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1 Overview

Corrosion is an important hidden danger that endangers the safe and smooth operation of refinery equipment. It can cause equipment leakage or failure. In severe cases, it can lead to equipment shutdown and accidents such as fires and explosions. Therefore, refineries generally take certain measures on Corrosion Monitoring Methods.

Traditional corrosion control methods mainly focus on preventing corrosion from the perspectives of material selection and process anti-corrosion. With the development of industrial automation technology and testing instruments, corrosion monitoring technology has gradually occupied an increasingly important position in anti-corrosion in the oil refining industry.

The so-called corrosion monitoring technology uses various instruments, tools and analysis methods to determine the corrosion rate of materials in the process medium environment, and provides timely feedback of equipment corrosion information to engineering and technical personnel, so as to take effective measures to slow down corrosion and avoid corrosion accidents.

Usually, corrosion monitoring mainly has the following purposes:

  • (1) Determine the extent and form of corrosion.
  • (2) Monitor the effectiveness of corrosion control methods (such as material selection, process anti-corrosion, etc.).
  • (3) Early warning of potential corrosion hazards.
  • (4) Determine whether technological measures need to be taken for anti-corrosion.
  • (5) Evaluate the usage status of equipment pipelines and predict the service life of equipment pipelines.
  • (6) Help formulate equipment and pipeline inspection and maintenance plans.

Therefore, through corrosion monitoring, factories can not only prevent corrosion accidents, but also adjust corrosion control plans promptly, reduce unnecessary corrosion control costs, and obtain maximum economic benefits.

2. Commonly used corrosion monitoring technologies

Since the 1970s, Chinese refineries have successively adopted corrosion monitoring technologies such as corrosion coupons, resistance probes, fixed-point thickness measurement, on-site corrosion test devices, and condensate water analysis, providing a large amount of data for equipment anti-corrosion in refineries.

2.1 Corrosion coupon monitoring

As one of the most basic methods of corrosion monitoring, corrosion coupon monitoring has the characteristics of simple operation and high data reliability, and can be used as an important basis for equipment and pipeline material selection. At present, there are two main methods of corrosion coupon technology used in Shengli Refinery. One is to use the device to shut down for maintenance, hang corrosion coupons in key corrosion parts inside the device, wait for a production cycle, and then take them out when the device is shut down for maintenance again, measure the corrosion weight loss of the coupons, and calculate the corrosion rate. This method is called On-site corrosion coupon monitoring. The monitoring period of this method is based on the operation period of the device, which is usually 2 to 3 years. The second method is to use coupon probe technology (Figures 1 and 2)

Figure 1 Schematic diagram of probe hanging piece

During the operation of the device, key corrosion areas are monitored, and the monitoring period is usually one to two months. The corrosion coupon monitoring operation cycle is relatively long, and the measured data is the average corrosion rate of the device over a period of time, which cannot reflect the corrosion rate of the equipment at a certain point, so it cannot be used for real-time online analysis. The corrosion coupon monitoring data is mainly used for equipment material selection and monitoring the application effect of process anti-corrosion measures. It can also be used as the basis for comparison of other corrosion monitoring data.

Figure 2 The application of probe coupons on the air cooler at the top of the atmospheric tower

(1) Structural type of probe hanger
The probe hanger is mainly composed of a probe rod, a sealed stuffing box, a locking ring, etc. It can be used with the valve installed on the on-site pipeline to perform pressure loading and unloading operations, so the monitoring coupon can be taken out at any time for corrosion measurement. The schematic diagram is as follows:
The loading and unloading of the coupon probe needs to be matched with a PN40, DN25 gate valve. There should be a large space at the opening position to facilitate monitoring operations. The schematic diagram is as follows:

In the picture:

  • 1 – Monofilament head, PN40DN25, made of the same material as the process pipe. The wireless end is connected to the process pipe and welded. One end of the wire is connected to the valve, tightened and then welded.
  • 2-Valve, Z11H-40-25 gate valve. Since the gate valves produced by various manufacturers are different, when selecting the valve, pay attention to the fact that it should be able to smoothly pass a round rod with a diameter of 20mm when the valve is fully open.
  • 3-The hanging piece probe is connected to the valve thread and can be disassembled and assembled at any time.

(2) Test piece specifications
Dimensions (length × width × thickness mm): 25.0 × 10.0 × 2.0, with an f4.0 hole 5mm from both ends.
The material of the test piece is selected as domestic No. 20 high-quality carbon steel (GB 699-65), and the chemical composition should meet the following standards:

(3) Data processing
The results of corrosion tests are usually expressed as the average value of uniform corrosion. Commonly used units such as: mg/m2·d) can visually indicate the speed of corrosion; millimeters/year (mm/a) can be used to estimate the service life of equipment, and its reliability is determined by Uniformity of corrosion. The general formula for calculating corrosion rate is as follows:

