Case Study: Increased Backpressure Improves Promass Measurement In Difficult Applications

It has long been known that among the most difficult applications for a Coriolis meters are viscous liquids with entrained air. Viscous liquids tend to hold entrained air for long periods of time, and air bubbles in a liquid can cause unstable measuring conditions for a Coriolis meter when the volume of air gets too high. A relatively large flow meter is often selected in order to minimize pressure drop when a viscous liquid needs to be measured. Accuracy can then be compromised by the resulting "oversized" meter, because any zero point problems for a Coriolis meter are more visible in the low part of a meter's overall range. A recent starch slurry application at a major soup producer illustrates this typical problem, and also demonstrates how backpressure can improve the measuring results.

The problem for a Coriolis meter with small air bubbles mixed throughout the fluid is that the air bubbles are compressed slightly by the vibrating action of the flow tubes. Another way of looking at it is the vibrating flow meter tubes compress the air bubbles and this compression will slightly dampen the tube vibration. When air entrainment increases, the meter's tube drive circuit then automatically increases, power output to drive the tubes to a desired amplitude for proper Coriolis phase shift measurement. At some point, as the amount of air increases, the drive circuit of the Promass meter saturates at 100 mA, and the meter fails to drive the tubes to the desired amplitude. It can be expected that before 100 mA drive current is reached, the measurement accuracy of a Promass meter would already have compromised to some degree, and this condition should be avoided. Repeatability of the measurement will also be poor when the fluid holds significant amounts of entrained air, and poor repeatability can make accurate batching impossible.

Symptoms of entrained air and low meter backpressure include:

• unstable flow rate display and output
• relatively high and sometimes unstable excitation current
• poor repeatability for successive batch flows - flow meter vs. a scale
• unstable and elevated flow rate from meter during no flow condition
• a different flow rate output on the display/output each time flow is stopped
• Promasss density measurement is different during flow and no flow conditions
• various error messages on flow meter: excitation current limit fluid non-homogeneous range current output tubes not oscillating