At present, there are many kinds of flowmeters in giant hydropower stations. There are more than 1900 flowmeters in total. According to the measuring principle of flowmeters, there are electromagnetic, differential pressure, thermal conductivity, and ultrasonic. The measuring medium mainly includes oil and water. The oil flow is mainly used to measure the upper guide, derivation, turbine oil pipe flow of the water guide bearing, and the oil flow of the high-pressure oil jacking system of the unit. The water flow is mainly used to measure the cooling water (river water), the main shaft sealing water (clean water), the generator pure water (non-conductive medium) flow of each bearing, and the main transformer of the unit.
Because the flowmeter is matched with units from different host manufacturers, with different specifications and models, there are many problems such as unclear functions of the flowmeter, unreasonable configuration, and installation that does not meet the product installation requirements, resulting in deviation in the accuracy of flow detection, which cannot meet the requirements of unit flow detection. For example, the straight pipe section of the electromagnetic flowmeter pipeline for measuring the cooling water of the unit is short, which causes the pipeline cannot to be measured in full.
The ultrasonic flowmeter for measuring the external circulating oil of the water guide of the unit is installed at the rear end of the oil filter. As the oil passes through the filter and bubbles are generated, the ultrasonic flowmeter cannot detect the flow for a long time; The thermal conductivity flowmeter is used to measure the cooling water of the derivation bearing of the unit. As the probe of the flowmeter has been immersed in river water for a long time, scaling occurs, resulting in the measured value of the flowmeter being far less than the actual value.
In this paper, the measuring principle of the main flow meters is analyzed in depth, and the function selection and correct installation methods of various flow meters are summarized, so as to create an optimized flow application scheme suitable for giant hydropower stations.
Probe into the measuring principle of the flowmeter
The measuring principle of the electromagnetic flowmeter is based on Faraday's law of electromagnetic induction. When the measured conductive current body flows in the magnetic field along the direction of the vertical magnetic line of force and cuts the magnetic line of force, an induced potential will be generated on the electrodes symmetrically installed on both sides of the flow pipe, which is proportional to the flow rate of the fluid (see Figure 1). The flow is calculated by the following formula:
Where, E - induced potential;
K - coefficient related to magnetic field distribution and axial direction;
B - magnetic induction intensity;
V -- average flow rate of conductive liquid;
D -- Electrode spacing (inner diameter of flowmeter tube).
The electromagnetic flowmeter has small pressure loss, a large flow measurement range, and quick response. The limitation of the application is that only the liquid flow of the conductive medium can be measured. In the hydropower station, the cooling water (river water) and main shaft sealing water (clean water) of each bearing and main transformer of the unit can be measured. Because the cooling water of each bearing of the hydropower station has the function of forward and reverse water supply, the electromagnetic flowmeter with forward and reverse measurement function can be selected, and because there are many impurities in the river water, it is easy to cause scaling and pollution of the detection electrode of the flowmeter, It will greatly affect the measurement, so the flowmeter with electrode cleanable can be selected.
Thermal conductivity flowmeter
The thermal conductivity flowmeter adopts the heat exchange principle. The probe of the flowmeter is equipped with a heating module and a heat-sensing module. The flow rate of the fluid is detected according to the proportional relationship between the heat conduction of the flow indicator and the flow rate of the measured fluid (see Figure 2).
Figure 2 Detection principle of thermal conductivity flowmeter
A thermal conductivity flowmeter is generally a plug-in installation, which is applicable to a wide range of flowmeter quality, with a range ratio greater than 10:1. The limitation of the application is that it is difficult to ensure measurement accuracy. As the measured thermal conductivity is polluted by scaling, the impact on the measurement accuracy is increased. In the hydropower station, the oil flow of each bearing of the unit can be measured as the basic flow indicator switch for oil flow in the pipeline. Because the measurement accuracy cannot meet the requirements, it is not recommended to use the oil flow conditions such as the logic control of the circulating oil pump, nor is it suitable for measuring the high-speed oil flow of the high-pressure oil system of the unit.
The measurement methods of an ultrasonic flowmeter can be divided into the time difference method, Doppler effect method, beam offset method, correlation method, and noise method. At present, the time difference method and the Doppler effect method are the most used.
The principle of the TDOA detection method is that the ultrasonic pulse travels back and forth between the sensors on both sides of the upstream and downstream. When there is fluid flow, because the signal propagation time in the downstream direction is shorter than that in the upstream direction, the velocity and direction of the fluid can be detected according to the proportional relationship between TDOA and fluid velocity, as shown in Figure 3.
Figure 3 Principle of ultrasonic flowmeter measurement
Figure 3 and Formula 2: D -- Instrument diameter;L -- sound path;
Vm -- medium velocity;Cm -- sound velocity in the medium;
α—— The included angle between sensor A/B and the horizontal centerline of the pipe;TB-A -- time from sensor B to A;
TA-B -- Time from sensor A to sensor B.
The ultrasonic flowmeter is a non-contact measurement method. The sensor does not contact the measured medium, has no pressure loss, has a long service life, and requires less equipment maintenance. It can be either a fixed installation of pipes or portable measurement (it must be metal pipes, plastic pipes, and pipes with other sound transmitting materials). It can be used for flow measurement of various water and oil pipelines in hydropower plants. The ultrasonic flowmeter is not applicable to the environment where the flow medium in the measurement pipeline has bubbles, so it is not suitable to measure the oil filter pipeline with fine mesh, nor is it applicable to measure the high-speed oil flow of the high-pressure oil system of the unit.
