|David W. Spitzer, P.E.|
There are times when there is no flowmeter installed, but it is necessary to determine the flow through a pipe. What techniques can be used to determine the flow?
A. Process calculations
B. Control valve calculations
C. None of the above
D. Both of the above
Process calculations can be used to calculate flowrate in many applications. In a simple application where two flows streams are combined, the combined flowrate can be calculated by summing the flow measurements from each individual flow stream. In one application, the unmeasured gas flow to certain nozzles was calculated and controlled in real time by mathematically adding the measurements from the two incoming gas flowmeters and subtracting the measurement from another gas stream flowing to other nozzles. This type of calculation is essentially a mass balance of part of the process.
More complex process calculations are also possible by performing an energy balance on a part of the process. For example, steam flow to a heat exchanger can be calculated using the specific heat, temperature rise and flowrate of the liquid being heated in conjunction with the heat value of the steam. In one application, the heat flow into a boiler was calculated and controlled in real time by mathematically adding the product of the fuel flows and their heat contents.
Another method to calculate the flowrate is to use a control valve as a flowmeter. The flowrate, differential pressure and fluid properties are typically used to calculate Cv to size control valves. This same relationship can be used in reverse to calculate the flowrate when the valve position, Cv at that valve position, differential pressure and fluid properties are known. Knowledge of these parameters is often limited, so the accuracy with which the flowrate can be calculated is also often limited. Nonetheless, this can be a useful tool when other measurements are lacking.
Additional Complicating Factors
The answer to the question depends upon a number of factors, including the process, availability of process measurements, and whether the control valve has a usable position indicator.
David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 30 years of experience in specifying, building, installing, startup and troubleshooting process control instrumentation. He has developed and taught seminars for over 20 years and is a member of ISA and belongs to the ASME MFC and ISO TC30 committees. Mr. Spitzer has written a number of books concerning the application and use of fluid handling technology, including the popular “Consumer Guide” series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.