We received two additional questions, so I thought it warrants a second post to bring more visibility to Marc’s responses to the questions.
- What is the max value of drive gain that you can believe the meter reading with confidence?
Any drive gain value below 100% means that the flow tube vibration in the Coriolis meter is still at the precise target amplitude (distance apart at the peak of cycle). This means you can trust the mass flow measurement as long as the drive gain is any number less than 100%. The meter will still function above 100%, but this indication means there is severe 2-phase flow that will start to adversely affect the meter measurement accuracy. Be aware that if the meter is indicating density or volume flow, any vapor in the stream will cause an error because the product is no longer all liquid. An unusually high drive gain value above where a meter operates when it is full, even if still below 100%, may mean you have vapor in a liquid stream and the density and volume measured are the density and volume of both the liquid and the vapor.
The reader’s second question:
- Could you describe the mathematical model of the drive gain? what affect it?
The drive gain is a calculation of the average power consumption that the meter is using to keep the tubes vibrating at the target amplitude. Bubbles in a liquid stream or mist in a gas stream will absorb energy because of damping in the vibration due to the relative decoupling motion of the two-phase fluid vibrating back and forth in the tubes. The meter will continue to supply more power through a feedback loop to keep the tubes vibrating at the precise target amplitude until the 100% drive gain level is reached. The 100% limit is defined as the maximum power that can be supplied while still keeping the senor unit of the meter intrinsically safe for hazardous area classifications. If the amount of 2-phase mixture continues to increase above the 100% drive gain mark, then the tube amplitude will start to decrease and measurement will be adversely affected because the lower tube amplitude is not as sensitive to flow and density as the target is designed to deliver. Even while still below 100%, elevated drive gain values can provide a good diagnostic indicator of a process problem that is either introducing vapor bubbles into a liquid flow stream or mist into a gas flow stream. Different size meters have different normal drive gain values. Some larger meters can have normal drive gain values close to 50%, so there is no fixed “normal” value that is the same for all meter shapes and sizes. The best practice is to observe the normal drive gain of your meter when you know the meter is full of liquid or dry gas (whichever is the normal condition for the application) and then, if the drive gain ever spikes above that value by more than 40%, you may want to investigate for pump seal leaks, product boil off, or other likely causes of vapor in the stream.
Visit the Micro Motion Coriolis Flow Meters for Mass, Volume, & Density section on Emerson.com for more this advanced phase measurement technology to provide accurate measurement and process diagnostics for complex processes and challenging multiphase flow environments. You can also connect and interact with other flow measurement experts in the Measurement Instrumentation group in the Emerson Exchange 365 community.