If the measurement requirement is higher than the above, indicating that the flow error is between 2-10%, install a flow meter. If the differential pressure instrument is selected according to the following selection sequence, it is not necessary to install a flow sensor such as an orifice fluid, and an elbow flowmeter or a portable ultrasonic flowmeter may be used.
1.2 Primary measurement distribution
After confirming that the flow meter must be installed, further detailed understanding of the use requirements and various conditions. First, in accordance with the type and characteristics of the fluid, the second-step in-depth consideration and analysis was adopted for a measurement scheme in which the exclusion method cannot and should not be used in the “primary tableâ€.
1.3 Analysis Factors
According to the selected programs of the primary selection, samples, technical data, and manuals are collected from the manufacturers of the primary instrumentation to fully understand the specification performance of the instrument; and then according to performance requirements and instrument specifications, fluid characteristics, installation sites, and environmental conditions, respectively. Economic considerations are based on five factors, which are analysed one by one in accordance with the issues raised in subsequent sections. Taking into account the order of measurement purposes and focus and different from the initial selection, generally start with the "performance requirements and instrument specifications", and then consider other factors as shown in Figure 1. If the applicable object thinks that “economic factors†are the main factors (such as low pumping costs for large pipe diameters and low measurement losses due to measurement errors in business accounting), when considering other factors of “performance requirements and instrument specificationsâ€, Sometimes it is also necessary to revert to considering migration factors.
2 Fluid properties considerations
2.1 General
The primary measurement method in Section 1.2 is based on the preliminary determination of certain applicable schemes such as fluid type, and detailed parameters and attributes of the fluid properties should be obtained from the process flow department to further consider the adaptability to the selected scheme. In addition to the fluid category, fluid temperature, pressure, density, viscosity and lubricity, corrosiveness, and abrasiveness are often considered, and some applications require hygiene. Some measurement methods also need to consider the special parameters of fluid properties, such as the use of electromagnetic flow meters to understand the conductivity of the liquid.
All kinds of flow meters are always affected by one or more parameters in the fluid properties, so the physical properties of the fluid largely dominate the type of instrument to be selected. The measurement method and instrument chosen must not only adapt to the properties of the fluid being measured, but also the amount of influence of a certain parameter change in the physical properties of the fluid during the measurement.
Common fluid densities, viscosities, vapor pressures, and other parameters can be found in the manual to evaluate the suitability of fluid parameters and selected instrument specifications under the conditions of use. However, the exact composition, temperature and pressure changes, flow characteristics, etc. of the fluid are often not fully understood. It is difficult for the user to determine the values ​​of various fluid properties. At this time, the manufacturer should be consulted to estimate whether the intended instrument can be used. .
2.2 Fluid Temperature and Pressure
The working pressure and temperature of the fluid must be carefully defined, especially when the gas is measured. Changes in temperature and pressure cause excessive density changes that may alter the selected measurement method. If temperature or pressure changes cause large flow characteristics that affect the accuracy of measurements, etc., temperature and/or pressure corrections are necessary. In addition, the structural design and material selection of the flow meter housing also depend on the temperature and pressure of the fluid, so the maximum and minimum pressure and temperature must be known exactly. When pressure and temperature vary greatly, they should be carefully selected.
When measuring the gas flow, must also be certain certain instruments (such as differential pressure type) flow limit temperature and pressure, in the working condition or in the standard state?
2.3 Fluid Density and Specific Gravity
In most liquid applications, the density and specific gravity are relatively constant. Unless the temperature changes greatly and causes a large density change, no correction is generally required.
In gas applications, the degree and linearity of some instruments depends on the gas density. Usually, the values ​​under standard conditions and use conditions are known for selection. There is also a conversion of the value of the flow state to some accepted reference value. This method is widely used in the storage and transportation of LPG. Low-density gases present difficulties for certain measurement methods, particularly those that use gas momentum to drive the detection elements (eg, turbine flow meters ).
2.4 Viscosity and Lubricity
Instrument performance tends to vary with Reynolds number and Reynolds number relates to fluid viscosity. Unlike gases and liquids, their viscosity does not change significantly due to changes in temperature and pressure, their values ​​are generally low, and the differences between gases are small. Therefore, the exact gas viscosity data is not as important as the liquid.
The effects of viscosity on the range of different types of flow meters vary, and the increase in viscosity of most positive displacement meters has increased, whereas the turbine and vortex type meters have reversed their viscosity increases.
When evaluating the adaptability of a meter, it is usually necessary to know the temperature-viscosity characteristics of the liquid.
Some non-Newtonian fluids (such as drilling mud, pulp, chocolate, and paint) are liquid and their flow conditions are complex, so it is not easy to determine their properties. Therefore, when selecting the instrument, be very cautious and negotiate with the manufacturer if necessary.
Lubricity is a property that is less easily evaluated. It is generally believed that high viscosity liquids also have good lubricity, but sometimes this is not the case. Lubricity is very important for instruments that have active measuring elements (for example, positive displacement and turbine type). Some liquids, especially solvents, have poor lubricity, which can shorten the bearing life of the instrument and affect the operating performance and range of the instrument.
