I really like to share questions I receive via email, so that others with similar questions can benefit through an online search. We’ve been encouraging our global subject matter experts to also do this in places such as our Emerson Exchange 365 community and LinkedIn groups and posts. In our era of digital transformation, this is “working out loud”.
In this spirit, here’s a question I recently received:
I am looking for concrete information on how oil drive gain is calculated and the overall meaning behind the numbers. I understand the Coriolis is giving me the number but I do not understand how or what it exactly means. The simple explanation I was given is that it has to do with the retention time of the oil in the separator and a high oil drive gain is not necessarily good when oil flow rate is zero. Any further explanation would be greatly appreciated from myself and my colleagues or if you could direct me to where I could find this information that would be great as well.
I turned to Emerson’s Marc Buttler for help. You may recall Marc from several earlier posts on oil & gas industry-related flow measurement. I’ve added some hyperlinks in Marc’s reply for you to find more information.
The Drive Gain diagnostic value that a Micro Motion Coriolis meter from Emerson reports is a very good indicator of when there is gas vapor in the oil leg. That is why is it important to monitor, because it can tell you very definitively if the oil leg on your separator is experiencing gas carry-under. Sometimes, if retention time is too short, there might be gas still trapped in the oil that dumps out of the separator. When that happens, it is very common for gas to break out of the liquid as the oil flow down the pipe and experiences pressure decrease due to the friction in the pipe.
The way the Coriolis meter Drive Gain indication works is that it measures and calculates on a scale from 0 to 100% of the energy budget just how much power is being consumed to keep the vibrating tubes inside the Coriolis meter vibrating at just the right distance from each other. If even a small amount of vapor phase is introduced into the liquid stream, this value will skyrocket, sometimes straight up to 100% in severe cases.
This can mean that the meter flow measurement accuracy of the oil is affected adversely compared to when the meter is kept full of liquid. When you see the Drive Gain increase when there is no flow (e.g., in between dump cycles), that probably just means that gas is coming out of solution and creating a gas pocket inside the meter. That may not be a problem for you if the gas pocket is immediately swept out as soon as the dump valve opens because the meter will start measuring the oil flow very accurately again as soon as the gas pocket is gone, and you can often set the low flow cutoff value in the meter high enough to ignore any zero noise in the meter that might be caused by the gas pocket when the dump valve is closed and there is no flow.
Here is a website that has some great information on how Coriolis meters work: https://www.whymicromotion.com/Home/MeasurementConfidence
Please enjoy watching some of the videos that explain how the meters work in more detail and how they perform with 2-phase flow.
Learn more about Coriolis flow meter technology for mass, volume & density measurements on Emerson.com. You can also connect and interact with other flow measurement and oil & gas industry experts in the Measurement Instrumentation and Oil & Gas groups in the Emerson Exchange 365 community.