Evaluation of Magnetic Meters for Irrigation Pipeline Measurement

Abstract

Magnetic flow meters are used to measure the flow rate of a liquid in a closed pipeline. This type of meter is becoming increasingly popular for measurement with agriculture applications. Electromagnetic meters were tested by the Irrigation Training and Research Center in pipelines located less than the 10 diameters upstream of disturbances with good results. Results show that location guidelines for placing a magnetic meter can be decreased even for turbulent conditions. This paper will discuss how a magnetic flow meter works, advantages and disadvantages of this type of meter, test results, and new guidelines for field applications. INTRODUCTION The accuracy and long-term success of any flow measurement program depends on many factors. Magnetic flow measurement is rapidly becoming the technique of choice in pipelines because of its simplicity and accuracy. However, the application of this technology has been limited in the past because of standard practice guidelines for magnetic flow meters, which have required installing the meters in a straight section of pipe at least 8-10 pipe diameters from any source of turbulence. In addition, standard practice recommends having at least two pipe diameters of straight unobstructed pipe downstream from the meter. However, in the case of many irrigation pumping plants, turnouts, and deliveries, these conditions can rarely be met without extensive and expensive modifications. Therefore, the Irrigation Training and Research Center (ITRC) at Cal Poly State University San Luis Obispo is currently investigating electromagnetic flow meters for potential applications under non-standard conditions. Several current manufacturers claim that their magnetic meters can perform well even in locations where only two diameter lengths of straight pipe are available. Testing was done by the ITRC on several magnetic meters at the Cal Poly Water Resources Facility to evaluate comments by the manufacturers and to make recommendations to irrigation districts and growers interested in this new technology. This research was conducted to determine: The accuracy of the magnetic meters in locations that are less than optimal The sensitivity of meters to increasing amounts of turbulence Since this research was conducted, there have been numerous field installations of magnetic meters in various irrigation applications in California. The general consensus is that they are working well. Presented at the May 17-21, 2009, World Environmental and Water Resources Congress in Kansas City, Missouri. http://www.itrc.org/papers/magmeter09.htm ITRC Paper No. P 09-002 Magnetic Meters According to Faraday’s Law, a voltage will be induced proportional to the velocity of a conductor as it moves at right angles through a magnetic field. The water in the pipe is the conductor. By simply measuring the voltage, a magnetic meter is able to calculate the volume of the liquid passing through a controlled section. Faraday's Formula: E is proportional to V x B x D E = The voltage generated in a conductor V = The velocity of the conductor B = The magnetic field strength D = The length of the conductor This principle works for irrigation water passing through the magnetic field generated by the magnetic meter since the water acts as the conductor. The basic operating principle for magnetic meters is illustrated in Figure 1. Note that this discussion is limited to full-bore or in-line magnetic meters. There are several other types of meters on the market but this type is the one that was evaluated. Figure 1. Operating principle for full-bore magnetic meter (Omega 2009) As seen in Figure 2, there are two electrodes located inside the magnetic meter to measure the induced voltage. The flow rate/totalizer indicator is located on the top of the unit. These units can be placed at any angle. It is recommended that they be rotated so that one of the electrodes does not sit on the bottom of the pipe. This helps prevent problems from sediment covering the electrode. Note that on the unit in Figure 2, the magnetic field is generated by an insert unit into the pipeline. Some manufacturers install the coils to generate the magnetic field outside of the spool piece. Presented at the May 17-21, 2009, World Environmental and Water Resources Congress in Kansas City, Missouri. http://www.itrc.org/papers/magmeter09.htm ITRC Paper No. P 09-002 Figure 2. View of typical magnetic meter (SeaMetrics 2009) Advantages of Magnetic Meters Highly accurate even with some flow disturbance. No headloss caused by meter. Is not impacted by trash, sediment, or sand. Can measure a wide range of velocity. This is a key criterion in areas that have seasonal high flows combined with very low flows. Has instantaneous and volumetric totalizing capability. Measurement accuracy is NOT affected by varying canal water levels. Minimal maintenance required. Temperature of the liquid has no effect on accuracy. Disadvantages of Magnetic Meters Cost is a major constraint. Recently the cost has dropped dramatically, but this is still the most expensive option in some cases. Pipe must be full (as is the case in most agricultural applications). Installation into existing sites is often difficult and expensive. Sensitivity to turbulence. Turbulence and Magnetic Meters Magnetic meters have been considered particularly tricky for pumping plants and turnouts, because of the meter’s sensitivity to turbulence. For example, one of the main problems associated with turnouts is the turbulence that occurs just downstream of the gate. Figure 3 shows a typical turnout application with a magnetic meter. Presented at the May 17-21, 2009, World Environmental and Water Resources Congress in Kansas City, Missouri. http://www.itrc.org/papers/magmeter09.htm ITRC Paper No. P 09-002 Figure 3. Conceptual sketch of magnetic meter installation for turnouts (not to scale) In 1998, Hanson and Schwankl published results from non-optimal flow meter testing in their paper Error Analysis of Flowmeter Measurements. Pipeline measurements were taken with different types of flow meters to determine the effects on error resulting from different degrees of turbulence caused by elbows, check valves, and a partially opened check valve. Measurements were made at 2, 5, 10, and 15 pipe diameters downstream from the source of turbulence. The results from Hanson and Schwankl (1998) indicated that for generally acceptable accuracy with propeller meters, pitot meters, and Doppler meters, measurements should be taken upstream of valves. The tests found that in some circumstances even with as little as 2 or 5 pipe diameters upstream of a flow meter, there were still large errors with all meters under conditions of severe turbulence. Since most applications require the flow measurement downstream of a valve or turnout gate, manufacturers have recently claimed that their magnetic flow meter technology is now able to work effectively in situations where there is as little as two diameters of pipe length available. ITRC tested several magnetic meters at the Cal Poly Water Resources Facility to investigate these claims.