The application of electromagnetic flowmeters in the water supply industry.

Keywords: Electromagnetic flowmeter, design selection, measurement accuracy, zero point

Topic: Discussing the application experience of electromagnetic flowmeters in the water supply industry

As a flowmeter with good linearity, wide range ratio, high reliability, and high accuracy, the electromagnetic flowmeter has been widely used in the water supply industry, especially in the measurement of source water and factory water at the inlet/outlet of water plants. Although the design and process technology of electromagnetic flowmeters are continuously improving, many factors in the actual application process at the user's site can directly affect the instrument's performance, failing to meet user expectations. Users need to ensure the basic conditions for the normal operation of the instrument throughout the design selection, installation, debugging, and maintenance processes, so that the instrument operates in a good working state.

1 Design Selection

1.1 Necessary conditions to ensure the measurement accuracy of electromagnetic flowmeters

(1) The measured fluid medium must be conductive;

(2) The measured fluid medium must fill the pipeline;

(3) The flowmeter measurement system must be well grounded;

(4) The flowmeter should meet the requirements for the length of straight pipe before and after it;

(5) Strong electromagnetic field interference should be avoided near the flowmeter.

1.2 General selection principles

(1) Determining the diameter

The electromagnetic flowmeter can continuously measure a wide range of flow rates, and within the specified flow (velocity) range (0.5 to 10 m/s), the measurement range can be adjusted arbitrarily. Generally, selecting a flowmeter diameter equal to the process pipeline diameter can meet the working conditions and is convenient for installation without pressure loss.

The relationship between flow, velocity, and diameter is as follows:

Q = 0.0028274 * D2 * v

v = Q /(0.0028274 * D2 )

Where: Q = flow (m3/h)

D = nominal diameter of the flowmeter (mm);

V = fluid medium velocity (m/s).

(2) Recommended flow velocity

a. From the perspectives of accuracy, economy, and durability, the recommended flow velocity range is between 1 to 5 m/s. Within this range, the flowmeter has high measurement accuracy, good linearity, low power loss, and less wear on the flowmeter lining and electrodes by the fluid medium.

b. For fluid media containing solid particles, the recommended flow velocity range is between 1 to 3 m/s. This choice helps avoid excessive wear on the flowmeter lining and electrodes caused by suspended solid particles at high flow velocities.

c. For fluid media that may cause sediment in the pipeline, the recommended flow velocity range is between 2 to 5 m/s. If the requirements cannot be met, a flowmeter with a diameter smaller than the pipeline diameter can be selected, and a reducer can be added, provided that pressure loss is allowed. Higher flow velocities help eliminate excessive sediment. Installing the flowmeter vertically or at a V-shaped bend helps eliminate excessive sediment.

(3) Corrosion resistance

The selection of electrode materials and lining materials for the flowmeter should be based on the corrosiveness of the measured fluid medium. For complex fluid media such as mixed acids, hanging piece tests should be conducted.

(4) Different diameters for the instrument and process pipeline

a. To ensure the instrument operates within an appropriate flow velocity range, when the flow velocity in the process pipeline is low, the process flow is relatively stable, and a certain pressure loss is allowed, a smaller diameter instrument can be selected, with reducers added before and after the instrument to increase the local flow velocity within the instrument.

b. For large-diameter process pipelines, when the flow velocity in the pipeline is low and the process flow is relatively stable, a smaller diameter instrument can be selected, with reducers added before and after the instrument. This can reduce instrument costs and also allow the instrument to operate within a better linearity flow velocity range.

c. To ensure the measurement accuracy of the instrument, the cone angle of the reducer should not exceed 15°, and there should be at least 5 times the process pipe diameter of straight pipe section on the upstream side of the reducer joint.

2 Installation

2.1 Installation site requirements (it is recommended to use the installation method shown in Figure 1)

The installation site and position of the electromagnetic flowmeter can be horizontal, vertical, or inclined according to the user's actual needs. To ensure stable and reliable operation of the electromagnetic flowmeter, the following requirements should be noted:

(1) The measurement pipe of the flowmeter must be filled with the fluid medium (i.e., no empty pipe or insufficiently filled pipe is allowed).

(2) The electrode axis of the flowmeter should be approximately horizontal.

