Flow meter technology and application in water supply industry

The flow meters used in the water supply industry are divided into two categories: the measurement of the handover and settlement of raw water entering the plant, finished water leaving the plant, and water supply to users, and the flow, measurement/control of the internal process flow of the water plant.

Before the 1950s, the water supply and handover settlement instruments all used traditional Venturi tube differential pressure flowmeters and mechanical impeller water meters for terminal user charges. In the 1960s abroad and the mid-1970s in China, large-diameter electromagnetic flowmeters appeared, which began to break the monopoly of Venturi tube meters on large-diameter water flow meters. In the mid-1970s, large-diameter flow meters with other measurement principles also appeared, such as insertion turbine flowmeters, average velocity tube flowmeters, ultrasonic flowmeters, etc. These meters have their own characteristics and are suitable for different application sites.

Installation and use of flow meters in the water supply industry

Before the 1970s and 1980s, the water supply industry in China was limited to internal process control in water plants (such as raw water/coagulant ratio flow control in the water purification process) and end-user water consumption metering because the public utility water plants, pipelines, and sales and distribution belonged to the same enterprise. Only a few water plants were equipped with metering instruments for finished water leaving the factory. With the separation of production and sales in the water supply industry to improve efficiency and control pipeline leakage, that is, the two independent enterprises of water plant production and pipeline distribution need to add handover and accounting metering flow meters; in addition, water plants need to pay water resource fees to the state instead of drawing water from rivers for free, so they also need to add flow meters. Therefore, the demand for large-caliber handover and accounting instruments in the water supply industry has increased significantly.

The Equipment Working Committee of China Urban Water Supply Association investigated the types of flow meters installed by water supply enterprises in 46 large and medium-sized cities in China in 2002/2003 (excluding impeller water meters). Among the 1,900 flow meters of the 46 enterprises, the most used were electromagnetic flow meters, accounting for 61.9%, followed by ultrasonic flow meters at 26.6%, insertion flow meters at 9.7%, and Venturi flow meters at only 1.8%. Compared with the flow meters used for water leaving water plants in the 1991 survey in the "Technical Development Plan for Urban Water Supply in 2000", the number of electromagnetic flow meters increased a lot (55% in 1991), while the number of insertion flow meters decreased a lot (20% in 1991). Compared with the flow meters reported by the Tokyo Metropolitan Waterworks Bureau in 2000 (1,083 samples, 30.1% of Venturi differential pressure flow meters, 45.2% of electromagnetic flow meters, and 18.4% of ultrasonic flow meters), our electromagnetic flow meters are more widely used, while Venturi differential pressure flow meters are less widely used. The Tokyo Metropolitan Waterworks Bureau's Venturi tube differential pressure instruments are widely used in the measurement of finished water leaving the factory and raw water entering the factory, as well as in flow control within the water plant.

Evaluation of the flow meters used by water supply enterprises

Domestic water supply enterprises believe in the long-term operation practice of various types of flow meters used: electromagnetic flowmeters have good performance, high accuracy, mature technology, and are recognized by most enterprises. Although the price is relatively high, ultrasonic flowmeters will be more widely used in the future. They have stable performance and medium accuracy, and multi-channel or pipe segment instruments are more accurate. Ultrasonic flowmeters with external clamped transducers can be installed without stopping the flow, which is its unique advantage. It is expected that due to its moderate price and convenient installation, it will have a large development space; the insertion turbine flowmeter has poor stability and accuracy, but the price is low. Under the condition of good maintenance and management, it is more suitable for use in small and medium-sized urban water plants; Venturi tube differential pressure flowmeters are currently installed by only a few companies. For example, the Dongshen Water Supply Project uses dozens of units to transport water to Hong Kong. They are generally of medium accuracy, and real-flow calibration can obtain higher accuracy. It has the advantage of only measuring the geometric dimensions of the Venturi tube for regular inspection without real-flow calibration. With technological progress, there is still room for development.

