An essential feature of modern chemical production is the large-scale and high degree of automation of production plants. Automation is the necessary condition and important guarantee for safe operation, smooth operation, and efficiency of production units. However, with the development of production and the improvement of automatic control, the corrosion problem of chemical instrumentation is becoming more and more prominent.
1. Main types of corrosion for chemical process instruments
Transmitters are commonly used on industrial sites as front-end collection and detection devices. The transmitters are made up of petroleum sensor variables and connecting wires, and these components are made of precious metals to improve the detection accuracy and sensitivity of the equipment. Unfortunately, these devices are exposed to the harsh environment of the industrial site, resulting in corrosion. Common types of pollution include.
1.1 Physical corrosion
External mechanical forces on the destruction of metal materials and physical dissolution of metal materials, and other reasons will cause the physical corrosion of chemical instruments.
1.2 Chemical corrosion
Instrument metal materials and external contact with the environment in the gas are prone to redox reactions, non-electrolyte solutions, and metal materials in contact with oxidation corrosion. These are chemical instrumentation of chemical pollution. For example, chemical plants produce toxic gases, chlorine gas, and instrument components in the element of an iron redox reaction to generate ferrous chloride, resulting in instrument corrosion.
1.3 Electrochemical corrosion
The most common corrosion in chemical instrumentation is electrochemical corrosion. In addition, there are strong acids, strong bases, and other common corrosive substances in the chemical environment. When the metal material of the chemical instrument is in contact with the electrolyte solution, the principle of primary battery reaction occurs, and the reductive active metal material loses electrons and is oxidized.
Steel materials are more prone to corrosion in humid air because the steel surface reacts in the moist air to generate electrolytes. Although the electrolyte solution and the iron and carbon in the steel form a primary battery device, the iron element loses electrons to be oxidized.
Damage to many metallic substances is caused by galvanic corrosion. Electrochemical corrosion erodes the surface and internal parts of the instrument within a short period. The precipitates caused by pollution can affect the accuracy and precision of the device.
2. Chemical instrumentation commonly uses anti-corrosion methods.
In the production process, corrosion causes the instrumentation failure rate to occur frequently. As a result, companies ensure the regular operation of their production equipment by often replacing instruments, thus increasing their operating and maintenance costs. As a result, the chemical industry is widely used in applying anti-corrosion methods: gas isolation, liquid isolation, and diaphragm isolation.
2.1 Gas isolation method
The principle of gas isolation is to fill the air in the pressure guide tube between the detection point and the detection element of the instrument to avoid direct contact between the detected pressure medium and the device and protect the tool from being damaged by the high pressure of the measured medium.
The gas isolation method uses pressure transmitters for low or absolute pressure measurements, also called the gas injection protection method. The air transmits the pressure change at the detection point in the pressure guide tube to the instrument transmitter, and the sensitive components of the instrument detect the results.
2.2 Liquid isolation method
Measurement of hydrogen chloride gas, nitrogen oxide, chlorine, and other media, with perfluorotributylamine or other isolation liquid filled in the isolation tank, the test instrument’s corrosive media and metal parts isolated.
The liquid isolation method has some disadvantages, such as increasing the liquid seal will appear fluid seal medium. In addition, the measured medium may occur with the liquid seal medium between the chemical reaction, thus appearing new corrosion problems to reduce the isolation effect. At the same time, the isolation liquid on the market is generally more expensive, filling method trouble, anti-corrosion effect is not good, in the practical application of liquid isolation method is not a large number of applications.
2.3 Diaphragm isolation
As PTFE has the below characteristics :
- High lubrication,
- High and low-temperature resistance,
- Weather aging resistance,
- Corrosion resistance,
- Small tension,
- Low friction coefficient,
- And resistance to most chemical solvents and drugs.
Therefore, the anticorrosive isolation diaphragms of chemical instruments are composed of PTFE.
The specific application method uses polytetrafluoroethylene, PTFE diaphragm adhesion, or spraying on the pressure transmitter bellows or pressure gauge spring tube. The corrosive medium, the sensing element phase isolation to achieve the purpose of corrosion protection.
The application shows that PTFE diaphragms can prevent the corrosion of traditional corrosive media, ensure the instrument’s accuracy during the measurement process and significantly extend the service life of the detection instrument. In addition, the price of PFP diaphragms is relatively low. The use of diaphragm isolation for corrosion protection can solve the corrosion problem on the one hand and reduce production costs on the other.
