Experimental study on vibration and noise of throttle valve in water-conveying systems

In the water conveying system, the throttle valve has a great impact on the vibration and noise, which needs to be controlled. In this paper, a noise test bench for throttle valve in water conveying system is established. The vibration, air noise and flow noise are measured under different pressure differences and flow rates for throttle valves with different structures and diameters. The results show that the throttle valve vibration and air noise increase with the increase of pressure difference and flow when the noise is above 70 dB(A). When the pressure difference is more than 0.5MPa, the flow noise of ordinary ball valve is mainly in the high frequency range. The vibration, noise and pipeline flow noise of the low-noise valve with multistage throttling are significantly reduced, and the flow noise is mainly in the low frequency band below 315Hz. The pipe flow noise in front of the throttle valve is broadband which decreases with the increase of frequency. Increasing the pressure behind the throttle valve can reduce the valve vibration under the same pressure difference.


Introduction
The valve with throttling function is the most influential noise source in the liquid transmission pipeline.According to the survey, the noise of throttle valve can generally reach about 100dB (A).Therefore, it is necessary to study the vibration and noise of the water medium throttle valve.
Domestic and foreign scholars have carried out a lot of research on the theory and test of throttle valve vibration and noise reduction.Y Liu et al. established single-stage orifice, two-stage orifice and three-stage orifice test models, and studied cavitation characteristics under different inlet pressures and the same outlet pressure test conditions [1].Biaohua Cai et.al proposed a multi-stage throttling noise reduction scheme based on the valve throttling cavitation noise mechanism, and carried out experimental verification [2].Chao Fang studied the noise control of the large differential pressure water injection system, and predicted the system pipeline noise based on BEM/FEM.The maximum error between the flow noise numerical calculation results and the test results is 3.3% [3].Rong Jin et.al conducted experimental research on steady state noise of artesian water injection and found that the vibration acceleration and flow noise are positively correlated with the pressure difference when the opening is below 60 degrees [4].
In this paper, a noise test system for water medium throttle valve was established in the laboratory.For throttle valves with different structures and diameters, vibration, air noise and flow noise were measured under different pressure differences and flow rates.

Noise sources
The flow noise of the valve mainly consists of three parts: a) The monopole noise source is formed by the pulsation of the flow through the valve caused by the pipe wall vibration; b) The dipole noise source formed by the interaction between fluid and pipe wall in the boundary layer; c) A quadrupole noise source formed by fluid turbulence near the valve outlet.
It is generally believed that the intensity of monopole sound source is related to the horizontal velocity of the valve wall and can be ignored [1]; The ratio of quadrupole noise source intensity to dipole noise source intensity is proportional to the square of Mach number: the flow velocity of the fluid medium in the water system is generally far less than the sound velocity, so it can be seen that the quadrupole noise source intensity of the valve is far less than the dipole noise source, so the main noise source of the throttle valve is the dipole noise source.

Air noise
There are two mechanisms for the throttle valve in the liquid transmission pipeline to generate steady state noise.One is the mechanical noise caused by the vibration of valve components.Except for the special cases that cause resonance, the mechanical noise is generally low; The second is fluid noise or cavitation noise.The bubbles produced by liquid vaporization release huge energy when they burst, producing strong noise.
According to the description of industrial process control valve noise related to the International Association of Electrical Engineering standard IEC60534, valve vibration noise is mainly related to the valve structure, flow, working pressure difference, and pressure difference before and after the valve.The relationship between valve differential pressure ratio and noise is shown in the figure below.

