Experimental and Numerical investigation of "Wet" and "Dry" critical speed of multistage pump
Funded by KaiQuan Pump Group
Recently, I am interested in the vibration measurement of multistage pump. Specifically, I try to investigate the influence of annular seal on
the dynamic characteristic of multistage pump, through CFD simulation and experiment. In simulation, we developed a improved dynamic
mesh method, which can be utilized to predict the rotor dynamic coefficient of annular seal, including stiffness and damping coefficient. This
dynamic mesh method is developed for small clearance simulation, considering that ordinary dynamic methods are not suitable for small
clearance where negative volume will appear. According to the predicted coefficient of annular seal, a rotor model is established and the
"dry" and "wet" critical speed of pump rotor are predicted.
In experiment, a new-type natural frequency exciter was developed to measure the critical speed of a nine-stage pump, both for static and full speed operation situation.
Recently, I am interested in the vibration measurement of multistage pump. Specifically, I try to investigate the influence of annular seal on
the dynamic characteristic of multistage pump, through CFD simulation and experiment. In simulation, we developed a improved dynamic
mesh method, which can be utilized to predict the rotor dynamic coefficient of annular seal, including stiffness and damping coefficient. This
dynamic mesh method is developed for small clearance simulation, considering that ordinary dynamic methods are not suitable for small
clearance where negative volume will appear. According to the predicted coefficient of annular seal, a rotor model is established and the
"dry" and "wet" critical speed of pump rotor are predicted.
In experiment, a new-type natural frequency exciter was developed to measure the critical speed of a nine-stage pump, both for static and full speed operation situation.
Introduction of experimental equipment
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Fig. 1 sketch map of measured multistage pump
The test-rig is a nine-stage pump with auxiliary pipeline system, which is provided by a pump group. A sketch map of this kind of pump can be seen in fig. 1.In the pump, large parts of rotor are covered by pump case, with only a short external part outside. This external part is connected with a coupling, which is drived by a electromotor. Actually, this close structure of pump presents a huge difficulty for measuring vibration of its rotor.
A electromagnetical exciter is installed under the coupling. This exciter is utilized to give the pump periodical impulse. Bearing this impulse, the pump will vibrate accordingly. Speed sensor is put in the pump case, and eddy current sensor is put in the coupling. The vibration signal is measured by a self-developed software (VibTest). Conducting Fast Fourier Transform (FFT) of measured signal, the spectrogram of pump vibration can be gotten. Theoretcially, the natural frequency of pump components can be excited when it is beared with periodical impulses. However, the practical vibration of multistage pump is usually large during operation, which makes impulse response has a low signal to noise ratio (SNR). It is challenging to recognize natural frequency of pump during operation. Now, I am trying to figure this problem out.
A electromagnetical exciter is installed under the coupling. This exciter is utilized to give the pump periodical impulse. Bearing this impulse, the pump will vibrate accordingly. Speed sensor is put in the pump case, and eddy current sensor is put in the coupling. The vibration signal is measured by a self-developed software (VibTest). Conducting Fast Fourier Transform (FFT) of measured signal, the spectrogram of pump vibration can be gotten. Theoretcially, the natural frequency of pump components can be excited when it is beared with periodical impulses. However, the practical vibration of multistage pump is usually large during operation, which makes impulse response has a low signal to noise ratio (SNR). It is challenging to recognize natural frequency of pump during operation. Now, I am trying to figure this problem out.
Experimental results
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Fig. 2 Spectrogram of multistage pump (static)
Now, the whole test-rig has already been established. Carefully examination of the whole rig has been conducted. That is, natural frequency of each component of the rig is measured. It is found that the pipeline system should receive important consideration. Because the pipeline system has abundant frequency, these frequency are easily mixed up with natural frequency of pump. Thus, vibration isolation measures should be carried out between pump and its pipeline system. Also, the foundation of pump should not be too soft, otherwise the pump will have a large vibration during operation, and the natural frequency of foundation is easily mixed up with frequency of pump components.
Fig. 2 shows a spectrogram of multistage pump when the pump is static. The signal is measured by speed sensor. It is found that the natural frequency of pump rotor can be clearly recognized by periodical impulse method. These frequency is the "dry" critical speed of pump. Further research need be done on "Wet" critical speed of pump.
Fig. 2 shows a spectrogram of multistage pump when the pump is static. The signal is measured by speed sensor. It is found that the natural frequency of pump rotor can be clearly recognized by periodical impulse method. These frequency is the "dry" critical speed of pump. Further research need be done on "Wet" critical speed of pump.