Critical
process control applications require highly accurate measurement instruments.
Coriolis flowmeters are one of the most accurate process measurement instruments
and commonly applied in some of these applications.
Working of Coriolis Flowmeters
The Advantages of Coriolis Flowmeters
You can also read:
Disadvantages of Coriolis Flowmeters
Working of Coriolis Flowmeters
To help you
understand the principle behind the Coriolis flowmeters, we will strive to make it
as simple as possible.
Coriolis
flowmeters works by shaking one or more tubes carrying the flowing fluid, then
precisely measuring the frequency and phase of that shaking.
The back and
forth shaking is driven by an electromagnetic coil, powered by an electronic
amplifier circuit to shake the tube(s) at their mechanical resonant frequency.
Since this
frequency depends on the mass of each tube, and the mass of the tubes depends
on the density of the fluid filling the fixed volume of the tubes, the resonant
frequency becomes an inverse indication of the fluid density whether or not the
fluid is flowing through the tubes.
-Tube
Frequency is inversely proportional to the Density
As the fluid begins to
move through the tubes, the inertia of the moving fluid adds another dimension
to the tube’s motion: the tubes begin to undulate i.e. twisting slightly
instead of just shaking back and forth.
This twisting motion is directly proportional
to the mass flow rate, and is internally measured by comparing the phase shift
between motion at one point on the tube versus another point: the greater the
undulation or twisting, the greater the phase shift between these two point’s
vibrations.
-Tube twisting is directly proportional to Mass Flow rate
Temperature changes
have the potential to interfere with density measurement that is why all
Coriolis Flowmeters are equipped with RTD temperature sensors to continuously monitor
the temperature of the vibrating tubes. The flowmeter’s microprocessor takes
the tube’s temperature measurement and uses it to compensate for the resulting
elasticity changes based on a prior modelling of the tube metal
characteristics. This temperature measurement happens to be accessible as an
auxiliary output signal, meaning that, Coriolis flowmeter may also work as a
temperature transmitter in addition to measuring mass flow rate, and fluid
density.
The ability of Coriolis
flowmeter to measure three process variables i.e. Mass flow rate, Temperature
and density makes it a very versatile instrument. This makes it easy to
communicate in digital environment involving Foundation
Fieldbus or Profibus Standard rather than the analog 4-20 mA signal.
Fieldbus communication allows multiple variables to be transmitted by the
device to the host system or on the same Fieldbus network.
The Advantages of Coriolis Flowmeters
Coriolis flowmeters are
very accurate instruments, and reliable. They are completely immune to swirl
and other fluid disturbances, hence they can easily be located anywhere in a
piping system with no need for straight run pipe lengths upstream or downstream
of the flowmeter. The ability of Coriolis flowmeter to measure true mass flow,
along with their characteristic linearity and accuracy, makes them ideally
suited for custody transfer applications, where the flow of fluid represents
product being bought or sold.
You can also read:
Disadvantages of Coriolis Flowmeters
The main disadvantage
of Coriolis flowmeters is the high cost compared to other flowmeters especially
for large pipe sizes. They have also more limited in operating temperature than
other types of flowmeters and may have difficulty measuring low-density fluid
-gases and mixed-phase i.e. liquid/vapor flows. The bent tubes used to sense process flow may
also trap process fluid inside to the point where it becomes unacceptable for
hygienic applications e.g. Food Processing, Pharmaceuticals. That is why; we
have new Coriolis tube design to try to overcome some of these problems. Straight-tube
Coriolis flowmeters are slightly better than U-shaped tubes however U-shaped
tubes aren’t as stiff as straight tubes, and so straight tube Coriolis
flowmeters tend to be less sensitive to low flow rates than U-tube designs.
Pretty good synopsis of the method. Several things to mention: 1)Temperature is measured accurately on the dry side of one of the tubes, or just in the meter body, to allow compensation for the effects of Young's Modulus of Elasticity. This effect on the stiffness of the flowtubes due to temperature change is a real problem if not actively taken into account, and all Coriolis devices must do this. Not really used with raw Density measurement so much (although it is essential to Concentration reading). 2)Bent tube meters, single or dual, are used all the time in hygienic applications and are made to be polished and drainable. 3) Standard duty Coriolis meters can be used up to about 400 F, with high temperature versions approaching twice that, so not too limited I would have to say! 4) Coriolis massmeters can read Helium and Hydrogen gas flows well, so no real issues with product density on the light side, right?
ReplyDeleteNot a bad article as it goes. Just some things needed clarification.