Quiz Corner: Calculating flow rate from tank level

What is the approximate flow rate into a circular 1,000-gallon vertical tank that is filled in 10 minutes?

A.    60 gallons per minute

B.    80 gallons per minute

C.   100 gallons per minute

D.   110 gallons per minute

One might think that the flow into the tank is approximately 100 gallons per minute (1000 / 10) or Answer C. However, tank sizes generally represent a nominal capacity so this 1,000-gallon tank could hold somewhat more or less than 1,000 gallons of liquid.

Note that the tank level transmitter is likely measuring 0% to 100% between the tangent lines of the vertical tank section but it might be measuring a smaller vertical range. Local sight glasses typically measure over an even smaller range.

Plant personnel might consider 20% and 80% levels to be empty and full respectively because operating below 20% or over 80% level might put the tank uncomfortably close to underfilling or overfilling conditions respectively. Therefore, filling the tank could entail using less than 60% of the total tank capacity. Under this scenario, the flow would be approximately 60 gallons per minute (Answer A). A similar case operating from 10% to 90% level could be made for Answer B.

Additional complicating factors

A more precise method is to use the tank mechanical information (dimensions and level transmitter location) in conjunction with operational information (starting/ending times and starting/ending levels) to calculate the flow rate.

David W. Spitzer is a principal at Spitzer and Boyes, LLC, which offers engineering, focused market research, writing/editing white papers, strategic marketing consulting, distribution consulting, seminars and expert witness services for manufacturing and automation companies. Spitzer has written more than 400 technical articles and 10 books about flow measurement, instrumentation and process control. He can be reached at 845-623-1830 or via spitzerandboyes.com.