The following tip is from the ISA book by Greg McMillan and Hunter Vegas titled 101 Tips for a Successful Automation Career, inspired by the ISA Mentor Program. This is Tip #11, and was written by Hunter.
Every instrument is well suited for some applications and perfectly awful for others. These next few tips discuss the more common types of flowmeters and provide an insight into how they work, when they should be used, and when they should be avoided. I will begin with a brief description of how each meter works, and then discuss the pros and cons.
Greg provides a good discussion of Coriolis meters so that type of meter is not discussed here.
Concept: Vortex flowmeters can be an excellent choice for a large variety of applications. However, certain limitations associated with this type of meter can make it a poor choice in some applications.
Details: Have you ever watched a flag wave in the wind? It actually waves because the flag pole generates eddies (whirlpools, or vortices) on alternating sides of the pole, which move past the flag. The eddies are relatively high pressure areas that “push” the flag away and because they form on alternating sides of the pole, the flag weaves between them and flutters in the wind. The vortex flowmeter works the same way. It employs some kind of vertical bar or “bluff body” that generates eddies (vortices) on alternate sides as the fluid flows past. Small sensors in (or behind) the bar detect the vortices and count them. The rate of vortex creation is chiefly dependent upon the rate of flow but is also dependent upon the viscosity of the fluid. (If the fluid is too viscous or the flow too low, the meter will not shed any vortices at all.) The flowmeter converts the meter count into a fluid velocity and determines a volumetric flow rate by multiplying the fluid velocity by the cross sectional area of the meter.
Here is a quick list of the pros and cons of this type of meter:
Watch-Outs: The low flow cutoff is the chief limitation of a vortex flowmeter, and it makes the meter unsuitable for any application where measuring low flows at high turndown is required. The low flow cutoff point is determined by the viscosity of the fluid and thus may vary with fluid temperature and composition. Highly viscous fluids cannot be measured with a vortex meter.
Exceptions: While most vortex flowmeters are poorly suited for measuring liquids which tend to polymerize, some meters employ a proprietary sensor design that is not easily plugged and has a constant flow of liquid around the sensors to help keep them clean. Such units can work where others tend to fail.
Insight: For high temperature applications, be sure to specify a remote mounted transmitter head. The electronics do not like being cooked, and a locally mounted transmitter will not last long. Be on the lookout for centering rings if the meter is a wafer type. The pipe fitters tend to leave these in the box and just bolt the meter between two flanges. If the meter is not centered, it will be inaccurate.
Rule of Thumb: Vortex flowmeters are a good choice for measuring the flow rate of any reasonably clean fluid where measurement at very low flows is not required. In line sizes of 6” or less, they are usually much cheaper than orifice plates, and they eliminate the need for impulse line heat tracing.
About the Author
Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.
Hunter Vegas, P.E., holds a B.S.E.E. degree from Tulane University and an M.B.A. from Wake Forest University. His job titles have included instrument engineer, production engineer, instrumentation group leader, principal automation engineer, and unit production manager. In 2001, he joined Avid Solutions, Inc., as an engineering manager and lead project engineer, where he works today. Hunter has executed nearly 2,000 instrumentation and control projects over his career, with budgets ranging from a few thousand to millions of dollars. He is proficient in field instrumentation sizing and selection, safety interlock design, electrical design, advanced control strategy, and numerous control system hardware and software platforms.