Every process control application includes measurement, control decisions, and action on the process. If you want to be a process control guru, you need to know how to measure flow.
Flow measurement is critical to most every continuous process. It is fundamental to establishing production rate, material ratios, and energy management. Not to mention that accurate flow measurement is often used to establish payments for what was produced.
There are many, many different types of flow measurement devices to choose from, and the choice depends heavily on the type of application. You’ll hear this theme again and again on the Process Control Guru blog…”Good process control starts with good process understanding”. So let’s look at the process, by asking a few key questions:
- Do you need to measure a solid? liquid? gas? mixture?
- Is the fluid clean? dirty? corrosive?
- Is the fluid abrasive? fibrous?
- Is it highly viscous? conductive?
- Will the density be constant over time?
- What are the temperature and pressure conditions?
- What changes during abnormal operation and shutdown?
- What are the highest and lowest flows that you need to measure?
- What level of accuracy is desired?
There are many different technologies for measuring flow. Listed from the most common to the least common, here are some of the technologies available, along with a summary of their strengths and weaknesses:
|Differential Pressure||Low Cost. Good at temperature extremes. Scales up easily.||Accuracy, plugged sensing lines. Avoid condensing vapors.|
|Magnetic Flow Meters||0.5% accuracy.||Cost. Requires conductive fluid.|
|Coriolis (mass flow)||100:1 turndown, 0.5% accuracy, handles variable density.||Must have full pipes.|
|Ultrasonic||Wide range of application, sizes.||Not good for abrasive, fibrous streams|
|Vortex Meters||Low cost.||Not good if fluid properties change.|
|Turbines||Fast response. 10:1 rangeability. Good for vapors.||Not good for dirty or viscous fluids. Beware overspeed conditions.|
An excellent reference for flow meter selection is “Process Measurement and Analysis” The Instrument Engineer’s Handbook, Edited by Bela Liptak. See it on Amazon.com