Application of average velocity tube flowmeter in power plants

With the rapid development of the power industry, various new and efficient technologies and processes are constantly being put into use. Advanced processes require high-precision control and measurement. Comprehensive and accurate measurement of the flow of various media such as steam, water, smoke, and wind in boilers and steam turbines is one of the necessary conditions to ensure the safe and economical operation of the unit.

There are many types of flow measurement products, which can be divided into: volumetric flowmeter, differential pressure flowmeter, float flowmeter, turbine flowmeter, electromagnetic flowmeter, vortex flowmeter and mass flowmeter according to the most extensive classification. Various flowmeters have their own characteristics, and no measurement method or flowmeter can adapt to all fluids and flow conditions. Different measurement methods and structures require different measurement operations, methods of use and conditions of use. Each flowmeter has its own advantages and disadvantages. Therefore, on the basis of a comprehensive understanding and comparison of various measurement methods and instrument characteristics, choose the best flow measurement product that is suitable for specific production characteristics, safe, reliable, economical and durable.

1 Comparison of differential pressure flow measurement orifice plate and average velocity tube

1.1 Types of differential pressure flow measurement devices

At present, differential pressure flow measurement is the most widely used in industrial measurement. The mainstream products of differential pressure are throttling type (orifice plate, venturi tube, etc.) and dynamic pressure head type (average velocity tube).

Using the differential pressure principle to measure flow is the most classic and reliable flow detection method in today's engineering field. The differential pressure flowmeter calculates the flow rate based on the differential pressure generated by the flow detection component installed in the pipeline, the known fluid conditions and the geometric dimensions of the detection component and the pipeline. The differential pressure flowmeter consists of a primary device (detection component) and a secondary device (differential pressure conversion and flow display instrument). Differential pressure flowmeters are usually classified in the form of detection components, such as orifice flowmeters, venturi flowmeters, average velocity tube flowmeters, etc. The
greater the pressure difference generated by the flow detection component in the pipeline, the more beneficial it is for the selection and cost of the differential pressure transmitter, and it is also beneficial for the measurement accuracy. However, since the flow measurement device usually selects the contraction ratio (orifice plate, nozzle) according to the flow size, the problem of increased pressure loss will also occur when the differential pressure is increased. The increase in permanent pressure loss in the pipeline requires an increase in pipeline power, which requires high-power fans, water pumps, etc., which increases energy consumption and increases the operating cost of the entire system. The

representative product of the differential pressure flowmeter is the orifice plate, because people are very familiar with its installation, use, and maintenance, and the orifice plate is easy to copy, simple, firm, stable and reliable, has a long service life, and is low in price, so it has been widely used for a long time. However, after the orifice plate is used for a long time, the accuracy will drop sharply, and regular inspection and cleaning are required.

1.2 Measurement principle of average velocity tube flowmeter The

average velocity tube flowmeter replaces the cross-sectional flow velocity of the orifice plate by measuring the flow velocity on a certain diameter. With the continuous development of average velocity tube cross-sectional technology and hole opening technology, its measurement accuracy continues to improve. The Deltaba average velocity tube flowmeter jointly launched by Systec-Controls of Germany and the famous Erlangen University of Germany adopts a streamlined tubular design. The flow detection is to detect the differential pressure generated by the fluid through two rows of unevenly distributed pressure holes on the average velocity tube inserted into the medium pipeline, and obtain an average differential pressure generated by the average flow velocity, and transmit this differential pressure to the transmitter through the interface. Through multiple uneven pressure holes, the average flow velocity in the pipeline can be reflected as realistically as possible. It not only maintains high precision, high strength and a large range ratio in flow measurement, but also greatly reduces the pressure loss of the pipeline. Compared with the use of orifice plates, the economic benefits of using an insertion flow probe are very obvious due to the reduction of pipeline pressure loss. At the same time, the unique probe design can meet the requirements of small pressure loss and large differential pressure at the same time, which is convenient for the selection of transmitters. The

insertion flow sensor (such as insertion turbine, vortex street, electromagnetic, etc.) with a single-point signal detects the flow velocity at a certain point in the pipeline, and then analyzes this detection point through the velocity profile on the empirical chart to infer the average flow velocity. The comprehensive conditions of pipelines and fluids are complex and changeable, and most of them are non-standard velocity profiles. It is inaccurate to use the measured value of a point to replace the average flow rate, which directly leads to poor measurement accuracy and low repeatability.

