Thursday, July 5, 2012
Flow Measurement-2
EPC-School 2012
I for Instrumentation
Coriolis Mass Flow Meter
I for Instrumentation
Coriolis Mass Flow Meter
Coriolis Meter by Micro Motion
Quick Facts:
Coriolis - for liquid &
gas service, size 1/6"-6", generally for clean service, very limited
use for dirty service.
Major
Advantages- High accuracy; Low maintenance; Insensitive to flow profile.
Major
Disadvantage- High initial cost, depending on size and model; Bent tubes
subject to
fouling; Not available for pipe sizes over
six inches.
Vibrating Tubes
Principal of Operation- The flow is guided into the U-shaped tube. When an osillating
excitation force is applied to the tube causing it to vibrate, the fluid
flowing through the tube will induce a rotation or twist to the tube because of
the Coriolis acceleration acting in opposite directions on either side of the
applied force. For example, when the tube is moving upward during the first
half of a cycle, the fluid flowing into the meter resists being forced up by
pushing down on the tube. On the opposite side, the liquid flowing out of the
meter resists having its vertical motion decreased by pushing up on the tube.
This action causes the tube to twist. When the tube is moving downward during
the second half of the vibration cycle, it twists in the opposite direction.
This twist results in a phase difference (time lag) between the inlet side and
the outlet side and this phase difference is directly affected by the mass
passing through the tube.
Twisting of the tubes due to Coriolis forces
A more rescent single
straight tube design is available to measure some dirty and/or abrasive liquids
that may clog the older U-shaped design. Power supply of 24 VDC is preferred,
else if 230 VAC is used it may introduce a power gap of max 250 ms by Automatic
Switch -over device of ACO board.
An advantage of
Coriolis flowmeters is that it measures the mass flow rate directly which
eliminates the need to compensate for changing temperature, viscosity, and
pressure conditions.
For working properly,
a minimum gas density of 4.5 kg/m3 is required.
Theoratically,
coriolis force, Fc= 2*m*w*V
m= mass (kg)
w= angular velocity
(rad/s)
V= velocity (m/s)
- 'U' and 'Ω' shaped coriolis meters cause pressure drop as the flow is increased, resulting in permanent pressure loss. Consequently, liquids near boiling point could introduced cavitation as the pressure across the meter drops below the vapour pressure of the liquid (forming of vapour bubbles). As soon as the pressure recovers above the vapour pressure the bubbles impode.
Cavitation will cause
the meter to malfunction and this should be avoided at all times.
The cross-section of the tube is normally smaller than the cross-section
of the line pipe to increase the velocity (v) in order to create higher
Coriolis forces. The latter holds good for Coriolis meters with straight
measuring tubes as well. Consequently, the pressure drop will increase
accordingly.
- Coriolis mass flow meters will measure the mass flow rate and actual density accurately irrespective of the nature of the fluid as long as the flow is single phase and the fluid is homogeneous. Being accurate, they are often used for custody transfer and critical reactor feed (ratio) control and also in streams where large variation in fluid composition will occur, which otherwise could not be measured.
A weak point is their requirement for Zero Adjustment at operation
conditions. the zero adjustment is easily upset by not fully stress free
installation of the meter.
The importance of zero adjustment is also demonstrated in the formula
for the uncertainity in the reading of the meter, expressed as:
+/-[(zero stability/ flow
rate)*100]% +/- x%
where x varies between 0.1 and
0.3% depending on design of the meter.
Largely advantages
& limitations of coriolis flow meter are:
ADVANTAGES:-
1. Coriolis flow meter is capable of measuring a
wide range of fluids that are often incompatible with other flow measurement
devices. The operation of the flow meter is independent of Reynolds number;
therefore, extremely vicious fluids can also be measured. A Coriolis flow meter
can measure the flow rate of Newtonian fluids, all types’ non-Newtonian fluids,
and slurries. Compressed gases and cryogenic liquids can also be measured by
some designs.
2. Coriolis flow meters provide a direct mass flow
measurement without the addition of external measurement instruments. While the
volumetric flow rate of the fluid will vary with changes in density, the mass
flow rate of fluid is independent of density changes.
3. Coriolis flow meters have outstanding accuracy.
The base accuracy is commonly on the order of 0.2%. In addition, the flow
meters are linear over their entire flow range.
4.
The rangeability of flow meters is usually on the
order of 20:1 or greater. Coriolis flow meters have been successfully applied
at flow rates 100 times lower than their full scale flow rate.
5.
A Coriolis flow meter is capable of measuring
mass flow rate, volumetric flow rate, fluid density and temperature --- all
from one instrument.
6.
The operation of the flow meter is independent
flow characteristics such as turbulence and profile. Therefore, upstream and
downstream straight run requirement and flow conditioning are not necessary.
They can also be used in installations that have pulsating flow.
7. Coriolis flow meters do not have internal
obstructions witch can be damaged or plugged by slurries or other types of
particulate matter in the flow stream. Entrained gas or slugs of gas in the
liquid will not damage the flow meter. There are no moving parts witch will
wear out require replacement. These design features reduces the need for
routine maintenance.
8.
The flow meter can be configured to measure flow
in either the forward or the reverse direction. In revere flow there will still
be a time or phase difference between the flow detector, but the relative
difference between the two detector signals will be reversed.
9. Coriolis flow meter designs are available witch
allow for use in sanitary applications, and for the measurement of shear
sensitive fluids. Materials are available to permit the measurement of
corrosive fluids.
DISADVANTAGES
1. Coriolis
flow meters are not available for large pipelines. The largest Coriolis flow
meter that is currently available has a maximum flow rating of 25,000 lb/min
(11,340 kg/min), and is equipped with 6 in. (15 cm) flanges. When larger flow
rates must be measured, two or more flow meters mounted in parallel are
required.
2. Some
flow meter designs require extremely high fluid velocities in order to achieve
a significant amount of time or phase difference between the flow detector
signals. This can result in extremely high pressure drops across the flow meter.
3. Coriolis
flow meters are expensive. However, the cost of a Coriolis meter is often comparable
to (or below) the cost of a volumetric meter plus densitometer used together to
determine the mass flow rate.
4.
Coriolis flow meters have difficulty measuring
the flow rate of low- pressure gas. Application with pressure less than 150
psig are marginal with the flow meter designs that are currently available.
Low-pressure gases have low density, and their mass flow rate is usually very
low. In order to generate enough mass flow rates to provide sufficient Coriolis
force to be measured, the gas velocity must be extremely high. This in turn may
lead to prohibitively high pressure drops across the meter.
Labels: coriolis, coriolis force, coriolis forces, custody transfer meter, density measurement, flow meter, mass flow meter, micro motion, Rahul Kapoor
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