Application of petrochemical industry

The sealing device is an important component of rotating equipment, and plays a key role in preventing leakage. In the petrochemical industry, the role of sealing devices is particularly evident because of environmental and safety requirements. From the point of view of sealing development, this paper introduces the historical evolution and application technology progress of packing seal and mechanical seal in petrochemical industry, and puts forward the prospect of sealing development in the future.

As we all know, rotating equipment is a very important part in the production of petrochemical industry. Its normal and effective operation plays an important role in the whole production system. But in actual production, the events that affect production due to rotating equipment failures are too numerous to list. Data show that in this type of failure, especially the leakage caused by sealing failure as the main form, accounting for more than 50% of the total failure. Therefore, it is of great significance to reduce the failure rate and ensure the safe operation of the equipment by understanding the history of various types of seal development and the factors used, and analyzing the leakage problems.

Packing seal

Packing seal, also known as compaction packing seal, commonly known as packing, English derived from the word "Packings". Packing seal is the oldest form of sealing, ancient Chinese water lifting machine used to fill the cotton yarn to plug the leak, the early steam engine has also adopted this form of sealing. By nineteenth Century, in order to adapt to the development of large-scale exploitation of petroleum energy, the material of packing and sealing was also developed. Enter after twentieth Century, because the packing structure is simple and low cost and wide adaptability, so the stuffing box is widely used in the petrochemical production in the stationary state, rotating and reciprocating seal seal, valve stuffing seal etc..

With the progress of science and technology, higher requirements have been put forward for the construction of packing seal. In recent decades, the development of new sealing materials has made it possible to develop new packing materials. Facing various kinds of packing materials, how to choose materials adapted to different working conditions is a new problem of great value. The following will be used to introduce common sealing packing, providing typical petrochemical enterprises using the status quo.

Asbestos packing

Because asbestos filler has a long history, the application experience is very mature, and the price is low. But because many of the weaknesses of natural asbestos materials: high temperature easy to dehydration, fiber penetration, friction coefficient, the asbestos filler regularly infiltration leakage, require frequent maintenance in use, and asbestos dressing a relatively short life span, the abrasion of pump shaft, the valve stem is relatively large and open. Asbestos packing, maximum temperature 450 degrees, maximum resistance 6 MPa. In actual production, it is usually used at a temperature less than 200 degrees centigrade. Most of them are placed on both ends of expanded graphite packing for combined sealing.

Flexible graphite packing ring

Flexible graphite packing ring, also called expanded graphite packing ring, relative to asbestos packing, has a short history of use. At the end of 1970s China began to carry out industrial production, due to the flexible graphite packing ring has excellent properties, so it quickly replaced the asbestos packing, especially in the original asbestos packing hard seal parts. The flexible graphite packing ring has wide application range, good sealing performance, long service life and convenient maintenance. Its disadvantage is its low strength and large number of specifications. At present, most of the petrochemical production is "single flexible graphite", that is, pure flexible graphite packing ring. Pure flexible graphite packing is used at both ends should be added braided packing or metal retainer to prevent extrusion.

Carbon fiber packing

Carbon fiber is made from synthetic fibers such as acrylonitrile at a certain temperature by oxidation. The carbonized fibers are carbonized at high temperature to become carbon fibers. Carbon fiber was successfully developed in the 60 generation in twentieth Century. In China, a large number of sealing fillers were used in the late 1990s. At present most is used as filler of carbon fiber impregnated with PTFE braided packing, the packing tolerance to acid, alkali and organic solvent etc. most medium was good, and low thermal expansion coefficient, good thermal conductivity, good self lubrication, so its applicability as wide, especially suitable for high speed pump.

PTFE packing

The advantages of PTFE is resistant to almost all known medium, and not easy to aging, good self lubrication; but the disadvantage is the small coefficient of thermal conductivity, thermal expansion coefficient, so in high speed pump will make the friction heat gathering. When the temperature reaches 200 degrees, the axial speed of the shaft is greater than 3.5 m/s, the PTFE will soften and stick to the shaft, resulting in sealing failure. To overcome this shortcoming, an American company has developed a woven fabric filled with graphite PTFE, which improves the poor thermal conductivity of polytetrafluoroethylene and enables it to be used in high speed pumps.

