Compressor failure and its coking

1 crack gas compressor process introduction

Shanghai Petrochemical Corporation 1# ethylene plant is a complete set of equipment imported from Mitsubishi Oil Chemical Co., Ltd., which has been rebuilt several times and has an annual production of 150kt. The C-201 cracking gas compressor model is 34M-6X5, using two cylinders. Three-stage compression process. After a period of compression of the cracked gas, the exhaust gas is cooled by seawater and separated into a two-stage suction tank. The oil layer is sent to the gasoline desorption tower, and the water layer returns to the process water oil separator, and the gas enters the second stage compression. After the second stage exhaust gas is cooled by seawater, it enters the three-stage suction tank for gas-liquid separation, and the condensate is controlled by the liquid level to return to the second-stage suction tank, and the gas enters the three-stage compression. After the three-stage exhaust gas is cooled by seawater, it enters the three-stage outlet tank for gas-liquid separation, and the condensate is controlled by the liquid level back to the three-stage suction tank, and the gas enters the alkali washing tower.

2 reasons for compressor coking

Since the cracking gas contains various unsaturated olefins, metal oxides and sulfides, especially sulfides, heterocyclic oxides, and a small amount of dissolved or suspended metal ions coexist, complicating the self-polymerization of olefins and diolefins. A complex composition of dirt, called so-called coking, is formed on the inner wall of the heated metal device. The factors causing the coking reaction of unsaturated hydrocarbons are analyzed as follows: Figure 1C-201 Cracking gas compressor system process flow diagram 1) Temperature. Temperature is one of the important reasons for the polymerization coking of unsaturated hydrocarbons. The interstage temperature of the cracking gas compressor gradually increases with the extension of the running time, thus creating favorable conditions for coking.

2) Oxygen content. The presence of trace oxygen can induce the production of oxide radicals, providing initiation conditions for polymerized coking.

3) Water. As a polar molecule, water has a certain promoting effect on polymerized coking.

4) Metal ions. The presence of iron and other metal ions acts as an initiator for the polymerization of the diene. The greater the amount of water, the higher the water temperature and the higher the tendency of metal ionization.

5) The concentration of unsaturated hydrocarbons in the material. The presence of unsaturated hydrocarbons is an intrinsic root cause of polymer coking.

3 compressor imbalance fault

3.1 rotor imbalance failure and characteristics

When the rotor rotates, if its center of mass does not coincide with the center of rotation, centrifugal force will be generated. Centrifugal force is the main cause of rotor and bearing vibration, which is the imbalance of the rotor. The centrifugal force generated by the mass imbalance of the rotor always acts on the rotor. Each revolution of the rotor produces a vibration response at a certain point of the rotor or bearing. Therefore, its vibration frequency is the rotational speed of the rotor. The main characteristics of the rotor imbalance fault are as follows: 1) on the spectrum of the radial vibration, the power frequency component is prominent; 2) the waveform of the radial vibration is basically a sine wave; 3) the two mutually perpendicular radial vibrations The time domain signals are substantially 90° out of phase, and the synthesized axis trajectory is relatively regular elliptical.

3.2 Causes of imbalance

The reasons for the rotor imbalance during unit operation are various. The usual factors are as follows: 1) uneven quality (shape, etc.); 2) structural asymmetry (keyway, etc.); 3) processing of different hearts (casting, machining, etc.) ); 4) assembly disequilibrium (with clearance, coupling is not right); 5) change of fit of fittings; 6) material (stress release, cold and hot state, etc.); 7) operation process (corrosion, wear, scouring , broken, dust, coke, etc.).

Therefore, which correct maintenance measures are taken to restore normal operation when the rotor is unbalanced depends on the cause of the imbalance.

For cracked gas compressors, the factors in the running process are one of the most common and major causes.

4 cracking gas compressor operation monitoring

4.1 Vibration monitoring

In order to monitor the mechanical vibration of the compressor operation, the C-201 compressor is equipped with the American BENTLY3300 shaft vibration monitoring instrument. The compressor shaft vibration value is displayed and output at the site, and is sent to the DCS system along with other parameters. The BENTLY eddy current displacement probe arrangement of the compressor is shown in Figure 2. It is one of the 23 key units of the joint-stock company. The professional technicians perform periodic spectrum and trend analysis on the shaft vibration signals of the instrument.

As the degree of coking of the rotor increases, the vibration of the rotor increases.

Uneven coking on the rotor will cause dynamic imbalance of the rotor, resulting in an unbalanced fault of the rotor, which is shown as a prominent component of the rotational speed power frequency component on the spectrum analysis diagram.

