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Introduction to the application of conical flowmeter in gas measurement

Gas flow measurement has always been a difficult point of instrument detection. Cone flowmeter provides a new measurement method for gas flow measurement. This paper introduces the use and characteristics of cone flowmeter in the field.
1 Conical flowmeter measurement principle
Since the wall friction reduces the velocity of the fluid flowing through the tube wall, the flow velocity distribution in the tube is such that the flow rate is near zero at the tube wall, the closer to the center of the line, the greater the flow rate is at the center of the line. When the fluid flows through the conical flowmeter, the conical flowmeter is throttled by suspending a cone at the center of the pipeline. The cone directly interacts with the high-speed center portion of the fluid, forcing the high-speed fluid in the center of the pipeline to be close to the wall. The low velocity fluid is homogenized, ie the flow rates of the two tend to be uniform.
Compared with the structure of various throttling devices, the standard orifice plate is a sudden contraction of fluid in the center of the pipe, which not only has a large loss of resistance, but also aggravates the unevenness of the fluid flow velocity distribution, the pipe edge is blocked, and the dirty media accumulates in the standard orifice plate. Before and after; the venturi tube (which is still a standard throttling device like the standard orifice plate) is gradually contracted at the center of the pipe, although the unevenness of the fluid flow velocity distribution still exists, but the resistance loss is slightly reduced, the dirty medium The situation of gathering is improved, but it is difficult to make; the annular orifice plate is a disc placed in the center of the pipeline, and the edge of the pipeline can be circulated, that is, the sudden contraction from the center of the pipe of the standard orifice plate to the sudden contraction of the edge of the pipe can make The flow velocity in the pipe is uniformized, so that the dirty medium does not gather before and after the annular orifice plate; and the conical flowmeter is placed in the center of the pipe with a cone, its structure is that the pipe edge is gradually contracted, the resistance loss is small, and the pipe can be inside. The flow rate is uniformized so that the dirty medium does not collect before and after the annular orifice. It is concluded that the conical flowmeter is the inevitable result of a series of innovations in the development of the throttling device.
2 Conical flowmeter features
(1) The requirements for straight pipe sections are low. Although all differential pressure flowmeters are measured according to Bernoulli's theorem, Bernoulli's theorem has a basic requirement that the fluid to be measured must be an ideal fluid. In the case of a throttling device of a conventional differential pressure flowmeter such as an orifice plate or a nozzle, in order to satisfy the Bernoulli's theorem as much as possible, it is necessary to have very long front and rear straight pipe sections in order to shape the irregularly flowing fluid as much as possible. Ideal fluid. The flow of the conical flowmeter adopts a unique streamlined throttling structure placed in the center of the pipeline. Before the flow is detected, the irregularly flowing fluid is rectified at the same time, and the fluid is solved without increasing the fluid rectifier. The problem of rectification. The streamlined cone suspended in the center can reshape the flow rate curve. In the narrower area upstream and downstream of the cone (0~3D, 0~1D), the liquid with irregular flow rate can be directly rectified into an ideal liquid. Satisfying the requirements of Bernoulli's theorem, achieving high measurement accuracy and repeatability. There is no need for very long straight pipe rectification, which is very important in the installation of the flowmeter. The petroleum liquefied gas cone flowmeter (caliber 270mm) used in Shandong Coking Gas Plant is in the process of installation. Due to the limitation of site conditions, the distance between the straight pipe before and after the flowmeter is 1000mm and 600mm respectively, but the flowmeter can still be stable. run.
(2) It is resistant to pollution and is not easy to block. The original use of orifice flowmeter and constant velocity tube flowmeter, because the gas contains tar, naphthalene, sulfur, ammonia and other impurities, the orifice plate or the constant velocity pipe must be cleaned regularly or steamed, otherwise the orifice plate The downstream end face and the upstream side right angle entrance edge will be polluted, and the instrument pressure guiding tube will not be unblocked or blocked, which will affect the measurement accuracy of the system; the water in the straight pipe section before and after the orifice plate will reduce the flow area of the pipe, increase the flow rate, and increase the differential pressure. The flow rate display value is too high. In severe cases, the transmitter cannot be operated normally due to blockage of the pressure guiding tube. In order to ensure the normal and stable operation of the instrument, the maintenance of the instrumentation is very large. The conical flowmeter has a self-purging structural design feature with no dead zone. When the medium flows through the cone, it will accelerate, continuously flushing the positive pressure to take the pressure hole, the outer wall of the cone and the wall near the cone, while the negative pressure pressure hole is separated by a small piece of non-flowing medium, and the dirty impurities can not enter. Therefore, the conical flowmeter can be used for various dirty media containing impurities and crystallization, such as coke oven gas, producer gas and the like. The flow rate of coke oven gas, producer gas and petroleum liquefied gas of five cone flowmeters used by Shandong Coking Gas Plant has been measured. Some have been used for three years and have not been maintained yet. No flow reduction has been found in the flowmeter. The pollution of the device, the blockage of the pressure guiding tube of the instrument, the accumulation of water in the pipeline, and the abnormal value of the flow rate greatly reduce the maintenance workload of the instrument and improve the accuracy and reliability of the gas flow measurement.
(3) Large differential pressure value and wide range ratio, suitable for flow measurement of low pressure and low flow medium. In the gas flow measurement process of the coking plant, the measurement object is characterized by large diameter, low pressure and slow flow rate. The orifice flowmeter is generally equipped with a differential pressure transmitter (Shandong Coking Gas Plant uses Rosemount's 1151DR), when the gas pressure is 3500Pa, the normal differential pressure varies between 100~1000Pa (the differential pressure of the full-scale return gas of the coking gas plant in Shandong Coking Gas Plant is 630pa), and the differential pressure signal of the flowmeter is small. When using the constant velocity tube flowmeter, due to the low flow rate of the pipeline gas, the differential pressure signal is very small, which has a great influence on the accuracy of the system. The city gas measurement of the Shandong Coking Gas Plant uses a constant velocity tube flowmeter, and the maximum differential pressure is only 120 Pa, the normal value. For 2~5Pa, the system accuracy is very poor. The differential pressure signal will affect the accuracy of the transmitter calibration and use; the orifice plate is polluted, the instrument pressure guiding tube is not smooth, and the water accumulated in the pipeline has a great influence on the system accuracy. The conical flowmeter is a throttler similar to the orifice plate, and the differential pressure value is much larger than that of the constant velocity tube flowmeter. Compared with the orifice plate, since the pipe edge is gradually contracted, the resistance loss is small, so the resistance loss can be the same. In the case of design, a conical flowmeter with a large differential pressure value may be selected in the design. As shown in Table 1, several conical flowmeters measure the differential pressure value of the gas when the differential pressure value is larger than that of the orifice plate. The differential pressure signal is increased, conditions are created for the transmitter calibration, and the measurement accuracy of the system is also improved.
Some measurement objects, such as factory gas, mixed gas flow measurement, due to the large difference between the maximum and minimum flow, usually the orifice plate can only reach a 3:1 range ratio, so the multi-tube parallel method is used to solve the measurement accuracy is not reduced The problem, while the cone flowmeter's turndown ratio can be greater than 10:1, so the measurement accuracy is higher at low range, and the scope of application is wider.

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