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About the evaluation of the influence flow

by:Sure     2021-08-14
1 Overview There are two types of influences to be considered, one is to affect the liquid in the primary device of the electromagnetic flowmeter, and the other is externally caused. Internal influences include changes in liquid temperature, flow rate distribution, and changes in liquid conductivity. External influences include changes in temperature, humidity, and atmospheric pressure, as well as changes in power supply current, voltage, or frequency. Unless otherwise specified, these 1 Overview    should consider two types of influencing quantities, one is to affect the liquid in the primary device of the electromagnetic flowmeter, and the other is externally caused. Internal influences include changes in liquid temperature, flow rate distribution, and changes in liquid conductivity. External influences include changes in temperature, humidity, and atmospheric pressure, as well as changes in power supply current, voltage, or frequency. Unless otherwise specified, these influence quantities shall be assessed individually. The method is to determine the deviation between the output of the flow meter and the result obtained under the reference condition. During the test, other unchecked conditions should remain unchanged under the reference value. Unless otherwise specified, the primary device and the secondary device connected to the public power supply should be evaluated under the equivalent liquid at a flow rate of 1 rn/s. If the speed range set by the secondary device can be adjusted, it should be set at 2M/s. The output (analog) load impedance should be the maximum value recommended by the manufacturer.  2 Internal influence 2.1 Liquid temperature The influence of liquid temperature changes should be measured at different liquid temperatures, while the ambient temperature of the flowmeter remains unchanged. The temperature range should be significantly different from the temperature under the reference test conditions in order to clearly show the effect of temperature, and there should be sufficient time in each case to reach a steady state condition. During the test, the lower limit of the range and the steady-state change of the range caused by the change in liquid temperature should be measured. The influence should be expressed as a percentage of the output range. The specific details of the test must be agreed by the manufacturer (see 4.3). 2.2 Liquid conductivity, the measurement of liquid conductivity change should be measured on 3 different conductivity, including the limit value specified by the manufacturer. Its influence is expressed as a percentage of the output range. Note: This test must be considered only when the conductivity of the liquid is less than 5rns/m (50scm). 2.3 Flow velocity distribution When the flow velocity distribution on the electrode plane is very different from the flow velocity distribution during initial calibration, the calibration of the electromagnetic flowmeter may show an offset. The configuration of the upstream piping components of the primary device is one of the factors that produce the characteristic velocity distribution. The test proposed below is recommended to study the flowmeter's response to the velocity distribution generated by some very common pipeline components, which are easy to find in practice. In order to form the actual flow velocity distribution profile immediately upstream of the tested flowmeter, it is recommended to conduct flow mapping according to the requirements of ISO3966. The results of each test point described in 5.2.3.1 to 5.2.3.3 can be expressed as the percentage deviation from the reference flow at that point. The flow rate under the reference condition should generally be obtained from the flowmeter installed on a long straight pipe section of the same caliber by using online wet calibration. 2.3.1 The test of the reducer should firstly install the reducer with a coaxial pipe section directly on the flange on the upstream side of the flowmeter, and then the reducer should be 5DN upstream from the electrode plane. place. The diameter of the reducer should be gradually reduced from 2DN to 1DN. For smaller diameter pipes, although it is recommended to use a 3DN length reducer, if the parties concerned agree, it is possible to use parts with other lengths available on the market, but its size should be measured and recorded in the test report. middle. The inner diameter of the outlet of the reducer should be compared with the inner diameter of the inlet of the flowmeter, and each position should be measured at least at two mutually perpendicular positions. The purpose of this test is to ensure that the inner diameter at the outlet of the reducer matches the inner diameter at the inlet of the flowmeter within the allowable tolerance range (see 4.2.1). The test readings at each recommended test point within the flow range of the flowmeter should be recorded (see 4.9). In special occasions, if an eccentric reducer (or a stepped shrink tube) is often used in practice, it may have to be used for testing. In this case, the structure of the reducer should be explained in detail, and the size should be measured and recorded in the test report. 2.3.2 The upstream valve should be subjected to a series of tests. First, install the gate valve at the upstream of the pole plane 2DN (if the flowmeter is longer than 4DN, install it at the flange next to the upstream side of the electromagnetic flowmeter), and then install it on the upstream side of the electromagnetic flowmeter. Upstream from the electrode plane 5DN (see Figure 9). In these two cases, the installation state of the two gate valves should be tested: a) The dashed line of its mandrel and the electrode axis are perpendicular to each other. b) Its mandrel and electrode axis are parallel to each other (referring to the case with a pair of horizontal electrodes). These two test configurations should be studied. First, use the upstream valve as a simulator to disturb the liquid flow to control the flow, and then fix the upstream valve at 25% and 50% closed positions respectively. In the latter arrangement, the downstream valve in the pipeline of the verification device is used to control the flow. Obtain test results. During these tests, the pipeline pressure at the valve should be kept high enough to avoid the risk of cavitation. In all test schemes, the test points should be determined based on at least four positions. These four positions are caused by the water in the terminal box or the dampness of the excitation coil and the decrease of insulation. When the water and dew are wiped off, the terminal block of the junction box is blown dry with a hair dryer, the resistance of the excitation terminal to the ground is restored from 5 to 6MQ to several tens of ohms, the offset zero point immediately returns to the zero position, and the instrument operates normally. The reason is that the insulation of the excitation coil circuit to the ground is reduced, so that a larger insulation resistance and the signal internal resistance to the excitation voltage are added to the electrode to form a larger common-mode interference signal, and the converter preamplifier The ability of the mode rejection ratio is limited, so that the converter has an output at the zero point.
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