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Misunderstanding of the normal operation of the flowmeter

Often users report that the meter is not measuring correctly or is not operating properly, but the site survey often finds that it is not a fault of the instrument, but a misunderstanding of the fault caused by other causes of the system. List the following examples.

1. Bypass tube leak
For ease of maintenance, the flow meter is usually equipped with a bypass tube, and the bypass tube shut-off valve leakage must reduce the meter reading only. The micro-leakage of the valve is not easy to detect, and it is often mistaken for the meter to be inaccurate. To ensure accurate measurement of trace leaks, the bypass valve can be closed as shown in Figure 17.3.
Some accounting or consumption can be used to save the rewards of the measurement site, but also on the bypass valve to falsify, artificially not closed, you can take precautions such as the line seal on the bypass valve hand wheel.

2. Check instrument flow with pump flow
If the operating personnel are suspicious of the flow meter, it is often compared with the rated flow rate of the “specified performance point” on the pump nameplate (or the flow reading of the pump's typical indenter-flow characteristic curve). If the meter is inconsistent, the meter is inaccurate. It is a misunderstanding. It can be seen from Fig. 19.10 that the pump delivery flow rate is determined by the intersection of the pump characteristic curve and the piping load characteristic curve, which varies with the operating load characteristics. The rated flow rate on the pump nameplate is the flow rate under a certain specified condition, which is not consistent in the case of large Shao. In addition, the rated flow rate of the pump is also allowed to have a tolerance of 4%-8%. The pressure characteristic curve of the indenter of each pump of the same specification is also different from the typical curve, and the output flow rate is also different. Even with the measured flow characteristics of the pump, the flow value may have an error of 2% to 3.5%. Therefore, the flow rate of the pump cannot be used to determine the basis for the accuracy of the flowmeter. But it can be cross-referenced in daily operation, if two values appear
Test pipeline.
Hardware
When there is an abnormal change in the difference from the daily operation, the pump, instrument, and piping should be checked as “signs of failure”. Below is an example.
Two water pumps of the same specification are delivered to two pipelines, respectively equipped with DN60Omm electromagnetic flowmeters, as shown in 19.10. The operating personnel of the water plant check the meter reading from the rated flow on the pump nameplate, and the meter A is connected to the meter A (ie, the valve C is closed), the instrument error is +10±1%, and the pump B is the B meter error of 5%. Both instruments are inaccurate. According to the above reasons, the operator can not accept it for a while, and the service personnel of the instrument factory use the favorable conditions of the valve G to test the flow of the pump A through B meter and the pump B through the A meter, and obtain the similar data. The two flow meters measured the same amount of water delivered by the same pump, which proved that apart from the difference in the load of the pipe network, the performance difference between the two pumps was mainly.

3. Misunderstanding caused by parallel operation of pumps
In order to save energy, multiple pumps are often connected in parallel according to different combinations of system load flow. There are two kinds of misunderstandings that are representative of the following two situations.

(1) Parallel operation with large head difference is mistaken for mutual interference between flow meters
A plant in Jilin uses several pumps to deliver liquids in parallel. Each pump is equipped with an electromagnetic flowmeter downstream, and then the manifold is found. The single pump operation (or several of them in parallel) is normal, but one pump is added and the human pipe system is added. The original instrument running the pump indicates that the flow rate is significantly reduced, and even the reverse flow phenomenon is indicated. The operating personnel believe that the electromagnetic flow of the specific pump
The meter interferes with other instruments in operation. After checking that the instrument is normal, the reason for this phenomenon is that the lift of the pump is much higher than the other, which causes the output of the depressed low-lift pump to decrease or even reverse.

(2) Misunderstanding of changes in output of multiple pumps of the same specification in parallel
A water plant in Luoyang, Henan Province, shown in Figure 19.11, A and B pumping rooms, each equipped with 7 pumps of the same specification, each collected into the DN700mm main pipe output. Each of the main pipes is equipped with an electromagnetic flowmeter, and the two main pipes downstream of the flowmeter are connected with a connecting pipe and a gate valve, and the gate valve is normally opened. Try to start different pumps in the two pumping stations and get the readings on the flowmeter as shown in Table 19.E. The number of pumps in the pump rooms A and B is reversed, and the readings are similar. The operation personnel of the water plant think that the flow meter is not linear, and the indication is low when the flow rate is low. It seems that the number of pumps to be increased and the amount of water to be pumped should be increased proportionally. Actually, this is a misunderstanding and is caused by the parallel operation of the pump. A brief explanation is given below.
Pump parallel operation example.
Hardware
Figure 19.12 shows the head-flow characteristics of eight sets of pumps of the same specification running in parallel and the head-flow characteristics of the piping load. PI is a single pump head-flow output characteristic curve, P2, P3, ....., P8 are characteristic curves of two, three, ..., eight pumps operating in parallel. The load line LA has a large water consumption, and each user has a large valve opening. The LB line user has closed the valve and the water consumption is small.

When the eight pumps are operated in parallel, the load characteristic curve and the pump characteristic curve are at point a, which is the pump head running and water delivery, and the water delivery capacity is 100%. At this time, the water volume of the pump room of A and B is 5D % o (The water delivery in the pump room of A and B in Table 19.2 is not completely consistent. It may be that there is some difference in performance between the pumps of the same specification and the load characteristics of the two pipes are different. The flow QA and QB are not completely equal, and the gate valve has flow q Caused by other factors). When the test number 2 is used, 7 pumps are opened. It is assumed that the user's valve opening is not adjusted in time, the load characteristic is still OA line, the water delivery is 95%; the test number 3 is 6 pumps, the water delivery is 87%. The pumping capacity of 4 pumps in the A pump room is 58%, and the pumping capacity of the 2 pumps in the B pump room is 29%. The set of data on the above operation is consistent with the trend of the flow change of the different number of pumps in Table 19.2, which indicates that the flow rate of the two pump rooms obtained in the test is a normal phenomenon of parallel operation.

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