​Application of corrosion monitoring and detection technology in petroleum and petrochemical industry

0 Preface

At present, the quality of global crude oil is showing a downward trend.

In order to grasp the corrosion situation of equipment and pipelines, a variety of monitoring and detection methods have been applied to the petroleum and petrochemical industry to qualitatively or quantitatively monitor the occurrence and development process of equipment and pipeline corrosion, so as to deploy additives, optimize process anticorrosion, and detect safety in time.

1 Ultrasonic technology

1.

Ultrasonic thickness measurement is a non-destructive testing technology and is currently the most widely used material thickness testing method

At present, the pulsed ultrasonic thickness gauge is generally used widely, and the frequency is generally 1~5 MHz

Figure 1 Principle of Ultrasonic Thickness Gauge

With the development of technology, the application of ultrasonic thickness measurement is more extensive, and the temperature range of thickness measurement is larger

The disadvantage of ultrasonic thickness measurement is that the measurement results are limited by the sensitivity of the instrument and the surface state of the test piece, and it is necessary to apply a couplant on the surface of the test piece to ensure the measurement effect

1.
2 Ultrasonic guided wave technology

The ultrasonic guided wave inspection system usually consists of a probe, a guided wave excitation unit, a guided wave receiving unit and a detection signal processing unit
.
When in use, the probe is installed around a specific part of the pipeline in an array, and the guided wave signal is generated by the guided wave excitation unit and amplified, and applied to the probe to generate the guided wave
.
The guided wave propagates in the pipe wall, and the discontinuity and deformation of the pipe will cause the change of the propagation velocity of the guided wave, resulting in the corresponding echo signal
.
It can be extracted and analyzed to judge the damage of the measured object, and determine the location and size of the defect
.

Figure 2 Schematic diagram of the propagation of ultrasonic guided waves

Compared with traditional ultrasonic technology, ultrasonic guided wave has the following advantages: (1) The commonly used guided wave frequency range is about 20~100 kHz, the frequency is low, and the attenuation along the propagation path is very small, which can be detected at one time in ideal pipeline inspection.
Over 100 meters, it overcomes the shortcomings of the point-by-point scanning method and reduces the detection cost; (2) No couplant is required, and no special treatment is required for the contact surface; (3) It can detect difficult-to-contact pipe parts, such as pipeline pressure.
, crossing areas, or areas with a high incidence of pipeline defects
.

The ultrasonic guided wave detection technology also has certain limitations: (1) the detection frequency must be obtained by experiments in advance; (2) it is impossible to accurately measure a certain part, and it cannot reflect the true residual wall thickness of the pipeline; (3) the circumferential direction The detection rate of defects is relatively high, and it is difficult to detect longitudinal cracks that do not exceed 70% of the wall thickness; (4) It is difficult to interpret the detection results of complex pipeline systems; (5) The coating layer such as asphalt will affect the echo signal, reducing the detection distance
.

Ultrasonic guided wave detection technology has been applied to a certain extent in China: Shengli Oilfield [13] used the electromagnetic ultrasonic guided wave detector jointly designed and developed with Tsinghua University to conduct a comprehensive inspection of a long-distance crude oil pipeline.
The verification compliance rate is 86.
7%
.
The long-term monitoring probe embedded in the ground can solve the shortcomings of repeated excavation for ultrasonic guided wave detection and save costs
.
In an oilfield in Bohai Sea, MsS (magnetostrictive sensor) ultrasonic guided wave detection technology was used to detect the corrosion of pipelines.
The two detection results at different positions and different frequencies showed that there was corrosion in the same position.
% of the area, verifying the detection results
.

2 On-line monitoring technology of corrosion coupons

Corrosion coupon detection is a kind of destructive detection, which can be installed in specific parts of the equipment in operation; it can detect the corrosion of the pipeline of the equipment within a period of time, and can also be used in the laboratory to simulate the corrosion environment, with simple operation and reliable data.
High characteristics; can also be used as an important basis for equipment and pipeline material selection
.
The corrosion coupon method can test multiple materials and multiple environments at the same time
.
The coupons can be installed directly in a representative process flow, or in a specific bypass, and the operating conditions can be changed according to demand
.
Corrosion coupons are generally divided into strip coupons, rod coupons, disc coupons and so on according to different shapes
.
In practice, it can be processed into various shapes according to the needs
.
The hanging piece generally needs to be installed with the help of hanging parts.
Common hanging parts include threaded connection hanging parts, flange hanging parts, and pressure-removing hanging parts
.

