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Flowmeter often encounters problems

First, the misunderstanding of normal operation
The meter itself is normally measured accurately, but it is misinterpreted as a fault of the flow meter due to the influence of other equipment. In this case, specific analysis is required, for example:
1. Misunderstanding caused by parallel operation of pumps
A unit uses several pumps to deliver liquids in parallel. Each pump is equipped with a flowmeter downstream. During operation, most of the meters indicate that the flow rate is significantly reduced, which is mistaken for the inaccurate measurement. The reason for this phenomenon is that the pump heads are inconsistent, and the high-lift pump will suppress the output of the low-lift pump, making the meter indication smaller.
2. Check the flowmeter with the rated flow of the pump.
If the rated flow on the pump nameplate is compared with the flowmeter, if it is inconsistent, the instrument is considered to be inaccurate, which is obviously a misunderstanding. The pump delivery volume varies with the operating load characteristics. The rated flow rate of the pump nameplate is the flow rate under a certain specified condition, with a tolerance of 4%-8%. In fact, when the pump is running, the pump is different due to the different piping loads. The output is often offset from the nominal flow on the nameplate. However, it can be cross-referenced in daily operation. If there is abnormal variation in the difference from daily operation, the pump, flow meter and piping load should be fully inspected.
3. Leakage of bypass pipe valve
If the flowmeter has a bypass pipe, the leakage of the bypass pipe shut-off valve will inevitably reduce the meter reading, and the valve leakage is not easy to detect, and it is often mistaken that the meter measurement is not accurate.
4. Differences in readings between the master and the sub-table
(1) Usually the instrument operation design runs around 80%-90% of full flow. If half of the meter is deactivated, that is, half of the branch flow is closed, it is possible that the total meter will run near 40% of the full flow. If the basic error of the meter is 1.5%, the maximum difference between the total table and the remaining sub-tables is 5.6% of the measured value. If fewer parts are running, the total table and the readings of the sub-tables will increase. Big.
(2) The value of the saturated steam manifold flowmeter installed at the boiler outlet is very different from the value of the workshop sub-meter because of the change in steam dryness. The dryness of the boiler outlet is generally higher (0.95 or more), and on the sub-meter, the dryness is seriously reduced due to the poor transportation and poor insulation of the pipe (the condensation is more), and some of the steam condenses into water, increasing the measured value. Poor.
It can be seen from (1) and (2) that the difference between the readings between the master meter and each meter table will exceed the basic error of the meter, and the meter cannot be considered inaccurate.
Second, common installation and use error cases
Although the installation technology of vortex street and electromagnetic flowmeter is not very complicated and precise, the following two kinds of bad installations often appear in actual use. The first type of bad installation is caused by the operator's carelessness; the second type of bad installation is the pipeline system. Objective conditions are limited (such as insufficient length of the upstream straight pipe), which is caused by bare installation. The installation of many new construction instruments is undertaken by the installation company. The installation work is handed over to the ordinary plumbers. There are no full-time instrument personnel. Due to the lack of knowledge of flow meters, they often fail to operate and cause installation failures. This is not uncommon.
Case number one
The gasket is not aligned with the center, partially covering the flow area and destroying the normal distribution of the flow rate
A unit uses a batch of DN50mm electromagnetic flowmeter to measure the flow of industrial water. In actual use, it is found that the flow difference of one of the instruments is as high as 4%-10%. The reason for the inspection is that the inlet gasket of the electromagnetic flow sensor is eccentric. .
Treatment: Raw new positive gasket, flow measurement returned to normal.
Case 2
The inner diameter of the process pipe is too different from the inner diameter of the flow meter
A unit uses a DN100 mm vortex flowmeter to measure compressed air. The actual error is found to be as high as 3.5%. The process diameter is 97mm, which exceeds the range of 0.98DN<=D<=1.05DN specified by the vortex flowmeter. The measurement error is too large.
Processing: Perform coefficient correction
Case three
When measuring high temperature steam, the vortex control head often breaks
A unit uses a vortex flowmeter to measure the saturated steam flow. The working pressure is 0.8Mpa. The process requires intermittent steam supply. Therefore, the valve is frequently opened. When operating, the valve opens too fast and generates additional impulses to damage the probe.
Handling: Slowly open the valve to full opening, it takes about 2 minutes
Case four
Leakage at the seal between the sensor and the flange
A unit uses a DN40mm vortex flowmeter to measure the water flow and mounts the sensor on a very long overhead pipe. After a long time, the seal between the sensor and the flange leaks due to the sagging of the center of gravity of the sensor.
Handling: If it is necessary to install, the pipe fastening device must be installed separately at the upstream and downstream 2D of the sensor.
Case five
The vortex flowmeter is installed at the outlet of the air compressor, causing the meter to display irregular
A unit uses a DN150mm vortex sensor to measure compressed air. The vortex sensor is installed on the pipe between the air compressor and the steam storage tank. Although the length of the straight pipe before and after is guaranteed, the vibration at the outlet of the air compressor is too Large, the flow velocity distribution is irregular, and the meter cannot work.
Handling: Install the sensor behind the gas tank and ensure that there is a certain straight pipe
Case six
Damage to the amplifier board during welding of the flange or pipe
When a vortex sensor is installed in a certain unit, the sensor is not removed during the welding process, and the sensor is still on the pipeline. The amplification plate is damaged due to too high welding temperature.
Handling: The sensor must not be on the pipe during the welding process.
Case seven
The flanges on both sides of the sensor are not parallel and leak
When a unit is installed, due to the high-altitude operation, the pipe will sag after being cut open, and the pipe will be difficult to level after the flange is welded. The sensor will be installed to cause the flanges on both sides to be uneven, and leakage will occur during operation.
Handling: Use a pipe or bracket to keep the pipes parallel during installation.
Case eight
Connect the grounding wire to the common grounding wire such as motor and transformer to introduce interference.
When a unit is installed with a sensor, it is convenient and convenient for the figure. The sensor grounding wire is connected to the common grounding line. When the instrument is put into operation, there is a large interference and the display value is unstable.
Disposal: Use a single point grounding to insert the angle iron or flat iron into the underground mud about 1 meter deep to ensure that the grounding resistance is less than 10 ohms.
Case nine
When the sensor is mounted on a vertical pipe, the flow direction is wrong.
When a unit is installed with a vortex sensor, the vortex sensor is installed on a vertical pipe to measure the liquid flow. The direction of the fluid is from top to bottom. In actual use, the meter displays unstable. Since the fluid is from top to bottom, the weight of the fluid itself will be Additional errors are added to the sensor measuring element.
Handling: Align the direction of the sensor with the direction of fluid flow.
Case ten
The length of the straight pipe before and after the sensor is not enough, causing measurement errors or even the meter cannot work.
When installing a vortex sensor, a unit did not fully consider the piping system structure, ignoring the two 90o elbows with different planes upstream, which will produce velocity distribution distortion, secondary flow and vortex flow blockers. When the instrument is actually running, Serious interference has occurred, resulting in malfunction.
Treatment: Ensure that there is a straight pipe section >=40D upstream of the sensor.

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