Principles of Mass Flow meters

1. PRINCIPLE OF MASS FLOW METER
PRESENTED BY
PREM BABOO
Sr. Manager(Prod)
National Fertilizers Ltd. India
An Expert for www.ureaknowhow.com
FIE institution of Engineers India

2. Mass Flow Operating Principle:
Contents:
1. How does a Mass flow meter measure mass
flow?
2. Why do the tubes vibrate?
3. How does the sensor detect mass flow
measurement?
4. What is the flow calibration factor and Delta-
T and how do these relate to the mass flow
measurement?
5. How does a flow meter measure volume flow?

3. PRINCIPLE OF MASS FLOW METER
A Mass Flow Meter operating on the "Coriolis principle" contains a
vibrating tube in which a fluid flow causes changes in frequency,
phase shift or amplitude. The sensor signal is fed into the integrally
mounted pc-board. The resulting output signal is strictly proportional
to the real mass flow rate, whereas thermal mass flow meters are
dependent of the physical properties of the fluid. Coriolis mass flow
measurement is fast and very accurate.A mass flow meter, also known as
an inertial flow meter is a device that measures mass flow rate of
a fluidtraveling through a tube. The mass flow rate is the mass of the fluid
traveling past a fixed point per unit time.
The mass flow meter does not measure the volume per unit time (e.g., cubic
meters per second) passing through the device; it measures the mass per unit
time (e.g., kilograms per second) flowing through the device. Volumetric flow
rate is the mass flow rate divided by the fluid density. If the density is constant,
then the relationship is simple. If the fluid has varying density, then the
relationship is not simple. The density of the fluid may change with
temperature, pressure, or composition, for example. The fluid may also be a
combination of phases such as a fluid with entrained bubbles. Actual density
can be determined due to dependency of sound velocity on the controlled
liquid concentration.[

4. If the fluid has varying density, then the relationship is
not simple. The density of the fluid may change with
temperature, pressure, or composition, for example.
The fluid may also be a combination of phases such as
a fluid with entrained bubbles. Actual density can be
determined due to dependency of sound velocity on
the controlled liquid concentration.

5. AMMONIA MASS FLOW METER

7. In a Coriolis mass flow meter, the “swinging” is generated by
vibrating the tube(s) in which the fluid flows. The amount of
twist is proportional to the mass flow rate of fluid passing
through the tube(s). Sensors and a Coriolis mass flow meter
transmitter are used to measure the twist and generate a
linear flow signal.
Coriolis mass flow meters measure the mass flow of
liquids, such as Ammonia, water, acids, caustic,
chemicals, and gases/vapors. Because mass flow is
measured, the measurement is not affected by fluid
density changes

8. Coriolis mass flowmeters measure the force resulting
from the acceleration caused by mass moving toward
(or away from) a centre of rotation. This effect can be
experienced when riding a merry-go-round, where
moving toward the centre will cause a person to have
to “lean into” the rotation so as to maintain balance.
As related to flow meters, the effect can be
demonstrated by flowing water in a loop of flexible
hose that is “swung” back and forth in front of the
body with both hands. Because the water is flowing
toward and away from the hands, opposite forces are
generated and cause the hose to twist.

9. There are two basic configurations of coriolis flow meter:
the curved tube flow meter and the straight tube flow meter.
This article discusses the curved tube design. The animations
on the right do not represent an actually existing coriolis flow
meter design. The purpose of the animations is to illustrate the
operating principle, and to show the connection with rotation
Fluid is being pumped through the mass flow meter. When there
is mass flow, the tube twists slightly. The arm through which
fluid flows away from the axis of rotation must exert a force on
the fluid, to increase its angular momentum, so it bends
backwards. The arm through which fluid is pushed back to the
axis of rotation must exert a force on the fluid to decrease the
fluid's angular momentum again, hence that arm will bend
forward.
In other words, the inlet arm is lagging behind the overall
rotation, and the outlet arm leads the overall rotation.

