What failures will air compressor blades appear?
Air compressor blades must be subjected to a variety of frequencies of exciting forces, including from the speed rate to the intake blade or nozzle through the frequency of the signal frequency, data accumulation installation method, the blade can have many natural frequencies or a group of natural frequencies in sequence.

They must not be excited to resonance, and there is no overstress and possible risk of failure.

In addition, many vane type air compressors must also operate in a wide speed range, and we can realize that the problem facing blade designers is trying to ensure that the response and excitation source water are far from overlapping.

In order to be able to identify how and why a blade fails, we must first understand the excitation source to which the blade is subjected. Starting with low frequencies, uneven blade or nozzle traces will stimulate the blade at the running frequency or its small frequency multiplier.

For example, a baffle mounted at an Angle will exert a larger force on the blade at the smallest interval and a smaller force on the blade at the largest interval.

As a result of this inclined Angle of the baffle, the blades are subjected to forces that vary in operating frequency.

Air compressor blade

If the baffle is tilted at an Angle in the center, it causes the blade to have a relatively high amplitude, non-synchronous frequency component close to the expected blade natural frequency in the external vibration characteristics.

The turbine was taken apart for inspection, and after several months, the frequency and amplitude of this component decreased and finally stabilized. A few months later, a few split leaves were found.

The observed situation is reproduced and the conclusion is as follows: the natural frequency of the blade has been excited by a low order harmonic frequency of the rotational speed at a little above the natural frequency.

The excitation at the natural frequency causes the blade to overload and open due to the growth of cracks, the natural frequency of the blade decreases and moves away from the excitation frequency until the stress level begins to crack. It is not sufficient to continue crack growth until the state becomes stable.

Another case is that the high frequency spectrum recorded from the steam turbine as shown in Figure 10-14, from the upper frequency of the figure because there are a large number of high amplitude harmonics between 12 kHz and 15kHz.

Although the spectrum is immediately recognizable as abnormal, the mechanism of harmonic generation and its transmission to the housing-mounted accelerometer are not yet known, but the visual data can be used to predict the cracking of the blade.

Later disassembly and inspection of the air compressor confirmed the above prediction. The features at the bottom of the figure are normal and were recorded after the rotor replacement.

In another controlled test, the amplitude of the rotor blade passing frequency produced by an air compressor with known blade faults was higher than the amplitude recorded from the same compressor with the same power level.

In contrast, the set of features recorded on one turbine was smaller than those recorded two weeks later after blade failure. Therefore, no method has been found to predict failure.

Based on these limited experiences, it can be seen that in some cases, there are external signs of impending blade failure. Although these characteristics cannot be quantified, their occurrence can focus attention on the underlying problem and then investigate it in other ways.

As mentioned earlier, the best method is undoubtedly to obtain the required data directly from the axle-mounted sensor through slip rings or radio telemetry methods.

With the increasing size of air compressors and the gradual rise in blade failure value, the focus on this field will also increase.