4.2 Monitoring factors

After the long-term continuous operation of the cracking gas compressor, the inlet and outlet temperatures of each section begin to rise, and the pressure also rises, and the capacity of the compressor will be insufficient. To this end, the United States Nalco / Exxon company proposed to use 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 larger, and the smaller the temperature rise (Ln(Td/Ts)), the higher the compression capability.

It can be seen from the above formula that the larger N is, the smaller the compression capability is, indicating that the degree of coking is more serious.

5 analysis of operating status of cracking gas compressor

5.1 vibration analysis

Figure 3 shows the variation trend of the double-peak vibration of the high-pressure side shaft of the low-pressure cylinder. There were two higher vibration values ​​at the end of May and November 1999.

Figure 4 is a spectrum analysis of the vibration signal on November 29, 1999. It can be seen that the apparent power frequency (5400RPM) component is similar in other probe signals. Vibration signal analysis indicates that the compressor has a rotor imbalance fault. After the maintenance in December, the shaft vibration returned to a low vibration level and was stable.

Figure 3 Low-pressure cylinder high-pressure side shaft vibration (X20111H) double peak trend

5.2 Monitoring factor analysis

During the period from the end of 1998 to the end of April 2000, the C-201 compressor was overhauled at the end of May 1999 and eliminated in December 1999. During this run, the inlet and outlet temperatures and pressures of the compressor sections are recorded and the N factor is calculated to generate a chart in chronological order.

1) The first segment of the N1 factor. Before the overhaul in May 1999, it reached the highest level. After driving in June, it fluctuated greatly. By November of 1999, N increased significantly. After the lack of maintenance in December, N1 fell back to a lower level.

2) The second segment of the N2 factor. It reached its highest level before the overhaul in May 1999 and has been relatively stable since its driving in June. After the lack of maintenance in December 1998, it fell further and stabilized.

Cracking gas compressor (C-201) first stage N1 factor diagram 5 cracking gas compressor (C-201) second stage N2 factor indication 3) third stage N3 factor (see Figure 6). Since the end of 1998, there has been a slow increase trend, which has declined after the lack of maintenance in December 1998.

5.3 Comprehensive analysis

It can be seen from Figures 3 to 6 that at the end of May and the end of November 1999, the axial vibration signals of the N-factor and the low-pressure cylinder of one section and two sections showed large values. The low-pressure cylinder rotor produces an unbalanced fault, which is precisely due to polymer coking. After the overhaul in May 1999 and the clearing of the inspection in December, the N factor and the shaft vibration decreased significantly. It shows that the comprehensive analysis method of N factor and shaft vibration can more accurately monitor the coking of the compressor and the resulting rotor imbalance.

6 coking prevention

In order to prevent coking of the compressor, a washing oil injection line is provided in the material pipelines of each section of the compressor, and continuous injection is controlled by the metering pump, which is also the most commonly used method at present. Washing oil has two functions. One is to wash the polymer attached to the impeller casing cover, and the other is to cool the cracked gas and slow down the polymerization coking.

However, washing oil does not fundamentally solve the problem of polymerized coking, and scouring will not be complete. From the C-201 operating history, there is still a certain degree of coking. For example, when the parking overhaul was completed on May 25, 1999, it was found that there were serious coking on the flow passages and impellers at all levels. When the maintenance was completed at the end of December 1999, it was found that the rotor was severely coked.

In order to improve the effect of preventing coking, on the basis of the injection of C-201 compressor, the AG3144A cracking gas compressor polymerization inhibitor of Nalco/Exxon Company of the United States was added on March 3, 2000.

7 Conclusion

The polymerization coking of the cracking gas compressor will cause the flow passages of all stages to be narrowed, the resistance to increase, and the temperature to rise, thereby reducing the load of the compressor and affecting the production operation of the device. Therefore, the analysis and monitoring of the coking condition of the C-201 cracking gas compressor is of great significance for ensuring the "safe and long-term" operation of the 1 # ethylene plant.

a. Using the N-factor calculation formula provided by Nalco/Exxon Company of the United States, and analyzing the vibration signal of the compressor shaft at the same time, it can monitor the operation of the compressor more accurately and effectively, and provide a reliable basis for the production and maintenance of the device.

b. It is required to accurately record the pressure and temperature changes of each section of the compressor on a regular basis and calculate the N factor. Regular trend and spectrum analysis of the compressor vibration signal, which requires strengthening the technical management of the compressor in various aspects.

c. From the N factor and vibration trend after adding EC3144A polymerization inhibitor on March 3, 2000, the compressor is in good working condition. For the effect of the EC3144A polymerization inhibitor, it is necessary to track and monitor the N-factor and vibration changes after a long period of operation, and finally make an appropriate evaluation at the next cover.