The patented technology to measure buildup can be used to optimize cleaning cycles and avoid unplanned shutdowns

To keep operations at peak efficiency, process systems must be kept as clean as possible. However, some of the normal byproducts of operating processes create deposits ranging from scales to sludge, precipitates and even metal deposits. These types of buildup can significantly reduce system efficiency, eventually causing plugging and unexpected downtime.

The flow measurement of an electromagnetic (EM) flowmeter is also directly affected by buildup in the measuring tube.

 

Electromagnetic flowmeters

To understand how buildup can be monitored, it is worthwhile to take a brief look at the operating principles of an EM flowmeter. EM flowmeters are suitable for measuring the flow of electrically conductive fluids. They operate on the dynamo principle of Faraday’s law of induction, which states that a metal rod moving in a magnetic field induces electrical voltage. Inside the flowmeter are two field coils. The coils are used to create a constant magnetic field over the entire cross section of the measurement tube. Two electrodes that pick up electrical voltages are installed in the wall of the tube at a right angle to the electromagnetic field. If there is no flow in the tube, no induced electrical voltage is measured between the electrodes, as the electrically charged particles are equally distributed in the fluid. When there is flow, the electromagnetic field generated by the two field coils exerts a force on the charged particles, separating the positive and negative charges. The induced voltage is picked up by the two measuring electrodes. This voltage is directly proportional to the velocity of the flow, and thus, with the cross-section area, to the flow volume. The magnetic field is generated by a pulsed direct current with alternating polarity. This ensures a stable zero point. This measuring principle is virtually independent of pressure, density, temperature and viscosity. Even fluids with entrained solids can be metered, for example, ore slurry or cellulose pulp. Because there are no moving parts, EM flowmeters are virtually maintenance-free. However, in some cases, the nature of the fluids measured with EM flowmeters make them susceptible to buildup. The flow measurement of an EM flowmeter is directly affected by buildup in the measuring tube. While the reduction of the inner diameter leads to a miscalculation of the volume flow, the voltage picked up by the measuring electrodes is altered by the electrical properties of the buildup. These two effects are unequal in magnitude and can either add up or subtract, depending on process conditions.  

Buildup measurement

With this in mind, a patented technology has been developed to measure buildup. The measurement is based on the fact that the electrical conductivity of the fluid and the buildup differ from one another. A voltage is applied to one of the measuring electrodes, either E1 or E2 (Figure 1). This creates a voltage distribution within the measuring tube, originating from the excited measuring electrode and the reference electrode ( REF). This voltage distribution (U2) is picked up by the opposing measuring electrode and empty pipe detection ( EPD) electrode. This measurement is repeated on the opposing electrodes and averaged to generate a stable signal.

 

Buildup index

A voltage ratio (index) is then calculated, taking into consideration the voltage distribution in the measuring tube and the electrical conductivity of the passing fluid. This voltage ratio is computed and compared continuously, making it largely independent of conductivity and temperature changes of the passing fluid. Best results, however, are achieved in the measuring range of 20 to 100,000 μS/cm. The calculated voltage ratio is subsequently normalized to the reference value of a clean pipe, which is determined during factory calibration. The resulting indexed value changes proportionally with the buildup thickness. This allows monitoring of an increase of buildup during operation, as well as a decrease of buildup during the cleaning process. The indexed value is displayed between 0 and 100%; whereby 0% corresponds to the reference values calibrated at production in a clean measuring tube, while 100% corresponds to the maximum detectable buildup — which is not necessarily a clogged tube. Buildup is measured periodically, replacing a flow measurement sample with a buildup measurement at a defined sampling rate. This measurement period is adaptable to process conditions. For instance, a slow sampling rate could be used if buildup increases over a long period, for example, months or years, while a fast sampling rate increases the monitoring accuracy over shorter periods (hours or days). The buildup measurement is performed continuously and without process interruption. As the measured buildup value is indexed, it is inherently process independent and can therefore be used in a wide range of applications and industries. Repeatability is highest with homogenous buildup, as layers of different types of buildup can lead to an incorrect interpretation of the voltage distribution. To derive a process-specific buildup thickness, the correlation between the buildup characteristics and the indexed value is necessary. By setting user-defined threshold values, the maintenance plan can be optimized, and the cleaning process shortened.  

Buildup in water wells

A water-treatment plant in Germany faced the problem of unexpected downtime due to clogged water wells. Buildup or clogging of water wells due to iron hydroxide, as well as manganese oxide deposition in groundwater, is a common problem for operators (Figure 2). Iron(II) oxide dissolved in water comes into contact with oxygen and oxidizes to iron(III) oxide, which is water-insoluble and thus precipitates in wells, drainage shafts, pumps and pipes, as well as in any flowmeter. If users don’t realize that buildup reaches critical levels, downtime due to unexpected interruptions of measurement signals or a clog in the process can occur. Moreover, iron hydroxide deposition causes additional pressure loss due to reduced pipe diameter. This in turn causes energy loss which leads to increased costs.

To solve these issues, a commercial EM flowmeter equipped with the patented buildup index monitoring technology was installed in the plant’s water-extraction pipe at the well. During two 12-month periods, the buildup of iron hydroxide in the user’s application was continuously monitored (Figure 3). Every second month, the buildup thickness was measured and compared with the buildup index value. Buildup index reached 20% (3-mm or 0.12-in. thickness) in the second month and 40% (10-mm or 0.39-in. thickness) after 11 months. After 12 months, a cleaning was conducted and during another 12-month-period, the buildup index values and buildup thickness were compared. The results of the first year could be confirmed, proving that for the same fluid, the values are identical and therefore repeatable.

Besides other parameters, such as pump performance loss and lower flowrate, the operators can now monitor the buildup index to estimate the optimal cleaning time of the well to avoid unexpected downtime. Furthermore, the feature reduces the duration of the cleaning process since the operator knows exactly when the device is clean. After evaluating the buildup during the first year, a more accurate maintenance plan for the well was established. The user was able to track and shorten the cleaning process. Additionally, the operator set a self-defined switch point that initiates planning of process maintenance in case buildup increases faster than expected. n

Edited by Gerald Ondrey

 

Author

Andrew Burg is a product manager for electromagnetic flowmeters at Endress+Hauser Flowtec AG (Christoph Merian-Ring 4, 4153 Reinach, Switzerland; Phone: +41-61-71561-11; Email: [email protected]). Burg holds a master of science, mechanical engineering degree from ETH Zurich.