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Throttle phenomenon and principle of Differential pressure type flowmeters

Author:vacorda    Source:vacorda.com    Date: 2018-01-23 16:41:08


As the earlier flow measuring instruments, Differential pressure type flowmeters are wildly use in types of working condition. But as the development of the new electronic type flowmeter, such as electromagnetic flowmeter, vortex flowmeter. This time we are talking about the differential pressure type flowmeters.


Differential pressure flow meter has been the most widely used in chemical production, and also the most familiar flow meter to operator. Its throttling device (1) is installed in the production process pipeline (2), and by the pressure pipe (3) and differential pressure transmitter (4) composed of three parts flow measurement system.

 Component of the differential pressure type flowmeters

Component of the differential pressure type flowmeters

 

Differential pressure flowmeter is based on the principle of throttling fluid flow, the use of fluid through the throttling device to achieve the pressure difference and flow measurement. Differential pressure flow meter is generally composed of a throttle device (orifice plate, nozzle) that can transform the flow meter into differential pressure signal and differential pressure gauge or differential pressure transmitter that used to measure the pressure value and a display instrument.


At present, this kind of flowmeter is widely used in chemical industry, oil refining and other industries, and has a long history of application. Therefore, it has accumulated rich practical experience and complete experimental data. For the commonly used orifice plate, nozzle and other throttling devices, they have been standardized at home and abroad, and it called "standard throttling device." Therefore, the standard throttling device can be directly put into manufacture and used according to the calculation results, and it is not necessary to experimentally separate the calibration. But for non-standard special throttling device, when used, should be individually calibrated.

 

Throttle phenomenon and principle


When the fluid flows through the pipe with the throttling device, the phenomenon that the static pressure of the fluid is different at the pipe bores before and after the throttling device is called the throttling phenomenon.

Throttle phenomenon when the fluid flows through the throttling device

Throttle phenomenon when the fluid flows through the throttling device

 

Now, let’s make the further analytics.


When continuous flow of fluid, in the face of the throttling device which installed in the pipeline, Since the cross-sectional area of the throttling device is smaller than the cross-sectional area of the pipeline, a sudden shrinkage of the fluid circulation area is formed, and under pressure, the fluid flow rate increases, squeezed through the orifice, the formation of the flow rate increases and decreases. At the same time, there is a difference in hydrostatic pressure at the pipe wall before and after the throttling device, then the formation of static pressure difference p (p = P1-P2, p1> p2)

 

Flow and pressure distribution near the orifice

Flow and pressure distribution near the orifice


From the top picture, the function of the throttling device is to create a differential pressure due to the partial contraction of the stream. Moreover, the larger the flow rate, the greater the differential pressure before and after the throttling device. Therefore, we can measure the flow by measuring the differential pressure.


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