EXPERIMENTAL RESEARCH ON CHARACTERISTICS OF CENTRIFUGAL PUMP NOISE IN WATER PIPE SYS- TEM

Transcription

1 The 21 st International Congress on Sound and Vibration July, 2014, Beijing/China EXPERIMENTAL RESEARCH ON CHARACTERISTICS OF CENTRIFUGAL PUMP NOISE IN WATER PIPE SYS- TEM Muze Huang, Hongling Sun, Qijun Zhou, Jun Yang, Jing Tian Key laboratory of Noise and Vibration Research, Institute of Acoustics, Chinese Academy of Sciences, Beijing, China Exhaust noise of centrifugal pumps exists in many civil and industrial applications. In this work, the noise radiation and transmission of a centrifugal pump in the water pipe system are investigated by experiments. The pipe system consists of two tanks, two sets of pipe and valves. The blade frequency of the centrifugal pump can be shifted by variable-frequency drive. Experiments are carried out for the sound fields in the pipe system with different pipe ends and opening positions of the valve at the end of the pipe. 1. Introduction Centrifugal pumps represent the primary acoustic source in many industrial piping systems. The pressure pulsations generated by centrifugal pumps propagate in the direction of pump discharge in the form of acoustic waves. The transmitted acoustic waves can be reflected by the system discontinuities (side branches, changes in cross-section etc.) and pipe ends. The superposition of reflected wave and incident wave forms standing waves in the fluid-filled pipe. Standing waves may strongly interact with piping system or its elements, leading to excessive vibrations [1-2]. Many cases of this kind have been reported in previous works, causing a lot of repairs and loss [3]. The control of centrifugal pump noise in fluid-filled industrial piping systems relies on the exact knowledge of the system acoustics, and the mechanism by which pressure pulsations are generated and transmitted. The acoustical energy is mainly radiated at low frequency, especially as a discrete excitation in the blade-passing frequency. Very little is known about the excitation mechanism within the pumps[4]. The amount of energy radiated may vary significantly between different pump designs. It depends on the size and driving frequency, and on the hydraulic design detail of the impeller and cutwater[5-6]. In this work, experiments are carried out for the measurement of pressure fields in the piping system with different pipe ends and the opening positions of the valve. Although the pressure field in the piping system is not free of resonance effects associated with the piping acoustics, the effect of different pipe ends on the transmitting properties of the pump noise is investigated in experiments. The variable-frequency drive is used to shift the running speed of the centrifugal pump. Besides the blade-passing frequency, there are other discrete excitations at low frequency. The pump radiation characteristics are investigated through varying the driven frequencies of the pump. ICSV21, Beijing, China, July

2 2. Experimental setup 2.1 Piping system The piping system consists of two tanks and two sets of pipes, forming a hydraulic loop cycle. One set of pipe is the test pipe. The other set is the return pipe. Both two sets of pipes connect with the two tanks. The experiment is carried out in the test pipe. The test pipe is shown in Figure 3. The 300 mm diameter uniform test pipe is made of steel. The overall length of the test pipe is 14.1 m. Each end of the test pipe is installed with a valve respectively. The opening position of the downstream valve (Valve B) can be altered to represent different boundary conditions. The centrifugal pump is in the upstream of the test pipe next to the upstream valve. There is one end section located in the downstream of the test pipe next to the downstream valve. The end section can be replaced by different pipe ends (T-section with hard plate, T-section with air cell, uniform pipe etc.). To monitor the sound fields in the test pipe, several hydrophones are installed along the discharge direction of the centrifugal pump. There exact locations are specified in Table 1. Figure 1.The schematic of the test pipe system Table 1. Locations in the test pipe (distance from pump center) Pump discharge side Pump suction side Location Distance (m) Location Distance (m) Point Point Point Valve A 1.5 Point Point Point Point 4 7 Point Point Point End section(center) 10.7 Valve B 12.5 ICSV21, Beijing, China, July

3 2.2 Test pump The test is conducted with a single-stage, double-volute pump. The pump is directly driven by a 30 kw electric motor. At the normal operational condition, it operates at a rated capacity of 515 m3/h and a hydraulic head of 11.5 m. The pump running speed is 1500 rounds per minute (25 Hz). Given that the impeller has 6 blades, the shaft frequency is 25 Hz and the blade passing frequency is 150 Hz. 3. Experimental investigation 3.1 Experiments with different pipe ends When the downstream valve opens wide, the sound pressures at the discharge and suction of the pump are monitored. The result is shown in Figure 2. It is found that the sound pressure at the discharge side contains many discrete excitations. The peaks at the shaft frequency (25 Hz) and the blade passing frequency (150 Hz) are the most prominent. Compared to the sound pressure at the discharge side, the discrete excitations at the suction side are not prominent, and the overall pressure level is more than 10 db lower. The coupling between the discharge and the suction is not strong, especially at discrete excitations. So we mainly focus on the sound field at the discharge side. 190 point 8 point Figure 2. The pump noise at discharge side (point 8) and suction side (point 10) when downstream valve is wide open The sound pressures measured at other points (point 3, point 4, point 5) at the discharge side are shown in Figure 3. The discrete excitations at shaft frequency (25 Hz) and blade passing frequency (150 Hz) are very predominant. The cross correlation factor is calculated between signals measured at different points. The result is shown in Figure 4. The cross correlation factor is close to 1 at low frequency, except for some troughs. The cross correlation factors at discrete excitation frequencies are higher than Some troughs of the cross correlation factor may derive from the local turbulent fluctuations. ICSV21, Beijing, China, July

