In the flow measurement of chilled water, the electromagnetic flowmeter has obvious advantages. The advantages of the electromagnetic flowmeter can also be revealed in the measurement of chilled water flow. Among them, corrosion resistance is particularly important for brine chilled water. In high-rise buildings, some air-conditioning water pressure is high. At this time, consultation with the instrument manufacturer can be made to specially manufacture electromagnetic flowmeters that can withstand pressure requirements. In the specific design fashion, the following problems need to be considered.
1. Measuring tube lining selection.
If the chilled water is fresh water, rubber lining is used and the price is lower. If chilled water is brine, PTFE liners must be selected. If you measure both freshwater chilled water and brine, you must choose PTFE lining.
2. Electromagnetic flowmeter tube excitation coil moisture problem.
There are two kinds of electromagnetic flow sensor in the method of enclosing the excitation coil sealed in the shell, one is to use the upper and lower halves of the shell, after the seal is pressed by the screw, to achieve the seal; the other is to use the shell welding seal method , isolate the coil from the outside world. The latter is more reliable and can completely eliminate the breathing phenomenon of the coil cavity and prevent the intrusion of external moisture. However, in the production process, the cavity around the coil is always filled with air, and the content of water vapor therein varies with the change of the atmospheric temperature and relative humidity during assembly. If the water vapor content is high, dew condensation may occur due to a decrease in the temperature in the conduit and cavity after the flowmeter is put into use, causing the coil to become wet and the insulation to be damaged. Thus, when the temperature of chilled water is low (some manufacturers are bounded at 0°C), the air in the cavities is replaced with nitrogen, and when the temperature is lower (such as -10°C or lower), Methyl silicone oil (some manufacturers use transformer oil) filling. Different manufacturers have different practices, but the total one is that the sealed dry media will not escape, and in the long-term use, the coil insulation will not vent the valve to reduce the pressure.
3. Lining is subject to negative pressure.
When the chilled water flow is measured normally, the electromagnetic flowmeter measuring tube itself does not bear negative pressure. Only when the water in the pipeline is discharged, the negative pressure will appear on the upper part of the pipeline because the vent valve at the highest point of the pipeline is forgotten to open. For the same reason, negative pressure can also occur in hot water pipes. If the negative pressure in the tube exceeds the specified value, the inner liner of the measuring tube may be detached instantaneously. Therefore, it should be avoided. Another way to solve this problem is to use a liner that can withstand vacuum. This liner can withstand absolute vacuum pressure due to the specially designed skeleton, of course, with a skeleton-type liner.
Another way to solve the negative pressure is to replace the original vent valve with a one-way valve (non-return valve). When a slight negative pressure occurs in the pipeline, the non-return valve opens automatically and sucks air from the atmosphere to ensure safety.
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