10. Coriolis type Mass Flow
Meters

11. The animation on the right represents how
curved tube mass flow meters are designed.
When the fluid is flowing, it is led through two
parallel tubes. An actuator (not shown) induces
a vibration of the tubes. The two parallel tubes
are counter-vibrating, to make the measuring
device less sensitive to outside vibrations. The
actual frequency of the vibration depends on
the size of the mass flow meter, and ranges from
80 to 1000 vibrations per second.
The amplitude of the vibration is too small to
be seen, but it can be felt by touch.When no
fluid is flowing, the vibration of the two tubes is
symmetrical, as shown in the animations.

13. The animation on the right represents what happens during
mass flow. When there is mass flow, there is some twisting of the
tubes. The arm through which fluid flows away from the axis of
rotation must exert a force on the fluid to increase its angular
momentum, so it is lagging behind the overall vibration. The
arm through which fluid is pushed back towards the axis of
rotation must exert a force on the fluid to decrease the fluid's
angular momentum again, hence that arm leads the overall
vibration.
The inlet arm and the outlet arm vibrate with the same
frequency as the overall vibration, but when there is mass flow
the two vibrations are out of sync: the inlet arm is behind, the
outlet arm is ahead. The two vibrations are shifted in phase
with respect to each other, and the degree of phase-shift is a
measure for the amount of mass that is flowing through the
tubes.

15. Coriolis forces Fc are generated in oscillating systems when a
liquid or a gas moves away from or towards an axis of
oscillation.
A Coriolis measuring system is of symmetrical design and
consists of one or two measuring tubes, either straight or
curved.A driver sets the measuring tube (AB) into a uniform
fundamental oscillation mode.When the flow velocity v = 0
m/s / 0 ft/s, the Coriolis force Fc is also 0. At flowing
conditions, i. e. flow velocity v > 0 m/s / 0 ft/s, the fluid
particles in the product are accelerated between points AC and
decelerated between points CB.

16. The Coriolis force Fc is generated by the inertia
of the fluid particles accelerated between
points AC and of those decelerated between
points CB.
This force causes an extremely slight distortion
of the measuring tube that is superimposed on
the fundamental component and is directly
proportional to the mass flow rate.
This distortion is picked up by special sensors.
Since the oscillatory characteristics of the
measuring tube are dependent on
temperature, the temperature is measured
continuously and the measured values

17. Density and volume measurements
The mass flow of a u-shaped coriolis flow meter
is given as:
where Ku is the temperature dependent stiffness of the
tube, K a shape-dependent factor, d the width, τ the time
lag, ω the vibration frequency and Iu the inertia of the tube. As
the inertia of the tube depend on its contents, knowledge of the
fluid density is needed for the calculation of an accurate mass
flow rate.If the density changes too often for manual
calibration to be sufficient, the coriolis flow meter can be
adapted to measure the density as well.

18. The natural vibration frequency of the flow tubes depend on
the combined mass of the tube and the fluid contained in it. By
setting the tube in motion and measuring the natural
frequency,
the mass of the fluid contained in the tube can be deduced.
Dividing the mass on the known volume of the tube gives us
the density of the fluid.
An instantaneous density measurement allows the calculation
of flow in volume per time by dividing mass flow with density.

19. Calibration
Both mass flow and density measurements depend on the
vibration of the tube. Calibration is affected by changes in the
rigidity of the flow tubes.
Changes in temperature and pressure will cause the tube
rigidity to change, but these can be compensated for through
pressure and temperature zero and span compensation factors.
Additional effects on tube rigidity will cause shifts in the
calibration factor over time due to degradation of the flow
tubes. These effects include pitting, cracking, coating, erosion or
corrosion. It is not possible to compensate for these changes
dynamically, but efforts to monitor the effects may be made
through regular meter calibration or verification checks. If a
change is deemed to have occurred, but is considered to be
acceptable, the offset may be added to the existing calibration
factor to ensure continued accurate measurement.

21. Application Cautions for Coriolis Mass
Flow meters
If the pressure drop is acceptable, operate a Coriolis mass flow
meter in the upper part of its flow range because operation at
low flow rates can degrade accuracy. Note that high viscosity
fluids increase the pressure drop across the flow meter. For
liquid flows, make sure that the flow meter is completely full of
liquid. Be especially careful when measuring gas/vapor flow
with Coriolis mass flow meters. Pay special attention to
installation because pipe vibration can cause operational
problems.