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Common and rare faults in the use of electromagnetic flowmeters

Jan 17, 2019


Common and rare faults in the use of electromagnetic flowmeters


Abstract The faults occurred in the use of electromagnetic flowmeters are divided into two categories: faults during commissioning and faults during operation. The causes are explained from the aspects of installation, environmental conditions and fluid characteristics. Finally, 10 examples of faults and solutions for the use of strong stray currents, strong radio wave interference, accumulated air, sludge deposits and electrode materials are listed.

 

1 Fault type

The instrument failure discussed in this paper does not refer to the failure caused by the electromagnetic flowmeter (hereinafter referred to as electromagnetic flowmeter (EMF) itself, but refers to the application technology such as improper installation and improper selection, as well as environmental conditions and fluid characteristics. Abnormalities such as output sway or damage to the instrument. The faults in the use of electromagnetic flowmeters (EMF) can be divided into two types: commissioning faults that occur during initial commissioning and run-time faults that occur after a period of normal operation. Some of these faults are common faults with high frequency, and some are rare faults that are rare. In rare faults, the cause of individual failures has only encountered one or two times for an electromagnetic flowmeter (EMF) manufacturer with more than 30 years of experience in after-sales service.

 

electromagnetic flowmeter (EMF


2 Debugging period failure

This type of failure occurs whenever the electromagnetic flowmeter (EMF) is installed and commissioned, but once it is corrected, it will not reappear under the same conditions. Common failures during commissioning are mainly due to three reasons: improper installation, environmental interference, and fluid characteristics.

 

2.1 Installation aspects

Usually the fault is caused by the incorrect installation position of the electromagnetic flow sensor. Common examples are the installation of the flow sensor at the high point of the pipe network that tends to accumulate the retained gas; the flow sensor has no back pressure, and the liquid is discharged straight into the atmosphere to form its measurement. The pipe is not full; it is installed on the vertical pipe flowing from the top to the bottom, and may be drained.

 

2.2 Environmental aspects

Mainly refers to pipeline stray current interference, space electromagnetic wave interference, large motor magnetic field interference, etc. Pipe stray current interference usually takes good separate grounding protection to obtain satisfactory measurement, but if there is strong stray current in the pipeline (such as electrolysis workshop piping), it may not be overcome, and the flow sensor and pipeline insulation measures must be taken (see the example below). 1). Space electromagnetic interference is introduced from the signal cable and is usually protected by a single or multiple layers of shielding, but shielding protection has not been overcome (see Example 2).

 

2.3 Fluid aspects

The liquid containing uniformly distributed fine bubbles usually does not affect the normal measurement. The measured volume flow is the sum of the liquid and the gas. The increase of the bubble causes the output signal to fluctuate. If the bubble is large enough to flow through the electrode to cover the entire electrode surface, the electrode signal The loop is momentarily disconnected and the output signal will produce more fluctuations.

electromagnetic flowmeter (EMF

 

Low frequency (50/8 ~ 50/4Hz) rectangular wave electromagnetic flowmeter (EMF) When the liquid contains more than a certain amount of solids, it will produce serous noise, and the output signal will also fluctuate to some extent.

 

When two or more liquids are used in the pipeline mixing process, if the conductivity of the two liquids (or the potential between the electrodes) is different, the flow sensor is measured before the mixing is not uniform, and the output signal also fluctuates.

 

Poorly matched electrode materials and measured media, chemical effects such as passivation or oxidation, formation of an insulating film on the surface of the electrode, and electrochemical and polarization phenomena can impede normal measurement (see Example 5).

 

3 Runtime failure

Electromagnetic flowmeter (EMF) A fault that occurs during operation after initial commissioning and normal operation for a period of time. Common causes of failure include: adhesion layer on the inner wall of the flow sensor, lightning strike, changes in environmental conditions

 

3.1 Inner wall adhesion layer

Since the electromagnetic flowmeter (EMF) measures much more with suspended solids or dirt than other flow meters, the probability of failure of the inner wall adhesion layer is relatively high. If the electrical conductivity of the adhesion layer is close to the liquid conductivity, the instrument can output the signal normally, but only change the flow area. If the thickness of the adhesion layer is not uniform, a hidden fault of measurement error will be formed; if it is a high conductivity adhesion layer, the electromotive force between the electrodes will be Short circuit; if it is an insulating adhesion layer, the electrode surface is insulated and the measurement circuit is disconnected. Both of the latter phenomena will make the meter inoperable (see examples 4 and 5).

