Problems that should be paid attention to in the selection and design of commonly used instruments in water treatment systems (2)
Flow measurement is divided into two types, one is used for flow detection, participating in process control, in order to achieve the purpose of increasing the level of production automation, improving production process conditions, and improving product quality and output. The other is used for flow measurement, not only to measure the output of products, but also to calculate the main technical and economic indicators of the water supply enterprise. Among the 8 most important economic indicators of water supply companies, 3 indicators are based on data measured by flow meters.
The selection of flowmeter should consider the following factors:
- Any type of flowmeter must have a certificate verified by the national metrology department before it can be used.
- The pressure loss of the flowmeter itself should be small.
- According to industry requirements, the accuracy of the flowmeter should not be lower than 2.5.
- The installation site conditions should meet the requirements of the selected flowmeter for straight pipe sections.
- The selected flowmeter should be able to adapt to the environmental conditions of the installation site, such as temperature, humidity, electromagnetic interference, etc.
- The selected flowmeter should be suitable for the liquid medium to be measured.
At present, in the design of water supply engineering, electromagnetic flowmeters and ultrasonic flowmeters are the most used.
The principle of electromagnetic flowmeter is to apply Faraday’s law of electromagnetic induction, which is composed of sensors and converters.
In the measurement, the liquid itself is a conductor, and the magnetic field is generated by two coils installed in the pipeline. The coil is excited by an AC or DC power source, and the magnetic field acts on the liquid flowing in the pipeline to generate a voltage in the pipeline corresponding to the average flow velocity V of the fluid to be measured, and the voltage has nothing to do with the flow velocity distribution of the fluid.
Two electrodes insulated from the pipe monitor the induced voltage of the liquid. The direction of the magnetic field, the direction of fluid flow, and the relative positions of the two detection electrodes are perpendicular to each other.
The advantages of electromagnetic flowmeter:
- The measurement is not affected by the temperature, pressure or viscosity of the liquid being measured.
- There is no pressure loss.
- Continuous measurement is possible, and the measurement accuracy is high.
- The caliber range and measurement range are large, and the measurement range is continuously adjustable.
- has nothing to do with the flow velocity distribution.
- The front and rear straight pipe sections are shorter, the front straight pipe section is 5D (D is the diameter of the meter), and the rear straight pipe section is 3D.
- The stability is good, and the output is a standardized signal, which can easily enter the automatic control system.
- The inner wall of the transmitter conduit is lined with material, which has good corrosion resistance and abrasion resistance.
- The converter is small in size, low in power consumption, and strong in anti-interference performance, which is convenient for on-site observation.
The lining material of the electromagnetic flowmeter used in the water treatment system is mostly neoprene because of its good wear resistance. When installing, pay attention to keep away from the external electromagnetic field source, so as not to affect the working magnetic field and flow signal of the sensor. When the sensor is installed horizontally, the central axis of the two electrodes are required to be in a horizontal state to prevent particle impurities from depositing and affecting the electrode work. The measuring tube should be full, and a large number of bubbles are not allowed to pass through the sensor. When the conditions cannot be met, corresponding measures should be taken.
In order to make the instrument work reliably, improve measurement accuracy, and not be interfered by external parasitic potential, the sensor should have a good separate grounding wire, and the grounding resistance should be less than 10Ω, especially when installed on a cathodic protection pipeline. For example, the electromagnetic flowmeter installed on the main pipe of Tianjin Water Source Factory, because the pipeline adopts cathodic protection, the inner wall and outer wall of the pipeline to protect against electrolytic corrosion are insulated, and the measured medium has no ground potential. Therefore, the sensor grounding ring is installed The two end faces of the sensor are insulated from the flange connecting the pipe. The sensor and the ground ring are connected with a ground wire and lead to the ground electrode. The pipe flanges are connected by cables but not connected to the sensor. Flange connection bolts are separated by insulating bushes and washers. Since the electromagnetic flowmeter was put into production and used, the effect has been good.
The converter should be installed in a place that meets the requirements of its protection level. Under the premise of meeting the requirements of the installation environment and use, the distance between the converter and the sensor and the connection cable should be as short as possible to save investment and reduce possible strong electricity Signal interference.
In the past ten years, due to the development of electronic technology, ultrasonic flowmeters have been applied to flow measurement. There are many methods for measuring with ultrasonic flowmeters, among which the most typical are the time difference method and the Doppler method. The time difference flowmeter is mostly used in water purification plants. The method is to install two transducers on the measuring pipeline. Due to the influence of the flow velocity difference between the downstream and the reverse flow, the time difference from launch to reception is measured, and the flow velocity is measured based on this.
The main advantages of ultrasonic flowmeter:
- Easy to install and maintain. With the widespread use of clamp-on sensors, there is no need to punch holes in the pipeline or cut off the flow when installing and maintaining the ultrasonic flowmeter. It can be easily installed in existing applications, especially suitable for large-diameter pipeline inspection systems. .
- The caliber range is large, and the price is not affected by the pipe diameter.
- High measurement reliability.
- No pressure loss.
- is not affected by fluid parameters.
- Output standardized DC signal, which can be easily entered into the automatic control system.
When selecting an ultrasonic flowmeter, special attention should be paid to the installation error of the sensor, the scaling of the inner wall of the pipeline, and the uniformity of the anti-corrosion layer. These factors have a great influence on the measurement result. In addition, according to the measurement principle of ultrasonic flowmeters, the accuracy of measurement can only be guaranteed when the flow rate is evenly distributed. Therefore, there must be enough straight pipe sections upstream and downstream of the flowmeter. Refer to various materials and the user manual of the flowmeter. Less than 10D, downstream is greater than 5D.
Because the tap water industry is continuous production, uninterrupted measurement is extremely important. Therefore, the flowmeter installed on the pipeline cannot be frequently disassembled and sent for inspection. The general practice is to use a portable ultrasonic flowmeter with high accuracy and send it to the country for certification on a periodic basis. The unit is calibrated as a standard appliance of the enterprise, and then the online flowmeter is checked regularly by means of comparison. This requires the designer to set aside space for comparison and measurement according to the requirements of the user during the design process and consider the needs of future production management to facilitate the user. The flowmeter well should be made slightly larger, except for the installation of fixed flowmeters. , Reserve a space for portable flow meter measurement.