The average corrosion rate can be calculated from the mass loss of the corroded coupon:

Among them, CR is the corrosion rate, mm/a; W is the mass loss, g; A is the area of ​​the corrosion coupon exposed to the corrosive environment, m2; T is the exposure time, a; D is the density of the coupon metal, g/ cm3
.

The average corrosion rate during the storage period can be calculated from the mass loss of the corrosion coupons, and the pit depth can also be measured to calculate the pitting rate.
By observing the pitting properties, the type of corrosion can also be judged
.
In addition, the scale type analysis can be used to determine the type of scale formation, and corresponding scale inhibition measures can be taken
.

The disadvantage of corrosion coupon monitoring is that the operation period is slightly longer, and the measured data is the average corrosion rate of the equipment in 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 cost of the coupon itself is low, but the installation and processing of the coupon take a certain amount of time
.
Therefore, corrosion coupon monitoring should be used in conjunction with other monitoring and detection methods to monitor the application effect of equipment material selection and process anti-corrosion measures
.

Changqing Oilfield adopts ring-shaped coupons to monitor the corrosion of 24 produced water reinjection stations with relatively serious corrosion and scaling.
The results show that the pipeline structure has a great influence on local pitting corrosion: as the scaling degree increases, Erosion became noticeably severe
.

3 In-line probe

On-line probes have gradually become the main development method of modern detection technology because of their advantages of providing real-time and continuous corrosion monitoring data
.
According to the different types of probes, it can be divided into pH probes, ion probes, resistance probes, inductance probes and linear polarization probes and so on
.
Usually a set of online probe corrosion monitoring system consists of corrosion probe, data collector, transmitter and data processing system.
The corrosion probe is installed on the process pipeline or equipment to obtain an analog signal related to the corrosion rate.
The transmitter is converted into a digital signal and sent to the computer, and the data can be analyzed after processing
.
With the help of the online probe, the relationship between the production process information and the equipment operating status can be obtained, and the process parameters can be adjusted in time to ensure the normal operation of the equipment
.

3.
1 pH probe

Usually, the pH value is closely related to the corrosiveness of the solution, so it is particularly important for pH analysis in the petroleum and petrochemical industry
.
A pH electrode is a sensor that can convert the hydrogen ion activity in a solution into a potential signal
.
The structure of the traditional glass electrode is shown in Figure 3, which is mainly composed of the following parts: (1) a sensitive glass film, which can convert the ion concentration in the solution to be measured into a potential signal; (2) an internal reference system, which can conduct the potential signal , consisting of an internal measuring electrode and an internal buffer solution; (3) a wire, which feeds the potential signal conducted by the internal conductor system to the instrument; (4) an external reference system, which measures the electrode potential; (5) a shell, which generally adopts high insulation , Made of materials with good chemical stability
.
When the pH electrode is immersed in the solution to be measured, a potential difference will be formed on both sides of the pH-sensitive glass film due to different H+ concentrations, which will be derived from the measuring electrode
.
After calculation and temperature compensation, the pH of the solution to be tested can be obtained
.
In order to cope with more complex industrial environments, solid-state glass electrodes and non-glass pH electrodes have been developed for different process conditions, which greatly enriches the application range of probes
.

Figure 3 Basic structure of pH electrode

3.
2 Resistance probe

After the metal wire is corroded in the corrosive medium, the length remains unchanged but the diameter decreases, and the cross-sectional area decreases, which increases the resistance
.
By measuring the change of resistance wire resistance, the diameter change can be converted, and then the corrosion rate can be calculated
.
When the material of the wire used is the same as the material of the equipment being measured, the corrosion rate of the wire can be used to approximate the corrosion rate of the equipment: this is the basic principle of the resistance probe (ER) testing technique
.
Since resistance is susceptible to temperature changes, a temperature compensation element is usually connected in series with the resistance probe to eliminate its influence, thereby reducing measurement errors
.
According to the difference of operating temperature, resistance probes can be divided into low temperature resistance probes and high temperature resistance probes, which are suitable for 0~260 ℃ and 0~450 ℃ respectively; Probes and tubular probes, etc.
, generally use filament probes due to their high sensitivity
.

