Flow measurement applications can be difficult because of physical properties, operating conditions, operating constraints or some combination of all three. For example, the fluid may be corrosive, abrasive and/or lethal. It may be operating near its vapor pressure or at an extremely high or low temperature or extremely high or low pressure. Selecting a flowmeter to measure even common fluids operating at nonextreme conditions can be challenging when operated under these conditions. That said, the operating constraints are often overlooked (at least initially).
Consider the measurement of combustion air to an incinerator. The ambient air was not corrosive, abrasive or lethal and operated at atmospheric pressure and ambient temperature. In other words, the air was benign and not operating at extreme conditions. Finding a flowmeter to measure the air flow should be easy, right?
Not so fast! The flowmeter must measure the combustion air flow over the entire firing rate of the incinerator — typically over a 10-to-1 turndown. However, this incinerator is different because one of the plant fumes is mixed with combustion air to provide better destruction, increase incinerator capacity and reduce energy consumption. Therefore, the flowmeter must control the air flow from about 30 to more than 500 flow units, so it would be appropriate for the flowmeter to measure from approximately 15 to 700 flow units, effectively resulting in a 45-to-1 turndown.
And by the way, it is possible for the fumes to flow backward through the combustion air fan to the atmosphere under certain conditions. This is not good, so the flowmeter must measure zero flow under reverse flow conditions (or utilize a separate flow switch that forces the flowmeter output to zero when reverse flow occurs).
We will select a flowmeter for this application next month.
David W. Spitzer is a regular contributor to Flow Control magazine and a principal in Spitzer and Boyes LLC, which offers engineering, seminars, strategic, marketing consulting, distribution consulting and expert witness services for manufacturing and automation companies. Spitzer and Boyes is also the publisher of the Industrial Automation INSIDER. He has more than 40 years of experience and has written more than 10 books and 350 articles about flow measurement, instrumentation and process control.
Spitzer may be reached at 845-623-1830 or via spitzerandboyes.com. Click on the “Products” tab to find his Consumer Guides to various flow and level measurement technologies.