 

3.2 lightning strike

Lightning strikes induce transient high voltages and surge currents in the line, and entering the meter can damage the meter. In the past few years, a production plant encountered several faults caused by lightning. Lightning damage The electromagnetic flowmeter (EMF) has three introduction paths: the power line, the flow signal line between the sensor and the converter, and the excitation line. However, from the analysis of damaged components in lightning faults, most of the induced high voltage and surge currents that cause faults are introduced from the control room power line. The other two approaches are unique. It was also learned from the electromagnetic flowmeter (EMF) lightning strike accident that not only the electromagnetic flowmeter (EMF) failed, but also other instruments in the control room often had lightning strikes at the same time. Therefore, the user must know the importance of setting up the lightning protection facilities of the power supply line of the control room. Several design units have now recognized and explored this problem, such as the Qilu Petrochemical Design Institute [1].

 

3.3 Changes in environmental conditions

The main reason is the same as the faulty environmental conditions during the commissioning period of the previous section, except that the interference source does not appear during the commissioning period and then intervenes during the operation. For example, an electromagnetic flowmeter (EMF) with a grounding protection is not ideal. During the commissioning period, the instrument is operating normally due to the absence of interference sources. However, new interference sources (such as pipelines near the measuring point or pipeline welding at a distance) interfere with each other during operation. The instrument is operating normally and there is a large fluctuation in the output signal.

 

electromagnetic flowmeter (EMF


4 Failure example

4.1 Pipe strong stray current

Two city water supply companies in Zhejiang Province installed two DN900mm large diameter electromagnetic flowmeters (EMF), which were opened in 1997. One of them is operating normally, and the other one is in an abnormal period of 0 to 50% full-scale fluctuation in the output of the instrument within 1 to 2 hours. Analyze the operation of the pipe network, eliminate the possibility of flow fluctuations, and consider it to be a problem in flow measurement. At the survey site, electromagnetic flowmeter (EMF) installation, electrical connection and grounding protection basically meet the requirements. The pipe connected to the flow sensor is a cement-lined steel pipe. The upstream and downstream of the flow sensor is replaced by a 0.5m long unlined short steel pipe instead of the grounding ring. The converter and sensor are approximately 10m apart. A three-phase transformer with a few hundred kVA is placed near the electromagnetic flowmeter (EMF), which is approximately 8m and 2m away from the electromagnetic flow sensor and converter, respectively.

 

There are two possibilities for analysis: 1 magnetic field interference generated by a high-power transformer; 2 stray current interference on the pipeline. To prove whether it is the influence of transformer magnetic field interference, because the transformer is to be closed, the second step is to check whether it is the stray current interference of the pipeline. The potential between the two electrodes is measured without applying an excitation current, and its value should be zero. In this example, the AC potential of the waveform distortion is measured, and the peak value V pp is as high as 1V. It is the main reason to initially determine the stray current on the pipe network.

 

Measures are taken to electrically insulate the electromagnetic flow sensor from its connecting pipe such that the flow sensor is electrically insulated from the pipe and at the same potential as the measured water flow. The electromagnetic flowmeter (EMF) is put into operation and the output shows stable and normal, and the possibility of magnetic field interference of the power transformer is eliminated. Indirectly verified the effects of hundreds of kVA power transformers separated by 8m on the electromagnetic flowmeter (EMF), which can be ignored under normal circumstances. This measure can also be used as a method to eliminate the influence of current interference on a cathodic protection current tube network.

 

This example is a fault during the commissioning period. Although it is a rare fault, in some similar applications, the same fault may occur.

 

4.2 Strong radio wave interference

A water meter in a city of Fujian Province installed several electromagnetic flowmeters (EMF) to measure the flow of water. One of the flow sensors is 50m away from the converter, and the two are connected by a shielded cable placed inside the iron pipe. The user reports that the electromagnetic flowmeter (EMF) output is greatly shaken. After checking the instrument itself is intact, but the common mode interference signal measured at the site is as high as 1.7V. One of the possible reasons for analyzing the source of interference is the large stray current on the pipeline. Firstly, the interference source is taken. The electromagnetic flow sensor is ungrounded according to the method of the previous example, and is electrically isolated from the pipeline. The upstream and downstream pipelines of the flow sensor are connected by thick conductors. The retest common mode interference signal is reduced, but there is still 0.6V. Obviously, there are other sources of interference that affect the normal operation of the instrument.