(3) The upstream straight pipe section of the flowmeter (measured from the electrode axis) should be at least 5D long, and the downstream straight pipe section at the outlet should be 2D long.

(4) The flow direction of the measured fluid should be consistent with the direction indicated by the flowmeter's flow direction mark.

(5) To facilitate installation, maintenance, and servicing, sufficient operational and maintenance space should be ensured near the pipeline flange.

(6) When the pipeline diameter does not match the flowmeter diameter, tapered pipes can be installed at both ends of the flowmeter, with a cone angle of less than 15°.

(7) The installation site of the electromagnetic flowmeter should avoid strong magnetic fields and strong vibration sources. Fixed supports should be provided on both sides of the flowmeter's pipeline.

(8) The converter of the separately installed flowmeter should be installed in a ventilated and dry place, avoiding rain exposure and water accumulation. This is to prevent the electrical components of the instrument from getting damp, which could lead to reduced insulation performance and damage.

2.2 Grounding requirements

To ensure that the electromagnetic flowmeter operates stably and reliably, and to guarantee that its measurement accuracy is not affected by external electromagnetic field interference, the flowmeter should have good separate grounding. If the pipeline connected to the flowmeter is coated with an insulating layer or is a non-metallic pipeline, a grounding (liquid) ring should be added to the flowmeter.

3 Measurement Accuracy and Error Curve

The measurement accuracy and error curve provided by the electromagnetic flowmeter manufacturer refer to the technical indicators under reference working conditions, and users should be aware that these differ from actual application working conditions.

According to the JB/T 9248-1999 "Electromagnetic Flowmeter" industry standard, the reference working conditions are as follows:

Ambient temperature: 20℃±2℃;

Relative humidity: 60%～70%;

Power supply: rated voltage ±1%;

Installation conditions: upstream straight pipe section length > 10D;

downstream straight pipe section length > 5D;

Preheating time: >15min.

3.1 Measurement Accuracy

Currently, most manufacturers express the measurement accuracy of instruments (also known as basic error limits) as a percentage of the indicated value. For example, a Class 0.3 flowmeter has a measurement accuracy of ±0.3%. More reputable manufacturers have their own controlled product flow calibration procedures. According to the Kaifeng Instrument Factory's procedures, under reference working conditions, the actual flow calibration measurement accuracy of the flowmeter is controlled within ±0.28%, which is better than the industry standard.

3.2 Error Curve

The error curve provided by the manufacturer indicates the trend of linearity variation of the flowmeter within its measurement range, corresponding to the given accuracy indicators (the error curve is shown in Figure 2). Taking the Kaifeng Instrument Factory's electromagnetic flowmeter as an example:

The accuracy is: ±0.3% of the indicated value (flow rate ≥ 1m/s); or ±3 mm/s (flow rate < 1m/s).

Therefore, at 0.5m/s, the flowmeter allows for an error of ±0.5%, and at 0.3 m/s, the flowmeter allows for an error of ±1.0%.

3.3 Working Condition

Under working conditions, due to various influencing factors, the measurement accuracy may differ from the actual flow calibration accuracy provided by the manufacturer under reference conditions. According to industry standards, for every 10℃ change in temperature, the measurement accuracy should not vary by more than 1/2 of the instrument's basic error limit.

When the temperature changes by 20℃, the ±0.3% measurement accuracy may change to ±0.6%.

Users should consider the impact of noise interference, installation condition limitations, changes in ambient temperature, humidity changes (in long-term humid environments, the insulation strength of the flowmeter decreases), and other factors on the measurement accuracy of the flowmeter during its operation. Generally, if a Class 0.5 flowmeter actually achieves Class 1, it should be considered that the user has maintained the flowmeter well.

4 Measurement Range and System Zero Point

4.1 Measurement Range

The flow range of electromagnetic flowmeters is relatively wide, corresponding to a flow velocity range of 0 to 10m/s, and some manufacturers' flowmeters range from 0 to 15m/s. Theoretically, changing the range does not affect the measurement accuracy and linearity of the flowmeter, but selecting an appropriate range helps improve the unit resolution of the 4-20mA analog output signal. If a 100m3/h range meets the user's requirements, there is no need to choose a 200m3/h range; otherwise, the unit resolution of the analog output signal will be halved.

