Problems that should be paid attention to in the selection and design of commonly used instruments in water treatment systems (1)

1. General requirements for instrument selection

  1. Accuracy: refers to the accuracy of the measurement result of the instrument under normal conditions of use. The smaller the error, the higher the accuracy. The accuracy of the physical detection instrument in the production process is ±1%, and the accuracy of the water quality analysis instrument is ±2% (the accuracy of the turbidity meter for measuring high turbid water is ±5%).
  2. Response time: When the measured value is measured, the indicator value of the meter will always be displayed after a period of time, and this period of time is the response time of the meter. Whether an instrument can reflect the change of parameters as soon as possible is a very important indicator. The response time required by the water quality analysis instrument should not exceed 3 minutes.
  3. Output signal: The analog output of the instrument should be a 4-20mA DC signal, and the load capacity should not be less than 600Ω.
  4. The protection level of the instrument should meet the requirements of the environment, and generally should not be lower than IP65. The detection instrument used in the pharmaceutical dosing system requires corrosion resistance.
  5. The power supply of the four-wire system is mostly 220V AC, 50Hz, and the power supply of the two-wire system is 24V DC.
  6. Digital display should be selected for field monitoring instrument.
  7. The working power supply of the instrument should be independent, and should not share the power supply with the computer, so as to ensure that the power supply does not interfere with each other during failure and maintenance, so that each can operate stably and reliably.
  8. In order to enable the computer to detect the abnormal signals of the voltage transformer and current transformer and give an alarm, the input signal of the voltage and current transducers designed and matched should be larger than that of the current and voltage transformers, that is, 0~6A respectively And 0~120V.
  9. An instrument manufacturer that can provide reliable services and has extensive experience should be selected.

2. Water level measurement

The following factors should be considered when selecting a liquid level timer: (1) The measurement object, such as the physical and chemical properties of the measured medium, as well as the working pressure and temperature, installation conditions, the speed of liquid level change, etc.; (2) Measurement and control requirements, such as Measurement range, measurement (or control) accuracy, display mode, on-site indication, remote indication, interface with computer, safety and corrosion protection, reliability and ease of construction.

The commonly used level gauges in water supply engineering and the key points of selection are as follows:

  • Floating ball level gauge
    Put a hollow floating ball in the liquid. When the liquid level changes, the floating ball will produce the same displacement as the liquid level change. The displacement of the float can be measured by mechanical or electrical methods, and its accuracy is ±(1-2)%. This level gauge is not suitable for high-viscosity liquids, and its output terminal has switch control and continuous output.
    In the design of water purification plants, this type of level gauge is often used for the level measurement of the collection well to control the automatic start and stop of the drainage pump.
  • Static pressure (or differential pressure) level gauge
    Since the static pressure of the liquid column is proportional to the liquid level, the liquid level can be measured by measuring the static pressure of the liquid column on the reference surface with a pressure gauge. Calculate the pressure or differential pressure range according to the density of the measured medium and the liquid measurement range, and then select a pressure gauge or differential pressure gauge with appropriate performance such as range and accuracy. The accuracy of this level gauge is ±(0.5~2)%.
  • Capacitance level gauge
    Insert an electrode into the container. When the liquid level changes, the internal medium of the electrode changes, and the capacitance between the electrodes (or between the electrode and the container wall) also changes accordingly, and the change in capacitance is then converted into a standardized direct current signal. Its accuracy is ±(0.5~1.5)%.
    Capacitive level gauge has the following advantages: the sensor has no mechanical moving parts, simple and reliable structure; high accuracy; low power consumption at the detection end, fast dynamic response; convenient maintenance and long life. The disadvantage is that the instability of the dielectric constant of the measured liquid will cause errors. Capacitance level gauges are generally used for liquid level measurement in regulating tanks and clean water tanks.
    When the measuring range does not exceed 2m, rod-shaped, plate-shaped, and coaxial electrodes are used; when it exceeds 2m, cable-type electrodes are used. When the measured medium is water, an electrode with an insulating layer (polyethylene can be used) is used.
  • Ultrasonic level gauge
    The sensor of the ultrasonic level gauge is composed of a pair of transmitting and receiving transducers. The transmitting transducer emits ultrasonic pulses facing the liquid surface, and the ultrasonic pulses are reflected from the liquid surface and received by the receiving transducer. The distance between the sensor and the liquid level can be determined according to the time from transmission to reception, which can be converted into liquid level. Its accuracy is ±0.5%.
    This kind of level gauge has no mechanical moving parts, high reliability, simple and convenient installation, non-contact measurement, and is not affected by the viscosity and density of the liquid, so it is mostly used in medicine pools, medicine tanks, mud drain pools, etc. Level measurement. However, this method has certain blind spots and is more expensive.
Runxin automatic valve F74A

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