Differential pressure (also known as throttling) flowmeter is based on the throttling principle of fluid flow, using the pressure difference generated when the fluid flows through the throttling device to achieve flow measurement. It is currently one of the mature and commonly used methods for measuring flow in production. It is usually composed of a throttling device (such as orifice plate, nozzle, Venturi tube, etc.) that can convert the flow rate of the measured fluid into a pressure difference signal, and a differential pressure flowmeter that can convert this pressure difference into the corresponding flow value for display.
The so-called throttling device refers to placing components in the pipeline that can cause local contraction of the fluid. The widely used ones are orifice plates, followed by nozzles, Venturi tubes, and Venturi nozzles. These types of throttling devices have a long history of use and have accumulated rich practical experience and complete experimental data. Therefore, their forms have been standardized and referred to as standard throttling devices both domestically and internationally. That is to say, standard throttling devices designed and manufactured according to unified standards can be directly used for measurement without the need for separate calibration. However, for non standardized special throttling devices, individual calibration should be performed during use.

Measurement principle of differential pressure flowmeter

When fluid flows in a pipeline with a throttling device, the phenomenon of the difference in static pressure of the fluid at the pipe walls before and after the throttling device is called throttling phenomenon. The throttling device includes a throttling element and a pressure sensing device. A throttling element is an element that causes local contraction of fluid in a pipeline. Commonly used throttling elements include orifice plates, nozzles, and Venturi tubes. The following will take orifice plates as an example to illustrate the throttling phenomenon.
The fluid flowing in the pipeline has kinetic energy and potential energy, which can be converted into each other under certain conditions. According to the law of conservation of energy, the sum of static pressure energy and kinetic energy possessed by a fluid, as well as the energy loss due to overcoming flow resistance, remains constant in the absence of external energy. Diagram the distribution of fluid velocity and pressure before and after the orifice plate. The fluid flows at a certain velocity v in front of section I of the pipeline. The static pressure at this time is P;. When approaching the throttling device, due to the obstruction of the throttling device, the fluid near the pipe wall is most affected by the blocking effect of the throttling device, thus converting a part of the kinetic energy into static pressure energy. This results in an increase in the static pressure of the fluid near the pipe wall at the end face of the throttling device, which is greater than the pressure at the center of the pipeline. That is, a radial pressure difference is generated at the end face of the throttling device, which causes the fluid to generate radial additional velocity, causing the flow direction of the fluid particles near the pipe wall to tilt with the central axis of the pipeline, forming a contraction motion of the flow bundle. Due to inertia, the minimum contraction of the flow beam is not at the opening of the orifice plate, but at section 11 at the opening. According to the continuity equation of fluid flow, the flow velocity of the fluid at section II is the highest, reaching v2. Subsequently, the flow gradually expanded and returned to a stable state after reaching section III, and the flow velocity decreased to its original value, that is, v1=v3.
Due to the local contraction of the flow caused by the throttling device, the flow velocity of the fluid changes, that is, the kinetic energy changes. At the same time, the static pressure that characterizes the hydrostatic energy of the fluid is also changing. At section I, the fluid has a static pressure P1. When reaching section II, the flow velocity increases to the maximum value, and the static pressure decreases to the minimum value P2, and then gradually recovers with the recovery of the flow beam. Due to the sudden reduction and expansion of the flow cross-section at the end face of the orifice plate, the fluid forms local eddies, which consumes some energy. At the same time, when the fluid flows through the orifice plate, it must overcome friction, so the static pressure of the fluid cannot be restored to its original value P; And a pressure loss of £ P1-P2 was generated. The pressure of the fluid before the throttling device is relatively high, called positive pressure, often marked with a "+"; the pressure of the fluid after the throttling device is relatively low, called negative pressure (different from the concept of vacuum degree), often marked with a "one". The magnitude of the pressure difference before and after the throttling device is related to the flow rate. The larger the fluid flow rate in the pipeline, the greater the pressure difference generated before and after the throttling device. As long as the pressure difference before and after the orifice plate is measured, the size of the flow rate can be reflected. This is the basic principle of measuring flow rate with a throttling device.
It is worth noting that it is difficult to accurately measure the pressures P1 and P2 at sections I and II because the position of section II, which produces low static pressure P2, changes with different flow velocities and cannot be determined in advance. Therefore, in reality, two fixed pressure points are selected on the pipe walls before and after the orifice plate to measure the pressure changes of the fluid before and after the throttling device. Therefore, the relationship between the measured pressure difference and flow rate is closely related to the selection of pressure measuring points and methods.