According to

the requirements of ISO9000 Quality Management System, measuring instruments must be operated under controlled conditions, and according to the provisions of JJG198-94 Velocity Flowmeter Verification Procedure, they must be calibrated regularly. There are usually two methods for checking the flow value of flow meters: offline inspection and on-site online inspection. Off-line flow calibration inspection removes the meter from the pipeline and sends it to the laboratory flow calibration device for real flow calibration. However, the water supply industry does not allow large-diameter water pipes to arbitrarily cut off the water flow, decommission and disassemble the flow sensor, and implement offline calibration, which is difficult to achieve due to the huge engineering costs. This problem became a hot topic of discussion at the "Flow Meter Application Technology Seminar" held by the Equipment Committee of the China Water Association in November 2003. In practice, each water supply company explores several on-site comparison or indirect inspection (check) methods according to their own use conditions to verify or evaluate whether the flow measurement value of the flow meter is still within or has exceeded the original calibration accuracy level, as a basis for whether it can still be used or needs further inspection. Eleven of the 24 papers submitted by 19 water supply companies involved on-site comparison or online verification of large-diameter flow meters.

The real-flow calibration of flow meters is a set of operation processes to establish the relationship between the indication value of the calibrated flow meter and the corresponding value of the original standard or transfer standard, and to transfer the calibration value to the calibrated meter, which has the legal metrological nature; on-site comparison is to compare the values ​​of instruments with the same (or similar) accuracy level under specified conditions, which is an operation without the legal metrological nature; online verification is to provide objective evidence on-site to show that the specified requirements have been met.

The methods of on-site comparison proposed in the seminar paper include the clear water tank volume method, the series pipe section instrument method, the external clamp (external sticker) transducer ultrasonic flowmeter method and the insertion turbine flowmeter method, and the online verification methods include the inspection and verification of electromagnetic flowmeters. [page]

The clear water tank volume method is a traditional method often used in the water supply industry. It can take advantage of the existing large-volume clear water tank in the water treatment plant to obtain a higher comparison accuracy. Three companies proposed papers on the implementation of this method at the seminar, among which the paper of Changsha Water Company briefly described the implementation process and the data of the comparison test case. Carefully measure the geometric dimensions of the water tank, and expect to obtain a volume uncertainty of 0.2%; operate for a long time (more than a few hours) to make the water level difference of the inlet tank exceed 1m to reduce the influence of various operating errors. Under such operating conditions, the total uncertainty of the clear water tank comparison method is expected to be between 0.5% and 1%.

The use of portable external clamp-on transducer ultrasonic flowmeter to compare flow is very common in the water supply industry. Among the 11 companies that proposed on-site online comparison papers, 9 have adopted this method. This method can obtain medium comparison accuracy, but when using this method, the uncertainty of the measurement system composed of the ultrasonic flowmeter installed in the field comparison should be evaluated. Because the system uncertainty includes not only the accuracy of the ultrasonic flowmeter itself (0.5% to 1.5% of the flow velocity), but also the additional errors caused by the measurement deviations of the flow area, sound path distance and sound path angle (such as a 0.5% measurement deviation of the inner diameter of the pipe, an additional error of 1% in the flow area; a 1% installation deviation of the transducer spacing, an additional error of 1%; a 1° deviation of the 45° sound path angle, an additional error of 1.7%). The incoming flow is deflected by the upstream deflectors (such as valves and elbows), and the insufficient straight pipe section causes the additional errors caused by velocity distribution distortion and rotational flow. The accuracy of the portable ultrasonic flowmeter is usually the accuracy of the measured flow velocity. Do not misunderstand or be misled into thinking it is the accuracy of the measured flow. The total uncertainty of the portable ultrasonic flowmeter comparison method is between 2% and 5%. If the measured geometric dimensions are rough or the exact flow area cannot be obtained (such as severe rust or accumulation on the inner wall), the system uncertainty may even exceed 5%. Most water supply companies have noticed the importance of comparing the installation position and pipe/installation size of portable ultrasonic flowmeters. For example, when Shijiazhuang Water Supply Corporation designed a new flowmeter well, it reserved pipe sections and space for the comparison flowmeter at a location with ideal measurement conditions in the instrument well. Some companies use portable ultrasonic flowmeters to measure and compare after installing new flowmeters, and record the data to facilitate future analysis of whether there are any changes in the compared fixed-installation flowmeters.