 

Further analysis of the on-site environmental conditions, what other sources of interference? Have learned that there are strong radio transmitters at tens of meters from the water plant, is strong electromagnetic wave another interference source? Sensors and converters have metal protection shells, signal cables are set In the iron pipe, it should have good protection. However, further analysis suggests that a 50m long signal cable may introduce interference signals. In order to confirm the existence of this analysis, move the converter to the vicinity of the sensor, the length of the connecting cable is shortened to only 3m, and the re-measuring common mode interference signal is less than 0.1V. Although it is still too large, the instrument operation tends to be normal. The fault was ruled out.

 

This example reveals that when a separate electromagnetic flowmeter (EMF) has large common mode interference in the field, the cause of the fault should be considered whether the strong radio wave interferes. This example is a rare fault during the commissioning period.

 

4.3 Raw water pipe accumulates air

In recent years, in order to strengthen water management, promote planning and save water, the state has to charge water and water users. Therefore, the use of electromagnetic flowmeters to measure the river's raw water is increasing. Experience has shown that the raw water extracted from the river will accumulate air bubbles in the upper part of the water pipeline. For example, when a DN1600mm electromagnetic flowmeter (EMF) was taken from a water plant in Shanghai from the Huangpu River in 1997, it was found that the top of the inner wall of the flowmeter measuring tube had a mark of 300 to 400 mm in circumference and a significant accumulation of gas layer, reducing the flow area by about 0.3%. The impact flow measurement is not too large.

 

People have noticed the problem of easy accumulation of gas in raw water, which is called the problem of “swelling gas”. In order to eliminate the influence of the gas flow on the flow measurement, it is required to install an exhaust valve at an appropriate position upstream of the flow meter (for example, 5 to 30 times the pipe diameter distance depending on the flow meter type) [3]. The Harbin Water Company has measured changes in flow measurements before and after gas emissions. The ultrasonic flowmeter is installed on the DN600mm pipeline. The front of the instrument is equipped with an automatic exhaust valve. The exhaust valve is automatically exhausted for several minutes at intervals, and is exhausted several times per hour. Before and after the exhaust gas flow value is actually 9.57% [4], the gas phenomenon is much more serious than the previous example. However, some people still do not understand the seriousness of this hidden fault. Many new design water plants have not installed gas traps and automatic exhaust valves upstream of the raw water pipeline measurement points.

 

4.4 Sedimentary layer

A water plant in Shanghai took a rectangular pipe from the upper reaches of the Huangpu River, which is more than 30 km away, and pumped raw water into the plant with two DN1600mm round pipes. The water was metered by two DN1600mm Huaheng electromagnetic flowmeters (EMF). It was put into normal use in 1991, but it felt that the metering was reduced in 1993. After checking, the cause of the failure of the open part of the meter was eliminated. Checking the resistance of the two electrodes of the flow sensor to the ground is asymmetrical, and the possibility of analyzing the flow sensor is greater. The flow is cut off and the hidden part cannot be checked. It was not until April 1997 that there was an opportunity to enter the pipeline to check the internal condition of the flow sensor. The thickest part of the inner wall was more than 10 mm, and the surface of the electrode was covered by a silt layer, which was flush with the surrounding silt layer. After cleaning by the shovel, the meter resumes normal operation. Confirm that the cause of the failure is caused by sludge deposits on the internal pipe wall. This example shows that the inner wall of the flow meter channel of raw water metering always deposits silt. Whether it affects the measurement is only a short period of time. In this case, the water quality condition has been reduced for 3 years. For this reason, the electromagnetic flowmeter (EMF) that measures the raw water of the river must be cleaned regularly. Other flow meters such as ultrasonic flowmeters and venturi flowmeters, at least the same deposition layer reduces the flow area affecting the measurement accuracy. The DN1600mm tube deposition 10mm flow rate should be changed by 1.2% to 2.5%. At the same time, the countermeasures for the long-term operation of sedimentary sludge should be considered in the engineering design, such as increasing the flow velocity of the measurement position to prolong the cleaning cycle; and presetting the inspection hole for cleaning in the pipe. This example is a common fault at runtime.

