The pulsation inherent in vane-type pumps turns out not to be a problem for Coriolis flowmeters—Coriolis flowmeters offer users many advantages over positive-displacement (PD) meters. But can they be used with PD pumps? Engineers from pump manufacturer Blackmer decided to investigate.
Mass flowmeters are hardly a new concept — they were developed in the 1830s by French scientist Gustave Coriolis and are still known by his name. But recently they have gained more penetration in many applications. For the transfer of liquid fuels, for instance, they have largely replaced positive displacement (PD) flowmeters.
Coriolis mass flowmeters do not measure flow in terms of gallons per minute (gpm) or cubic meters per hour (m3/h). Instead, they measure the Coriolis forces acting on a moving tube containing fluid, and the density of the fluid, and use these to calculate mass flowrate directly.
A typical Coriolis meter has two measuring tubes, each formed with either a slight bend or a more dramatic ‘U’ shape. An external actuator forces the tubes to vibrate at their resonant frequency in a plane perpendicular to the direction of fluid flow. The tubes vibrate in opposite directions, and when no fluid is flowing through them they remain in phase. When fluid flows, however, it experiences the Coriolis effect — a “pseudo-force” created when Newton’s laws of motion are translated to a rotating frame of reference. This causes each tube to twist slightly, creating a phase difference that can be measured and used — along with the fluid density — to calculate the mass flowrate.
The same meter can also measure the fluid density. The resonant frequency of each tube is a function of fluid density: the higher the density, the lower the resonant frequency. Comparing the resonant frequencies of full and empty tubes allows the mass of the fluid in the tube to be calculated. Since the volume of the tube is known, the fluid density can be calculated.
The oscillation inherent to Coriolis flowmeters presents no problems in systems using rotodynamic pumps, which generate a smooth flow. However, positive-displacement pumps such as piston, gear, and sliding-vane pumps generate pulses during normal operation. It has been questioned whether these pulses interfere with the operation of mass flowmeters.
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