Wastewater Flowmeter-FAQ 

1. What’s wastewater？

Wastewater is the discharge water from domestic and production that is contaminated somehow. Water that has lost its original function is referred to as wastewater. It can also be described as water that has deteriorated from its original use due to the addition of new substances to the water or due to changes in external conditions.

From the point of view of the source of sewage, it can be defined as a waste-carrying liquid or water mixed with groundwater, surface water, stormwater, etc., discharged from a residential, institutional, commercial, or industrial area. There are many categories of wastewater and, accordingly, many techniques and technologies to reduce the environmental impact of wastewater.

2. Why do we need wastewater treatment?

Currently, China is a country with severe water scarcity. The shortage of water resources has become a severe constraint to sustainable development and can be said to have become a bottleneck for the sustainable development of our country. Water resources are thoughtfully unevenly distributed in time and space. The large population and the rapid growth of the traditional industrial economy have exacerbated the water shortage problem; severe water pollution has also made the water shortage increasingly prominent. Therefore, water pollution prevention and control is the most urgent task of water resources protection. Water conservation is the quantitative protection of water resources, pollution prevention and control are the qualitative protection of water resources. These two are indispensable and complementary. Water conservation itself reduces the total amount of pollution; pollution prevention and control reduce unavailable water, equivalent to water conservation.

3. How to treat wastewater？

Wastewater treatment uses physical, chemical, and biological methods to treat wastewater to purify it, reduce pollution, to the point of recycling and reusing it, and make full use of water resources.

The construction of wastewater treatment plants is one of the essential means of managing wastewater. And the liquid flow meter becomes the central measuring apparatus in the wastewater treatment plant.

In practice, to ensure the relevant benefits and work quality improvement, the technical staff needs to strengthen sewage flow measurement and improve its control accuracy and reliability. This paper mainly analyses the working principle of several standard sewage flow meters, their installation and use of the problems, and specific analysis and interpretation of the selected design and treatment process of the relevant content.

4. How many types of wastewater flowmeters are available?

1)Electromagnetic flowmeter

Electromagnetic flow meters were developed in the middle of the 20th century and were widely used in industrial processes in the 1970s and 1980s. The principle of operation is based on Faraday’s law of electromagnetic induction. The measured medium flows perpendicular to the magnetic lines of force, thus generating an induced electric potential in the direction perpendicular to the medium flow and the magnetic lines of force. The relationship between the induced electric potential and the measured medium flow is proportional. Therefore, the flow rate and the pipe’s cross-sectional area can be derived from the instantaneous flow.

The conductivity of the water is one of the prerequisites for the electromagnetic flowmeter to work correctly. For example, wastewater has many contaminants and salts with a specific conductivity so that electromagnetic flowmeters can measure wastewater. On the other hand, pure water cannot be measured using an electromagnetic flowmeter.

Advantages：

① There are no moving parts and resistive fluid in the measuring tube. Thus the pressure loss is minimal, with no mechanical inertia, so the response is sensitive.

②Wide range of measurement: the range ratio is generally 10:1, up to 100:1, the flow rate range is generally 1-6m/s, can be extended to 0.5-10m/s; flow rate range can be from 90mL/h to more than 100,000m3/h; pipe diameter range can be from 2mm to 2400mm or 3000mm.

③Volume flow of liquids containing solid particles, suspended matter or acid, alkali, and salt solutions with specific conductivity can be measured, as well as pulsating flow and bi-directional measurement.

④ Flow signal and fluid volume flow rate between the linear relationship, so the instrument has a uniform scale. In addition, the volume flow rate of the fluid and the physical properties of the medium flow state, so the electromagnetic flowmeter only needs to be calibrated with water, can be used to measure the volume flow rate of other conductive liquids without correction.

⑤ Compared with most other flow meters, the front straight pipe section requirements are lower.

⑥Small size, lightweight, easy installation, low maintenance.

⑦ Measuring range is comprehensive, and measurement is not affected by changes in fluid temperature, density, pressure, viscosity, conductivity, etc.

⑧ It can be installed in the old pipeline by opening a hole, simple construction, and installation, small amount of work

⑨Adopting advanced processing technology, solid-state encapsulation, long life, so that the instrument has good measurement accuracy and stability, as well as the solid anti-interference ability

Disadvantages：

① The temperature and pressure of use must not be too high.