2.5 Chemical corrosion and scaling
Fluid chemistry is sometimes the deciding factor in selecting measurement methods and instruments. Some fluids can cause contact with parts of the instrument to corrode. Surface fouling or precipitation of crystalline metal surfaces produces electrolytic chemistry. These phenomena will all reduce the performance and service life of the flow meter. The meter manufacturer will take measures to provide a number of variant products or special instruments to adapt. For example, for certain fluid anti-corrosion materials or structural anti-corrosion measures, such as metal float meter lining anti-corrosion engineering plastics, orifices made of ceramic materials. However, those meters with complex measuring structures and shapes (such as positive displacement, turbine, etc.) are not easy to handle and are used to corrode liquids.
Some flow meters are, in principle, corrosion-resistant or susceptible to corrosion. Ultrasonic transducers mounted on the outer wall of the pipe that are not in contact with the fluid being measured are essentially anti-corrosive. Electromagnetic flowmeters only have a simple pair of electrodes and the measuring tube lining is in contact with the liquid, so it is easy to choose the appropriate material for the anti-corrosion measures. ""
The fouling or precipitation of crystals on the instrument chamber and the measuring element will reduce the clearance of the moving parts and reduce the sensitivity or measurement performance of the sensitive elements. Another example is scaling that obstructs ultrasonic emission in ultrasonic instrument applications, and insulates the electrode's signal detection surface from use in electromagnetic instrument applications. Instrument manufacturers often use external measures such as the flow sensor to prevent the precipitation of crystals or the device descaler.
A zinc smelter originally used a zinc smelting solution in a pipeline plant. Not only did the scaling of flow meters of various types of pipes seriously affected the work, but even if the fouling of the pipeline was removed within 3-5 months, the difficulty and the engineering were all great. Later, the whole plant pipeline was changed to open channel transportation, and the flow rate was measured with a helium-type instrument to facilitate the descaling work.
2.6 Compression System and Other Parameters
The measurement gas needs to know the compression factor value to find the density of the fluid in the working state. The fluid with the fixed composition calculates the density through the pressure, temperature, and compressibility; when the composition changes fluid and works close to (or in) the supercritical area, the on-line density measurement should be considered.
Some flow measurement methods need to consider the specific fluid toughness. For example, the thermal instrument should have adaptable heat conduction and heat capacity, and the electromagnetic instrument can only work with certain conductivity of the liquid.
Fluids that may cause combustion or rapid reaction with certain substances should be designed and verified for the intended use of the instrument. For example, it has been known that when an acetylene gas flow rate is measured, sudden pressure and velocity gradients inside the flow meter suddenly burn.
2.7 Multiphase and Multi-component Flows
Measuring multiphase and multicomponent flows should be treated with great care. Experience shows that for multi-phase or multi-component flow, the measurement performance will be greatly changed, and some situations are still unknown. Flow meters generally evaluate their measurement performance under single-phase flow conditions. There is no single-phase flow calibration meter to assess the system change criteria for dual-phase systems. Whenever possible, separate the phases and measure the phase separation to ensure the lowest measurement uncertainty. However, in some applications, this method is not feasible or desirable.
Single fluids sometimes appear to be biphasic. Common examples are wet steam and the water particles flow with steam (wet steam quality: the fraction of gas mass flow in the two-phase flow of wet steam that accounts for the share of the two-phase mass flow). Ambient temperature and medium temperature and pressure change dryness may deviate from the original flow state, the instrument can not adapt. Although these application site instruments can also be applied, it is more prudent to propose instrument specifications than when they are applied to a single phase.
Slurry can be used electromagnetic flowmeters , specially designed mass, ultrasonic or differential pressure meter can also be measured, can obtain a stable flow and signal between the closed and the meter's meter coefficient or the determination of the outflow coefficient can not be resolved. The interplay of energy, momentum, and mass in the complex flow of multi-phase flow is extremely complex. Only in some cases can satisfactory results be obtained. Use it with extreme caution. To fully understand the flow conditions. That is, the particle size, solid content, and solid properties are clearly defined. The solid nature may be abrasive, fibrous, or sized particles. The abrasive mixture produces a stable wear rate, which leads to a steady increase in the instrument error at a certain rate, thereby estimating the age of the instrument damage.
In liquid-liquid and gas-phase mixtures in flow, the characteristics of the phases and their possible interphase transitions will affect the flow characteristics. The flow pattern depends on the relative occupancy of liquid and gas and the direction of the pipeline (horizontal or vertical). Different flow types require their own applicability and instrumentation, and the exact inter-phase distribution is often unclear. It is very difficult to estimate the measurement performance obtained in this case, but its measurement error is usually several times larger than that of a single-phase fluid measurement.
Mixtures of two or more liquids are mixed and the flow rate of the mixed liquid is measured. If the liquids are fused with each other, there is no problem; but if they are not fused, there are often problems with the uniformity of the flow (line), flow, or become Laminar flow or lump flow depends on the relative content and density difference. Meters that measure gas-liquid flow may be in highly variable flow characteristics, and flow characteristics depend on the installation design.
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