Selection of Caliber and Range
The full scale scale of the flowmeter can be selected according to the principle of not less than the expected maximum flow value, and the normal flow should preferably exceed 50% of the full scale flow. In this way, higher measurement accuracy can be obtained.
The diameter of the flowmeter is usually the same as or slightly smaller than the inner diameter of the process pipe, which shall be determined by combining different measuring objects, flow velocity in the measuring pipe, etc. The flow rate of liquid and pipeline with low viscosity, such as water for process industry, is generally 1.5-3m/s, and the diameter of flowmeter is the same as that of pipe. For some pipeline systems with low flow or low flow rate at the initial stage of operation of new projects, the instrument diameter should be smaller than the pipe diameter and connected with a reducer in consideration of measurement accuracy.
For slurry and other fluid with particles and high abrasiveness, the flow rate should be lower than 2-3m/s in common use to reduce the wear of lining and electrode, but not too low, so as to avoid small signal or particle deposition and asymmetric flow velocity distribution in horizontal pipeline. For the fluid easy to settle, adhere and scale, the common flow rate is not less than 2m/s, It is better to raise it to 3-4m/s or above to play the role of self-cleaning and preventing adhesion and deposition.
Conductivity of Liquid
The premise of using electromagnetic flowmeter is that the measured liquid must be conductive and the conductivity cannot be lower than the threshold value. The use of the universal electromagnetic flowmeter also depends on the length of the signal line between the transmitter and the converter and its distributed capacitance. The manufacturer's instructions usually specify the length of the signal line corresponding to the conductivity.
According to the use experience, the actual liquid conductivity should be at least one order of magnitude greater than the threshold value specified by the instrument manufacturer. Because the lower limit value specified in the manufacturer's instrument specification is the lowest value measured under various conditions of good use, it is limited by some conditions of use, such as conductivity uniformity, connecting signal lines, external noise, etc., otherwise, output jitter will occur.
Material of Parts in Contact with Fluid
For measuring tubes, electrodes, grounding rings and sealing gaskets made of lining or insulating materials in contact with the fluid, the corrosion resistance, wear resistance and upper limit of service temperature of the materials affect the adaptability of the instrument to the fluid. Due to fewer parts, simple shape, flexible material selection, electromagnetic flow transmitter has strong adaptability to fluid.
The lining must be electrical insulating material. Because it directly contacts the measured medium, it must be considered comprehensively according to the physical and chemical properties of the medium. Common lining materials of fluoroplastics include polytetrafluoroethylene, perfluoroethylene propylene, polyurethane rubber, chloramine rubber, neoprene rubber, glass fiber reinforced plastic and ceramics.
Neoprene rubber and glass fiber reinforced plastic are used for non corrosive or weak corrosive liquids; Polyurethane rubber has excellent wear resistance, but poor corrosion resistance to acid and alkali; Alumina ceramics have excellent wear resistance and corrosion resistance to strong acids and alkalis. The electrode is made of non magnetic conductive materials. When selecting the type, the corrosivity of the medium is mainly considered, followed by whether there will be surface effects such as passivation and noise.
At present, acid resistant steel and keyed stainless steel are mostly used for general fluid. For strong corrosive medium, Hastelloy alloy, titanium alloy, platinum iridium alloy or acid resistant steel are used to coat gold.
Different electrode structures can be selected according to different flowmeter diameters, withstand voltage requirements and medium properties. For example, it is suitable for medium caliber and pressure resistant insert type, small caliber and high pressure resistant insert type, tip type for measuring adhesion and scaling media, and inverted cone structure for severe wear. In the worst case, the structure of integral sintering of platinum electrode and metal ceramic lining can also be considered.
The grounding ring is connected at both ends of the flow transmitter of the plastic pipe or insulated lined metal pipe. The grounding ring shall be separately and well grounded to make the fluid in the pipe connect with the earth electricity and have zero potential, otherwise the flowmeter will not work normally. If the metal process pipe is in direct contact with the fluid, no grounding ring is required.
The material of grounding ring shall be determined according to the corrosivity of the medium, generally molybdenum stainless steel; For highly corrosive liquid, it can be selected by referring to the selected electrode material or a lower level.