② The limited range of application, which cannot be used to measure the flow of non-conductive fluids such as gases, steam, and petroleum products and liquids containing a high number of more giant air bubbles.

③ When the flow rate and velocity distribution do not meet the set conditions, significant measurement errors will occur.

④ When the flow rate is too low, it is more difficult to amplify and measure the induced potential with the interference signal in the opposite order of magnitude. The meter is also prone to zero-point drift.

⑤ Electromagnetic flowmeter signal is relatively weak, the outside world slightly interference can affect the accuracy of the measurement.

2)Ultrasonic flow meter

The working principle of the ultrasonic flowmeter is that, with the help of the time difference measuring guide, the signal is transmitted through a pair of ultrasonic probes through the intermediate medium to reach the other side of the pipe wall where another examination receives it. The same signal is transmitted by the different searches received by the first probe. There is a limited time difference in this process as the flow rate of the medium influences the signal transmission time. Based on this, the ultrasonic flowmeter calculates the corresponding flow value with the help of the relevant formula.

Generally speaking, ultrasonic flow meters can be divided into tri-clamping type，pipeline and handhold. As technology is constantly renewed and escalated, ultrasonic flowmeters have evolved from mono-channel to multi-channel. Nowadays, the 8-channel ultrasonic flowmeter is commonly used. It can reach the accuracy of electromagnetic flowmeters and can be used to overcome the problem of bad mounting requirements. If the length of the straight pipe section before the flowmeter can not reach the installation requirements, a multi-sound path ultrasonic flowmeter can be installed.

Advantages：

①Ultrasonic flowmeter can be a non-contact measurement. A clamping ultrasonic flowmeter can be installed without stopping the flow carrier tube, as long as the pipe to be measured outside the installation of the transducer can be, that is, you can not break the flow or can not punch the existing pipeline with an ultrasonic flowmeter to measure the flow.

② Ultrasonic flowmeter is suitable for no flow resistance radial measurement, no additional pressure loss.

③ Measuring meter instrumentation coefficient can be calculated from the actual measurement pipeline and sound channel geometry to obtain, that is, the use of dry calibration, in addition to with the measurement pipe section type generally do not need to make the actual flow calibration.

④ Ultrasonic flowmeter is suitable for large round and rectangular pipes. The diameter of the tube does not limit the principle, and its cost is irrelevant to the diameter of the pipe.

⑤ Doppler ultrasonic flowmeter can measure the solid phase content of more or contain liquid bubbles.

Disadvantages：

① Propagation time method in the ultrasonic flowmeter can only be used for clean liquids and gases, can not measure suspended particles and bubbles more than a certain range of liquids; conversely, Doppler method LSF can only be used to measure drinks containing certain heterogeneous phases.

② External clamping type ultrasonic flowmeter can not be used for lining or scaling very thick pipeline, as well as can not be used for lining (or rust layer) and the inner tube stripping (if the sandwich layer with gas will seriously attenuate the ultrasound signal) or rust serious (change the ultrasound path) pipeline

③ Doppler method ultrasonic flowmeter, the measurement number is not high in most cases.

④ It can not be used for pipe diameter less than a DN25mm pipeline.

3)Open channel flowmeters

Most of our wastewater treatment plants are discharged naturally. That is to say, the flow of sewage in the pipeline or the channel is not a full pipe. In this non-full tube state, the waterway’s flow, that is, with a free surface of the waterway, is called “open channel.” Even in the so-called “culvert” waterways, as the liquid naturally flows down, it is also open channel flow. Traditional total pipe flow meters with pressure cannot measure flow in such watercourses. The measured wastewater has a wide range of contaminants and other organic chemical components. Even though the effluent is purified before discharge, if a flow meter in direct contact with the effluent medium is used, the inside of the flow meter will inevitably become scaled or eroded over time, which will affect the smooth performance of the measurement.

Wastewater discharge, especially sewage treatment plant sewage discharge metering instrument, is one of the vital monitoring data of the environmental protection department. Therefore, it must have the following characteristics.

