In this blog post we will go through an Emerson Exchange presentation that was presented by Tonya Wyatt, Martin Schlebach, and Thomas McCulloch on gas flow measurement. This will give an overview of theory of operations for Coriolis, vortex, differential pressure and ultrasonic flow meters and the limitations and advantages of each when measuring gas. Of course, this will be a summary of the presentation so if you have any questions feel free to reach out to us!

Why is Gas Difficult to Measure?
Gas is a compressible substance which makes it difficult to measure. Some of the attributes of gas that make it difficult to measure include:

• Changes to temperature and pressure ultimately change the density of the gas
• Density of the gas affects the volume that a fixed mass of gas occupies

Check out this image for an illustration of changing gas properties:

 


Constant Mass_01dec15

 

 

 

 

 

Four popular gas flow measurement technologies will be discussed, but it is helpful to first understand the Principle of Operation for each of these technologies:

  • dP Meters: dP Meters are based on the Bernoulli Principle (conservation of energy for flowing fluid), the Continuity Equation (conservation of mass), and experimentally determined correction factors. They operate on the premise that a change in kinetic energy (velocity) results in a pressure drop. The pressure drop across the primary element (such as an orifice plate to constrict the flow) is proportional to the square of the flow rate (velocity).
  • Coriolis Meters: Coriolis mass flow meters work on the principle that fluid flowing through oscillating tube(s) causes the tube(s) to twist in proportion to the mass flow rate. A drive coil vibrates the tube(s) at their natural frequency and the pick-off coils (located on the inlet and outlet sides of the tube) generate a signal. The twist causes a time difference between the motion of the inlet and the outlet which is directly proportional to mass flow. Check out this video for more information (link to Theory of operation video)
  • Vortex Meters: When a fluid passes a blunt object, vortices are created. A vortex is an area of swirling motion with high local velocity and a lower pressure than the surrounding fluid. The frequency of vortex formations is proportional to the velocity of the fluid. A vortex flow meter measures the frequency of the vortex shedding to provide an inferred flow rate.
  • Ultrasonic Meters: Transit time ultrasonic meters emit high frequency sound waves into and against the direction of flow and measure the transit time from the transmitting transducer to the receiving transducer. The difference in transit time in and against the flow direction is proportional to the average velocity along that path. Many ultrasonic meters use multiple paths to get a better overall average of the velocity measurement.

So what the advantages and limitations of these technologies? See below for a summary on each technology (note: click on the image for a close up view).

 

Meter Comparisons

 

 

 

 

 

 

 

 

For each of these technologies, Emerson also offers some unique advantages including:

  • Daniel Ultrasonic Flow Meters can offer +/-0.3% accuracy in short straight-run installations (as little as 5D upstream and 2D downstream)
  • Micro Motion Coriolis Flow Meters offer Smart Meter Verification to check the health of the meter without taking it offline
  • Micro Motion Coriolis Flow Meters offer Zero Verification to help improve performance at high turndown ratios
  • Rosemount 8800 Series Vortex Flow Meters have no ports or crevices eliminating the potential for plugging and reducing potential leak points
  • Rosemount 8800 Series Vortex Flow Meters have an isolated flow sensor allowing for replacement without interrupting the process
  • Rosemount 8800 Series Vortex Flow Meters have a meter verification option to check the health of the electronics without removing the meter from the line
  • Rosemount Conditioning Orifice Flow Meters offer +/-0.5% accuracy in short straight-run applications (2D upstream and 2D downstream) allowing for significant installation savings compared to traditional orifice meters
  • Rosemount Conditioning Orifice Flow Meters with integral transmitter significantly reduce the potential leak points compared to traditional orifice meters

Best Practices

So how do you go about choosing the right technology for your gas application? There isn’t one technology that will always be the best selection. It will depend on your application. A few key considerations to keep in mind:

  • Sizing is very important and will help determine performance and pressure drop for the application
  • Materials of construction that are required for the application
  • Reliability and diagnostics

Once a technology is selected, proper installation will help to provide the best results. Be sure to consider the following and follow the manufacturer’s recommendations to maximize performance:

  • Orientation
  • Piping requirements including any needed straight-runs, flow conditioners, or support guidelines

As you can see, there are many choices and many things to consider when choosing a technology for your gas application. For more information, or for success stories, go to: