Problems that should be paid attention to in the selection and design of commonly used instruments in water treatment systems (3)
Turbidity is a measure of the turbidity of water, that is, the degree to which the presence of finely dispersed suspended particles in the water reduces the transparency of the water. Turbidity meter is an instrument for measuring the turbidity of water bodies, mainly used for monitoring and management of water quality.
The water purification plant is responsible for the supply of domestic and industrial water for residents. The quality of water supply directly involves the health and safety of the people, as well as the normal production and product quality of various industries such as food, brewing, medicine, textiles, printing and dyeing, and electricity. Turbidity is a very important water quality indicator, so the choice of turbidity meter is particularly important. Turbidity meters can be divided into two categories: visual turbidity meters and photoelectric turbidity meters. The photoelectric turbidity meter can be divided into process monitoring (continuous measurement) turbidity meter and laboratory (including portable) turbidity meter in terms of its use, and it can be divided into transmission light turbidity meter and scattered light turbidity meter in terms of its design principle.
Because the scattered light turbidity meter has high sensitivity to low water turbidity, high accuracy, small relative error, good repeatability, the chromaticity of water does not show turbidity, and the intensity ratio of scattered light to incident light can be linear Therefore, the “Drinking Water Quality Guidelines” published by the World Health Organization in September 1992 stipulated that the scattered light turbidity meter should be used as a measuring instrument. At the same time, the “Technical Progress Development Plan for the Water Supply Industry in 2000” has clearly stipulated that the water turbidity index of the first-class water pipe network is 1NTU.
The 1720D, SS6 series turbidity meter (belonging to scattered light turbidity meter) of HACH Company is commonly used in the design of water purification plant.
In the measurement of filtered water and factory water, the 1720D (formerly 1720C) series turbidity meter is generally used. When in use, the water sample continuously flows into the turbidity meter, flows through the deaerator to empty the bubbles in the water stream, and then enters the center column of the turbidity meter, rises to the measurement chamber and overflows its edge into the discharge port. The focused beam is projected downward from the sensor head assembly to the water sample in the main body of the turbidity meter. The photoelectric tube immersed in the water sample measures the scattered light of the suspended solids in the water at 90°, the amount of scattered light and the turbidity of the water sample The degree is proportional. The 1720D does not need to use a sample cell, which can reduce stray light and improve measurement accuracy. The accuracy of the 1720D is: ±2% in the range of 0-40NTU, ±5% in the range of 40-100NTU, resolution of 0.001NTU, and response time of 75s.
The turbidity meter for measuring filtered water is mostly installed in the pipe gallery of the filter station, which can be wall-mounted or cabinet mounted. The measurement of factory water is generally set in the water quality meter room of the water pump room, and the turbidity meter and other water quality detection meters are placed in the meter. In the room, then lead the signal to the monitoring station.
Although the measurement range of 1720D is 0~100NTU, it is best not to measure the pre-filtered water, because although 100NTU can be measured optically, it will bring a lot of inconvenience in production and use. SS6 series surface scattering turbidity meter is often used to measure source water and water before filtration. It shoots the beam on the liquid surface to measure the scattered light from the liquid surface, avoiding direct contact between the optical system and the water sample, and eliminating the need to clean the flow cell. Signal loss caused by time,
The measurement range of SS6 series is 0~9999NTU. Generally, the source water of surface water plants is within this range. It has an accuracy of ±5% in the range of 0~2000NTU, and an accuracy of ±10% in the range of 2000~9999NTU.
The selection of the sampling point of the turbidity meter should be closely integrated with the process specialty, and the most representative point should be selected. The sampling hole is best not to be opened on the top of the sampled pipe to avoid drawing bubbles in the pipe into the sampling pipe to affect the turbidity meter. It is best to use a small sampling pump to sample the water sample to ensure that there is a certain flow rate in the sampling pipe, and it is not easy to scale on the inner wall of the pipe. The diameter of the sampling pipe should be determined according to the total demand for sampling water from the instrument.
The choice of display instrument
General water purification plant projects mostly use intelligent display instruments, which have complete functions, can carry out digital signal processing, realize control functions, and the measured values are displayed by liquid crystals, which are convenient to operate, can save data, and have self-diagnostic functions. Although its advantages have not been fully utilized after being connected to the computer system and have been replaced by the computer system, in the current construction of the water purification plant, the use of intelligent display instruments is used in the uncommissioned stage of the computer system or malfunctions The auxiliary instrument at the time can also meet the requirements of on-site control and display.
In some cases, both local display and remote transmission are required. At this time, it is not advisable to adopt the signal serial mode, but a signal distributor, that is, one input, two outputs, one output is sent to the display instrument, and the other output can be input to the PLC
Grounding and lightning protection of instrument system
Grounding can be divided into protective grounding and working grounding. Protective grounding is to avoid electric shock hazard and protect the safety of the equipment when the equipment is damaged or the insulation performance is degraded. Working grounding is to ensure the stable and reliable operation of the instrument. Generally, the grounding of the instrument system of the water purification plant adopts the TN-S system, that is, 3 phase wires A, B, and C, and 1 neutral wire N is the protection wire PE. The exposed conductive part of the electrical equipment is connected to the PE line. The advantage is that the PE line does not present current during normal operation. Therefore, the exposed conductive part of the equipment does not present a voltage to ground and it is easy to cut off the power in the event of an accident. Strong electromagnetic adaptability avoids the interference of higher harmonics.
The principle of working grounding is single-point grounding. Due to the existence of the potential difference to the ground, if more than one grounding point appears, a ground loop will be formed and interference will be introduced into the instrument. Therefore, there can only be one grounding point for the same signal loop and the same shielding layer. The working ground of the instrument can be set separately or share the same grounding body with the protective grounding. From the engineering practice experience, the grounding resistance should generally not exceed 1Ω.
Generally, the facilities of the water purification plant are scattered, the structures are low, and the terrain is flat and open. In particular, some flow meter wells are located outside the plant area. In this case, the lightning strike rate of the instrumentation equipment increases. In practice, the author has repeatedly encountered incidents where the instrument was damaged by lightning or the instrument was damaged for unknown reasons. Therefore, installing a lightning arrester with good quality and reliable action is an indispensable protective measure. For example, the use of the ESP series lightning arrester of the German Pepperl+Fuchs company for the protection of the signal and power supply of the flowmeter has good results.