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Using Corrosion as a Process Variable

 Let’s say you’re a process engineer trying to optimize a particular unit. You think to yourself that it would be really helpful if you knew what the temperature is at the outlet of a reactor. How would you solve the problem? You would add a temperature sensor and transmitter at that point and tie it in to the DCS where the reading would be tracked along with every other process variable. If the value is worth knowing, it’s worth monitoring and historizing.

Jake Davies says this thinking should apply to corrosion in his article in the March 2019 issue of Chemical Engineering, Using Corrosion as a Process Variable. His point is that many refiners are at risk of potential safety incidents because feedstocks carrying corrosive contaminants are eating equipment away from the inside.

Some petroleum refiners looking for ways to improve profitability have lately turned to buying opportunity crudes from secondary sources. These crudes, while priced less than more premium feedstocks, often contain various contaminants, including solids and corrosive compounds. Such contaminants can damage processing equipment by corroding and eroding it from the inside out. In the worst case, a pipe wall may become progressively thinner over time until the pressure causes it to break open — hot hydrocarbons that are perhaps higher than the auto-ignition temperature can escape and mix with air, resulting in a potentially serious safety and environmental incident.

If you pay attention to safety incidents in refineries, you can probably think of a disaster or two caused by corrosion weakening piping until it bursts, with predictable results. But just like trying to determine the process temperature at a strategic point, is it possible to measure the effects of corrosion? Fortunately, the answer is yes.

For plant operators, corrosiveness has to be viewed as another process variable — it has to be measured and managed. However, in the past, there was little instrumentation available to characterize it and measure the impact that the corrosiveness of the process fluids has on process equipment. Fortunately, this situation is improving with the advent of more advanced corrosion-monitoring technologies, including wireless systems.

Jake explains several strategies operators can use to mitigate the problem, but they all benefit from knowing what is going on inside the pipe, particularly the extent to which the metal is being eaten away, or better yet, if the mitigation is effective and the metal is not being eaten away. It is possible to determine both types of situations if the metal thickness is being measured just like temperature or pressure.

Wireless wall-thickness measurement sensors can be installed throughout a unit on pipe and vessel walls to monitor metal thickness continuously. Data from these sensors can be transmitted over a wireless network to automation systems and process historians. Engineers can use these data to make decisions in real time, and the data can also be historized, just like data from a temperature or pressure transmitter or sensor, and retrieved for analysis. Just as a pressure transmitter can warn when the equipment has exceeded a safe value, a thickness monitor can trigger an alarm when wall thickness has reached a safety threshold. This is an important function, but the ability to watch metal loss over time can provide even deeper insight into the process, and assist with other corrosion-mitigation strategies.

So there it is: Users can monitor metal thickness continuously in real time using Rosemount Corrosion & Erosion Monitoring, and thereby determine how much metal has been lost to corrosion. Ultrasonic thickness measurement probes, communicating via WirelessHART, provide basic data. Historized data identifies those times when metal loss is excessive and when it is not, so it is possible to determine conditions that make it better or worse so the process can be understood, characterized and optimized.

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 Corrosion Monitoring and Digital Transformation Groups and other specialty areas for suggestions and answers.

Posted by Deanna Johnson, Director Integrated Marketing Communications for Machine Automation Solutions