Aramid braided packing

Aramid fiber is an artificial fiber which can be divided into several different types according to its composition. Used as filler, it is a bright yellow counterpoint aramid fiber. The advantage of this kind of fiber is its high strength. Under the same quality, the strength of the fiber is five times of that of the ordinary steel, and it is rather wearable. It has good thermal stability and begins to decompose at more than 427 degrees centigrade. Its thermal expansion coefficient is smaller. Aramid woven fabric is made of this kind of fiber and is made of PTFE emulsion. The utility model is especially suitable for the sealing of a rotary pump or other rotating equipment containing granular media.

Mechanical seal

Although after years of development, has been widely used and packing the continuously updated, but as people on the growing requirements of environmental protection, at the same time, the sealing medium is more and more complicated, packing has been unable to meet the sealing requirements of some equipment, thus gradually replace the mechanical seal packing seal on history the stage.

As a sealing device, the main advantages of the mechanical seal is reliable, small leakage and long service life, less power consumption, so it is widely used in pumps, mixers, compressors, centrifuges, reactor, rotary tower and filter process equipment etc.. According to statistics, about 95% of the rotating equipment in foreign petrochemical industry employs mechanical seals.

The historical evolution of mechanical seals at home and abroad

The earliest mechanical seals appeared in Britain in the 1885, in the form of patents, and were not formally used until 1900. At that time, the appearance of the mechanical seal structure is very simple, but it also solved the problem of shaft seal in the machine manufacturing industry. Compared with the usual packing seal, mechanical seal has advanced both in principle and function. However, due to the lack of suitable materials and processing technology, although mechanical seals have many advantages, they are still not widely used. It was not until 1920 that mechanical seals gradually became more useful in refrigeration plants because of the advent of new technologies and new materials. Since 1930, mechanical seals have been scaled up for the shaft seal of water pumps for internal combustion engines.

After the Second World War, mechanical seal in the United States has been developed by leaps and bounds. The middle of the twentieth Century, due to the development of petroleum and chemical technology, new materials, graphite ceramic and cemented carbide in mechanical seals used in manufacturing, processing and control technology to enhance the level of the late 50s the new balanced mechanical seal and intermediate ring seal. In the early 60s, hydrodynamic seals and hydrostatic seals were developed due to the demand of the nuclear industry. In the early 70s, due to the special requirements of aerospace and nuclear power, a new type of multi-stage sealing, as well as new materials such as silicon carbide and carbon graphite, appeared. 80 90s - along with people's increasing awareness of environmental protection, a "zero leakage" as the goal of the mechanical seal; and technical personnel based on hydrodynamic seal principle, developed the thermo hydrodynamic mechanical seal and wedge model upstream pumping mechanical seal.

Since entering the new century, with the sealing technology gradually mature, researchers have paid more and more attention in the development of new types of mechanical seal, also gradually to the rotating machinery fault analysis and maintenance, with the rapid development of computer simulation software, there are many simulation studies.

The application research of mechanical seal in our country started late. After the founding of new China, in 1965, Lanzhou refinery, Shenyang water pump plant and other units took the lead in trial production of mechanical seals for pumps. In 1966, Tianjin mechanical seals factory began to produce mechanical seals. In 1970, the Ministry of machinery industry, the Ministry of petroleum and chemical industry by the Ministry of scientific research, production and use of units deployed researchers formed a joint design team, advanced technology abroad and the joint design of mechanical seal pump series".

In 1975, the Ministry of machinery industry approved and promulgated the standard of mechanical seal for pumps. In 1978, the Ministry of chemical industry approved the promulgation of the standard for mechanical seal used in kettle. In order to improve product quality, accelerate the development of sealing technology, in 1980 after the introduction of advanced technology, the United States, Germany, Britain, Japan and other countries, through research, digestion, mechanical seal in varieties, materials and technical parameters are improved to a new stage. In 1993, the state promulgated the national standard "GB/T14211 - 1993 mechanical seal test method". In 1999, the State Bureau of machinery industry revised the technical conditions of mechanical seals and classified methods of mechanical seals, which provided a guarantee for the mechanical seal products to be close to the advanced level of foreign countries.

Application technology of mechanical seal

In more than a century, along with the increasing awareness of the sealing mechanism, it also successfully designed and produced all kinds of new mechanical seal, and apply it to the petrochemical, power generation, aerospace and nuclear industry. At present, the mechanical seal application technology mainly has surface modification technology, combination sealing technology and controllable mechanical seal technology three kinds.

