Accurate Flow Meters

Late last year, I made a presentation to a room full of gas transmission pipeline company executives. The presentation included a proposal to create a center of excellence that would serve as a gas industry focal point for natural gas measurement technology development, standards development and maintenance, and technical training. I was about one hour into my presentation and had covered nearly all of my key points when one of the executives in the audience asked why I had not yet mentioned the need for more accurate flow meters. It seemed to me from the tone of his delivery that he felt that was a glaring omission from my proposed plan.

I quickly responded to his question, and my reply may not have been what he expected. The following is a paraphrased version of that reply. In essence, there is a difference between accurate flow meters and accurate flow measurement. The difference may be subtle, but it is very important if you want to get the most “bang for your buck.”

Meters versus measurement systems

I told the audience that improving gas meter accuracy is a relatively low priority compared to other changes and advancements that are needed to substantially improve flow measurement accuracy for the gas transmission pipeline companies. The Technology Roadmap laid out by the PRCI also supports this perspective. The Roadmap lists the following as the top five priority projects:

• Smart technology
• Evaluate gas quality measurement technology
• Improve physical property equations
• Better fluid property measurement methods
• Assess direct energy measurement.

More accurate flow meters did not even make the PRCI top ten, and the reason is that the flow meter is just one element of many in a flow measurement system. Whether your flow meter of choice is an orifice, a turbine, an ultrasonic, a Coriolis, or some other type, there are many other components that make up a flow measurement system in total, and each component creates an opportunity for an inaccuracy or error in the final measurement.

I’m sure you can quickly and easily name some of the “other” system components, such as pressure and temperature sensors. However, some of the “other” system components that are critically important may not be so obvious. How about meter station piping? As noted in a past column, research has proven that some flow meters can be adversely affected by the piping configuration upstream (and, sometime, downstream) of the meter. Or, how about the way natural gas properties, such as density, are measured or derived? In many metering systems, fluid density is required to determine the volumetric flow rate or totalized flow volume. For natural gas flows, oftentimes, a compositional assay is acquired via a gas chromatograph and used as input to an equation of state calculation of the gas density. What if the GC measurements or the equation of state calculation (which is usually a computer algorithm) are not accurate? Or, what if the GC is functioning properly and the equation of state calculation is being done correctly, but the gas sample analyzed by the GC was not withdrawn correctly from the pipeline, and is not truly representative of the flowing gas stream? Then the density and, consequently, the flow rate or totalized gas volume, will be in error – even if the flow meter is functioning correctly and accurately. Systematic errors from sources other than the flow meter almost always make up the majority of the total measurement error observed in most flow measurement systems in operation today.

Closing thoughts

Most all of today’s commonly used flow meters can produce accurate measurements, if the meters are installed, operated, and maintained correctly. This is especially true if the meters have been properly flow calibrated (prior to field use) using a flow reference that is traceable to recognized national or international standards. When flow measurement accuracy is poor, the primary culprits are usually improper installation, improper operation, or improper maintenance (or some combination of the three) of the measurement system, rather than the inherent accuracy (or inaccuracy) of the flow meter. Once we reach the point that improved meter accuracy does truly become an industry priority, we will have already made great strides in significantly improving measurement system accuracy overall.