Travis Richards welcomes back Adam Hawley for Part 2 of the podcast. They move on to discussing how to meter and measure gas and the different flow meters he recommends for listeners. Topics discussed include: Metering and measuring gasDifferent flow meters for custody transferTradeoffs in the different meters When figuring out what flow meter or size meter to get, Adam advises to consider the required accuracy or uncertainty of measurement when measuring natural gas. It could be a custody transfer meter, which is a meter that’s being used to buy or sell gas between companies that needs a high accuracy and does a good job of measuring to avoid discrepancies that could result in millions of dollars of gas that’s unaccounted for, or just a check meter in a plant that determines whether there’s flow or none. Some good choices for custody transfer applications include the Coriolis flow meter. This uses the Coriolis force to measure flow and what happens is that the gas flows through vibrating tubes and based on how the gas flows through the tube as well as the geometry of it, it imparts the Coriolis force on these tubes which makes them bend and twist proportional to the flow rate; the higher the flow rate, the more they’ll bend or twist. Through that bending and twisting, they can get a high accuracy of measurement. What’s unique about the Coriolis meter is that it measures mass flow and not volume flow which is advantageous for natural gas flow movement because it cuts out required steps to get back to your volume or standard flow. How the mechanism works is that there are oils on each of the tubes that measure vibrations and the raw signal that goes into the computer is a wave that shows the pulsing of the vibrations. As the bending and the twisting of the tubes happen, it shifts that vibration waves between the tubes and the computer would transmit the difference in timing between the wave it’s measuring and the time it’s proportional to. For an ultrasonic meter, there are multiple paths that send ultrasonic pulses back and forth the gas and each of those paths measure the velocity of the gas in a defined spot in the pipe. The meter integrates those paths and informs you of the velocity of the gas in the pipe. Based on the geometry of the pipe, it can convert it into the standard volume. One great thing about both technologies is that they have diagnostics associated with them to inform you whether they’re healthy or if there’s a need to check on the operation of the meters. Another tried and tested meter is the turbine meter that still also fits in the custody transfer bucket and was heavily used prior to the Coriolis and ultrasonic meters available in the market. An example of how a turbine meter works is if when there are fan blades in the pipes that are on a shaft and as the gas flows over the blades, it causes the rotor to turn, and the meter counts the number of revolutions of the fan, blades, and rotor in the flow proportional to the volume flow. However, since turbine meters are mechanical, there aren’t many diagnostics as much as the Coriolis and ultrasonic to determine the problem. The gas should be clean with little to no contamination because the mechanical components are more susceptible to that versus the other two technologies. Still, turbine meters have great accuracy and the best repeatability out of any flow meter. Thermal mass meters are more on the side of low-cost flow meters. There is a probe in the flow that maintains the current at a certain temperature and as the gas flows over the probe, it cools the probe, and the meter can determine the flow rate based on how much cooling is happening or how much energy it’s putting in to try to keep the probe at a given temperature. The grandfather of all flow measurement is the orifice meter and is still used in some custody transfer applications because it’s cheap, reliable, and has been used for so long, but are also the most susceptible to issues with cleanliness of the gas. The orifice meter is essentially just a plate with a hole in it and as the gas runs into the plate and through the hole, it exits at a high velocity and causes a differential pressure across the plate. Upstreaming the plate brings higher pressure while down streaming the plate causes low pressure which is the fundamental measurement that an orifice meter would use to measure flow. When it comes to tradeoff, orifice meters are susceptible to the overall geometric tolerance of the meter. There is a gas industry standard called The American Gas Association Report No. 3 that details exactly how an orifice meter should be designed down to the pipe roughness finish and the geometry of where the plate is. If an orifice meter is designed down to the letter of the law, it’s going to perform within its uncertainty. Where things go awry is when the tolerances are so tight and specific on fabricating this meter that when there’s a mistake or ...
