June 2019

Process Control and Instrumentation

Mitigating corrosion challenges from opportunity crudes

High-quality crudes are not always readily available, or at a price that is acceptable to the producer’s budget.

Wold, K., Emerson Automation Solutions

High-quality crudes are not always readily available, or at a price that is acceptable to the producer’s budget. Therefore, the trend toward using opportunity crudes—or spot crudes—is growing, despite the corrosion challenges that come with them.

The financial case for opportunity crudes—purchased at a significant discount per barrel—is compelling. Even a 1% use of opportunity crudes, at a discount of $6/bbl in a medium-size refinery of 300,000 bpd, can lead to crude oil cost savings of up to $6.5 MM/yr. In addition, these crude oil cost savings are conservative, considering that the typical Western Canada Select (WCS) crude discount to West Texas Intermediate (WTI) is about 25%.

However, the quality of the crude often fluctuates, depending on the field the crude originated from—with oil sand fields, in particular, producing a crude with much higher acidity, or total acid number (TAN) levels.

According to Natural Resources Canada, Canada exported 3.3 MMbpd to the US in 2017, with much of the crude coming from the Alberta oil sands. This crude is characterized as being heavy, sour and acidic, with high TAN levels and high sulfur. These characteristics—along with nitrogen and aromatics content and high viscosity—are often key elements of opportunity crudes and pose significant dangers to refinery equipment.

Such challenges are also compounded by storage tanks, which can have residue left in them from previous crudes stored there, and by the issues around blending. Opportunity crude oil properties often do not match the specifications of the refinery and process equipment and, therefore, have to be blended. With numerous crude oil feedstocks and varying properties, as well as the danger of blending too much opportunity crude into the mix, it is vital to have modern and flexible corrosion monitoring.

Finally, the aging infrastructure that characterizes many refineries brings with it another set of challenges when it comes to opportunity crudes. Maintaining any system requires deliberate planning and scheduled maintenance to remain safe and meet regulatory standards, while also remaining profitable.

While the optimal blending of opportunity crudes can have a significant impact on finances in refineries, it must be backed up by the latest corrosion monitoring and measurement technologies. This article will examine some of the recent advances in this area.

Opportunity crude corrosion challenges

What impact can opportunity crudes have on corrosion, and what are the challenges in detecting corrosion?

The causes of corrosion and the problems that are associated with it can vary. The most common forms of corrosion in refineries are caused by high TAN levels in opportunity crudes, leading to naphthenic acid corrosion (NAC), sulfuric acid corrosion, hydrochloric acid corrosion and hydrofluoric acid corrosion.

A key challenge is that these different types of corrosion often behave differently and are found at different plant locations. For example, NAC corrosion—the result of a mixture of naturally occurring cycloaliphatic carboxylic acids recovered from petroleum distillates—takes place in the high-temperature parts of a plant, typically in primary and vacuum distillation units and associated pipework, and increases when there is higher TAN and sulfur content in the crude.

NAC is also often triggered by velocity, and is, therefore, found in bends. Another challenge with NAC is that the impact is localized and the exact impact point is difficult to predict.

It is also important to note that the types of acids in a crude oil vary and that the boiling point distribution can differ dramatically among different crudes. The TAN value for a crude oil indicates the potential for corrosion problems, but this one acid number measurement does not provide all the details on acids within the crude oil and the impact on refinery assets.

Furthermore, because opportunity crudes can accelerate corrosion issues that might not have been detected earlier, the risk of process upsets increases if no instruments are in place to monitor for corrosion at various locations and critical points throughout the process.

The purpose of corrosion monitoring and the intended information value of the system should be considered when designing a monitoring program. Monitoring data can be used for process tuning or alarms if corrosion rates increase. If so, fast response and high-resolution monitors will often be preferred. Corrosion monitoring can also be part of an integrity management program, in which case longer term trends for metal loss would be more important than immediate response to changes.

Available monitoring technologies have different benefits and limitations with respect to the objectives indicated above. If a combination of process optimization and integrity management is required, a combination of monitoring technologies would provide the best result.

In addition to selecting technologies suitable for the purpose, data handling and reporting should reflect the objective of monitoring and the actionable information needed. Data processing should reflect if information is intended for detecting rapid changes, or whether long-term corrosion damage can be expected. Many examples exist in industry where the full information value of the monitoring system is not achieved due to a lack of understanding of the data and how that data should be processed for best value.

An integrated corrosion monitoring strategy

A good corrosion management system will ensure that line and equipment conditions are known at all times, so that problems can be anticipated and surprise process upsets avoided. However, what constitutes a good corrosion management system and how will it fit within existing systems?

What is needed is to make sure that opportunity crudes remain economically attractive without causing additional—unforeseen—expenses on the back end. Unfortunately, a one-size-fits-all solution does not exist. The ideal setup depends on the age of the facility and the condition of the infrastructure, as well as the types of metals used in the piping and how corrosion resistant they might be, and whether or not the facility is already set up for digital and wireless communications.

