 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

Commentary
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.