Differential pressure flowmeter
The measurement principle of the differential pressure flowmeter is to equip throttling components in the middle of the pipeline of the flowmeter. When the full pipe flows through the throttling components, the flow beam will form local contraction, the flow rate will increase, and the static pressure will decrease, resulting in a certain pressure difference before and after the throttling components. According to the proportional relationship between the fluid velocity and the differential pressure value, the fluid velocity is detected. Figure 4 shows the measurement principle of the differential pressure flowmeter.
Figure 4 Measuring principle of differential pressure flowmeter
1 - Throttle element; 2 - Impulse pipeline; 3-3-valve manifold; 4 - Differential pressure gauge
The differential pressure flowmeter is the most widely used, with stable and reliable performance and long service life. Its limitation is large pressure loss, which is not suitable for a small flow environment. Its range is narrow, only 3:1~4:1. It is especially suitable for large oil flow measurement of circulating oil pump pipelines in hydropower plants, but it is also not suitable for accurate measurement of oil flow analog quantity of tens of MPa high-speed oil flow in the high-pressure oil system of units.
Piston-type flow switch
The detection principle of the piston-type flow switch is that when the fluid flows through the pipeline in the indicated direction, it pushes the piston supported by the spring and installed with a permanent magnet to drive the reed pipe switch magnetic switch to act on the set flow and output the switch signal.
The piston-type flow switch is widely used for measuring water, gas, oil, and other media due to its large setting flow range and convenient adjustment. It has the advantages of low-pressure drop, high-pressure resistance, etc. The state only depends on the flow, but has nothing to do with the pressure, and is no longer sensitive to the peak value of the pressure. It can solve the high-speed oil flow detection difficulty of the high-pressure oil of the unit in the hydropower plant.
Installation standard of flowmeter
At present, in addition to unclear functions and unreasonable configurations, the main reasons for the operation of flow meters in huge hydropower stations under the low economy and added value are that the installation does not meet the product installation requirements and relevant standards, resulting in the high failure rate of flow meters and inaccurate measurement.
Selection of diameter and range of flowmeter
First, the flow meter needs to determine its path diameter and measurement range, that is, determine the flow rate range of the fluid in the sensor. The selection of the range of the flow meter has a great relationship to improve the reliability and accuracy of the flow meter. Select the full range according to the principle of not less than the expected maximum flow value. The normal common flow should not exceed 50% of the full range so that higher measurement accuracy can be obtained.
The flow meter usually uses the same or slightly smaller diameter as the process. When the flow is selected, the diameter is determined according to different measurement objects and the flow rate in the flow meter measurement pipe. For the flow rate of the fluid measured by the flowmeter, the relationship between the flow rate of the fluid in the pipeline and the pressure head loss shall be considered. After the flow rate is determined, the diameter of the flowmeter can be determined according to the following formula:
Where: d -- diameter of flowmeter, mm;
Q -- Volume flow, m3/s.
Flow meter groundingThe correct and reliable grounding of the flowmeter is critical to the normal operation of the flowmeter:
(1) Zero potential for signal source reference;
(2) Connect the measured fluid with the reliable grounding body;
(3) Meet corresponding safety requirements (such as lightning stroke);
(4) Anti-external interference measures (such as shielding).
The distance from the grounding point to the sensor shall be as short as possible, and it must be directly connected to the reliable grounding body. It cannot be connected to the grounding point of other equipment. The grounding resistance shall be ≤ 10 Ω. The shell of the electromagnetic flowmeter, the cable shield wire, and both ends of the measuring pipeline must be reliably grounded separately. It must not be connected to the public ground wire such as the motor or the water supply and drainage pipes.
The accurate grounding is shown in Figure 5;
If the converter part of the flowmeter has been grounded through the cable, no additional grounding is required to avoid interference due to potential differences. Grounding method for flowmeter equipped with grounding ring: if the process pipe is made of insulating material, it must be equipped with a grounding ring for reliable grounding to stabilize boundary conditions and improve measurement stability and accuracy, as shown in Figure 6.
Installation pipeline standard of flowmeter
The accurate measurement of the flowmeter must ensure the full pipe measurement state. In the hydropower station, the diagram of the common wrong installation position of the flowmeter pipeline that causes the non-full pipe is shown in Figure 7. A in the figure shows that the flowmeter is installed directly in front of and behind the gate valve, B in the figure shows that the flowmeter is installed on the long horizontal pipeline of the vent pipeline, and C in the figure shows that the flowmeter is installed at the height and vertical discharge section of the vent pipeline. These three types will cause the medium in the pipeline to be not full, thus affecting the measurement accuracy.
Figure 7 Flow meter pipeline installation error diagram
It is specified in GB/T 18659-2002 Measurement of Conductive Liquid Flow in Closed Pipelines - Performance of Electromagnetic Flowmeters that upstream 10D (D is the diameter of the flowmeter) and downstream 5D.It is stipulated in DL/T 1107-2009 Basic Technical Conditions for Automation Components of Hydropower Plants that:
(1) Mechanical flow switch: 5D upstream and downstream;
(2) Thermal conductivity flow switch: upstream and downstream 4D;
(3) Electromagnetic flowmeter: 10D upstream and 5D downstream.
The correct and standard pipeline of the flowmeter is shown in Figure 8. A in the figure is to ensure that there are enough straight pipe sections upstream and downstream of the flowmeter; In the Figure 8, B refers to the venting on the long horizontal venting pipeline, with the pipeline part designed to rise slightly at the downstream side of the flowmeter; C in the figure refers to open water supply or discharge, and the flowmeter is installed at the lower or climbing part of the pipeline. There are three installation methods to ensure that the pipeline through the flowmeter is fully filled with liquid, so as to achieve measurement accuracy.
Figure 8 Pipeline installation diagram of Flowmeter Standard Specification