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During “The Natural Gas Coriolis Measurement Revolution” webinar that was conducted on July 24th we came across a great question from one of the viewers and wanted to share with you our thoughts.
Question: “Several times during the webinar you said that one of the benefits with Coriolis measurement is that you don’t need to worry about pressure and temperature. With that said, I am hearing a lot about adding PT’s and TT’s in Coriolis-based meter runs for NGLs (natural gas liquids) for compensation. One of our process engineers was telling me that the Coriolis units themselves allow for a pressure compensation input to help reduce overall error for custody transfer applications. I’m confused between what you said during the webinar and what I’m hearing from our process folks.”
Answer: In the Webinar, I talked about the conversion from mass to standard volume for natural gas. This equation is standard volume = mass divided by base density. The base density is the density of the gas at standard reference conditions, so it is entirely dependent on gas composition and will not change as pressure and temperature change. Therefore, none of the terms in this conversion are effected by pressure and temperature.
This statement is not the same when you are trying to convert NGL actual volume to volume at reference conditions. Because Coriolis meters can measure liquid in either volume or mass units, it is not at all uncommon for people to use the volume measurement and convert to volume at reference conditions using API MPMS volume correction factors for pressure and temperature the same way they would with any other volume metering device. This is often done because people are more comfortable with proving and measuring in volume, but they still want to enjoy all the other benefits of Coriolis meters even if they are not comfortable with mass units. It is possible, and rather popular to measure NGL directly in mass units; though, not everyone does it this way. If you do measure NGL in mass units, you could apply base density of the mixture to convert to NGL standard volume without pressure and temperature, but I don’t know anyone who does this because if they have NGL quantity in mass already, they generally prefer to just keep it in mass units while it is still a mixture. I am attaching a Natural Gas Liquids Mixture Measurement paper that explains how to use direct mass measurement and composition analysis to compute final total volume of each individual, pure component of an NGL mix. This is done in such a way to avoid problems that occur with solution mixing when you try to measure NGL in volume units.
As far as the pressure compensation for the effect of pressure on the meter, this can be a factor for some meter sizes and designs, but not for all. So, it is important to be careful and work with the manufacturer to see if you have a meter that is even effected. When it is present, the effect is typically linear with pressure and very small, so fine tuning can be done in processes that have fairly steady line pressure by simply configuring the average line pressure, which the meter then uses to apply a constant correction factor. In cases where the line pressure can vary significantly during operation, it is possible to fine tune for the pressure effect by wiring a pressure transmitter to the Coriolis meter transmitter input. The Coriolis meter can then correct itself with the live pressure reading. The compensation for the effect of pressure on the meter is very different than the compensation for the effect of pressure on the gas. It is possible to get an accurate measure of mass and thus standard volume with a Coriolis meter without the fine tuning of the pressure effect compensation, but it is not at all possible to convert to any measure of standard volume (or energy) without pressure and temperature if starting with a measurement from a volume or velocity meter.
We hope this explanation has helped resolve any confusion, and if you have a question, feel free to ask away in the comment section below!