Pumps and Systems, March 2009
Any time a measuring instrument is used in a plant, it is expected to provide reliable results.
This is especially true in closed pipeline systems in which it is not possible to check the flow visually. Electromagnetic flowmeters, which exceed mechanical measuring instruments in terms of accuracy, can perform this task. The accuracy of measuring devices in process measuring technology is becoming increasingly important; flow volumes and diagnostic values must be determined and documented more comprehensively and more accurately.
In addition, measured values are continually required for precise process control in plants and must be available at all times. Also, information pertaining to the condition of the device, the quality of the measured value and any possible application errors is needed for process monitoring.
A variety of information is required on an ongoing basis for state-of-the-art process control, which places high demands on the converter. This article briefly explains the most important requirements, and two applications will illustrate how a standard measuring device can still provide reliable results under difficult conditions.
Independent Diagnostic Function
Once started, the signal converter first checks that its microprocessor, memory and outputs are functioning properly. This self-diagnosis is the basis for the reproducibility of the measured values and assures the operator that the device is functioning smoothly within the scope of his specifications.
Readability and User-Friendliness
A standard, yet state-of-the-art signal converter adjusts quickly to respective applications and is ready for use in a short period of time. As flowmeters are sometimes used in areas of the plant which are difficult to access, signal converters are available in both a compact version (installed on the sensor itself) as well as a remote version for separate installation. Easy-to-understand, intuitive menu navigation facilitates on-site operation and reduces the risk of faulty entry.
Information for Process Control
It is crucial for the plant operator to be able to use the information provided by the measuring device to control his processes directly. The device is able to display a change of medium or low conductivity using an integrated conductivity measurement. It is also possible to detect gas bubbles or solids in the medium using an application test, which tells something about the installation location or helps with batching processes.
Other possible indications, e.g. corrosion, scaling, flow profile, empty pipe detection, etc. are also detected and displayed by the device independently. The signal converter supports the operator in a meaningful way in the event of error diagnosis. In an emergency, the operator can quickly intervene in the process based on the displayed information.
A signal converter is one solution for this complex task. A signal converter combined with an electromagnetic sensor offer a reliable solution for various applications.
Reliable Under Difficult Conditions
Signal converters have proven their reliability in multiple situations. In the city of Ostrava (population 300,000) in the Czech Republic, the signal converter together with a flow sensor is used to measure the flow of the lime water in the drinking water treatment plant. The lime water is added to the water to set the pH value. The measuring device is attached one meter behind a hose pump in the application. The pressure in the system reaches up to 400-kPa, the measuring range is between 0- and 10-m³/h. At the beginning of the first test month, a flow of 1.4-m³/h could be measured with the flow switch, but by the end of the month the value was only 0.9-m³/h. Opening the pipe revealed a thick layer of lime on the interior walls of the pipes and of the sensor. Despite this presence of lime, the combination of the flowmeter and flow switch continued to function, providing reliable results.
During the course of the test month, there were no measuring errors or faults. The plant operator therefore decided to continue to test the flow switch. The result after three months was impressive: no measuring errors were registered, and the device showed no failures even though the sensor was not cleaned during this time. Despite the deposits, the flow measurement was reliable and remained within the operator's error tolerance limits. The operator was pleased with the performance of the converter considering the difficulty of the application for measuring devices. He was also impressed with the simple navigation and user-friendliness of the device.
Following the test, the manufacturer received the order from Ostrava for the delivery of 13 more flow switches in combination with flow sensors of various sizes.
Solid Content in Medium
Another project took place in the Rainitza mine water purification plant near Großräschen, Saxony. A suction excavator is used to pump approximately 10,000-m³ of watery slurry from the bottom of the slurry tank. The slurry is then fed via a pipe system into an abandoned quarry where it is rinsed. The flow volume was between 70- and 120-m³/h. The total amount of slurry required during one month is what was measured. Flow measurement is then performed using a flow sensor and a signal converter. The solid content in the slurry can interfere with electromagnetic measurement. The mixing ratio of water to solid was somewhat above the expected maximum. Even so, throughout the four-week long test phase of this not-so-simple application, no measuring errors or device failures were recorded and the operator was satisfied.