Selection and installation of flow meter

Abstract: The selection and installation information of flow meters are provided by excellent flow meter and flow meter manufacturers. Flow measurement of gas-liquid two-phase flow The phenomenon that liquid and its vapor or gas and liquid with different components flow together is called gas-liquid two-phase flow. The former is called single-component gas-liquid two-phase flow, and the latter is called multi-component gas-liquid two-phase inflow gas-liquid two-phase flow in power, chemical industry, and stone. More flowmeter manufacturers choose models and price quotations. You are welcome to inquire. The following are the details of the selection and installation of flowmeters. Flow measurement of gas-liquid two-phase flow The phenomenon that liquid and its vapor or gas and liquid with different components flow together is called gas-liquid two-phase flow. The former is called single-component gas-liquid two-phase flow, and the latter is called multi-component gas-liquid two-phase flow. Gas-liquid two-phase flow is common in industrial equipment such as power, chemical industry, petroleum and metallurgy. There is a flow velocity difference between the liquid phases, and this relative velocity should be taken into account when measuring flow. Manufacturers usually claim that liquids containing a few percent by volume of free gas have little effect on the flow indication, but there is no specific data on its influence. However, the information provided by the relevant literature indicates that the free gas in the liquid has an effect on the flow indication. , Different designs of instruments are very different, and the pressure, flow rate, viscosity of the fluid and the distribution state of the gas in the liquid are different, and the effects are also different. Therefore, it is best to install an air eliminator upstream of the flow meter. When the liquid contains a small amount of gas, the distribution of the gas in the liquid is in the form of tiny bubbles, which can be measured by an electromagnetic flowmeter, but the measured volume flow of the bubble mixture. When the amount of gas contained in the liquid increases, the geometric size of the bubble gradually increases, and then transitions to a slug-like structure. When the size of the bubble is equal to or larger than the size of the end face of the electrode of the flowmeter and passes over the electrode, the electrode may be covered by the gas, causing the circuit to be disconnected instantaneously and the output shaking phenomenon. Flow measurement of liquids with high saturated vapor pressure The following takes liquid ammonia as an example to discuss the flow measurement of liquids that are easy to evaporate, such as ethylene, propylene, and vinyl chloride. There are two major differences between liquid ammonia flow measurement and water flow measurement and oil flow measurement: ① The saturated vapor pressure of liquid ammonia is high, and its boiling point is -33.41 under standard atmospheric pressure conditions, so it must be transported and stored under pressure conditions. ; ② In the flow measurement of this kind of fluid, it is easy to generate cavitation and accompanying cavitation at the outlet of the flowmeter due to the pressure loss of the instrument, causing the flowmeter to indicate a high value and the flow primary device to be damaged. The same situation is encountered in the flow measurement of liquid ethylene, propylene, etc. The problems encountered in the liquid ammonia flow measurement and their treatment methods analyzed here also have reference value for the flow measurement of other liquids with high saturated vapor pressure. 1. Characteristics of liquid ammonia flow measurement 1) The gas-liquid interface of liquid ammonia stored in the storage tank is generally in a gas-liquid equilibrium state. Figure 4-46 shows a typical flow measurement of liquid ammonia flow in an ammonia plant: the synthesis gas, gaseous ammonia and liquid ammonia mixture from the ammonia condenser are separated in the ammonia separator, and the liquid ammonia is adjusted by the flow meter and the liquid level. The valve is sent to the low pressure ammonia intermediate tank. Obviously, at the gas-liquid interface between the ammonia separator and the intermediate tank in the figure, the gas-liquid two phases are in equilibrium. Two-phase flow should be avoided as much as possible in the flow measurement of liquid ammonia. However, liquid ammonia that is close to the gas-liquid equilibrium state, when flowing through the flow meter, if the pressure loss is large, it is easy to cause partial gasification, which affects the measurement accuracy. 2) The temperature coefficient of fluid density is large. From the p=/(i) function table of liquid ammonia, it can be seen that under normal temperature conditions, the density of liquid ammonia changes by more than 0.2% for every 1 °C change in the temperature of liquid ammonia. Therefore, the measurement of liquid ammonia must be temperature compensated 3) High accuracy is required. According to the relevant documents of the former Ministry of Chemical Industry, the measurement of liquid ammonia should reach level 1 accuracy. If effective measures are not taken, it is difficult to achieve this requirement 04) Fluid is flammable and explosive. Explosion-proof instruments should be selected when selecting instruments, and explosion-proof regulations should be followed during instrument installation, use and maintenance. 5) The measured medium is corrosive. Ammonia has a strong corrosive effect on copper and other materials, so’The part of the instrument that is in direct contact with the measured substance should be able to withstand the corrosion of ammonia, and the electronic circuit part of the instrument should have a protection capability of 1P67 or above to prevent the corrosion of the electronic part by corrosive gases in the surrounding environment. 2. Cavitation and cavitation and preventing fluid vaporization problems, in the pipeline where liquid flows, if the pressure in a certain area drops below the saturated vapor pressure of the liquid’Then the liquid in this area will generate air bubbles, and these air bubbles will accumulate near the low pressure area, and air pockets will be formed. Lai Wei is in charge of the ship—A gas appeared in the area where the bowl remained unchanged. In the water flow pipeline, the bubbles mainly contain water vapor, but because a certain amount of gas is dissolved in the water, a small amount of gas precipitated from the water is also entrained in the bubbles. When the bubbles follow the water flow to an area of ​​high pressure’The vapor in the bubble will re-condensate into a liquid. At this time, the bubble will deform and rupture. The surrounding liquid flows to the center of the bubble, and a violent impact occurs. The g force increases sharply, and its value can reach hundreds of atmospheres. The material on the wall of the runner is continuously impacted, which causes the material to be eroded. If a flow meter is installed in the pipeline, the cavitation phenomenon will cause the measurement error to increase’and can damage—secondary device.

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