Can The Water Pump Manufacturer Explain The Common Sources Of Water Pump Noise

Aug 21, 2024

Pump noise has always been a headache for customers. Whether it is caused by a malfunction or the inherent noise of the pump itself, I believe many customers will encounter these problems when using the pump. Today, Lutsee will explain to you the common sources of pump noise.
Mechanical noise originates from vibrating components or surfaces that produce audible pressure fluctuations in adjacent media. For example, pistons, unbalanced vibrations caused by rotation, and vibrating pipe walls.
In positive displacement pumps, noise is generally associated with pump speed and the number of pistons in the pump. Liquid pulsation is the main mechanical induced noise, and conversely, these pulsations can also excite mechanical vibrations in pump and pipeline system components. Incorrect crankshaft balance weights can also cause vibration according to the rotational speed, which may loosen the foundation bolts and produce a knocking sound of the foundation or guide rail. Other noises are related to the sound of worn connecting rods, worn piston pins, or piston strikes.
In centrifugal pumps, improperly installed couplings often produce noise (misalignment) at twice the pump speed. If the speed of the pump approaches or passes the critical speed of the level, high vibration caused by imbalance or noise generated by bearing, seal, or impeller wear can occur. If wear occurs, its characteristic may be the emission of high pitched whistling sounds. Electric motor fans, shaft keys, and coupling bolts may all produce clearance noise.
Liquid noise source
When pressure fluctuations are directly generated by liquid movement, the noise source is proportional to fluid dynamics. Possible fluid power sources include turbulence, liquid flow separation (vortex state), cavitation, water hammer, flash evaporation, and the interaction between impeller and pump separation angle. The pressure and flow pulsations caused may be either periodic or broadband in frequency, and may generally excite mechanical vibrations in pipelines or pumps themselves. Then, mechanical vibrations can diffuse noise into the environment.
Generally, there are four types of pulsation sources in liquid pumps:
(1) Discrete frequency components generated by pump impeller or piston
(2) Broadband turbulence energy caused by high flow velocity
(3) Intermittent oscillation of broadband noise caused by cavitation, flash evaporation, and water hammer constitutes impact noise
(4) When the liquid flow passes through obstacles and lateral tributaries of the pipeline system, periodic vortices may cause flow induced pulsations, which may result in secondary flow spectrum changes of pressure fluctuations in the centrifugal pump.
This is especially true when operating under non design flow conditions. The numbers shown on the streamline indicate the positioning of the following flow process principles:
Due to the interaction of the boundary layer between the high-speed and low-speed regions in the flow field, most of these unstable flow patterns generate vortices, for example, caused by liquid flow around obstacles or through stagnant water zones, or by bidirectional flow. When these vortices impact the sidewall, they transform into pressure fluctuations and can cause local oscillations in pipelines or pump components. The acoustic response of pipeline systems may strongly affect the frequency and amplitude of eddy current diffusion. Research has shown that when the resonance of sound in the system is consistent with the natural or preferred frequency of the noise source, eddy currents are strong.
When the centrifugal pump operates at a flow rate less than or greater than the optimal efficiency, noise is usually heard around the pump casing. The level and frequency of this noise vary from pump to pump, depending on the pressure head level generated by the pump at that time, the ratio of required NPSH to available NPSH, and the degree to which the pump fluid deviates from the ideal flow. When the angle of the inlet guide vanes, impeller, and casing (or diffuser) are not suitable for the actual flow rate, noise often occurs. The main source of this noise is also considered to be recirculation.
Before the liquid flows through the centrifugal pump and is pressurized, it must pass through an area with a pressure not greater than the existing pressure in the inlet pipe. This is partly due to the acceleration effect of the liquid entering the impeller inlet, as well as the separation of the airflow from the impeller inlet blades. If the V flow rate exceeds the design flow rate and the accompanying blade angle is incorrect, high-speed and low-pressure vortices will form. If the liquid pressure drops to the vaporization pressure, the liquid gas will flash off. The pressure inside the passage will increase later. The subsequent implosion causes noise commonly known as cavitation. Usually, the rupture of air pockets on the non pressure side of impeller blades not only causes noise, but also poses serious hazards (blade corrosion).
The noise level measured on the casing of an 8000hp (5970kW) pump and near the inlet pipeline during cavitation.
The generation of cavitation can excite broadband impacts of many frequencies; However, in this case, the common frequency of the blades (the number of impeller blades multiplied by the number of revolutions per second) and its multiples dominate. This type of cavitation noise typically produces very high-frequency noise, best referred to as "explosion noise".
The noise of cavitation may also be heard when the flow rate is lower than the design condition, or even when the available inlet NPSH exceeds the NPSH required by the pump, which is a very puzzling problem. The explanation proposed by Fraser suggests that this very low irregular frequency but high-intensity noise originates from the backflow at the inlet or outlet of the impeller, or at two locations, and every centrifugal pump experiences this recirculation at a certain flow rate decrease condition. Operating under recirculation conditions damages the inlet and outlet of the impeller blades (as well as the pressure side of the casing guide vanes). The increase in the loudness of impulse noise, irregular noise, and the increase in inlet and outlet pressure pulsation when the flow rate decreases can all serve as evidence of recirculation.
Automatic pressure regulators or flow control valves can generate noise related to both turbulence and airflow separation. When these valves operate under severe pressure drop, they have high flow rates that generate significant turbulence. Although the generated noise spectrum is very wideband, its characteristics are centered around a frequency with a corresponding Strouhal number of approximately 0.2.
Cavitation and flash evaporation
For many liquid pumping systems, there is generally some flash evaporation and cavitation related to pressure control valves in the pump or delivery system. Due to the significant flow loss caused by throttling, higher flow rates result in more severe cavitation.
In the suction line of a positive displacement pump, the piston may generate high amplitude pulsations and be enhanced by the acoustic performance of the system, causing the dynamic pressure to periodically reach the vaporization pressure of the liquid, even if the static pressure at the suction port may be greater than this pressure. When the circulating pressure increases, bubbles rupture, producing noise and impacting the system, which may lead to corrosion and also produce unpleasant noise.
When the pressure of hot pressurized water decreases through throttling (such as flow control valves), flash evaporation is particularly common in hot water systems (feed pump systems). The decrease in pressure causes the liquid to suddenly vaporize, i.e. flash evaporation, resulting in noise similar to cavitation. To avoid flash evaporation after throttling, sufficient back pressure should be provided. On the other hand, throttling should be applied at the end of the pipeline to disperse the energy of flash evaporation into a larger space.