EMF Flow Measurement in Partially Filled Pipes
By combining advanced microprocessor technology with a well-designed electromagnetic flowmeter, a leading manufacturer has tackled a difficult application and come up with a solution that will reduce your installation and maintenance costs--and still deliver the performance you want.
Blake Doney, ABB Automation, Inc.
TThe configurations of fluid piping systems aren't always suited for flow measurement. To minimize piping alterations, we often push traditional metering devices to their operational limits. This is particularly true in the wastewater industry, where piping systems are designed to handle a wide operating range, from low flows at night, to higher flows during the day, to very high flows in a rainstorm. Given these demands, the pipes are typically partially full.
Environmental concerns and accountability for the waste processing costs are increasing the demand for metering devices with better accuracies, wider measurement ranges, and flexible installation parameters. Traditional electromagnetic flowmeters (EMFs) have been favored because of their high accuracy, ability to easily handle fluid with high solids content, wide rangeability, obstructionless flow path, minimal pressure loss, and low-maintenance design. To capitalize on these benefits and avoid the inaccuracies caused by a partially filled pipe, traditional EMFs require piping modifications to ensure that they remain full of liquid at all times. A U-shaped invert is the typical solution. But these configurations have several disadvantages, such as added pressure drop, a tendency to collect sediment, and added cost. In large pipelines, the price of an invert generally exceeds the cost of the flowmeter.
ABB Automation, Inc. has developed the Parti-Mag II for this type of flow measurement. The physical format of the device is similar to that of a traditional magnetic flowmeter. The meter primary is mounted in the pipeline and contains the fluid being measured as well as the devices necessary to make the raw measurements (see Figure 1). The raw signals are transmitted via an interconnection cable to a converter, where the data are used to calculate the desired flow measurements and then are translated into engineering units.
The calculations necessary to measure flow in a partially filled pipe are much more complicated than those performed by standard magmeters. A microprocessor lets the Parti-Mag II perform complex calculations at high speed, increasing the unit's accuracy and responsiveness.
Parti-Mag II has incorporated advances in the measurement of both variables necessary to calculate volumetric flow. The basic formula is:
Q = v x A
Q = actual volumetric flow rate
v = average fluid velocity
A = cross-sectional area of the flow
Under full pipe conditions, the meter functions as a traditional EMF. Faraday's law of induction determines the flow velocity. The law states that a conductive medium moving through a magnetic field will induce a voltage in the medium that is proportional to its average flow velocity. A full pipe provides a constant cross section, which is easily calculated from the geometric configuration of the flow tube. Here, the flow calculation is straightforward.
In a partially filled pipeline, the unique construction of the Parti-Mag II comes into play. As the fluid level drops, the full pipe electrode senses the change and activates a correction algorithm. This mathematical correction alters the measurement method for both the velocity and the cross-sectional area and allows accurate flow measurement down to a 10% fill height.
With traditional EMF technology, the voltage sensed at the electrodes is a direct measurement of the average flow velocity. Once a minimum threshold of fluid conductivity has been met, the measurement is independent of fluid characteristics (e.g., density, viscosity, and molecular weight). The typical limit for standard magmeters is 5 µs/cm.
The Parti-Mag II requires a fluid with slightly more conductivity (50 µs/cm), but this limitation does not significantly reduce the application spectrum. Additionally, under partially filled conditions, the unit must correct the measured voltage before the flow rate is calculated. The correction factors are a function of the fill height and are empirically determined during the meter's factory calibration. The data sets for each group of measurement electrodes are stored in EEPROM on the converter, eliminating the need for on-site calibration.
In the Parti-Mag II, three pairs of measurement electrodes are positioned in the flow tube at different fill heights. The converter selects the optimum measurement electrode pair in contact with the liquid and makes the voltage measurement. Varying the height of the measurement electrodes helps minimize flow profile effects. The use of multiple electrode locations has reduced the adjacent straight-run requirements to five pipe diameters upstream and three downstream. Considering the large diameters typically found in wastewater lines, the reduction of installation requirements directly translates into substantial savings when considering installation cost.
Fill Height Determination
A high-frequency signal with a sinusoidal waveform of fixed amplitude is applied through the bottom pair of electrodes. The resultant signal, as read by the three pairs of measurement electrodes, is adjusted by the converter and translated into an accurate measurement of fill height. The change in amplitude of the detected signal is the source of the information. Normalized ratios of the received signal (Urec) to the injected signal (Uinj) are plotted to determine both the fill height and the correction factor for the velocity calculation. An example is shown in Figure 2.
The three sets of measurement electrodes generate data that are well dispersed and easily interpreted. Careful electrical design avoids data groupings and slopes approaching zero or infinity in the meaningful interpolation zones. This directly correlates with increased measurement accuracy and is a significant advance over previous technologies.
Updated data points for fill height and flow velocity are required for an accurate measurement. Because both parameters use the same set of electrodes to derive the two signals, continual alternation of the measurement function is required. The coil excitation for the velocity measurement is set at a regular frequency, typically 71/2 Hz (or 1/8 of the line frequency). A signal generator for the height measurement is capacitively coupled to the lowest electrode pair and operates between the DC pulses to the coils. This strategy allows the separation of the low-voltage signal induced by the flow velocity from the high-frequency voltage signal containing fill height information and enables the microprocessor to capture the raw measurement data. The desired outputs are then calculated, as shown in Figure 3.
Cost Savings and Operational Benefits
Accuracy, installation considerations, and rangeability become more critical when considering partially filled pipelines. The extremes of flow rates are greatly extended, and the low-end measurement limits are taxed more often.
Parti-Mag II's full-pipe accuracy is 1% of the indicated flow rate, and the accuracy in the partially filled mode is 3%5% of that rate, depending on the fill height. At first glance, these raw numbers seem high for an electronic flow measurement device. With further investigation, you can see that the numbers represent approximately a two-fold advance when compared with a flume. You also gain additional application flexibility through extended rangeability. By maintaining the ±5% error limit of the indicated flow rate throughout a 10001 operating range, Parti-Mag II extends the measurement limitations to almost 10 3 that of a flume.
Installation parameters for free surface flow measurement devices are typically quite restrictive. Tight limitations are placed on pipeline slope, upstream/downstream channel configuration, and flow profile. Parti-Mag II will measure sub- or super-critical flows and therefore function with pipeline slopes up to 5%. This flexibility eliminates the necessity of costly design considerations, such as stilling ponds and elevation changes. Additionally, the high limit on pipeline slope allows the metering run to adhere to the ATV 110 standard, which details the minimum slope required to eliminate particulate settling. By minimizing settling, the maintenance budgeted for clean-out can be eliminated.
Parti-Mag II is a good example of advanced microprocessor technology providing a solution to a long-standing problem.
Blake Doney is Senior Product Manager--Magnetic Flow Products at ABB Automation, Inc., 125 E. County Line Rd., Warminster, PA 18974; 215-674-6325, fax 215-674-6394, firstname.lastname@example.org