Abstract: Through the analysis of fluid noise, combined with the investigation of the noise after using the flow control valve in the field, the theory of flow rate, differential pressure and multi-stage noise reduction is used to produce low noise products.
Our company's leading product, the self-operated flow control valve, has been in existence for ten years since its inception in 1993. During this decade, the heating industry's peers never realized the understanding and learned the system application from a one-sided basis. The self-operated flow control valve plays a certain role in heating.
During this decade, our company's self-operated flow control valves have undergone two major improvements. From the use of Type II products in 1993 to 98 years, in the first five years, due to the large size and limited installation position of the old Type II, we improved the old products in 1999 to meet the needs of users. Production can guarantee control performance and can be installed at any angle, and the volume is reduced by 40%; by the second five years in 2002, many users reported that the noise increased after installation and use, especially in outdoor overhead pipelines. on. In response to this situation, we organized technical personnel to conduct special research and development. After nearly four months of trials in the past four months, we have produced low-noise, lock-up, and easily adjustable flow control valves. The noise is reduced from 65 to 75 dB to 45 to 55 dB, which can meet the needs of different users. The following is a summary of the experience we got in the development process for the reference of peers.
First, the noise source analysis
Pumps, pipes, and valves are indispensable in heating systems, and these are all facilities that generate noise sources.
1. Let's talk about the pipeline first. When the liquid flows through the pipeline, noise will be generated due to the pressure disturbance caused by turbulence and friction, especially when the Reynolds number Re> 2400 is turbulent, this turbulent flow containing a large number of irregular small vortices, It can be said that it is in a state of "noisy". Especially when flowing through a throttling or pressure reducing valve, a pipe with a sudden cross section, or a sharply bent elbow, the turbulent flow interacts with these portions that block the passage of the fluid to generate eddy current noise, and the sound power level (dB) can be expressed as a function of the flow rate. It is: â–³Lw=60lg, if the pipeline is not properly designed, it can also generate cavitation noise;
2. Say the valve, the valve with throttling or pressure limiting, is the most influential noise source in the liquid transmission pipeline. When the flow rate of the fluid in the pipeline is sufficient, if the valve is partially closed, a large area of ​​turbulence is formed at the inlet of the valve, and the liquid flow rate is increased and the internal static pressure is decreased in the turbulent region, and the static pressure is decreased when the flow rate is greater than or equal to the critical speed of the medium. Below or equal to the evaporation pressure of the medium, bubbles are formed in the fluid. As the bubble flows with the liquid, the flow velocity gradually decreases in the downstream of the turbulent zone of the valve, the static pressure rises, and the bubbles are successively crushed, causing irregular pressure fluctuations in the fluid. This special deuteration phenomenon is called cavitation, resulting in The noise is called cavitation noise. In pipelines with high flow and high pressure, almost all throttle valves can generate cavitation noise. This cavitation noise can travel far down the pipeline and can propagate the valve or pipeline. The natural vibration of the movable parts in the middle, and through these parts acting on other adjacent parts to the surface of the pipe, producing a tuned sound similar to the collision of metal. The acoustic power of cavitation noise is proportional to the seventh or eighth power of the flow rate, so a multi-stage series valve can be used to reduce valve noise in order to reduce the flow rate step by step. As we often use the shut-off valve, the flow direction is low and high, so when the fluid flows through the valve cavity, a low-pressure high-speed zone is formed under the control valve flap (ie, the turbulent flow zone), and bubbles are generated. After the valve flap is formed, a high-pressure low-speed region is formed, and the bubbles are successively crushed to generate cavitation noise.
According to the above analysis, it can be seen that the pipeline noise and the valve noise are related to the state of the liquid flow, in other words, the pressure difference and the flow rate.