Sources of pump noise and discussions thereon

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This post was last edited by yinkuilin6868 on 2015-12-21 10:22. Sources of mechanical noise: Mechanical noise originates from vibrating components or surfaces, which generate sound pressure waves in the surrounding medium. Such as pistons, unbalanced rotational vibrations, and vibrating pipe walls. 　　In positive-displacement pumps, noise is generally related to the pump speed and the number of pistons in the pump. Liquid pulsation is the main mechanically-induced noise; conversely, these pulsations can also induce mechanical vibrations in the components of the pump and pipeline systems. Incorrect crankshaft counterweights can also cause vibrations depending on the speed, which may loosen the foot bolts and result in knocking sounds from the foundation or guide rails. Other noises are related to the sound produced by worn connecting rods, as well as the knocking sound of worn piston pins or pistons. 　　In centrifugal pumps, an incorrectly installed coupling often generates noise at twice the pump speed (misalignment). If the pump’s speed is close to or exceeds its critical speed, high vibrations caused by imbalance, or noise resulting from wear in the bearings, seals, or impeller can occur. If wear occurs, its characteristic may be the production of a high-pitched whistling sound. Motor fans, shaft keys, and coupling bolts can all generate gap noise. 　　Liquid noise source: When pressure fluctuations are generated directly by the movement of liquid, the noise source is the corresponding hydrodynamic force. Possible hydrodynamic sources include turbulence, flow separation (vortex states), cavitation, water hammer, flashing, and the interaction between the impeller and the pump’s hydraulic angle. The resulting pressure and flow pulsations may be either periodic or broadband in frequency, and they can generally induce mechanical vibrations in the pipeline or the pump itself. Then, mechanical vibration can spread noise into the environment. 　　Generally, there are four types of pulsation sources in liquid pumps: (1) discrete frequency components generated by the pump impeller or piston; (2) broadband turbulent energy caused by high flow velocities; (3) impact noise resulting from intermittent oscillations of broadband noise caused by cavitation, flashing, and water hammer; (4) flow-induced pulsations due to periodic vortices when the liquid flows around obstacles and lateral branches in the piping system, which may lead to changes in the secondary flow spectrum and pressure fluctuations in centrifugal pumps. 　　This is especially true when operating at flow rates other than those designed for. The numbers indicated on the flow lines represent the principle underlying each of the following flow patterns: (1) Separation, (2) Recirculation (secondary flow), (3) Circulation, (4) Leakage, (5) Unstable flow fluctuations, (6) Wake (vortex), (7) Turbulence, (8) Cavitation. Due to the interaction between the boundary layers in the high-speed and low-speed regions of the flow field, most of these unstable flow patterns give rise to vortices; for example, these can be caused by the flow around obstacles or through stagnant areas, or by bidirectional flow. When these vortices strike the side walls, they are transformed into pressure fluctuations, which can cause localized oscillations in the pipeline or pump components. The acoustic response of the piping system can strongly affect the frequency and amplitude of eddy diffusion. Research has shown that eddies are most intense when the resonant frequency of the system matches the natural or dominant frequency of the noise source. 　　When a centrifugal pump operates at a flow rate that is less than or greater than the flow rate at which its efficiency is optimal, noise is usually audible around the pump casing. The level and frequency of this noise vary from pump to pump, depending on the head generated by the pump at that time, the ratio of required NPSH to available NPSH, and the degree to which the fluid flow within the pump deviates from ideal flow. Noise often occurs when the angle of the inlet guide vanes, as well as the impeller and casing (or diffuser), are not suitable for the actual flow rate. Additionally, recirculation is also considered to be the main source of this noise. 　　Before the liquid flows through the centrifugal pump and is pressurized, it must pass through a region where the pressure is not higher than the existing pressure in the inlet pipe. This is partly due to the acceleration of the liquid as it enters the impeller inlet, and also due to the separation of the airflow from the inlet blades of the impeller. If the V-flow exceeds the designed flow rate and the accompanying blade angle is incorrect, high-speed, low-pressure vortices will be formed. If the liquid pressure drops to the vaporization pressure, the liquid flashes. The pressure in that channel will increase later. The resulting implosion causes noise commonly known as cavitation. Cavitation usually occurs on the non-pressure side of the impeller blades; in addition to causing noise, it can also lead to serious damage (blade corrosion). 　　The noise level measured on the casing of an 8000 hp (5970 kW) pump, near the inlet pipeline, during cavitation. 　　Cavitation can generate broadband shocks at many frequencies; however, in this case, the blade common frequency (the number of impeller blades multiplied by the revolutions per second) and its multiples dominate. This type of cavitation noise typically generates very high-frequency noises, which are best described as “explosive sounds”. 　　Cavitation-related noise can also be heard when the flow rate is below the design value, or even when the available inlet NPSH exceeds the NPSH required by the pump; this is a rather puzzling issue. The explanation proposed by Fraser suggests that this very low-frequency, irregular noise of high intensity originates from recirculation at or near the impeller inlet or outlet, or at both locations, and that every centrifugal pump experiences such recirculation under conditions of reduced flow rate. Operation under recirculation conditions damages the pressure-bearing sides of the impeller blades at the inlet and outlet (as well as the shell guide vanes). An increase in impact-type noise and random noise levels, as well as an increase in inlet and outlet pressure fluctuations when the flow rate decreases, can all serve as evidence of recirculation. 　　Pressure automatic regulators or flow control valves can generate noise related to both turbulence and flow separation. When these valves operate under high pressure drops, they have high flow rates that result in significant turbulence. Although the generated noise spectrum is very broad-band, it is characterized by frequencies centered around a corresponding Strouhal number of about 0.2. 　　Cavitation and flashing: For many liquid pumping systems, there is usually some degree of flashing as well as cavitation related to the pressure control valves in the pump or conveying system. Due to the significant flow losses associated with throttling, higher flow rates result in more severe cavitation. 　　In the suction line of a positive displacement pump, the piston can generate high-amplitude pulsations that are amplified by the acoustic properties of the system, causing the dynamic pressure to periodically reach the vaporization pressure of the liquid, even if the static pressure at the suction inlet is higher than this pressure. As the cycle pressure increases, the bubbles burst, generating noise and impacting the system; this can also lead to corrosion as well as annoying noise. 　　Flash evaporation occurs quite frequently in hot water systems (feed pump systems) when hot pressurized water has its pressure reduced by throttling (such as through a flow control valve). This reduction in pressure causes the liquid to vaporize suddenly, that is, to flash, resulting in noise similar to cavitation. To avoid flashing after throttling, sufficient backpressure should be provided. On the other hand, throttling should be applied at the end of the pipeline to disperse the energy of flashing over a larger volume.