In the production process, the flow of working fluid needs to be measured in real time to control the working conditions in the production process and to measure the efficiency of the equipment and important indicators of economic accounting. Therefore, flow measurement is an important means of energy management in industrial enterprises. Differential pressure flowmeter is one of them. The differential pressure flowmeter is one of the most used flowmeters in the industry, and the orifice plate is the most commonly used throttle element for differential pressure flowmeters. It has the characteristics of simple structure, convenient installation, and low price. Second, the composition of differential pressure flowmeter It consists of throttle element, differential pressure gauge (differential pressure transmitter is generally used), impulse conduit and flow display recorder (our company uses BSD-98 multi-function flow totalizer). 1. Throttling element A fixed barrier is installed in the pipeline, and a small hole is opened in the middle. When the fluid flows through the cross section of the opening, the contraction of the stream will occur. At this time, the flow speed of the fluid is accelerated, and the pressure is reduced. A large pressure difference is generated before and after. This pressure difference changes with the flow rate. The greater the flow rate, the greater the differential pressure. Therefore, the size of the flow rate can be calculated by measuring this pressure difference. This process is called the throttling process, and the element that causes the contraction of the stream is the throttling element. Our company's gas flowmeter uses a differential pressure flowmeter, the throttle element used is a standard orifice plate, the pressure tube is φ18 × 2 seamless steel pipe, and the differential pressure transmitter is Rosemount's 3051 differential pressure transmitter . And equipped with BSD-1151 pressure transmitter and thermal resistance as temperature and pressure compensation. 2. Pressure extraction method of throttle element (standard orifice plate) There are many pressure-relieving methods for the throttle element, such as corner pressure and flange pressure. Our standard orifice pressure-receiving method uses ring chamber pressure, that is, the front and rear ring chambers are installed on both sides of the orifice plate, and The ring chamber, orifice plate and gasket are fastened together by the flange. 3. Installation of differential pressure flowmeter It includes four parts: 1. Installation of throttle element (standard orifice plate) 2. Installation of differential pressure signal pipeline (pressure pipe) 3. Installation of differential pressure gauge (3051 differential pressure transmitter) 4. Installation of flow display recorder (BSD-98 multi-function flow calculator) 4. Analysis of measurement error in use of differential pressure flowmeter When the differential pressure flowmeter is actually applied in the field, its measurement error tends to increase, sometimes reaching 10% -20%, especially when the differential pressure flowmeter is used as the material (water, steam, In the measurement of gas and raw materials), it is particularly important to reduce measurement errors when performing economic accounting and material balance. But it must be noted that not only reasonable selection, accurate design calculation and processing and manufacturing, but also correct installation, maintenance and compliance with the conditions of use, etc. are needed to ensure that the orifice flowmeter has sufficient actual measurement accuracy. The following analyzes several causes of measurement errors: 1. The working state of the measured fluid changes If the working state of the measured fluid (such as temperature and pressure) and the corresponding fluid density, viscosity and pipeline roughness in actual application are inconsistent with the design, if the flow rate is still calculated according to the original instrument constant, it will be different from the actual If there is an error in the flow rate, it can be corrected or redesigned according to the relevant calculation formula. 2. The orifice is not installed correctly When our company just put into production test, the gas flowmeter showed too small, and the orifice plate was reversed after inspection. In fact, the sharp side of the orifice plate should face the fluid flow as the inlet end, and the flared side is the outlet end. Pay attention to the direction. In addition, the center of the opening of the orifice plate is not concentric with the center line of the pipe during installation, which will also cause measurement errors. The clogging of the pressure pipe and the appearance of protrusions such as gaskets are also the causes of the error. 3. The edge of the orifice entrance is worn Due to the long use time of the orifice plate, especially in the case where the measured medium contains solid particles and other impurities, or is chemically corroded, it will cause changes in the geometry and size of the orifice plate. If the sharpness of the inlet edge of the orifice plate is due to It becomes blunt due to the impact or corrosion of the medium, so that the pressure difference ΔP generated when an equal amount of fluid passes through is reduced, which causes the display value to be low. In severe cases, the orifice plate needs to be replaced. 4. Scaling on the surface of the orifice plate and changes in the flow cross-sectional area In the field use, the surface of the orifice plate may be contaminated with a layer of dirt, or due to the accumulation of impurities in the front and back corners of the orifice plate, or due to the strong corrosion effect, the flow cross-sectional area of ​​the pipeline will be gradually changed, and the pressure conduit Leakage and blockage of the pipeline will cause measurement errors. Due to the dirty gas used by our company (including tar), the pressure ring chambers in front and back of the orifice plate are blocked, there is no pressure difference at the pressure port, and the flowmeter has no flow display. After removing, clean the two ring chambers and replace the pressure guide tube , The flowmeter shows normal. 5. Transmitter zero drift and improper range setting can also cause measurement errors Due to the long time, the zero point of the transmitter will drift. If the drift is negative, the output current of the transmitter is less than the standard 4MA, and the flow rate is displayed low. If the drift is positive, the output current of the transmitter is greater than the standard 4MA, the flow rate is The display is high. If the range setting is large, the flow rate is displayed low, and if the range setting is small, the flow rate is displayed high. (Organized by China Educational Equipment Purchasing Network)
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