Like corrosion coupons, resistance probes can be used in various media such as liquid and gas phases, but with faster response
.
And the measurement process has nothing to do with the conductivity of the process material, and the principle is intuitive
.
The disadvantage of resistance probes is that the data fluctuates greatly, the test piece is more rigorously processed, and the measurement results are affected by conductive corrosion products (such as iron sulfide)
.

4 Magnetic flux leakage detection

Magnetic flux leakage detection (MFL) uses the principle of magnetic flux leakage: the device is used to saturate magnetize the DUT and generate a magnetic field inside it
.
When there is a defect, the magnetic field near the defect will be distorted, and part of the magnetic field lines will pass through the test piece to generate magnetic flux leakage.
The pipeline defect can be found by detecting the leakage magnetic field with a magnetic sensitive probe
.

Figure 4 Schematic diagram of magnetic flux leakage detection

Magnetic flux leakage inspection can not only find surface and near-surface defects and their sizes, but also has the advantages of non-contact characteristics, fast inspection speed and low cost, and is suitable for online inspection during production
.
However, because it does not contact the surface of the test piece, the lift-off value inevitably exists, which reduces the magnetic field strength; and the existence of the coating will also reduce the detection sensitivity
.
In addition, magnetic flux leakage detection is generally not suitable for components with complex shapes
.

5 Infrared thermal imaging

In nature, objects whose temperature exceeds absolute zero (-273 °C) will radiate electromagnetic waves due to the thermal motion of molecules, and the wavelength of infrared rays is between 0.
72 and 1000 μm
.
Infrared thermal imaging monitoring is to use the infrared imaging technology of thermal induction cameras to collect infrared radiation emitted by objects onto the detector through optical elements, convert it into electrical signals, and then process them into visible images, which are displayed on the display screen with grayscale differences.
Or the form of false color reflects the distribution of the surface temperature of the measured object
.

In the petrochemical industry, due to production needs, many equipment pipelines operate under high temperature conditions.
When thermal insulation falls off, corrosion and thinning occur, the surface heat distribution will be abnormal
.
Through the analysis of temperature distribution, it is possible to monitor the operation of reactors, furnaces, pipelines, radiators and other equipment, evaluate lining damage and thermal insulation effect, predict the operation of equipment, and at the same time guide maintenance and increase the predictability of detection
.
The picture shows the thermal image of the pipeline and air-cooling pipe taken by the staff using the Fluke Ti300 portable thermal imager in the refinery.
It can be seen that the temperature in the local area is abnormal (shown in red), and corrosion and thinning may have occurred
.

Figure 5 Thermal image

6 Conclusions and Outlook

(1) Corrosion monitoring and detection technology provides a strong guarantee for the safe and smooth operation of the petroleum and petrochemical industry, and has been paid more and more attention
.
However, each technology has its own characteristics and shortcomings, and usually requires multiple methods to be used in combination and complement each other to achieve better results.
At the same time, new technologies such as ultrasonic diffraction time difference technology and ultrasonic phased array technology are gradually entering the market
.

(2) The online monitoring technology is still the future development direction
.
At present, the analysis of oil products and process condensate is still mainly based on laboratory analysis.
The online analysis technology has problems of high cost and insufficient precision.
The development and replacement of related products still need to invest a certain amount of manpower and material resources
.

(3) The combination of multiple technologies is a requirement for building an intelligent factory
.
With the help of the Internet of Things and RBI technology, a complete set of corrosion data collection and analysis system is built to realize real-time monitoring of process data and corrosion monitoring data, intelligently judge the operation status of equipment, provide early warning and provide solutions in time; cooperate with automatic filling, etc.
Anti-corrosion technology to complete the intelligent "treatment" of corrosion problems
.

References (omitted)

Author | Zhu Guoli

(CNOOC Changzhou Coating Chemical Research Institute Co.
, Ltd.
, Changzhou, Jiangsu 213016)