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The speedometers that we normally use, regardless of the size of the caliber, and whether they are impellers, electromagnetic or ultrasonic water meters, have encountered such martyrdom more or less. The user does not actually use water, but the water meter has a phenomenon of cumulative flow. We call this the water meter rotation or idling.
Faced with the rotation of the water meter, the user is resentful, the water supply department is anxious, and the meter is accompanied with care. When we talk about the rotation of water meters, what are we talking about?
Is it talking about user experience?
The user said, I do not believe this reading.
Is it about the quality of water meters?
The Water Secretary said, "What form is this for you? Don't leave the dripping water and go without water."
Is it about measuring the environment?
The meter meter silently thought that the water meter should not be counted unless the water in the pipeline is still.
From the point of view of hydraulics or fluid mechanics, the flow state of the liquid in the pipeline can be divided into two major categories: stability and instability. When the flow state changes from a stable state to another steady state, the unstable flow state between them is called transitional process or transient flow. In the water industry, it is commonly called water hammer. Under the influence of water hammer, the water meter is disturbed by unstable transient flow. In contrast, if the user is using water, the size, direction, and pressure of the flow in the pipeline will not change over time. We call this flow state constant flow.
Formation of rotation of water meter
The water hammer pressure wave propagates in the pipeline at a sound speed a. The propagation speed is about 1000 m/s, and the generated high pressure may exceed 10 MPa, which is ten times the maximum allowable working pressure of the water meter; and its duration is only a few hundredths of a second. Because the pressure wave velocity is so fast, the pressure gauge on the pipeline cannot show the instantaneous elastic pressure fluctuation in the pipeline system at all, and the water meter is often too late to measure the rapid flow changes.
When rotation occurs, the transient flow oscillates back and forth in the pipeline, and the pressure, velocity, and flow direction change rapidly. The forward, reverse, oblique wear, and whirlpool flow through the water meter. Transient flow pressure and flow rate exceeds the rated working conditions of commonly used water meters in many cases. Not only the pressure is excessive, but also the flow rate may exceed the standard, which is smaller than the minimum flow rate of the water meter Q1, or greater than the overload flow rate Q4. For example, a common 20 meter has a range of 50 liters/h to 5 m3/h and a sensitivity of 6 liters/h, so less than 6 liters/h and more than 5 m3/h of flow are not recorded, and flow rates below 50 liters/h are also Rough record, no accuracy requirements.
Set the downstream of the water meter is closed end, there will be no water outflow. The upstream of the water meter is an oscillation flow environment caused by the superposition and reflection of various transient pressure waves. When the water meter is just in the transient flow environment, rotation is inevitable. Due to the limitation of the response time, sensitivity and effective range of the water meter, the flow of these transient flows is only partially measured by the water meter, and the accumulated flow is accumulated. The cumulative flow is either positive or negative, which we know is mostly positive. Because the same amount of water in the case, the general water meter forward measurement is always more than the reverse measurement; but also because only the positive cumulative flow will cause cost disputes.
The reason for the rotation of the water meter
When the general water supply system is designed properly, the transient process is within an acceptable range. The water supply pipe network, including the secondary water supply system, has a stable supply pressure, and the maximum and minimum pressures are within the specified limits. The reason for the rotation of the water meter may be:
• Improper installation of the water meter is precisely where the transient flow is active and no protective measures are taken;
· A stable water supply system may cause rapid fluctuations in piping pressure due to sudden changes in boundary conditions, resulting in sudden changes in traffic conditions, such as the addition of a new branch line to the main pipeline, new pumps in the vicinity, and large underground construction.
· When the design of the water supply system itself is not standardized enough, or many details such as exhaust valves, pressure-regulating valves, pressure reducing valves, check valves, etc. are lost during construction, or because the straight pipe section is not enough, the quality of the pipe itself, etc. May cause water meter rotation.
· The water supply pipe network that generates rotation needs to correct its system layout or system parameters, or install various control equipment and analyze the system again. Repeat this process until it is satisfactory. There are many causes of rotation, but when the phenomenon of rotation is generally found, the water supply system layout is not easy to change. We try to improve the system through a one-way valve, a pressure-regulating valve, an exhaust valve, and an anti-rotation valve to maintain a stable measurement environment for the water meter. , Reduce pressure fluctuations and transient flow.
How to prevent rotation
The reason for the rotation is transient flow. Then we can think of ways to eliminate the transient flow, and we cannot eliminate it by weakening it. For transient flow with particularly large pressure and flow, beyond the metering range of the water meter, we can only reduce the amplitude of the pressure wave, reduce the flow, and then record the exact amount of the process, so that the accumulated flow of the water meter is accurate. For extremely small transients, where the frequency of occurrence is high and there is no way to avoid it, and the accurate measurement cost is too high, we can choose to ignore it: reduce the sensitivity of the water meter so that small transients have no effect on the measurement.
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Does the water meter in your house rotate? Xiao Bian to tell you why
[Chinese Instrument Network Encyclopedia Knowledge] If you come from the water supply department, this scene is not familiar with: in the front of the watch valve, the valve is closed after the state of the watch, the water meter accumulated a few tons of water per day, during this period, the user There is no actual use of water and therefore the amount of water and costs for this period of time are not recognized, and water meters are required for inspection.