July 2018

Environment and Safety

No excuse for cutting corners on monitoring corrosion

Corrosion is a major challenge and risk to contain for any oil and gas operator.

Yule, P., Cosasco

Corrosion is a major challenge and risk to contain for any oil and gas operator. Loss of containment can mean loss of process and revenue, which can lead to expensive repairs and a potential major safety hazard. While a breach in a cross-country pipeline can be a major incident and environmental risk, it is another matter downstream. Various plants and processes are tightly packed together, with all manner of hydrocarbons and other combustible fluids boiling, cooling or flowing. In this environment, loss of containment could be catastrophic. 

When it comes to the downstream processing industry, corrosion control is too important to ignore. However, in conjunction with increased risk comes increased complexity—the number of different processes and the variety of different mediums and hydrocarbons entail a diverse and heterogeneous environment for monitoring. One monitoring system does not fit all, so it can be difficult for operators to avoid a pick-and-mix of systems with incomplete patchwork coverage.

How can you manage the risk of corrosion within your plant? Most, if not all, refineries and petrochemical plants employ talented and knowledgeable corrosion engineers with the expertise to manage corrosion. However, they need the best data to monitor corrosion efficiently. This requires fully integrated monitoring systems that, historically, have been in short supply.

An aggressive environment

The difficulty involved in downstream corrosion monitoring cannot be underestimated. The refining industry has many elements that can contribute to increased corrosion rates. Corrosive substances can be found in the feedstock (e.g., amines and sulfuric acid) and, often, further elements (such as oxygen, nitrogen, trace metals, salts, carbon dioxide and naphthenic acids) can be added and produced during the refining process itself. Refinery processes involve extreme temperatures and velocities, which can contribute to elevated corrosion rates.

With aging assets, extended operating windows and high demands on production rates, some firms have adopted crude blending techniques—mixing different qualities of conventional and higher total acid number (TAN) crudes to a level they had not been in the past. For operators, TAN crudes can be a third of the cost of conventional crudes. However, they also make corrosion less predictable and increase the risk of corrosion due to their higher acid content. Crude oil prices have recovered, but not to the level witnessed several years ago. Therefore, crude blending is still common practice. If refineries continue to blend crudes, they must ensure that a robust and accurate integrated corrosion monitoring system is in place.

Keeping tabs

In a refinery or petrochemical plant environment, corrosion is unavoidable. The key is to effectively monitor the issue. If corrosion is underestimated, it can quickly become a safety and business risk. However, if operators err too much on the side of caution, they risk unnecessary maintenance downtime, premature replacement of equipment or an overzealous corrosion inhibitor program—all potentially expensive and avoidable mistakes.

Although the accurate and effective monitoring of corrosion is key to safe and reliable operations, it is easier said than done. Different pipework and processes require different approaches to corrosion. For example, one of the most tried and tested methods for corrosion monitoring is the use of corrosion coupons. These are small samples of metal inserted into the flow at selected locations, which are then subjected to the same corrosive factors as the pipework. At regular intervals (approximately every few months), the corrosion coupons are removed and engineers measure how much of the metal has been corroded. These samples provide a representation for pipeline corrosion. Corrosion coupons can also provide valuable data on the types of corrosion and potential localized pitting corrosion issues. This is appropriate in many instances, but not in all.

Another commonly used monitoring method is to insert electrical-resistance (ER) probes inline into the pipework. These are capable of real-time monitoring and can detect changes in corrosivity within hours, making them ideal for highly changeable applications. High-resolution ER probes allow operators to directly monitor levels of corrosion and to react quickly to process changes in a system, rather than using a coupon as a proxy. This approach is extremely effective and, since it allows operators to adjust injected volumes based on live, granular data, it is the only viable method to accurately monitor and control corrosion inhibitor injection programs. This method has a huge cost-saving potential.

However, as with coupons, there are certain processes within the downstream industry where intrusive monitoring cannot be used due to extreme process conditions. In those cases, corrosion engineers have turned to external ultrasound thickness (UT) monitoring systems that affix directly to the outside of the pipe. These systems require no downtime or intrusion to install. However, using only the UT monitoring system is no silver bullet. The trade-off for these advantages is that operators have to settle for a lower level of sensitivity, resolution and accuracy. Modern ultrasonic technology has made great progress on this front, but still does not match ER probes for accuracy and response times.

In the downstream processing industries, there is no one-size-fits-all perfect solution for corrosion. Most likely, operators will have a patchwork of different systems, as they select the best option for each process. This often results in the best possible corrosion monitoring performance at the individual application level, while carrying inherent risks at the facility-wide scale.

The optimal path to monitor corrosion is for the engineering team to have an overall view of corrosion risk across the facility. Understanding which closely positioned equipment and/or processes are suffering from near-problematic corrosion levels helps give a more accurate gauge of overall risk to the operator and personnel. Similarly, understanding if one process is due to undergo downtime for corrective maintenance helps operators plan more effectively. For example, if one process has knock-on effects on another, it may be best to schedule maintenance for both at the same time, even if one is not quite at its corrosion limits. This action would prevent having to shut down a second time in a couple of months to repair the second unit.

The only way for corrosion engineers and operators to effectively monitor the entire plant is through assimilating those disparate systems into one broader integrated system. This operation would include incorporating corrosion coupons, ER probes and UT devices to simultaneously feed data into a central platform. This approach would provide a holistic, facility-wide view of risk. Corrosion engineers are then empowered to make the most informed decisions, guarding safety while maximizing asset profitability. In the past, this complex monitoring and decision-making might have been unfathomable. However, companies have accumulated decades of experience with these individual technologies and have invested in platforms to bring them together in a fully integrated way.

For corrosion engineers at refineries and petrochemical plants, there is no excuse not to implement an integrated, multi-pronged corrosion monitoring strategy. The risks to safety and revenue are too high to ignore, and the technological limitations that may have hampered such a program are no longer insurmountable. A modern system utilizes intrusive ER probes and state-of-the-art, highly accurate, nonintrusive ultrasonic ones. Feeding that data back into a central view of risk is the logical next step in keeping the downstream sector safe and profitable. HP

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