09,22,2026 27 views
1、 Liu's physical characteristics
The fluid passing through the flowmeter in the industrial field cannot be as clean as the fluid used in the laboratory, as they may have sediment and corrosiveness. After using for a period of time, scale, wear, and corrosion may still occur on the pipe edge inspection parts. Sediments on the pipe wall will alter the wall thickness and roughness of the pipeline. For standard orifice plates, it will change the beta value, causing an error of ± 3-10%; Wear and corrosion may occur to the moving parts of the turbine, rotor, and volumetric flowmeter, ranging from minor errors to severe malfunctions; The contamination of the electrodes of electromagnetic flow meters, transducers of ultrasonic flow meters, and thermal resistors of thermal flow meters will reduce their sensitivity and increase errors; Blocking the pressure tapping hole of the differential pressure flowmeter, etc. Of course, this process is slow, but it must not be taken lightly. Generally speaking, as long as regular maintenance is valued, its impact can be reduced (or eliminated).
2、 Physical properties of fluids
The commonly used media in experiments are water, air, and oil, while in field applications, there will be tens of thousands of various fluids with different physical properties (such as density, viscosity, electrical conductivity, thermal conductivity, sound velocity, composition, etc.), which will affect the accuracy of the flowmeter to varying degrees. But the physical properties of these fluids can be found in some engineering manuals and corrected to mitigate their impact. This is also the intelligentization of flow meters
3、 Characteristics of Flow
In the laboratory, flow meters should be in a relatively ideal flow state, generally including the following types:
1. Newtonian fluid: In the process industry, except for the food industry, Newtonian fluid is commonly used.
2. Steady flow: a flow state in which the flow rate in a measuring pipeline does not change over time, but slow changes are allowed. In industry, pulsating flow (a flow state in which the flow rate changes rapidly over time) is also known as unsteady flow. In industrial sites, the oscillation of pumps, compressors, blowers, certain regulators, and valves will generate pulsating flow, which will bring significant errors to flow meters. As early as 1956, Head raised this issue and introduced the concept of pulsation coefficient Ip to define the impact of pulsating flow on flow measurement. He believed that when Ip is less than 0.03, it can be considered as steady flow; Greater than 0.03 should be given due attention.
Pulsating flow can cause errors in flow meters, which is particularly severe for differential pressure flow meters. Pulsating flow in turbine flow meters can cause changes in rotor speed; For vortex flowmeters, if the frequency of pulsation is close to the vortex frequency, a so-called "synchronization phenomenon" will occur, which will also result in significant errors. For fluids, the compressibility of gases is better than that of liquids, and pulsating flow will be quickly attenuated in the flow, with less impact on flow meters than liquids. People have been concerned about the impact of pulsating flow on flow meters for nearly 60 years. Although many studies have been conducted to make corrections, there is still a lack of sufficient data to date. The current common method is to use filters in the pipeline to eliminate (or mitigate) its impact.
3. Single phase fluid: There is a significant difference in the measurement results of flow meters for single-phase and multiphase fluids. Here, we only discuss the measurement of single-phase flow meters, and flow meters are only calibrated in single-phase flow laboratories. In field applications, it is inevitable to encounter problems with multiphase fluids.
In process engineering, due to the flow of fluid through various resistance components, friction and separation will inevitably occur, as well as changes in cross-section and pressure drop, causing the gas phase dissolved in the liquid phase to separate and produce voids. For throttling devices, the probability of void formation due to the severe flow changes in the orifice plate will be 8 times that of Venturi tubes and 3 times that of Dower tubes. If the generation of voids can be limited within a certain range, the measurement error can be as high as 20%. If it develops out of control and becomes too large, it may damage the instrument.
For gas-solid and liquid-solid two-phase flows, the measurement of flow meters will also bring significant errors and hazards. The avoidance method is to install the flow meter above the vertical pipeline as much as possible to avoid solid deposition.
*The velocity distribution in the pipeline affects the accuracy of most principle flow meters (excluding Coriolis and volumetric), so ISO TC30 stipulates that flow meters must be installed in fully developed turbulent flow to maintain high accuracy. Of course, flow laboratories should also have fully developed turbulent flow to verify the flow coefficient. Generally speaking, as long as there is a straight pipe section length of 30D (where D is the inner diameter of the pipeline), sufficient development of turbulent flow can be achieved.
However, with the trend of increasingly modern and large-scale engineering, the diameter of pipes in engineering is increasing day by day. The process design has never considered the length of straight pipes necessary for flow meters to maintain high accuracy from the perspective of saving space. In addition, there are thousands of types of resistance components on site, and the flow velocity distribution in pipelines is very complex. The flow coefficient verified under ideal laboratory conditions cannot be accurately transmitted to the flow meters on site due to flow field differences, making it difficult to obtain high accuracy.