1.3 Advantages of Deltaba flow probe

(1) It can measure a variety of media, has a wide range of applications, and is minimally affected by physical properties and pipeline characteristics. The caliber range can be from 3mm to 15m; high-temperature and high-pressure probes can work in a steam environment of 650℃ and 28MPa. If you choose 1.4828, X15CrNiSi20-12 type probes, the operating temperature can be as high as 1040℃; you can also choose 1.4539 (A904 alloy, X1NiCrMoCu25-20-5) and HC4 type probes to work in flue gas with SO, SO2 or strong acid and alkali media.


Deltaba flow probes can be used for: ① various gas measurements, including air, humid gas, various flue gases before and after purification, high-temperature flue gas, compressed air, primary air, secondary air, etc.; ② various steam measurements, including steam, saturated steam, superheated steam; ③ various liquid measurements, including water, fuel oil and benzene, etc.

(2) High accuracy, good repeatability, and large range ratio (1:30). The openings of Deltaba flow probes are not evenly distributed. The opening points are based on complex calculations of the calculation software, and the manufacturing parameters are determined according to the pipe diameter, medium and main process parameters. The improvement of the probe measurement accuracy makes the adjustment of the measured parameters more accurate.

(3) The probe pressure tapping hole is designed with anti-blocking characteristics. The maximum probe opening is 8mm. As long as the dust or particles in the medium are not greater than 130mg/m3, no maintenance and purge are required. The installation of a nitrogen purge pipeline can accurately measure high-dust gases such as blast furnace gas, coke oven gas, converter gas, sintering flue gas, etc. in large pipelines.

(4) The streamlined probe cavity design ensures stable measurement signals with small fluctuations. The number and distribution of the front and rear pressure holes are completely symmetrical, so there is no need to worry about reverse installation, and bidirectional measurement can be achieved.

(5) The permanent pressure loss of the pipeline is very small, and the straight pipe section requirements are low. The insertion flow probe can guarantee an accuracy of 1% in a very short straight pipe section (4D in the front and 3D in the back), and adopts a non-contraction throttling design, which is at least 90% lower than the permanent pressure loss of the orifice plate.

(6) The installation cost is low, basically maintenance-free, and can be installed and repaired online.

2 The use of Deltaba flow probe in main steam measurement and heat network transformation

Deltaba flow probe is widely used and has been widely used in 300MW/600MW large thermal power units. The measurement of saturated steam at Jiangsu Tianwan Nuclear Power Plant has further proved its safety and measurement accuracy.

Deltaba flow probe has its unique advantages of micro-pressure loss and energy saving in main steam measurement and heat network transformation. [page]

(1) The accurate measurement of main steam flow is to meet the requirements of unit detection and control, and to adjust the output of the steam turbine through accurate steam measurement. Generally, the main steam flow of 300MW/600MW units is measured indirectly, that is, measuring the steam turbine regulating stage pressure, and deriving the steam work capacity according to the mathematical model provided by the main engine manufacturer. The main steam flow measurement accuracy depends not only on the correctness and accuracy of the mathematical model and the accuracy of the sensor, but also on the structure of the steam turbine equipment and the changes in its operating conditions, the installation location of the parameter measurement points, and whether the measured parameters are representative, which will affect the accuracy of the measurement. Its calculation accuracy is difficult to meet the operation requirements.

