

The liquid is mixed with gas (Bubble)Error on electromagnetic flowmeter

by：Sure     2020-06-16
Bubbles mixed in liquid is one of the reasons for the high frequency of faults such as measurement error and unstable output of electromagnetic flowmeter. In addition to the gas bubbles in the liquid mentioned in the previous section, there are several other ways to enter the air or generate free gas in the liquid (Aerosol or bubble). The following engineers from professional electromagnetic flowmeter manufacturers will continue to introduce the phenomenon of several liquids mixed with bubbles. The liquid level in the air storage container such as vortex drops slightly higher than the inlet end of the suction tube, or only 1-2 times the distance of the inlet diameter D (To be greater than 1-2D, depending on the suction flow rate) At the same time, a vortex will be generated, which will easily roll the air at the liquid-gas interface into the pipeline. In practice, there are many examples of such mistakes, which may be the reason why the air entering the pipeline is the most common and the air intake is the most. In the process industry, mixing air with mixing containers is also often encountered in practice. The air in the raw water has been used to measure the raw water since the management of water resources was strengthened and the water was charged to the rivers. However, the raw water from the river is pumped to the storage tank. Experience shows that the air mixed in the raw water is easy to accumulate in the water pipe in this section, which is called the 'gas' problem. If the flow is measured under the condition of nest air, the accuracy of the measurement is affected by the reduction of the flow area of the gas. Therefore, it is required to install an exhaust valve at an appropriate position upstream of the measurement point to solve the problem of nest air. Harbin bailaishui company used ultrasonic flowmeter to measure the flow value before and after air emission in DN6UUmm pipeline, and the difference between the two was over 9%. The exhaust is set in the upstream of the flow meter automatic exhaust valve exhaust valve automatically exhaust several minutes every hour several times. The severity of this hidden error can be seen from this example, so in the newly designed water plant (Or technical transformation of existing units) Exhaust measures must be taken upstream of the raw water pipe flow measurement point, such as installing automatic exhaust valves. If the pipeline is not fully filled with liquid and there is still air left, the overhaul of the pipeline system must first drain all the liquid and refill the liquid after the end. However, it is sometimes quite difficult to completely fill the pipeline system, because at all high points of the pipeline system (Such as inverted U-shaped top) And the gas mass breaks down and is taken away by the liquid. This is often one of the reasons for inaccurate measurement of flow meters at the initial stage of pipeline operation. A small amount of bubbles will bring considerable errors. Experiments show that bubbles containing 1% volume ratio of liquid can produce 10 5% errors when entering the turbine flowmeter. In addition, too many bubbles will block the suction tube of the differential pressure instrument, making the measurement impossible. A good pipe system design has only a few high points, and exhaust valves are provided at these high points to manually discharge residual gas. If there are many high-point piping systems, discharging gas will bring a lot of work. An alternative method is to install a gas separator upstream of the flow meter to separate the entrained gas. Seal leakage, air viscosity is far less than liquid, a hydraulic seal test can keep the liquid in the pipe from leaking out, but it may not ensure that the gas in the pipe does not leak out or suck people. There is a slight mistake in sealing the joint of the negative pressure pipe, which makes it easy to suck air into the pipe. Poor sealing of negative pressure pipes such as the suction end of the pump will easily remind people of the suction of air. However, if the pipe pressure is slightly higher than atmospheric pressure and pulsating flow occurs, the phenomenon of instantaneous pressure being lower than atmospheric pressure and absorbing air will also occur. The connection of all negative pressure pipes of the flow measurement system without air separator should be checked frequently. Once the seal is found to have signs of damage, the gasket should be replaced immediately. Cooling the bubbles formed by shrinkage is a relatively hidden way of mixing liquid with human gas. The pipe system filled with liquid stops running, and the inlet and outlet stop valves gradually cool down after closing. Due to different thermal expansion coefficients, the volume shrinkage of liquid is much larger than that of the pipe system, forming a vacuum shrinkage space in the pipe. Dissolved gases in the liquid are separated to form free bubbles that accumulate at the high point of the pipeline system. These gases must be excluded when re-driving, otherwise positive measurement errors will occur at the initial stage of operation. The dissolved gas in the liquid forms free bubbles due to changes in temperature and pressure, and the gas dissolved in the liquid will separate into free aerosol or bubbles when the liquid pressure decreases or the temperature increases. For example, water at an atmospheric pressure of 0℃ can dissolve at most about 0. If the water temperature of air with a volume ratio of 3% rises to 30℃ in the process, it can only dissolve about 0. 15%; Petroleum products dissolve much more air. If the temperature difference is only 15℃, the bubble volume formed by free air is 1%-1. 5%. This is one of the main reasons why gas separators are usually installed upstream of petroleum product flow measurement points. The gas separator used for gas separator is also called air eliminator, which was mostly used together with positive displacement flow meter in the past. When the liquid enters the air from the above several ways and affects the measurement, it must also be equipped with a gas separator, especially the measurement environment for intermittent operation and the medium with higher measurement value such as petroleum products, and places with strict trade accounting requirements. The liquid in the continuous operation system of the process industry does not often contain gas, so in the past, gas separators were rarely installed in front of flow meters; However, due to the increase in requirements, there is a tendency to increase the installation of turbine-type and other flow meters in the continuous operation system. The gas discharged by the separator may be air, saturated vapor of liquid and mixed droplets. If it is a liquid point of flammable liquid, it should be properly disposed to ensure safety.
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