cracking during times when the relative humidity is decreasing. Thus, long-term monitoring of these environmental conditions could potentially be used as an indicator of structural damage. Surrogate sensors that directly measure corrosion rates are most useful in process streams. For example, these sensors could be used in a feedback loop to indicate the need for inhibitor injection.”
Corrosion monitoring in water systems is not necessarily predictive maintenance, agrees McElroy. “It is necessary to control the addition of corrosion inhibiting chemicals,” he explains. “If the corrosion monitoring is not done, then it’s likely that inhibiting chemicals aren’t being added. In this situation we can predict that there will be corrosion problems that require maintenance. The two basic methods of measuring corrosion are to measure the corrosivity of the gas or liquid and to measure the thickness of the wall of the pipe or vessel containing the liquid. Method 1 is more amenable to control. Method 2 would be better suited to predictive maintenance, since the wall thickness gets thinner with corrosion and, at a certain point, the pipe or vessel must be replaced and maintenance can be scheduled.”
The essence of corrosion monitoring is to predict the retirement dates for individual pieces of fixed based equipment such as tanks, vessels, and piping, says Keener. “Some damage mechanisms, such as cracking, are not easy to predict, but general corrosion and uniform thinning can and should be accurately monitored,” he explains. “When necessary equipment replacements can be planned, the cost savings factor can be nine times greater than unplanned changeouts.”