If the main steam flow measurement is set, the flow measurement point should be placed in front of the steam turbine. With high temperature, high pressure and high flow, if the probe breaks, the damage to the turbine behind will be disastrous. The highest application of Deltaba high temperature and high pressure flow measurement elements is 69MPa and 1200℃. The probe selection is automatically generated by online calculation through the corresponding software based on the highest pressure and highest temperature of the pipeline, and has a high safety factor. Under high temperature and high pressure conditions, the material of Deltaba probe is 1.4828. Its material and manufacturing process ensure that the design life can reach 200,000 hours. It can be used for high temperature steam occasions below 650℃ for a long time, and can directly measure the main steam flow in front of the steam turbine. For the main steam flow measurement, a condenser is equipped at the outlet of the probe to quickly cool down the high temperature steam in the condenser (the lower end is condensed water and the upper end is steam). When the normal operating medium temperature reaches 550℃, it can protect the stop valve and differential pressure transmitter from the influence of high temperature and ensure the reliable flow measurement. Since the condenser is welded and pressure tested before leaving the factory, users only need to install the three-valve group and transmitter for the integrated product, and weld the split pressure pipe after the stop valve for the split product, which reduces the leakage caused by high temperature and high pressure and the workload on site. Due to the small pressure loss, the Deltaba flowmeter increases the steam production of the boiler and makes the operation of the boiler and steam transmission pipeline safer and more reliable. The

Deltaba flowmeter directly measures the high temperature and high pressure main steam flow and has been widely used in Europe. Since it was put into use, there have been hundreds of high temperature and high pressure application examples. It has also been gradually recognized and widely used in China.

(2) The measurement of flow meters is the basis for the metering and charging of heating systems, so the flowmeter is required to have a high measurement accuracy. In the past, orifice flowmeters and ultrasonic flowmeters were more commonly used in heating network metering, but recently Deltaba flowmeters have been increasingly used in heating network steam and circulating water. Not only can its metering accuracy fully meet the metering requirements of heating network steam and circulating hot water, but also when the pipeline temperature is lower than 100℃ and the pressure is lower than 1MPa, it can implement pressure tapping and installation operations on existing pipelines, which is very suitable for technical transformation projects such as heating networks. Deltaba flowmeter can choose online pluggable flow probes, which meets the needs of online removal and maintenance of probes for pipelines that cannot be shut down for a long time, and solves the problem of difficult installation and maintenance of metering devices such as orifice plates.

The main performance indicators of several common flow meters in heating network applications are compared in Table 1.

3. Installation and use of flowmeter

The average velocity tube flowmeter is a high-precision sensor that can work normally under quite complex working conditions and maintain good performance. In order to achieve the best performance, the flowmeter needs to be reasonably installed and maintained. When selecting this product, the inner diameter of the measuring tube must be accurately provided. Because the distribution of the design probe pressure holes is calculated according to the center of the pipeline, if there is a large error in the inner diameter of the pipe, it will cause the probe and the measured pipeline to be non-concentric and cause a large measurement error. As a flowmeter for measurement, it is recommended to use a probe with full pipe insertion, especially when the straight pipe section does not meet the requirements. The average velocity tube replaces the cross-sectional flow velocity of the orifice plate by measuring the flow velocity on a certain diameter, and the flow velocity on lines of different diameters is not exactly the same (unless the straight pipe section at its location is long enough). Therefore, if the installation position and direction are not suitable, it will lead to invalid measurement or inaccurate measurement. Therefore, when selecting, it is necessary to perform theoretical analysis and calculation according to the installation conditions, and select the appropriate installation position and installation method to achieve the best effect. It is recommended to install the flowmeter probe under the guidance of the manufacturer to prevent large measurement errors caused by the installation of the probe.

References:
[1] Zhou Qunfeng. Deltaba integrated average velocity tube flow measurement scheme and application in large thermal power plants [Z]. (end)