Application of Condition Monitoring System on Reciprocating Compressor
I. Introduction
A reciprocating compressor model VF-2.4/4-240 in an oil field has a motor power of 150KW, a speed of 980RPM, a nominal volume of 2.4 m2/min, and natural gas as the working medium. The suction pressure is 0.4Mp, and the rated discharge pressure is 24Mp. The structure of the compressor is shown in Figure 1.
Figure 1 The structure layout of the compressor
Since the compressor was installed, the vibration of the compressor has been great, until the unit was shut down due to the rupture of the pipeline due to excessive vibration. During this period, the concentricity of the unit was adjusted, and the crankshaft, oil scraper ring, piston rod, seal packing and cylinder of the compressor were replaced, and then returned to the factory for refurbishment, and rubber gaskets were added to the unit. After starting up and running, the vibration value of the unit still reaches 60mm/s. In response to this situation, we conducted a comprehensive system of vibration detection and fault diagnosis analysis of the reciprocating compressor.
2. "Knock test" diagnostic test before starting up
According to the situation provided by the customer, first, when the unit is shut down, perform a "knock test" test on its base, cylinder, bracket and other parts to confirm the natural frequency of each measuring point as shown in Figure 2.
Figure 2 "Knock test" test chart
According to the test results of the "knock test", after calculation, we found that the natural frequency of 32.5Hz and its harmonic frequency existing in the unit coincide exactly with twice the compressor's rotating power frequency. Based on this, we doubt whether the unit has a resonance frequency at 32.5 Hz.
3. "Speed-up test" diagnostic test when starting up
During the compressor startup process, we conduct a "speed-up test" test on the unit to confirm the vibration of the unit at different speeds. As shown in Figure3.
Figure 3 "Speed-up test" test chart
According to the test results of the "speed-up test", as shown in the above figure, we can clearly see that the unit has a vibration trend of 2 times the power frequency, but at 800rpm, the vibration trend of the unit is obviously smaller. Based on this, we suspect that the unit has resonance at 2 times the power frequency.
4. Data collection during normal operation
When the compressor is operating normally, we collect vibration data of the unit to confirm the potential failure of the unit. As shown in Figures 4 and 5.
Figure 4 Vertical frequency spectrum of the 3-stage cylinder
Figure 5 Vertical frequency spectrum of 4-stage cylinder
It can be seen from Figures 4 and 5 that the vibration is mainly dominated by double power frequency, and the total value of vibration reaches 67.83mm/s. According to the two-step confirmation of "knock test and speed-up test", we analyzed that the higher 2 times power frequency should be caused by misalignment or resonance of the unit. According to the information provided on site, the basic lock is caused by resonance, and the unit is loosened due to large vibration.
5. Phase analysis
In order to further analyze the root cause of the resonance, we carried out a phase test on the unit. The test position and angle changes are shown in Figures 6 and 7.
Figure 6 Location of phase test
Figure 7 Changes in the angle of the phase test
According to the analysis of the phase test results, we found that there is a significant phase change between the compressor base and the ground. Based on this, we judged that the cause of the resonance was insufficient foundation stiffness.
6. Frequency conversion application
In order to eliminate the vibration caused by resonance, we recommend using a frequency conversion device to avoid the resonance point without affecting the process and production. The customer directly calls the spare frequency conversion device on site to run the speed avoiding the resonance point, and finally lock it at 650rpm, and the vibration value is less than 9mm/s. The results of the changes are shown in Figures 8 and 9.
Figure 8 Vertical direction trend diagram of 3-stage cylinder
Figure 9 Vertical direction trend diagram of 4-stage cylinder
7. Conclusion
Although the vibration of the reciprocating compressor is reduced by frequency conversion without affecting the process and production, we believe that the lack of foundation strength is the root cause of resonance. To completely solve the resonance problem, the foundation needs to be reformed.