4.2 System Zero Point

Under normal operating conditions, the system zero point of the electromagnetic flowmeter may change and drift over time due to factors such as component aging, reduced insulation strength of the excitation coil, electrode polarization and contamination, and increased system grounding resistance (potential). Users should regularly check the system zero point of the flowmeter and make adjustments and maintenance. For a flow corresponding to 1m/s, if the system has a zero point of ±5mm/s, it will cause an additional error of ±0.5%. Generally,

the smaller the flow, the greater the additional error caused by the system zero point.

Since the system zero point always exists to some extent, manufacturers will adjust it to the minimum during the actual flow calibration of the flowmeter before leaving the factory. Users also need to make adjustments on-site. Therefore, the error curve provided by the manufacturer objectively reflects the existence of the flowmeter's system zero point, thus causing non-linearity of the instrument. If only the measurement accuracy of the electromagnetic flowmeter is provided without the error curve, it cannot clearly reflect the measurement accuracy of the flowmeter across the entire measurement range.

5 Signal Reference and DC Noise

5.1 Signal Reference

The electromagnetic flowmeter is a type of flowmeter based on Faraday's electromagnetic induction law (when a conductive fluid crosses a magnetic field, a voltage is induced in the conductive fluid that is proportional to the fluid flow velocity). To effectively capture the mV-level flow signal induced on the two measurement electrodes and suppress interference signals, the flow signal is transmitted differentially from the sensor to the differential amplifier signal input of the converter, using the "zero resistance" fluid medium as the signal ground for the differential amplifier. The differential amplifier amplifies the difference of the flow signal and suppresses and attenuates the common-mode interference signals superimposed on the flow signal. When there are common-mode interferences such as grounding loop currents, electrode polarization voltages, and electrostatic coupling voltages between the excitation circuit and the electrode measurement circuit, as long as the working parameters of the differential amplifier are symmetrical, common-mode interference will not affect the amplifier's amplification of the flow signal, unless the common-mode interference voltage exceeds the common-mode rejection range of the amplifier.

Users should fully recognize the importance of system grounding during the installation and maintenance of the electromagnetic flowmeter. Some manufacturers clarify the distinction of grounding by referring to the ground used as the flow signal reference as the liquid contact ground (liquid contact components: grounding electrodes or grounding rings, etc.) and the physical ground as the earth. The liquid contact ground not only requires the grounding resistance to be as low as possible but also needs to have good stability and reliability.

The earth is mainly to reduce external environmental interference on the flowmeter system and to provide protection, such as electromagnetic field radiation, loop currents between the fluid medium ground and the earth, lightning strikes, etc.

5.2 DC Noise

In cases where the flow is relatively stable, if the flowmeter output fluctuates significantly, the main influencing factor is the DC noise superimposed on the measurement signal. If the fluid medium is well grounded, the cause of the DC noise is the polarization voltage present on the electrodes. Between the electrodes and the electrolyte fluid medium, the directional movement of positive and negative ions in the liquid generates a certain electric field, thus forming a drift polarization voltage between the electrodes and the liquid contact point. The polarization voltage is superimposed on the flow signal in the form of common-mode interference, blocking the differential amplifier, preventing the flow signal from being amplified, and causing the flowmeter to fail to complete normal signal sampling, or due to its drift, resulting in fluctuations in the flow signal. In the water supply industry, polarization voltage generally does not occur, but in the chemical industry, due to the corrosion of the fluid medium and the different materials of the electrodes and liquid contact points, polarization voltage can easily occur. Therefore, it is required that the liquid contact material be consistent with the measurement electrode material, so that even if polarization voltage occurs, the potential generated by the same material will also be the same, minimizing the common-mode voltage between the electrodes and the measurement reference.

6 Online Calibration of Electromagnetic Flowmeters with Ultrasonic Flowmeters

Manufacturers use standard container methods, weighing methods, or standard meter methods for value transfer to calibrate electromagnetic flowmeters in real flow. Currently, there are no widely recognized online calibration devices for electromagnetic flowmeters. ABB provides a device called 'CalMaster' to users, which can be considered an expert evaluation system for electromagnetic flowmeters. It can perform online evaluations of electromagnetic flowmeters manufactured by Kent, comparing the historical parameter records from the product's factory with the operational status parameters during online use, to determine the potential measurement accuracy error compared to the original factory calibration. If users feel there are issues with measurement accuracy during the use of the flowmeter, they should send it to the relevant metrology department for verification. Using the online comparison method with ultrasonic flowmeters can qualitatively assess whether the operational status of the electromagnetic flowmeter is good, eliminating the possibility of significant measurement errors.