A good refinery corrosion monitoring setup will consist of a combination of technologies—all with different means of tackling the unique corrosion monitoring challenges that opportunity crudes bring (FIG 1).

Fig. 1. An overview of corrosion monitoring technologies.
Fig. 1. An overview of corrosion monitoring technologies.

 
For example, online information from corrosion monitoring sensors can form part of a comprehensive monitoring program, which includes metallurgy upgrades, chemical inhibitors, crude oil (blend) feedstock selection, inspections during turnarounds and the repair/replacement of assets damaged by corrosion.

Such a system might also include corrosion probes that provide the highest sensitivity and fastest responses to changing corrosion rates, with their sensitivity highly valuable for the fast-track monitoring of process changes and for tuning remedial processes, such as corrosion inhibitors.

For example, electrical resistance (ER) probes are based on measuring changes in electrical resistance, as the thickness of the probe’s measurement element decreases due to corrosion. ER probes are particularly effective for correlating with other process data, identifying how process changes (such as crude selection, including blends) influence corrosion conditions, and providing input into the amount of chemical inhibitors to add.

In addition, inline corrosion coupons calculate corrosion through weight loss with the coupon surface analyzed with respect to localized attacks (pits) and possible deposits. Such inline probes and coupons deliver fluid corrosivity data and rapid information about changing corrosion rates, giving operators a more accurate and timely understanding of asset integrity.

Another non-intrusive technology is the field signature method (FSM), which is based on feeding an electric current through a monitored section of a pipe, pipeline or vessel, where the applied current sets up an electric field that is monitored as voltage drop values between a set of sensing pins installed on the external pipe wall (FIG 2).

FIG. 2. FSM sensing pins installed on the external pipe wall.
FIG. 2. FSM sensing pins installed on the external pipe wall.

 
The initial measurement sequence, called the field signature, within the FSM measures the voltage drop between all pairs of sensing pins. Measurements are compared to the field signature, where general corrosion can be seen as a uniform increase in voltage drops between all pin pairs, and localized corrosion can be seen as a local increase in the values. The FSM is particularly adept at differentiating between localized corrosion—small areas or zones on the metal surface—and generalized corrosion, where corrosion is uniformly distributed over a much larger area.

Finally, wall thickness monitoring via ultrasonic (UT) measurements is a fast and easy corrosion monitoring tool that provides direct metal thickness measurements at refinery temperatures of up to 600°C. The limited maintenance requirements and ease of installation of wall thickness measurements also allow for a wide distribution of sensors for integrity and corrosion management, as well as process insight across the refinery.

Ultrasonic systems also deliver wall thickness measurements continuously from locations where access is costly, dangerous or physically restricted. The data from ultrasonic systems can be combined with inline probes, leading to the high-sensitivity/fast-response technologies of the probes for the immediate tracking of corrosion rate changes alongside direct wall thickness change monitoring for confirmation and verification of asset conditions. FSM corrosion monitoring can also work alongside ultrasonic measurements, combining total area coverage for localized corrosion with the direct assessment of pipe and vessel wall thickness.

The further merging of corrosion monitoring technologies could cover the ability to differentiate between localized and generalized corrosion and prioritize comprehensive monitoring solutions for critical conditions (such as the distillation area) against less-costly monitoring solutions spread over the refinery.

Within the plant, such a corrosion monitoring strategy must also be closely linked to other maintenance and reliability strategies related to inhibitor injection control, flow measurements and risk-based inspection and reliability maintenance programs.

The growth in wireless

One solution that makes tracking plant health and collecting data more efficient is wireless technologies. Not only can the right wireless tools make data collection more effective, but they can also collate data, and create current and actionable reports that help streamline operations. Similarly, the ability to identify problems early can get slowed down by data bottlenecks if equipment is unable to efficiently provide data or if there is no way to sift through the data in a timely manner.

Whereas many older corrosion monitoring systems were based on offline data collection, the standardization of wireless communications (such as through the WirelessHART standard) has led to unparalleled quality and frequency of corrosion monitoring data.

At present, a large number of applications (for pressure, temperature, vibration, etc.) are available with IEC 62591-standardized (WirelessHART) communications and can be combined through the same gateway, providing online communication to a range of data management solutions or control systems.

The growth in wireless technologies also means that corrosion monitoring sensors are viable options for future investments to enable digital transformation strategies and solutions in manufacturing and refining, such as big data, the Industrial Internet of Things (IIoT) and Industry 4.0.

A more flexible and responsive approach

As refineries continue to focus on costs and margins, it is essential for operators to learn which crude oils and blends work best in their refineries, and how any corrosion resulting from this can be preempted and mitigated.

A combination of inline, fast-response and non-intrusive corrosion monitoring technologies, alongside the latest in wireless, are ensuring a more flexible and responsive approach to opportunity crudes and more financially sustainable refineries. HP

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