Clamp-on or insertion ultrasonic flowmeter sensors are often installed to measure the flow through large pipes to avoid the expense of a (large) full-bore spool piece. The ultrasonic sensors for most pipe sizes are similar, so flowmeter cost is almost independent of pipe size. These attributes of ultrasonic flowmeters often make this technology less expensive and more convenient to install.

Ultrasonic flowmeters measure liquid velocity, from which the volumetric flow rate is inferred. The measurement is linear with liquid velocity and exhibits a relatively large turndown. In addition, the range of flow measurement is relatively large.

Ultrasonic flowmeters can be applied to gas flows, particularly stack gas, flare gas, and natural gas. Some ultrasonic flowmeters can measure the flow of liquids in partially filled pipes.

Excerpted from The Consumer Guide to Ultrasonic and Correlation Flowmeters.

We used to 'know' that flowmeters require 10 pipe diameters of upstream straight run and 5 pipe diameters of downstream straight run.  Based upon experimental test results, we 'know' that the flowmeter will measure accurately if a minimum straight run is installed upstream of the flowmeter.  We now 'know' that the minimum upstream requirement is often much more than the 10 pipe diameters that we used to 'know' that we needed. 

By inference, we 'know' that the number of pipe diameters required is independent of pipe size.  For example, 2-inch and 48-inch flowmeters requiring 6 pipe diameters of upstream straight run would require 1 and 24 feet of upstream straight run respectively.  Stated differently, in this example, 1 and 24 feet of upstream straight run dissipate velocity profile distortions to the extent that they are small enough to not affect the flow measurement. 

However, recent flowmeter data taken on four continents shows that this dissipation does not occur as quickly in larger pipes as it does in smaller pipes.  This appears to be because the fluid in larger pipes does not have as extensive contact with the pipe wall as do fluids in smaller pipes and due to the large amount of momentum present in the fluid flow.  As a result, more pipe diameters of straight run are required to dissipate flow profile distortions in larger pipes. 

One can conceptualize this phenomenon by imagining a large pipe with swirl in its center that has little or no contact with the pipe wall.  It is not likely that this swirl involving of tons of fluid per second will be sufficiently dissipated in (say) 6 pipe diameters.  In other words, flowmeter performance can be adversely affected in this application and the effect will likely be large due to the large pipe size and large flow rate involved. 

This is something that I 'know' (for now).

This article originally appeared in Flow Control magazine. 

1. Process calculations
2. Control valve calculations
3. None of the above
   
4. Both of the above

Process calculations can be used to calculate flow rate in many applications.  In a simple application where two flows streams are combined, the combined flow rate 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 flow rate 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 flow rate is to use a control valve as a flowmeter.  The flow rate, differential pressure, and fluid properties are typically used to calculate C_v to size control valves.  This same relationship can be used in reverse to calculate the flow rate when the valve position, C_v at that valve position, differential pressure, and fluid properties are known.  Knowledge of these parameters is often limited so the accuracy with which the flow rate 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. 

This article originally appeared in Flow Control magazine.