Application case study: Upgrading flowmeter stabilizes control of process slurry flow

-- Control Engineering, 7/17/2008

Replacing a faulty magmeter with new advanced design helped eliminate erratic flow signals, resulting in improved control of catalyst slurry flow.

Problem: An electromagnetic flowmeter measuring catalyst slurry containing 30% solids flowing through a 3-in. pipe continually produced erratic and erroneous flow readings, thwarting flow control efforts. The plant maintenance group tried various methods to correct its performance with no success. Meanwhile, operation under automatic control was impossible.

The graphic shows intermediate steps in the manufacture of catalyst, including the slurry flow control system. A centrifugal pump with variable-speed drive controls flow at a rate set by the DCS. Slurry goes to a belt filter that removes excess liquid and feeds moist catalyst cake into a gas-fired rotary dryer.
 
The magmeter sends flow measurements to a flow controller in a DCS control room. The operator sets the desired flow rate and the controller is supposed to keep the filtration and drying operations running smoothly and continuously at an optimum production rate. However, the original meter's flow signals were erratic and completely unpredictable. The signals could call for excessive rates of slurry flow that caused the dryer to flame out and shut down the line. The control room operator could not depend on the measured flow readings, which forced reliance on manual control.

Solution: Plant maintenance decided to substitute the new FSM4000 magnetic flowmeter from ABB Instrumentation. Upon installation, the new meter performed exceptionally well, producing reliable flow signals that allowed operation via automatic control.

ABB says the FSM4000’s success in dealing with noisy applications originates with its use of 70 Hz ac excitation of the primary's magnetic field, combined with advanced digital signal processing. The measurement signal has an accuracy of +/- 0.5 percent of rate, and offers fast response. With dependable flow control readings, the process is now on automatic control, improving production and eliminating troublesome shutdowns.

—Edited by Peter Welander, process industries editor, peter.welander@reedbusiness.com,
Control Engineering Process Instrumentation & Sensors Monthly
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