1. Requirements for Straight Pipe Section
When installing the instrument, ensure that there are enough straight pipe sections. Generally, follow the principle of upstream 5D and downstream 3D (D is the pipe diameter). When installing as far as possible away from valves, elbows, tees, reducers that affect the flow pattern. If it is unavoidable, install the flowmeter in front of these fittings. Generally, 10D straight pipe section is required after reducers or valves, and 5D is required after 90 ° elbow or tee.
2. Installation Direction and Location
The vertical installation of the sensor is the most ideal, but it should be noted that the fluid flow direction should be downward in and upward out. When installed horizontally, the axis of the electrode should be horizontal. If the axis of the electrode is perpendicular to the ground, the electrode on the top is easy to collect gas, and the electrode below is easy to scale or be covered by dirt, which affects the contact between the electrode and the fluid and causes errors. The installation position should be selected where the measuring tube can be filled with liquid at any time to prevent errors caused by the display not being at the zero point because there is no liquid in the measuring tube. Because of the throttling effect of the valve, the pressure in the pipeline is reduced, resulting in bubbles. In the piping design, ensure that the liquid does not generate bubbles, and the flow meter should be installed as far upstream of the valve as possible. There should be no large interference electromagnetic fields (such as high-power motors, transformers, etc.) around the sensor to avoid measurement errors.
The electromagnetic flowmeter measurement circuit regards the potential of measured liquid as zero potential. In most cases, the measured liquid is ground potential. The midpoint of the input end of the converter is electrically connected with the measured fluid through the grounding point of the sensor output signal. In addition, the output signal of the electromagnetic flowmeter sensor is very small, only 2.5 ~ 8.0mV at full scale, and the output is only a few microvolts when the flow is very small. A slight external interference will affect the accuracy of the instrument. In order to ensure the normal operation of the electromagnetic flowmeter and reduce electrical interference, good grounding measures are required.
When the electromagnetic flowmeter is grounded, you should pay attention to: (1) The measuring tube, housing, shielding wire, converter, and secondary instrument need to be grounded; (2) The sensor and converter should be grounded separately, and never connect to the motor or process pipeline. The grounding resistance should be less than 10Ω; (3) The grounding of the sensors and converters should be on site, and the shielding layer of the secondary meter should be grounded on the side of the control room. Do not ground at multiple ends to avoid interference due to different potentials. (4) The sensor is installed on the metal pipe, and the grounding wire of the sensor is connected to the flanges of both sides of the pipe according to the requirements of the manufacturer to form a reliable grounding loop (see Figure 1). The grounding point of the meter should be independent, and it is not allowed to connect with the grounding wire of other electrical equipment.
4. Problems to be paid attention to when installing reducing pipes
In order not to have too much influence on the flow pattern after the installation of the reducer, the central cone angle of the reducer is required to be no more than 15°, and the smaller the better, this is also a problem that is often overlooked in practice.