Why measure vibration displacement for large-scale units

It’s probably due to the effects of the power generation system; the displacement of turbines is measured

This post was last edited by gjn1970 on 2016-5-24 at 14:43. 1. Standards for bearing vibration: Bearing vibration standards (double amplitude, in mm) – Excellent, Good, Acceptable: 1500 r/min: ≤0.03, ≤0.05, ≤0.07; 3000 r/min: ≤0.02, ≤0.025, ≤0.05; ≥5000 r/min: ≤0.01, ≤0.025, ≤0.05. 2. For generator sets operating at 3000 RPM, the ISO 3945 vibration standards are as follows: Vibration intensity Vf (mm/s) – Support type: Rigid support, Flexible support: 0.45 (Good), A (Good); 0.71, 1.12, 1.8 (B – Satisfactory), B (Satisfactory); 2.8, 4.5, 7.1 (C – Unsatisfactory), C (Unsatisfactory); 11.2, 18 (D – Immediate shutdown), D (Immediate shutdown); 28, 45, 71. The relationship between vibration intensity Vf (mm/s) and the peak-to-peak vibration displacement Sp-p (mm) is given by Sp-p = 2√2 × Vf/ω, where angular velocity ω = 2πf, and f is the frequency. Therefore, whether measuring the bearing vibration velocity or displacement, the meaning of both is the same

Shell vibration involves measuring vibration displacement, velocity, and acceleration; in large machinery, it is the shaft vibration that is measured, with all values expressed in μmm

Any measurement has a purpose; a measurement without a purpose is meaningless. For large-scale units, what we are primarily concerned with is the motion trajectory of the rotor within the stator, in order to determine whether this motion will cause static and dynamic friction. This requires the use of displacement values for measurement, and it is best to employ eddy current probes for this purpose.

In the literature on condition monitoring, it is recommended to use low frequencies (0–100 Hz) to measure vibration displacement, medium frequencies (100–1000 Hz) to measure vibration velocity, and high frequencies (1 K–10 KHz) to measure vibration acceleration. Generally speaking, there is nothing wrong with this statement. Because for the same vibration intensity, the lower the frequency, the less sensitive the velocity is, and the acceleration is even less sensitive; whereas displacement is relatively more sensitive. Similarly, for the same vibration intensity, the higher the frequency, the more sensitive the acceleration is; velocity is less sensitive, while displacement is very insensitive. Therefore, it is relatively better to use acceleration as a measure. Why are vibration displacements measured for all three compressors in our factory? Their rotational speeds are 14,738 rpm (245.6 Hz), 5,550 rpm (92.5 Hz), and 16,465 rpm (274.42 Hz). As for a specific piece of equipment, whether to use displacement, velocity, or acceleration to monitor its operating condition actually depends on the type of bearings it has and the mode of failure that may occur. For example, in the case of sliding bearings (which are generally used when the product of the rotational speed and the nominal diameter of the shaft is greater than 500,000 or 350,000), displacement is usually chosen, as it has little to do with the rotational frequency ； For rolling bearings, especially with regard to their radial vibration, velocity or acceleration is commonly used. When measuring the axial vibration of a shaft in general, displacement is often used.

The higher the unit’s speed, the greater the requirement for balance. So, it is necessary to measure the displacement value. Otherwise, it will pose significant safety risks!

Axial vibration and axial displacement are two different concepts; axial vibration detection involves measuring the radial vibration of the shaft. The compressor rotor is generally subjected to axial thrust, and the thrust bearings are required to bear this axial thrust. Under normal conditions, the thrust bearing shows no wear. It’s just being compressed. However, if force wear or other damage occurs, the axial play of the rotor will increase. It is necessary to call the police to shut it down. Monitoring the shaft displacement becomes particularly important at this time. So it’s not the case that shaft displacement needs to be measured within certain speed ranges, or that shaft vibration detection is required within specific speed ranges. It’s best to have both

I have a question regarding the condition monitoring of a reciprocating compressor. I have doubts about the new monitoring data – it shows fluctuations in the data before and after shutdowns over a month’s worth of historical data. The monitoring system indicates large variations in the data, with a sawtooth pattern, whereas the compressor operates quite smoothly. I hope experts can help explain whether these data are normal, and why there were changes in the data despite no maintenance being carried out during the shutdown periods. This is a real-time curve showing the longitudinal settlement of the piston rod; the situation is similar in the transverse direction as well. This is real-time monitoring of the crosshead acceleration; the gas flow rate and operating conditions are relatively stable, and no changes have been observed in the compressor’s operation. But why is the curve looking like this? Moreover, now that the unit is running, the curve remains unchanged. The unit’s motor drives the machine at a speed of 1000; it features two-stage pressure enhancement, with the pressure increasing from 8 MPa to 28 MPa, and its output volume is 2 million per day. How should one interpret these graphs?