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Study on the failure of screw air compressor and its coking

time:2017-09-26 12:45:28source:Ziqi compressorauthor: 赵疆昆Clicks: 0
   Screw Air Compressor Process Profile Shanghai Petrochemical Co., Ltd. 1 # ethylene plant from Japan's Mitsubishi Petrochemical Co., Ltd. introduced a complete set of equipment, after repeated transformation, now has an annual output of 150kt. C-20

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      A brief introduction to the process of 1 screw air compressor
The 1# ethylene plant of Shanghai Petrochemical Company is a complete set of equipment imported from MITSUBISHI oil and Chemical Corporation of Japan. After many times of transformation, it has been produced 150kt. for many years, of which C-201 cracking gas compressor is 34M-6X5, and adopts two cylinder and three stage compression process. After a section of compressed gas is compressed, the discharge gas is cooled by the sea water and separated into the two section of the suction tank. The oil layer sends the gasoline desorption tower, and the water layer returns to the process water and oil separator, and the gas enters the two section. The two stage exhaust gas is cooled by seawater, and then enters the three stage suction tank for gas-liquid separation. The condensate is controlled by liquid level to the two stage suction tank, and the gas enters the three stage compression. The three stage exhaust gas is cooled by seawater, and then enters the three stage outlet tank for gas-liquid separation. The condensate is controlled by liquid level to the three stage suction tank, and the gas enters the alkali washing tower.
The cause of the coking of the 2 screw air compressor
The pyrolysis gas contains a variety of unsaturated olefins, metal oxides and sulfides, especially heterocyclic sulfides, oxides, trace amounts of metal ions dissolved or suspended coexist, the olefin, self polymerization copolymerization of complex double olefin, forming a layer composed of dirt on the metal complex device wall heating, the so-called coking. The factors that cause the polymerization of unsaturated hydrocarbons are analyzed as follows: the temperature of the process flow of the 1C-201 cracking gas compressor system is 1). Temperature is one of the important reasons for the polymerization and coking of unsaturated hydrocarbons. The intersection temperature of the cracking gas compressor gradually increases with the prolongation of the operation time, thus creating favorable conditions for the coking.
2) oxygen content. The presence of trace oxygen can induce the generation of oxide free radicals, which provide the initiating conditions for the polymerization and coking.
3) water. Water, as a polar molecule, has a certain promoting effect on polymerization and coking.
4) metal ions. The presence of iron and other metal ions acts as an initiator for the polymerization of dienes. The more water, the higher the water temperature, the higher the tendency of metal ionization.
5) the concentration of unsaturated hydrocarbons in the material. The existence of unsaturated hydrocarbons is the underlying cause of the polymerization and coking.
3 compressor unbalance fault
3.1 rotor unbalance faults and characteristics
When the rotor rotates, the centrifugal force will be generated if the mass center does not coincide with the rotation center. The centrifugal force is the main cause of the vibration of the rotor and bearing, which is the unbalanced fault of the rotor. The centrifugal force generated by the mass imbalance of the rotor always acts on the rotor. When the rotor rotates for a week, a vibration response is generated at a certain measuring point of the rotor or bearing. So the frequency of its vibration is the speed of the rotor speed. The main features of the rotor imbalance fault: 1) spectrum in the radial vibration, frequency components are prominent; 2) the radial vibration waveform is a sine wave; 3) signal of two radial vibration perpendicular to each other, the basic difference of 90 degrees in phase, the core axis trajectory synthesis oval like more rules.
The cause of the 3.2 imbalances
The cause of the unit operation of rotor imbalance is various, usually the following factors: 1) uneven quality (shape); (2) the structure of asymmetric key slot); 3) processing is not concentric (casting, machining); 4) assembly is not concentric with the gap (coupling.); 5) with moderate changes in the material (6); cold stress release, etc.); (7) the operation process of corrosion, abrasion, erosion, breaking, dust, dirt and other coking).
Therefore, what kind of correct maintenance measures to restore normal operation in the rotor imbalance depends on the cause of the imbalance.
For the cracking gas compressor, the factors in the operation are one of the most common and most important reasons.
Operation monitoring of 4 cracking gas compressor
4.1 vibration monitoring
In order to monitor the mechanical vibration of the compressor, the C-201 compressor is equipped with the American BENTLY3300 type axle vibration monitoring instrument. The vibration values of the compressor shaft are displayed and output at the site, and are also sent to the DCS system with other parameters. The layout of the BENTLY eddy current displacement probe of the compressor is shown in Figure 2. It is one of the 23 sets of key units in the joint stock company. The frequency spectrum and trend analysis of the axle vibration signal of the instrument are regularly carried out by professional technicians.
The vibration of the rotor increases with the increase of the coking degree of the rotor.
The uneven coking on the rotor will lead to dynamic imbalance of the rotor, resulting in the imbalance of the rotor. It is shown that the power frequency component of the speed is prominent in the spectrum analysis chart.
4.2 monitoring factor
After a long-term continuous operation of the cracking gas compressor, the inlet and exit temperature of each section begins to rise, and the pressure will rise, and the capacity of the compressor will be insufficient. To this end, the United States Nalco/Exxon company proposed the use of N factor to monitor the operating state of the compressor.
N= Ln (Pd/Ps) / (Ln (Td/Ts))
Ln (Pd/Ps) /Ln (Td/Ts) -1 Pd, Td - compressor outlet pressure, outlet temperature, Ps, Ts - compressor inlet pressure, inlet temperature.
The compression ratio (Ln (Pd/Ps)) of the compressor is greater, and the higher the temperature rise (Ln (Td/Ts)), the higher the compression ability.
From the above, the larger the N, the smaller the compression ability, indicating the more serious the degree of coking.
Operation state analysis of 5 split gas compressor
5.1 vibration analysis
Figure 3 is the change trend of the Shuangfeng value of the high pressure side shaft of the low pressure cylinder. There are two times higher vibration value in May 1999 and the end of November.
Figure 4 is the spectrum analysis of the vibration signal in November 29, 1999, which shows the obvious 5400RPM components, similar to other probe signals. Analysis of vibration signal

 

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