4 point 3 point 4 point Figure 3. The pump noise at discharge side (point 3, point 4, point 5) when downstream valve is wide open 1 Cross-correlation factor point 3-point 4 point 5-point Figure 4. The cross-correlation factor between point 3 and point 4, and between point 5 and point 8 Altering the downstream valve to half-open position, the pump noise at point 8 is compared to the noise when the valve is wide open. The result is shown in Figure 5. The discrete excitation at blade passing frequency is much lower when the valve is half-open. And the overall pressure level is 8 db lower. ICSV21, Beijing, China, July

5 open half-open Figure 5.The pressure measurement at discharge side (point 8) with different downstream valve open positions The end section is installed with T-section with hard plate in the above experiments. The T- section s side branch pipe is less than 0.5 m, and the side branch is sealed by a thick hard plate. The results in Figure 6 illustrate the sound pressures at point 8 with different end sections.the length of the side branch is much shorter than the wavelength at low frequencies (the wavelength is about 3 m at 500 Hz), so its discrepancy with the uniform section can be negligible. While above Hz, the broadband noise of T-section with hard plate rises compared to uniform pipe. The hydraulic turbulent fluctuations deriving from the discontinuity of the side branch may contribute to the discrepancy. The T-section with air chamber replaces the hard thick plate with an air chamber, which can be considered as a soft boundary. It is equivalent of decreasing the overall length of the test pipe, thus the resonance frequency of the pipe acoutics increases compared to the T-section with hard plate. T-section with hard plate T-section with air chamber uniform pipe Figure 6. The pressure measurement at discharge side (point 8) with different end sections ICSV21, Beijing, China, July

6 3.2 Experiment with different driving frequencies To further investigate the pump radiating characteristics, we use a variable-frequency drive to shift the running speed of centrifugal pump. The electric driven frequency varies from 40 Hz to 50 Hz with an interval of 2 Hz (shaft frequency of the pump varies from 20 Hz to 25 Hz). The result is shown in Figure 7. The blade passing frequency shifts proportionally with the shaft frequency, while the discrete excitation frequencies above the blade passing frequency decrease as the shaft frequency increases. The mechanism is not known yet. 50 Hz 48 Hz 46 Hz Hz 42 Hz 40 Hz (a) (b) Figure 7. The pressure measurement at discharge side (point 8) with different driving frequencies: (a) driving frequencies from 46 Hz to 50 Hz and (b) from 40 Hz to 44 Hz 4. Conclusion The centrifugal pump represents the primary noise source in industrial piping systems. Very little is known about the excitation mechanism within the pump. In this work, the pump characteristics in the piping system are investigated through monitoring the sound field at the discharge side of the pump, as the coupling between the suction side and the discharge side is not strong. The magnitude of the discrete excitation at blade passing frequency varies with different open positions of the downstream valve. The discrete excitation frequency may vary with different end sections. As the running speed changes, the shaft frequency and the blade passing frequency shift proportionally, but the higher discrete frequencies shift in an opposite direction. REFERENCES 1 De Jong, C. A. F., Analysis of pulsations and vibrations in fluid-filled pipe systems, Ph.D. Thesis,Eindhoven University of Technology, Endhoven, The Netherlands,(1994). 2 Tilsseling, A.S., Fluid-structure interaction in liquid-filled pipe systems: A review, Journal of Fluids and Structures, 10, , (1996). 3 Rzentkowski, G., Forest, J. W. & Martin, D. G., Acoustical characterization of the heattransport system of Darlington nuclear generation station: analysis technique and results,proceedings 1st International Symposium on Pump Noise and Vibrations, Clamart, France,pp , (1993). ICSV21, Beijing, China, July

7 4 Rzentkowski G., Zbroja S., Experimental characterization of centrifugal pumps as an acoustic source at the blade-passing frequency, Journal of Fluids and Structures, 114(4), , (0). 5 Guelich, J. F. & Bolleter, U., Pressure pulsations in centrifugal pumps, ASME Journal of Vibration and Acoustics, 114, ,(1992). 6 Rzentkowski,G., Generation and control of pressure pulsations emitted from centrifugal pumps:a review, ASME PVP Conference Montreal, Canada; PVP-vol. 328, pp , (1996). ICSV21, Beijing, China, July