The installation position or method of the portable ultrasonic flowmeter transducer can also be changed to detect the flow conditions of the on-site pipe section. For example, move the two transducers along the circumference of the pipe and check the linear average velocity at different positions. The maximum velocity may be the position closest to the actual average velocity, because the average velocity reading formed by the velocity distortion at the most asymmetric position is the smallest. This is the evaluation method recommended by all published experimental reports. Compare the velocity measured by the transducer installed according to the Z method and the V method. If the difference between the two is large, it indicates that there is a serious lateral flow, that is, there is a sign of rotational flow, which should be paid attention to and measures should be taken.

There are 3 papers proposing the implementation of the series pipe section flow meter comparison method. Guangzhou Water Company has used one DN800 and one DN1200 electromagnetic flowmeter for five years. The sending and receiving parties have disputes over the measurement accuracy of the instrument, so they temporarily connect the same or different caliber electromagnetic flowmeters in series for a long time to run and compare. The difference is only between 0.1% and 0.2%, and the dispute is settled.

Changchun Water Supply Company connected an electromagnetic flowmeter in series downstream of an ultrasonic flowmeter with a transducer inserted into the pipe wall for comparison, and found that the ultrasonic flowmeter undermeasured by 8%. Nanchang Water Supply Company connected two electromagnetic flowmeters in series on the factory water pipeline for mutual comparison. If the difference between the measured values ​​of the two meters is abnormal, they can be sent for inspection in steps. Once one of the meters fails, the other will work as usual, and there will be no disputes due to metering interruption.

Two companies use inserted turbine flowmeters to compare flow with fixed installed flowmeters in use. When Nanchang Water Supply Company updated three flowmeters on the DN1000 pipeline, it reserved an insertion hole and valve on one side to insert the turbine flowmeter for regular flow comparison. The company believes that the turbine flowmeter is stable and reliable in short-term use, easy to operate, and cost-saving. When Xiangfan Water Supply Company updated the original inserted turbine flowmeter measurement point to a pipe segment flowmeter, it retained the original insertion installation port so that the turbine flowmeter can be inserted regularly for comparison in the future. Although the measurement accuracy of the insertion flowmeter in the large-diameter system is low (2.5% to 5%), because even if the turbine flow sensor has a higher accuracy (0.5% to 1% of the flow rate), it also includes the measurement error of the flow area and the uncertainty of the velocity distribution coefficient/blockage coefficient, etc., but it has excellent repeatability (0.25% to 0.5%). If the turbine flowmeter is regularly compared with the flowmeter in use at the same location, and compared with the first comparison data after the new installation, it can be evaluated whether the meter is operating normally and accurately.

Shanghai Water Supply Company/Raw Water Company and Guangzhou Water Supply Company have explored methods for on-site online inspection and verification of electromagnetic flowmeters. They have adopted methods to check the resistance/insulation of the flow sensor excitation coil and the excitation current to evaluate whether the magnetic field strength has changed, measure the electrode contact resistance to evaluate the electrode surface condition, check the insulation of each part to determine the degree of component degradation, etc., and use analog signal devices and other general instruments to check the converter.

Through these inspections, it is verified whether the instrument maintains its original performance. In the past few years, Shanghai has accumulated experience in inspection and verification of more than 300 units, and is formulating the "Online Verification Specifications for Electromagnetic Flowmeters".