Pulsation in pipes and other hydraulic systems
Pumping and system pulsations are an important factor in the suitability and reliability of a design. Finite element modelling can predict the natural waves and forced waves based on the expected pump harmonics, geometries, and speeds. However, modelling can only go so far, and oftentimes a measurement setup is required, such as ENSURE, for physical validation of the key assumptions. The key design strategy for reducing the risk of pulsation-induced component failures (which is to ensure that pulsation natural frequencies of the pumping systems are sufficiently detuned from the induced stimulus occurring at specific harmonics of the pump speed) can be verified by measuring the pulsation natural frequencies to ensure they are properly detuned.
One method of identifying the system's natural frequencies is based on the instantaneous pressure at various points. This method is preferred because of its advantages over vibration analysis. A data acquisition system based on analog signal detection is key. It must be ensured that it can handle the varying pressure pulsations and that the high frequencies can be obtained effectively. Owing to the simultaneous data acquisition of these signals, the information is captured at the same instantaneous time at the various points of interest in the system. Thus, the instantaneous pressures can be post-processed to derive the resulting pulsation excitations.
The signals that are fed to the data acquisition system are typically from high-frequency response pressure sensors. Sensors (based on piezo technology, for example) are strategically placed so that damping effects are minimized for maximum frequency response and amplitude quantification. The amplitude and shape of the delivered signal will vary with the pumping speed and accuracy may be affected mostly at low or high pumping speeds. Other types of sensors are based on a compensation strategy and are equipped with miniaturized electronic circuits to condition the output to deliver an optimal output signal. Proper sensor selection is key to good data analysis and proper experimental conclusions.
Once the desired experimental validation data is acquired and logged to the file, it is then ready for analysis with specialized software such as FloINT. There are several types of frequency spectra that are commonly used. Each of the three main methods will be covered.
The Joint-Time Frequency spectrum provides pulsation information on a data set divided up into time intervals. For rotating hydraulic machinery, order analysis is very effective for examining pulsation magnitudes correlated to the unit under test's pump speed. Joint-Time Frequency analysis divides a data set into time intervals and performs a Fast-Fourier Transform (FFT) on the data in each time interval separately. This enables inspection of how the frequency content of a signal develops over time.
In the Order Analysis Frequency Spectrum method, the magnitude of the frequency content is examined slightly differently. Frequency is displayed on one axis, pumping Speed on a second axis, and frequency magnitudes on a third axis. This enables analysis of pulsation magnitudes tracked with pumping speed. For example, as a pump motor is run from startup (0 RPM) to nominal speed (1000 RPM), the pulsation the system experiences changes at different RPM levels. Order analysis allows identification of the critical pumping speeds that drive the pulsation of the UUT, perhaps to examine natural frequencies or other design problems.
The Order Analysis Order Spectrum method is very similar to the Order Analysis Frequency Spectrum method. The main difference is that a quantity called "Order" replaces frequency on one axis. Order simply represents the frequency of the vibration divided by the instantaneous pumping frequency of the unit under test. This analysis easily identifies relative overtone amplitudes of the pump speed as the pumping speed develops.
Many issues have been solved or even prevented by the consistent application of pulsation measurement, analysis, and design improvement. ATA engineers are available for consultations on methods, software tools, and hardware systems to eliminate unwanted pulsation vibration.
Torsional Vibration in shafts and other mechanical systems
The torsional vibration of a crank or motor shaft is an important factor in the suitability and reliability of a system design. Finite element modelling can predict the natural vibration and forced vibration based on the expected harmonics and speeds. However, modelling can only go so far, and oftentimes a measurement setup is required, such as ENSURE, for physical validation of the key assumptions.