①The measuring instrument must be a non-contact measuring instrument. Most of the sewage contains a variety of dirt and corrosive substances, only non-contact measurement, to avoid the impact of the physical and chemical properties of the medium to ensure the accuracy of size and even inertia.

②The detection component must be clipped to the pipe on the outside. And it can be moved to any position for measurement without the need to cut off the pipe. As a result, the gauge has a wide range ratio, good linearity, and is moderately priced.

③Measuring instrumentation low power consumption, in line with today’s trend of miniaturization of instrumentation.

Ultrasonic open channel flowmeter is a non-contact instrument with the above characteristics. One of the advantages of the available channel flow meter is the extensive measuring range. And its flow measurement is unaffected by return water on the tributary surface, floating objects in the water, air bubbles, and significant changes in water level. The flow sensor also creates drag on the water flow. It is simple in construction, small, and easy to install. It will be used in the PRD region, the standard channel does not need to be modified, can be installed directly, and the installation and construction costs are low.

The Bashar flume is an open channel water weir. It is generally a constriction section, a rough throat channel section, and a lower throat channel section. It can be divided into large, small, and standard type troughs. It has the characteristic that solids do not settle in the water., the water level raised small fall characteristics, suitable for use in the drainage ditch for the measurement of flow. The nullah flow meter also relies on the ultrasonic level meter to measure the flow in the actual operation process. In addition, the device can be used in harsher environments due to its ability to perform non-contact measurements during essential operations.

Overall, the complex shape of the marshall tank is more expensive than a weir, and the dimensions of the measuring tank are required to be accurate to improve accuracy. In addition, when installing an open channel flowmeter such as a bar shall tank in the Pearl River Delta, it is essential to avoid the effects of high and low tides on the flowmeter and ensure that the flowmeter has a sufficient fall to confirm accurate measurement. Its advantages are mainly small water level loss (about 1/4 of the weir), almost no precipitation in the water even if there is a solid material, small impact of approaching flow rate, relatively small impact on the water level on the downstream side, etc. Therefore, it is suitable for measuring the flow of sewage discharged from wastewater treatment plants.

4)Orifice plate flowmeters

Since its application in the industry, the range of applications for orifice flowmeters has expanded. Orifice plate flow meter application in sewage treatment is also feasible. Wastewater has the characteristics of significant changes in flow, impurities, corrosive and small, and a certain degree of electrical conductivity. Thus the orifice plate flowmeter measures the flow of wastewater in the direction of flow of the measured medium vertically to the magnetic line of force. Therefore, the flow direction and the magnetic force line in the media perpendicular to the induction electric force EX and the measured medium flow are positively proportional.

Orifice plate flow meter is not subject to temperature, pressure, viscosity, can choose neoprene lining, containing stainless steel electrodes of the orifice plate flowmeter, can meet the requirements of sewage flow measurement.

The existing orifice flowmeter mainly includes an integrated orifice flowmeter and intelligent orifice flowmeter two.

The difference between the two is that.

①Integrated orifice plate flow meter measures the flow of differential pressure generation device, with a variety of differential pressure meter or differential pressure transmitter can measure the flow of various fluids in the pipeline, orifice plate flowmeter throttling device including ring chamber orifice plate, nozzle, etc.

②Intelligent orifice plate flow meter is a set of flow, temperature, pressure detection functions in one. It can be temperature, pressure automatic compensation of a new flowmeter generation, the orifice plate flowmeter using advanced microcomputer technology and micropower consumption of new technology, muscular function, compact structure, simple operation, easy to use.

Principle and structure：

In the fluid that fills the pipe, when they flow through the throttling device in the tube, the flow rate will form a local contraction at the throttling device, thus increasing the flow rate and reducing the static pressure, so the pressure drop or pressure difference is generated before and after the throttling device. The greater the flow of the medium, the greater the differential pressure generated before and after the throttling device, so the differential pressure can be measured to measure the size of the fluid flow.  This measurement method is based on the law of energy conservation and the rule of continuity of flow.

Advantages

① The standard throttle is fully operational and approved by the International Standards Organisation, and can be commissioned without the need for accurate flow calibration, and is unique among flow sensors.

②The structure is easy to replicate, simple, robust, stable, and reliable in performance and low price.

