Industrial wastewater discharge includes pressure piping and open channels, pressureless pipelines and other forms of non-pressure discharge.The sewage flow meter discharged in the form of a pressure pipe is convenient to measure, and can be measured by a pipe flow meter such as an electromagnetic type, a laser type, or a venturi differential pressure type. At present, the sewage discharge in China mainly adopts the form of discharge without pressure open channel, and the sewage open channel flow meter is designed to solve the problem of sewage measurement in this form of discharge.
1. Working principle and composition of open channel flow meter
Pasteur trough and thin-walled weir are commonly used as measurement methods for open channel sewage flow.Its principle belongs to the throttling flow measurement technology. The direct object of measurement is the water level difference between upstream and downstream of the throttling device. According to the water level difference, the instantaneous flow on the cross section can be calculated.
The open channel sewage flow meter is composed of a measuring weir (flow measuring flume), liquid level sensor and secondary instrument. The work of forming the head (standard flow), measuring the height of water level and calculating the flow rate are completed respectively.
2. Types and scope of weirs and troughs
A weir is a device that intercepts the flow of water and allows it to overflow from the top. There are many kinds of weirs: According to the thickness, it can be divided into thin-walled weirs (whose thickness is less than 0.67 times the geometric head), suitable weirs and wide-crested weirs, etc.According to the cross-section shape, it can be divided into triangular weir, trapezoidal weir, equal-width weir, etc. According to the connection form of jet and downstream water level, it can be divided into submerged weir and non-submerged weir.Thin wall triangular weir, thin walled trapezoidal weir and thin wall rectangular weir are commonly used.Under certain conditions, for a certain thin-walled weir, the flow rate has a certain function relationship with the geometric head height, and the sewage flow can be measured accordingly. When measuring the flow with water weir, the loss of water head is large, which is suitable for measuring the sewage with little suspended substance and non-fibrous substance.
Wide-crested weir with lateral contraction is called flume when the height of weir bar is zero. It is a special deformation of weir. It can be divided into short throat flume (Pasteur flume), no throat flume (Sun flume) and long throat flume.Commonly used in open channel sewage flow metering is Pasteur trough.Under certain conditions, for a certain Pasteur trough,there is a certain functional relationship between the flow and the upstream water level height,according to which the sewage flow can be measured. When measuring flow with flow measuring trough, head loss is small and water quality conditions are wide. When measuring sewage, aquatic organisms on the trough wall should be cleaned up in time to keep the trough wall smooth.
3. Liquid level sensor
There are mainly ultrasonic liquid level sensors, pressure type liquid level sensors, capacitive liquid level sensors, float type liquid level sensors and so on.These level sensors have their own characteristics and different application ranges.
Ultrasonic sensor is a specific application of ultrasonic ranging method. Its characteristics are to achieve non contact flow and wide application range, but the required liquid is reflected well by ultrasonic wave. If there is abnormal reflection of foam, debris and biological mud, it will bring greater error. In addition, ultrasound is also affected by weather factors such as temperature, air pressure and wind speed. Although these factors can be compensated, the structure of the instrument is complex and the cost is high.
The characteristic of pressure type liquid level sensor is simple installation. However, the specific gravity of the tested liquid is basically constant, so its application in sewage with large variation of specific gravity is limited. In addition, attention should be paid to the smooth opening of the pressure measuring hole when used in sewage.
The characteristic of capacitive hydraulic sensor is that there are no moving parts, but the mechanical strength and dielectric properties of insulating medium are difficult to take into account, and the change of capacitance caused by the influence of temperature reduces the measurement accuracy. In addition, the electrode surface should be cleaned up in time when used in sewage.
The float liquid level sensor has the characteristics of low cost, not affected by water quality and wide application range, but it requires that the specific gravity of water body is basically constant, and attention should be paid to the effect of surface adhesives on the specific gravity of float.
Secondary instrument is an automatic device which can receive the voltage (current) signal of liquid level sensor and accumulate sewage flow based on it. It can not only give instantaneous flow, but also get cumulative flow. It can connect the recorder to observe the change of flow rate and the computer to realize the automatic flow monitoring.