⑴ The measurement accuracy of ultrasonic flowmeters is lower than that of electromagnetic flowmeters because electromagnetic flowmeters measure the area-averaged flow velocity over a circular cross-section, and the nominal diameter and non-circularity errors of the electromagnetic conduit have been corrected during factory calibration. In contrast, ultrasonic flowmeters measure the line-averaged flow velocity across the axial cross-section of the pipeline, and the effects of flow velocity distribution distortion should be considered, making it difficult for users to provide quantitative values for the actual pipeline diameter and non-circularity errors.

⑵ The straight pipe section length requirement for ultrasonic flowmeters is 10D upstream and 5D downstream, and the actual length for single-channel ultrasonic flowmeters should be longer, which is often not achievable under field conditions.

⑶ Factors such as wall rust, protective coatings, and thickness also affect the measurement accuracy of ultrasonic flowmeters.

7 Pump Flow Cannot Determine the Accuracy of Electromagnetic Flowmeters

Users often assess the accuracy of electromagnetic flowmeters based on the size of the pump flow during use. However, pump flow cannot determine the accuracy of electromagnetic flowmeters.

⑴ The flow rate and head values specified on the pump nameplate should represent the nominal values of the pump's capacity. Under actual working conditions, the pump flow rate varies due to changes in head, efficiency, power, and pipeline load.

⑵ Class C pump flow may have a tolerance of ±8%.

⑶ When multiple pumps operate in parallel, the flow rate entering the main pipe may not equal the sum of the flow rates of each pump.

8 Measurement Electrode Pollution Protection

It is difficult to quantify the extent to which electrode pollution affects the measurement accuracy of electromagnetic flowmeters.

⑴ When the fluid medium may cause sediment that pollutes the electrodes, appropriate flow rates (selecting the pipe diameter) and installation methods should be chosen in design and installation to avoid excessive sedimentation.

⑵ Electrode pollution physically manifests as an increase or decrease in the signal input impedance of the electrode measurement circuit. As long as the impedance does not exceed a certain limit, it will not adversely affect the measurement of the electromagnetic flowmeter. High input impedance (10^11Ω) differential amplifier design parameters can avoid the impact of electrode pollution (impedance increase). Well-known brands of electromagnetic flowmeters can meet this technical specification; therefore, in the water supply industry, electrode pollution is not a concern in the application of electromagnetic flowmeters.

9 System Lightning Protection

There are mainly two forms of lightning: one is the discharge between differently charged cloud layers, and the other is the discharge from the cloud to the ground, the latter being the main source of lightning damage. Direct lightning strikes, lightning-induced static electricity, and electromagnetic induction are the main factors causing instrument damage. The purpose of instrument lightning protection is not to avoid lightning strikes but to protect instruments from damage caused by lightning waves.

When designing electromagnetic flowmeter products, manufacturers consider certain protective measures for lightning protection and anti-interference in the flowmeter system. For example: adding transient suppression diodes or discharge tubes to the power supply section; electrical isolation of power and signal input/output; using lightning-resistant components for digital communication interfaces; employing triple shielding for signal connection cables between sensors and converters; and ensuring good grounding of the flowmeter system. However, in areas with frequent and intense lightning, users should take further measures:

⑴ Install a 1:1 transformer and lightning arrester at the power input end to prevent the power supply from being broken down.

⑵ Install a lightning arrester on the signal analog output.

⑶ The grounding of the flowmeter system must be good, ensuring that the liquid contact point is well connected to the ground to prevent lightning current conducted through pipelines and fluid media from flowing through the instrument body.

References:

1 Industry Standard · JB/T 9248-1999 'Electromagnetic Flowmeter'

2 Kaifeng Instrument Factory · E-mag Electromagnetic Flowmeter User Manual

3 ABB Ltd · CalMaster Operating Instructions

4 Cai Wuchang · Flow Measurement Methods and Instrument Selection · Beijing: Chemical Industry Press, 2001.3