This article was printed in CONTROL's April 2009 edition.
“Ask the Experts” is moderated by Béla Lipták, process control consultant and editor of the Instrument Engineer’s Handbook (IEH). Preparation of the next edition of Vol. 3, “Process Software and Networks,” is in progress. If you’re qualified to update an existing one or prepare a new chapter, or if you would like to participate this column, send an email to firstname.lastname@example.org.
Q: I have a problem in flowmeter selection. We have a process that requires an accurate flowmeter, which can be installed with only 3D-2D of pipe diameter straight run. I wonder if you could help me solve this problem?
Another question, what does the rangeability of a flowmeter mean?
A: Of the flowmeters I’ve listed in Table 1, the Coriolis and variable area designs require no straight runs at all, while the V-cone and the magnetic flowmeters can operate with five or less pipe diameters of straight run. The straight run requirements of others, like the vortex or the Venturi types, can be reduced to about eight diameters upstream and five diameters downstream if upstream straightening vanes are used. For details refer to Chapter 2 in Volume 1 of my handbook.
A: Try a four-hole “Rosemount” orifice which only requires 2D/2D up to DN200 if a 4:1 rangeability is adequate. The correct question to ask here is how accurate a flowmeter you need.
Ian H. Gibson
A: Several flowmeter types operate reasonably accurately in such a short run of straight pipe. Vortex processing meters require no straight run. V-Cone devices and magnetic flowmeters require short, straight runs. Multiple-port pitot tube devices, such as Annubar or Verabar, can be characterized and linearized for flow, even when inserted in an elbow. Any positive displacement flowmeter will also work, again, depending on pipe size. Another tack to try is to buy a flow conditioning insert, and use a differential pressure insert immediately downstream of the flow conditioner. However, this usually requires at least five to seven diameters.
A: I’m also a fan of magnetic flowmeters for this type of application, but a potential problem is the wide flow range needed. If you use a 75-mm (~3 in.) meter, the maximum velocity is about 9.5 mps (30 fps), which is on the high side for a magnetic flowmeter. This might cause liner wear or damage, but if it’s infrequent, it might be okay.
If you use a 100-mm (4-in.) meter, then maximum velocity is about 5.5 mps (17 fps), which is much better, but the minimum velocity is only .17 mps (0.57 fps). Most magnetic flowmeters start losing accuracy under 1.0 fps to 1.5 fps. You should check the accuracy versus velocity curves for the specific flowmeter you intend to use to make sure it will meet your needs.
Curt Wendt, CAP
Q: With our control systems, we serve an international market and have trouble switching between languages and alphabets, using overlays and other primitive methods. If it was your job to design a multi-language keyboard, how would you go about it?
A: There are switches with character displays built into their tops that could be used. Maybe a custom design based on the Blackberry Storm touchscreen would be worth looking at. I understand it has touch feedback.
A: I would suggest LED labels.
A: I have worked in Germany, Japan, Korea and Indonesia. During that time, I saw keyboards in all of these local languages. The keys are just pop-off caps on pushrod activators.
Robert E. Sherman
Q: I’m pursuing my engineering in electronics instrumentation and control. I’m now in my last semester of engineering. Are there websites where I can get all the study materials on DCS, PLCs, field instrumentation, etc.?
A: You may wish to check http://tinyurl.com/c8equ8.
Q: What is the down-side of a vibrating reed viscosity control? The application would be inline, maximum 90 °F, maximum viscosity of 150 cp. There are two fluids it would potentially be working in; it would be dedicated to measuring the viscosity of one or the other. One fuild is an aqueous base with a pH in the range of 8-11. The other, a solvent containing the most aggressive MEK and Toluene.
A: In a vibrating reed sensor, the amplitude of probe vibration is a function of viscosity. The measurement error is 1% full scale, and for 150 cp you will probably pick a 10-1,000 cp probe, so the error will be up to 10 cp. Pressure and temperature must be constant or compensated and flow be laminar, so you will need a bypass sample loop. The installation requires complete immersion, so the probe should be inserted in a vertical upward pipe section.
Vibration amplitude changes with age, therefore periodic recalibration is needed. If you expect buildup of coating, this sensor should not be used at all. Naturally, the materials of construction should be selected to match the corrosiveness of your process fluid.
Connecting wires should be shielded and power supply and signal wires run in separate conduits.