Surface modification technique

The surface modification technology refers to the technology of improving the lubrication condition of the end face by slotting on the end face of the seal so as to realize the long period operation of the mechanical seal. According to the difference of groove depth, it can be divided into two categories: open deep groove and open shallow groove.

The opening groove refers to a groove with a depth of millimeters (usually 1~2 mm) at the end face of the mechanical seal. According to the sealing principle is different, the end of the deep groove, mechanical seals can be divided into fluid static mechanical seal and hot fluid power wedge mechanical seal.

Shallow trench refers to micron grooves opened at the mechanical seal, seal on the relative movement and groove steps and conveying effect, resulting in the hydrodynamic pressure, and to balance the closing force, to achieve non-contact seal face. Relatively advanced dry gas seals and upstream pumping seals are all end face open slot mechanical seals. The end face is opened with a shallow groove, and the mechanical seal can be used in gas or liquid medium.

Combination sealing technology

With the rapid development of modern industry, high temperature and high pressure and a variety of new media, industrial production has more and more high requirements for sealing, many times a single seal is difficult to meet the harsh working conditions. Therefore, several kinds of seals are combined to make full use of the inherent advantages of various kinds of seals, and the combination seal emerges as the times require.

Although the form of combination seal is a lot, but its essence is nothing more than non-contact sealing and contact sealing combination, as well as contact sealing and contact sealing combination of two major categories. In combination seals, mechanical seals are used primarily to prevent leakage, while other forms of sealing are primarily throttling throttling.

In the medium and low pressure process gas compressor, it is an important technology trend to replace the traditional floating ring seal with mechanical floating ring combined seal. According to the data, in twentieth Century, after the 70 generation, imported from abroad large petrochemical installations, mechanical - floating ring combination seal occupies an increasingly large proportion, the scope also involves the various fields of petrochemical.

The combination is widely used in medium and high pressure under the condition of mechanical seal is the dry gas seal, this seal is usually divided into two levels, the first level is the seal mechanical seal, the sealing medium is liquid, second is sealed dry gas seal, the sealing medium for the first stage leakage trace medium and external import inertia gas. Mechanical dry gas combination seal is characterized by mechanical seal as the main seal, sealing the process medium, and the dry gas seal is used as an auxiliary seal to reinforce the sealing form.

Controllable mechanical seal technology

After the high parameter design and manufacture of the mechanical seal, the researchers have turned the research focus to the simultaneous control of leakage rate and wear. Leakage rate and wear are two parameters to describe mechanical seal performance. The leakage rate is high, causes the seal between the liquid film thickness, wear small end face mechanical seal work, long service life; and the leakage rate is low, causes the seal wear thin, work surface, mechanical seal life is short. Therefore, it is very important to find the best working point of mechanical seal, that is to say, the key of performance control of mechanical seal.

In the practical application are used to seal failure monitoring early acoustic emission technology, using piezoelectric or eddy current sensor to monitor the change of the thickness of the film, the relationship between the test torque resistance strain gauge to monitor the operating parameters and the friction coefficient, the leakage quantity monitoring to monitor the working condition of the seal. In addition, some key equipment's mechanical seal even uses the intelligent monitoring system, uses the computer control technology, the continuous automatic adjustment seal each kind of parameter, achieves the best operating condition. For example, the United States and Japan and other countries of the acoustic emission, ultrasonic, microelectronics and sensor technology used in nuclear reactor mechanical and aerospace condensate pump liquid pump seal monitoring system, domestic petroleum university developed mechanical seal phase monitoring system and industrial applications.

Although at this stage the controllable sealing techniques have not been widely used in large scale, but with the development of computer, electronic sensor technology and theory of seal, controllable mechanical seal technology in the future will be widely used in various fields.

summary

After years of research and development, the sealing device of rotating equipment from the initial simple packing become advanced combined seal, controllable sealing, to reduce leakage from "zero leakage", new materials continue to improve the reliability of this device. Although because the operating conditions and operating conditions have become increasingly demanding, the petrochemical industry there are still seal failure events, but I believe that with the rapid development of advanced processing technology and sensor electronics, new materials, and truly environmentally friendly and efficient and safe sealing technology will go on the stage of history.