3. Routine corrosion prevention and maintenance of chemical instruments
3.1 Rational choice of materials
In many cases, the corrosion of chemical instruments is due to the poor corrosion resistance of the materials used in chemical agents, which leads to the decline of the devices. Therefore, it is necessary to choose some materials with strong corrosion resistance, which is also a meaningful way to solve the phenomenon of corrosion of chemical instruments completely. Furthermore, as each corrosion phenomenon has different causes, it is necessary to analyze each and take the correct protective measures to deal with it.
For the prevention and treatment of chemical materials in the prevention of corrosion, to alleviate the time of chemical materials corrosion, they must be able to choose appropriate chemical materials. The use of some corrosion-resistant materials for the production of chemical instrumentation can effectively play a role in preventing corrosion. However, the choice of materials for chemical instruments should not be based on corrosion resistance alone but on a good performance in terms of the production environment, the cost of the device, and the corresponding availability.
The performance of the material is critical. Therefore, its mechanical and physical properties will have a decisive role in the instrument’s working. The main reason for this is the different chemical composition and substance content, while another point is the careful analysis of the working environment of the chemical instrument. In particular, the humidity and the content of corrosive substances in the working environment will affect the service life of the appliance as well as the daily work efficiency, so careful consideration must be made.
3.2 Protection layer
Protective layers on instrument parts or components are a prevalent method of corrosion prevention in industry. According to the protective layer’s different materials and forming principles, it can be divided into the following three kinds.
A protective metal layer, including spraying, electroplating, hot dipping, carburizing, etc.
Non-metallic protective layers include paint, acid-resistant cement, rubber, plastic, enamel, etc.
Non-metallic protective film, chemical treatment on the metal surface to generate oxide film, phosphate film, and other protective films.
3.3 The use of isolation fluid
It is an effective method to prevent direct contact between corrosive media and the instrument. In the inability to choose a suitable corrosion-resistant mechanism, isolation fluid can achieve the purpose of isolation.
Isolation fluid is often used in corrosive media pressure, flow, and level measurement. The isolation fluid must be not only insoluble and chemically incompatible with the medium being measured but also not corrosive to the measuring parts of the instrument. The density of the isolation fluid should be different from that of the measured medium and the instrument’s working medium. Its density and viscosity should not change significantly when the ambient temperature changes; it should also have good fluidity.
In the event of accidental mixing of the isolation fluid into the measurement line, it should not affect the use of the measured medium. For example, glycerin aqueous solution is suitable for oil, water gas, semi-water gas, C1, C2, and other hydrocarbons; ethanol is ideal for propane, butane, and other media. Methyl silicone oil is suitable for gases and liquids other than wet chlorine gas bodies.
3.4 Diaphragm isolation
The use of corrosion-resistant diaphragms to isolate the isolation fluid or filling fluid and the measured medium to achieve the purpose of corrosion protection. It is suitable for corrosive solid media, challenging to use in-tube isolation or container isolation occasions, usually applicable to pressure measurement, and not ideal for differential pressure measurement. In addition, the isolation diaphragm should have elasticity and impermeability, such as the commonly used diaphragm type manometer, mono flange anti-corrosion pressure transmitter, etc.
3.5 Blowing method
It is used to blow into the air (or nitrogen and other inert gases) to isolate the measured medium on the instrument, measuring parts of the corrosive effect. The blowing method is usually used for atmospheric pressure or low-pressure level measurement system. Blowing into the gas should not act with measured corrosive media.
According to the constant pressure blowing method principle, the blowing liquid (water and other clean liquids) method is also used in flow and level measurement systems. For example, they are blowing in steam condensate to isolate the medium from the corrosive measuring parts of the instrument and to eliminate the blockage of the pressure guide tube.
Chemical instrumentation is essential monitoring equipment. On the one hand, chemical instrumentation maintains the smooth operation of chemical production lines. But on the other hand, it also plays a role in life safety and property protection.
For field instruments, they are exposed to the sun and rain for a long time, which makes them prone to failure. Then we need to do an excellent job of instrumentation anti-corrosion work. Strengthening the instrumentation’s management and maintenance can improve the instrumentation’s accuracy and extend the service life of the instrumentation.