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Application Impossible: The Automation Affair by Walt Boyes
Many years ago, I assisted in the design, specification, and installation of a highly sophisticated chlorination/dechlorination system for a tiny wastewater utility in Northern California. The plant discharged directly to the Eel River, so its NPDES (EPA discharge) permit was very strict as to the amount of chlorine residual they could discharge (0.0 percent).
The plant had a single, part-time operator, who was moonlighting from his job at another wastewater utility. This operator was very good, and the engineer and I had some hope that by installing and tuning this advanced control system, we could make it possible to make the plant virtually unattended, and still have it work. (We included plenty of alarms and an autodialer, just in case.)
I got an invitation to the official grand opening and since I happened to be in the area, I went. During the festivities, the chairman of the board of the wastewater district pulled me aside, and asked me in total seriousness, "Now, this fully automatic thingy here, this means we can fire our operator and save all that money, right?"
Sometimes it just isn't possible to change the rules. Sometimes you have to walk away.
Flowmeter Slippage: Avoiding the Avoidable by David W Spitzer
Every now and then, a portion of a published article or internet post catches my eye --- sometimes because it is well-thought-out and sometimes for the opposite reason. A recent article referring primarily to positive displacement flowmeters stated, under the leakage heading, that fluid leaking past the moving parts "rather than turning them... can occur when a meter starts from a zero-flow condition. To avoid slippage at low flow rates, installers may have to increase flow rate at startup, then back it off once the right momentum is achieved."
Notwithstanding that leakage and slippage are different but appear to be used synonymously, installers may have little interest in whether the flowmeter operates because they are typically long gone before operation begins. In addition, having to increase the flow rate every time flow is started is an excellent reason to consider another flowmeter or flowmeter technology.
Some years ago, I noticed that a particular flow control valve was manually opened for about 30 seconds every day to remove accumulated solids. This was not acceptable because we wanted the operators to focus on the operation, not on minutiae and busy work. Within a few weeks, a different style valve was installed that did not plug.
In this case, installation of a flowmeter that requires more than normal attention is not advised unless it is the only flowmeter that will work in the application. In addition, operators do not necessarily have the luxury of increasing the flow during startup because the additional amount of the material may create a hazard in some operations and/or waste a valuable raw material or product. In some plants, no suitable location may exist to store the additional material.
It is bad enough to inherit a problem installation such as the above flow control valve, but efforts should be made to not install new inconveniences that you or your successors will need to address.
Which of the following flowmeters could be used to measure 10 to 100 gallons per minute (gpm) of liquid with a specific gravity of 1.20 and a viscosity of 500 centipoise (cP) in a 3-inch pipe?
Coriolis mass
Differential pressure
Magnetic
Positive displacement
Thermal
Turbine
Vortex shedding
Commentary
Thermal flowmeters are not applicable because they are used in gas service (Answer E).
Reynolds number at maximum flow can be calculated as 3160 x 100 x 1.2 / 500 x 3, or approximately 250. Operation in the laminar flow regime with Reynolds numbers of 25 to 250 generally precludes the use of differential pressure (Answer B), turbine (Answer F) and vortex shedding (Answer G) flowmeter technologies.
Coriolis mass (Answer A), magnetic (Answer C) and positive displacement (Answer D) flowmeters remain as potential technologies to apply in this application.
Additional Complicating Factors
Many liquids with high viscosities, such as hydrocarbons, are not conductive, so magnetic flowmeters (Answer C) can sometimes be eliminated from consideration, simplifying the selection process.
In addition to over 40 years of experience as an instrument user, consultant and expert witness, David W Spitzer has written over 10 books and 450 articles about flow measurement, level measurement, instrumentation and process control. David teaches his flow measurement seminars in both English and Portuguese.
Spitzer and Boyes, LLC provides engineering, technical writing, training seminars, strategic marketing consulting and expert witness services worldwide.
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