2.2 Resistance probe monitoring

Resistance probe corrosion monitoring is to calculate the corrosion rate of metal in the process medium by measuring the change in resistance value of the metal component (called the probe) after corrosion in the process medium. When the probe is corroded, the cross-sectional area decreases, causing the resistance to increase. The increase in resistance is directly related to metal loss. Therefore, through a certain formula, the corrosion rate of the metal can be calculated.
Resistance probe technology is the most widely used online monitoring technology in refineries. It installs multiple probes at different locations and displays changes in corrosion rates through monitoring instruments. In addition, the probe measuring elements can be made of different materials depending on the needs of the site. Another advantage of the resistance probe is its wide range of applications, which can be used in almost all media environments in the refinery, including gas phase, liquid phase, solid phase and flowing particles.
The resistance probe signal feedback time is short and the measurement is rapid. It can reflect the corrosion situation of equipment pipelines in time, so that the corrosion of equipment pipelines is always under monitoring. Therefore, this method is an indispensable monitoring and control method for severely corroded parts and parts with sudden severe corrosion in a short period of time. However, due to the limitation of the measurement sensitivity of the instrument, the measured data are greatly affected by changes in the corrosion rate of the process medium, and the measurement results sometimes deviate.
(1) Single-point monitoring resistance probe
The single-point monitoring resistance probe can only monitor the corrosion rate of one point at a time. The resistance probe monitoring instrument used by Shengli Refinery is the self-developed and designed DF-type resistance probe corrosion monitor. The corrosion monitor consists of two parts: a resistance probe (sensor) and a corrosion monitor (see Figure 3). In order to ensure that the resistance probe can be easily disassembled on site, the probe part is designed to be retractable and equipped with a probe loader and unloader. Resistance probes are mainly used in Shengli Refinery for corrosion monitoring of the low-temperature system of atmospheric and vacuum units, corrosion monitoring of process anti-corrosion tests, and corrosion monitoring during catalyst regeneration of hydrogenation units.

Figure 3 Resistance probe corrosion monitor

(2) Multi-point online monitoring resistance probe
Online corrosion multi-point monitoring technology is to realize multi-channel online automatic corrosion monitoring by converting the corrosion signals of multiple resistance probes through analog-to-digital conversion, remote transmission, data processing, software centralized control, etc.

3. Formulation of corrosion monitoring plan

Due to the complexity of the process medium environment in refineries, simply using a certain corrosion monitoring method generally cannot fully grasp the corrosion status of equipment and pipelines. To obtain good corrosion monitoring results, a corresponding corrosion monitoring plan must be formulated. Usually a corrosion monitoring plan includes the determination of corrosion monitoring locations, corrosion monitoring methods and corrosion monitoring frequency.

3.1 Determination of corrosion monitoring locations

The determination of the corrosion monitoring location directly determines the quality of the corrosion monitoring effect. Generally speaking, the real threat to equipment pipelines is local corrosion. Therefore, how to monitor the relatively serious parts of equipment pipelines, that is, the selection of corrosion monitoring locations, is very important. These parts vary with the process conditions, materials, structures, etc. of equipment and pipelines. Usually, you need to pay attention to the following parts with serious corrosion:

(1) Parts where water condenses, especially the parts where water condensation begins, such as the air cooler outlet and water cooler inlet and outlet of the atmospheric and vacuum tower top condensation cooling system.
(2) Parts where corrosive media are concentrated, such as circulating cooling water systems.
(3) High turbulence areas in equipment pipelines, such as elbows in pipelines.
(4) Parts with serious corrosion caused by high temperature and high pressure.
(5) Equipment and pipelines where accidents occur frequently.
(6) Equipment pipelines planned to be replaced in the next cycle.

3.2 Determination of corrosion monitoring methods

In the actual production process, a single corrosion monitoring method cannot meet the user’s requirements. It is usually necessary to use multiple methods at the same time to obtain more accurate and reliable corrosion monitoring information. For example, resistance probe corrosion monitoring data usually need to be corrected with corrosion coupon data to prevent data deviation caused by factors such as probe contamination. In addition, process medium analysis and corrosion product analysis are also very important, which can reflect the main causes of corrosion and corrosion conditions. After being associated with corrosion monitoring data, these data can be used to predict the possible corrosion and its extent. The atmospheric and vacuum tower top condensation cooling system of the atmospheric and vacuum unit of Shengli Refinery adopts a variety of corrosion monitoring methods including corrosion coupons, resistance probes, condensate water analysis, etc.

3.3 Determination of corrosion monitoring frequency

Corrosion monitoring can be carried out periodically or continuously. The monitoring frequency is determined by the corrosion monitoring method, the corrosion degree of the monitored part and the cost of corrosion monitoring. The corrosion monitoring method determines the response time of the corrosion rate. For example, corrosion coupons usually require a monitoring cycle of more than one month, while the monitoring cycle of a resistance probe can be shortened to a few hours to a few days. In addition, the frequency of corrosion monitoring should be increased when the corrosion of the monitored parts increases, while the monitoring frequency of parts with relatively mild corrosion should be reduced accordingly. The cost of corrosion monitoring is also very important for refineries. If high-cost corrosion monitoring methods are used too frequently, the cost is quite huge. At the current stage, the cost of continuous online corrosion monitoring is higher than that of periodic corrosion monitoring. Therefore, refineries mostly adopt the latter method and try to reduce the monitoring frequency where possible.