A Reader Writes:
We do system automation work for the installation of weight-based and flow-based specialty blending equipment for batch plants. Charging accuracy for some very expensive liquids isn't good enough. We've been asked for suggestions for an upgrade to be bid out next year. What can we do to improve performance with the flow charging, metering, load cell systems?
-From September 2002 Control Design
Closely Coupled Coriolis
We have used two or even three-stage flow metering on expensive liquids by sequencing successively smaller Coriolis flowmeter outputs during metering cycle. Smallest "trickle flow" completes metering after larger flow rates that expedite metering event times shut down prior to completion.
...Close monitoring of the liquid's specific gravity information provided by the Coriolis flowmeter stops batching when entrained gas in the liquid stream is detected and sends an alarm to the operator.
...Each metering link should have a "purge back to hold tank" option to recirculate liquid streams with entrained gas and purge each flowmeter until correct specific gravity for process liquid is attained.
Karl Holz, I&C Design Engineering
AEP Pro Serv, Dallas
Simpler Design, More Consistency
Currently accepted practices of material feed control force design engineers to trade speed for accuracy, or visa versa. Besides forcing a performance tradeoff, these designs can consume large amounts of enterprise capital by wasting money on raw material over feeds (trading accuracy for speed), and by consuming significant capital investment (and space) on complex piping, and/or variable position valve systems, as well as the automation hardware and programming required to drive such multispeed material feed designs (trading speed for accuracy).
...Our predictive-adaptive control (PAC) solution (globally patented and exclusively licensed to us by Procter & Gamble) demonstrated improvements in automated material delivery accuracies by factors of up to 10 times, and decreased batch cycle times by up to 30%.
...Using model-based predictive-adaptive control embedded on our Q.i platform, the measurement and distributed control system predicts the future and then uses this information to accurately add the formulated amount of material. The PAC allows the use of simple and inexpensive on-off flow control devices, reducing not only the installed cost of a system but also its long-term maintenance costs.
...While the process industry has available many adaptive strategies for continuous systems, there have been fewer choices for batch material delivery systems, until now.
...Figure 1 provides four examples of how PAC technology reduced a delivery system's variability.
...PAC technology allows the manufacturer to use simple and inexpensive on/off control equipment in the material delivery systems and achieve higher accuracy results than realized by using the traditional dribble/flow control elements used by most of the industry as represented in Figure 2 below.
...The predictive part of PAC continuously monitors the process, determining a material's actual flow rate at all times during the material delivery. Based on the flow rate and the PAC's knowledge of the past performance of the delivery system, it continually predicts the optimum point at which the delivery system should stop (cutoff) so that the desired amount is transferred.
...The adaptive part of PAC monitors the performance of each delivery and updates the factors used by the predictive part. This allows the PAC to adapt to and compensate for normal process variations that can cause a material's delivery characteristics to change over time (e.g., humidity, which affects dry materials, or viscosity, which affects liquids).
...By using the flow rate of a material and monitoring batch-to-batch deviation from setpoint, the PAC can determine the amount of "dynamic spill" that it can expect. Dynamic spill is the amount of material that will still be transported after the command to stop has been issued.
Rodger Jeffery, Business Development Manager
Tom Tait, Market Manager, Process Control
Mettler Toledo, Columbus, Ohio
Fast Response Coriolis
Inaccuracies with batch processes using flowmeters are generally due to issues at the beginning and end of the batch.
...Coriolis technology is certainly the ideal way to measure the flow of high-value fluids because of its inherent accuracy, as well as the fact it measures mass rather than volume. However, ramp-up and ramp-down of flows have to be slow enough to accommodate the rather sluggish response times of many existing Coriolis flowmeters. Traditional Coriolis flowmeters also cannot deal with starting from empty, so elaborate means are taken to keep the flowmeter full at the beginning and end of a batch. This often wastes some of the fluid and increases batch time.
...Our new CFT50 digital Coriolis system improves the speed of response by at least a factor of 10 over previous Coriolis systems, allowing batch ramp-up and ramp-down times to be reduced. The new flowmeter technology can also deal with the flowtube starting from an empty condition, eliminating the need for elaborate and often costly workarounds. These digital Coriolis capabilities allow for accurate batching of even small amounts of expensive additives and ingredients.
Wade Mattar, Senior Flow Consultant
Invensys/Foxboro Measurement & Instrumentation Group, Foxboro, Mass.
Should We Use Laser-Based Sensors?
We have a possible application to use laser-based sensors in a high-precision (10 um) triangulation solution with a range of about 2 ft. It seems this is a borderline range, and perhaps a less-expensive, LED-based solution makes more sense. I'd like some advice on the pros and cons of both, or an alternative to these solutions, if there is one.
Send us your comments, suggestions, or solutions for these problems. We'll include them in the May 2003 issue. Send visuals, too,a sketch is fine. Have a problem you'd like to pose to the readers? E-mail us at CDTheAnswer@putman.net or mail to The Answer to Your Problems, CONTROL DESIGN, 555 W. Pierce Rd., Suite 301, Itasca, IL 60143. You can also fax to 630/467-1124. Please include your company, location, and title in the response.