 

4.5 Example of short-circuit effect of conductive deposition layer

If the conductive flow is deposited on the surface of the insulating lining of the electromagnetic flow sensor, the flow signal will be short-circuited and the meter will fail. Since the conductive material is gradually deposited, this type of fault usually does not appear in the commissioning period and will not appear until after a period of operation.

 

The DN80mm SCLDE series electromagnetic flowmeter (EMF) is used to measure and control the flow rate of saturated salt electrolyte in an electrolytic cutting process test device of a diesel engine tool shop to obtain the best cutting efficiency. At first the instrument was operating normally. After two months of intermittent use, the flow indication was found to be getting smaller and smaller until it was close to zero. On-site inspection revealed that a thin layer of yellow rust was deposited on the surface of the insulation layer, and the instrument was operating normally after cleaning. The yellow rust layer is caused by the deposition of a large amount of iron oxide in the electrolyte.

 

This example is a fault in the running period. Although it is very faulty, if the ferrous metal pipeline is seriously corroded, there is a possibility of this type of fault.

 

4.6 Electrode material is not suitable for medium examples

The choice of the electrode material for the suitability of the medium is first considered to be corrosion resistance. Sometimes an electrode material is impeccable to the corrosion resistance of a medium, but it cannot work due to the surface effect of the electrode. Other electrode materials are required. This class instance is a common fault during the commissioning period. The following examples of non-corrosive electrodes are not applicable.

 

(1) Hydrochloric acid - Hastelloy B

Hastelloy B is corrosion-resistant to hydrochloric acid, which is not high in temperature and concentration. There are several good examples, but when the concentration of hydrochloric acid exceeds a certain value, noise is generated. We have done tests to change the concentration of hydrochloric acid on the spot, and the concentration gradually increases. When the temperature exceeds 15% to 20%, the output of the instrument will sway, and the concentration will reach 25% and the output slosh will be as high as 20%. The failure to use this type of use site after switching to the electrode is solved. Acids such as nitric acid and sulfuric acid also have similar effects.

 

(2) Aluminum sulfate solution - Hastelloy B

The water plant usually uses aluminum sulfate solution mixed with raw water to agglomerate the suspension. The mixing ratio is measured by Huaheng electromagnetic flowmeter (EMF) to measure the aluminum sulfate solution, and the acid-resistant steel electrode can be used to obtain satisfactory results. We have encountered an example of accidentally using a Hastelloy B-electrode electromagnetic flowmeter (EMF) for measuring 15% aluminum sulfate. The output sloshing phenomenon also occurs during use, and then the acid-resistant steel electrode is used, and the work becomes normal.

 

(3) Hydrochloric acid - platinum rhodium alloy

The platinum-rhodium alloy electrode or the platinum electrode has good corrosion resistance to hydrochloric acid, and the platinum electrode electromagnetic flowmeter (EMF) is used for measuring hydrochloric acid to obtain satisfactory results. However, the measurement of higher concentrations of hydrochloric acid (more than 10%) produces severe noise and should be replaced by a ruthenium electrode.

 

(4) Hydrogen peroxide (hydrogen peroxide) - platinum

The platinum electrode is used to measure low-pressure hydrogen peroxide with a pressure lower than 0.3 MPa. The catalyst will cause an aerosol on the surface of the electrode, which blocks the electrical path of the flow signal and affects the work.

 

(5) Water solution and "non-acid" liquid -- tantalum

Tantalum is absolutely corrosion resistant to water.However, if the tantalum electrode electromagnetic flowmeter (EMF) is used to measure the flow of water solution, the surface of tantalum electrode will form an insulating layer, which will make the instrument malfunction or make a lot of noise after a short period of operation.Nor can tantalum electrodes be used in alkaline solutions such as sodium hydroxide.In the process, even if the tantalum electrode is in contact with water or "non-acid" liquid for a very short time, if the pipeline is cleaned with water, it will affect the normal use of the meter.