Figure 1. The relationship between x F and air noise
In the figure, x Fz represents the critical pressure ratio, that is, the pressure ratio corresponding to the cavitation incipient state; Δp=p 1 -p 2 represents the upstream and downstream differential pressure; p v is the saturated steam pressure at the inlet temperature.The noise inside the valve in laminar flow state is small and can be ignored, while the noise generated by the valve in turbulent flow state will increase significantly.When the pressure increases further and the valve cavitation occurs, the cavitation collapse noise will be the main part, and the noise is far greater than the non-cavitation state.The sound power level and air noise in the frequency range of 500Hz-8000Hz can be evaluated: (1) Non-cavitation state The radiated sound power level Lwi inside the valve can be estimated as follows: wi F F 120 10 lg 10 lg 10 lg 10 lg The reference power is calculated as 10 -12 W, and the sound effect coefficient η F is the ratio of sound power to flow power of non-cavitating flow, which is determined by the geometric conditions at the throttle of the valve; ρ F is the liquid density; Q represents mass flow; Δp represents the inlet and outlet pressure difference. ( where ΔL F represents the corrected value of cavitation flow.

Test bench
The diagram below shows the schematic diagram of the test bench.The test system ensures the constant water pressure in the pressurized water tank by supplying air and exhausting air to the pressurized water tank, and uses the pressure difference between the pressurized water tank and the open water tank to generate the pressure difference.Flexible nozzles are set at the inlet and outlet of the valve test area to isolate the impact of upstream and downstream vibration.Pressure sensors and hydrophones are installed at the inlet and outlet of the measured valve to measure the pressure and flow noise during the system operation.Axial and radial vibration acceleration sensors are installed on the inlet and outlet flanges of the measured valve to measure the vibration of the valve body (10Hz-8kHz).A microphone is arranged 1m downstream of the valve to measure the air noise of the valve (20Hz-10kHz).Two types of throttle valves were used in the test.First, ordinary ball valves were used, and then multistage throttle low-noise valves were used [5].

Effect of different throttling valves
As shown in the following table, the throttling noise of the low-noise valve is reduced by 9~15dB compared with the ordinary ball valve, and the effect is significant.Figure 3 shows the comparison between the total level of system flow noise and the total level of low frequency band (Q=120m 3 /h) of the two valves under different water tank pressures, and the frequency distribution is shown in Figure 4.In general, after the improvement, the total flow noise level is reduced by 8.4 dB to 11.8 dB, and the reduction amplitude in low frequency range is between 5.3dB and 5.6 dB.The total level of flow noise of ordinary ball valve is quite different from that of low frequency range, with high frequency flow noise accounting for the main part.The total level of flow noise of low noise valve is very close to that of low frequency range.It can be seen that under the same pressure difference and flow rate, the air noise of DN125 valve is lower than that of DN50 valve, but the vibration is greater.In addition, when the valve is fully open, the vibration of the valve will increase significantly.

Effect of increasing outlet pressure
In order to observe the difference between the same pressure difference and different pressures, the vibration test was carried out with low-noise valves with two diameters.The results are as follows: It can be seen that the vibration acceleration is significantly reduced after increasing the valve outlet pressure, and this measure is effective.

Conclusion
In this paper, a noise test bench for throttle valve in water conveying system is established.The vibration, air noise and flow noise are measured under different pressure differences and flow rates for throttle valves with different structures and diameters.The results show that the throttle valve vibration and air noise increase with the increase of pressure difference and flow when the noise is above 70 dB(A).When the pressure difference is more than 0.5MPa, the flow noise of common ball valve is mainly in the high frequency range.The vibration, noise and pipeline flow noise of the low-noise valve with multistage throttling are significantly reduced, and the flow noise is mainly in the low frequency band below 315Hz.The pipe flow noise in front of the throttle valve is broadband which decreases with the increase of frequency.Increasing the pressure behind the throttle valve can reduce the valve vibration under the same pressure difference.At the end, the low-noise valve with multistage throttling is chosen.

Figure 3 .
Figure 3. Flow noise under different pressures

Figure 4 .
Figure 4. Frequency domain distribution of flow noise under different pressures4.2.Influence of pipeline diameterIn order to compare the impact of pipe diameter, low-noise valves with different diameters are designed and produced.The test results are as follows: Cavitation stateThe influence part of cavitation can be added on the basis of (1):

Table 1 .
Effect of different throttling valves 4

Table 2 .
Comparison of throttling effect of different nominal sizes

Table 3 .
Effect of increased outlet pressure