③ A wide range of applications can be measured, including all, single-phase fluids (liquid, gas, steam), part of the mixed-phase flow, general production process pipe diameter, and operating conditions (temperature, pressure).

④ Detection parts and differential pressure display instruments can be produced separately by different manufacturers, facilitating specialized scale production.

Disadvantages

①The repeatability and accuracy are of medium level in the flow sensor. Due to the complex influence of many factors, the accuracy is difficult to improve.

② Narrow range, due to the flow coefficient and Reynolds number, the available range of only 3:1 to 4:1.

③ The orifice flowmeter has high requirements for the length of the straight pipe section, which is generally difficult to meet. Especially for bigger tube sizes, this problem is more prominent.

④ Large pressure loss.

⑤ Orifice plate within the hole sharp angle line to ensure accuracy, so the sensor is sensitive to corrosion, wear, scaling, dirt. Long-term precision is difficult to ensure the need to remove the vigorous inspection once a year.

⑥The use of flange connection, easy to produce running, bubbling, dripping, leakage problems, significantly increasing the maintenance workload.

5. How to choose a wastewater flowmeter？

The measurement of flow is complex technology. In terms of the fluid to be measured, it includes three different physical properties: gases, liquids, and mixed fluids. From the measurement conditions, and a variety of, to the metallurgical industry, for example, the production of liquid – water used in the measurement of the different production systems, divided into various media such as net ring water, turbid ring water, rolling steel wastewater, smelting wastewater, domestic wastewater. The selection and application of flowmeters for various wastewater quality are also different.

① Electromagnetic flowmeter measurement stability, a wide range of applications, can measure various media but is vulnerable to electromagnetic wave interference. Furthermore, with the increase in the diameter of the pipe, its price will also become more and more expensive.

②Ultrasonic flowmeter cost is relatively low. It has high measurement accuracy and is also more stable in operation. The product is easy to install and easy to maintain. It does not get more expensive as the pipe diameter increases, but it does get more costly as the sound path increases.

③ The open channel flowmeter has a broad spectrum of measurement. Medium changes do not impact its flow measurement, but the size of the measuring tank constrains it. Generally speaking, in the case of meeting the open channel installation conditions, the flow data is large and can be conveyed through the available channel sewage flow operations, using a weir flowmeter and trough flowmeter for related processes.

④ Orifice plate flowmeter is a large number of early use of metering devices, its most extended history, and the most use. Now standard for the round orifice plate type and conical inlet plate type, its working principle is to add an orifice plate throttle in the fluid, through the introduction of pressure transmitters to measure the pressure difference between the throttle up and downstream, after the calculation that the instantaneous value of the flow. However, due to the immobility of the water in the guide tube, the outdoor installation of the orifice plate in winter takes the pressure tube, which is easy to freeze and crack, so that the differential pressure instrument can not work correctly. This situation has been encountered in the actual work. In the measurement of wastewater, the orifice plate needs to be cleaned frequently. Otherwise, it will affect the measurement accuracy, which will seriously cause the instrument not to work correctly. There are also disadvantages such as high-pressure loss and high maintenance when measuring flow using an orifice plate.

6. What are the installation requirements for the wastewater flowmeter？

(1) The sensor can be installed either in a straight pipe, a horizontal, or an inclined line, but the centreline of the two electrodes is required to be in a horizontal position.

(2) The medium should flow in a full pipe at the installation, avoiding a full tube and gas adhering to the electrode.

(3) Vertical installation is preferable for liquid and solid phase fluids. This allows the sensor lining to be worn evenly and prolongs the service life.

(4) Flowmeter installation position media dissatisfaction, can take the method of lifting the back end of the flowmeter pipeline so that it is the whole pipe, is strictly prohibited in the highest point of the channel and the outlet installation of the flowmeter.

(5) Modified pipe installation method: a smaller diameter flowmeter should be used when the medium does not meet the flow rate requirements. This should be used to change the diameter of the tapered pipe or amend part of the pipeline so that it is the same caliber with the sensor, but before and after the straight pipe section must meet at least before the straight pipe section ≥ 10DN, after the straight pipe ≥ 5DN (DN for the diameter of the tube).

(6) Electromagnetic flowmeter before and after the straight pipe section for the flowmeter before ≥ 10DN, the back end ≥ 5DN.