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On - line calibration method for large - diameter water flowmeters

Date:2020-01-08 source:

The large diameter water flowmeter with nominal diameter greater than DN300 is widely used in the water supply of reservoir and water supply company. This kind of flowmeter is used for trade settlement, which involves a large quantity and amount of money. According to the provisions of the metrology law, it must be tested compulsivly.Since most of the large-diameter water flowmeters are HKE electromagnetic flowmeters or the inserted HKU ultrasonic flowmeters, the instrument structure is mostly connected with flanges or inserted pipes, and disassembly is allowed only when the flow is stopped, and disassembly and transportation are very troublesome;On the other hand, the working conditions of such flowmeters differ greatly from those verified in the laboratory, and the accuracy deviation of flowmeters cannot be determined.Therefore, it is necessary to calibrate large diameter water flowmeter on line.

On - line calibration method for large - diameter water flowmeters

1. Observe the installation location of the tested flowmeter and select the correct installation location of the standard flowmeter

Although there is a clear requirement of various flowmeter installation, such as straight section of long enough, but in practice, the site conditions can not meet the installation requirements are common, the condition of limited by installation conditions, the straight pipe is not long enough, valve, elbow and pump flow meter too close, flow meter diameter and pipe size inconsistent reason caused by error is very large.Therefore, in many cases, it is impossible for the tested flowmeter to reach the calibration accuracy of the laboratory, which is why the actual calibration error of the tested flowmeter is much larger than that of the laboratory calibration error.Therefore, field real flow calibration becomes more important.

The correct installation location of the standard flow meter: away from upstream and downstream disturbance sources to avoid the most harmful vortices. The transducer should be installed at the position of 10D in upstream and 5D in downstream.The upstream disturbance sources are spiral welded tubes and various stop-flow fittings, such as elbows, profiled tubes, branch tubes and valves.Upstream disturbance sources may cause velocity distribution distortion and rotational flow.Downstream side disturbances, such as those caused by bend valves, can propagate upstream, affecting distances of several diameters.When there are pumps, valves and other equipment upstream, the length of the straight pipe section should be 30D.

2. Set the pipe measurement parameters of the standard and tested flowmeters correctly

The on-site pipeline is very complicated, not necessarily the standard pipe, but most of the installation of the tested flowmeter fails to measure the on-site pipeline parameters, so it is customary to install according to the standard pipe.Because the incorrect setting of pipeline parameters will bring great error to the tested flowmeter, it is necessary to measure pipeline parameters on site.

Input the correct pipe parameters, and measure the outside diameter of the field pipe with the outside diameter ruler. The measuring error of the outside diameter is ±1mm.The thickness of the pipeline wall was measured with the HKU ultrasonic thickness gauge, and the measurement error was ±0.1mm.Select the correct pipe material input, pay special attention to whether there is lining or scaling phenomenon, if there is, input lining parameters (1 ~ 2) mm.

Check the parameters of the flowmeter under test. If the original pipe parameters are not consistent with the actual measured parameters, input the measured parameters to prevent unnecessary human error.

3. The transducer installation of the standard flow meter shall adopt the v-type method as shown in figure 1

The veracity of measurement can be improved by increasing the sound range by one time with v-type method.In addition, when installing with v-type method, two transducers are installed on the slide rail of a fixture, so it is easy to install two transducers on the center line of the pipe, so as to ensure the sound path on the pipe diameter plane as far as possible.However, when installing with the z-method, the transducer installed on both sides is not easy to realize the sound path on the pipe diameter plane, which is easy to bring measurement error.

4. Severe scaling on the inner wall of the pipeline

If no signal can be found in the v-type transducer installation of the standard flow meter, please install the transducer by the z-type method as shown in figure 2.

5. Eliminate zero drift

Using butter or vaseline with good acoustic performance as coupling agent of transducer, standard flow meter adopts static or dynamic zero adjustment when measuring flow signal to eliminate zero drift.

6. Check the display value and signal value of sound velocity of standard flow meter

If there is a big difference between the sound speed display value and the actual water temperature, please check the setting of pipe parameters again. Most of the reasons are that the pipe parameter setting is not consistent with the actual pipe parameters, resulting in measurement error.If the signal value is too weak, check again if the transducer is installed correctly.If there are abnormal signals, check for interference sources and avoid strong magnetic fields and vibration interference.

7. During the verification process, the pipeline velocity should not be less than 0.2m/s

In short, strictly follow the instructions for the use of the flow meter for the correct installation, try to avoid the unnecessary error caused by human error operation.

The HKU ultrasonic flowmeter is used as the standard table for quantity value transfer, which not only realizes the periodic verification of the large-diameter water flowmeter, but also reduces the inspection fee of the enterprise, protects the interests of both parties, maximizes the paid use of water resources, and scientifically and reasonably protects water resources.


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