Brabender Technologie

Brabender Technologie
Flow Metering
2
Fzp = m * vr
M = m * w * r²
TECHNOLOGIE
Bulk Solids Flow Meter and Additive
Feeders in PE and PP Plants
MBS Conference Zurich 2004/2005
Brabender Technologie - Global Partners to the Bulk Handling Industries
Brabender Technologie of Duisburg, Germany, renowned as one of the leading manufacturers of bulk
solids and powder feeding and discharging equipment with a world-wide sales and service network,
was founded in 1957 and has been supplying metering feeders and vibratory bin dischargers to the dry
material handling industries ever since - 45 years of experience to the benefit of our world-wide
customers. Our delivery programme includes:
• Flow meters for bulk materials
• Gravimetric metering feeders for continuous and discontinuous feeding and weighing
applications
• Weighing systems with high-resolution fully digital deflectionless load cell Digi MASS-2
• Volumetric metering feeders
• Microcomputer controllers for automatic system and recipe control
• Discharging devices
®
®
The Brabender FlexWall Feeder and the further developed Brabender FlexWall Plus Feeder are
universal metering feeders suitable for virtually all kinds of bulk ingredients.
Our main field of business is the plastics industry:
-
Polyolefine:
Compounding:
PP, PE, (LDPE, LLDPE, HDPE)
Engineering plastics, masterbatches, cable compound, powder coating,
chemical fibres, PVC, recycling, wood plastic composites
Dipl. Ing. Rolf Welsch
Sales and Marketing Department Manager
Mr. Welsch is 52 years old. He studied electrical engineering at the University of Duisburg and has
been involved in material handling for 28 years. During this time he has conducted a lot of
presentations at technical institutes and congresses.
0
Bulk Solids Flow Meter and Additive Feeders in PE and PP Plants
Written by Terry D. Fahlenbock and Rolf Welsch
Presented at MBS Conference Zurich 2004/2005 by Rolf Welsch
1) Introduction
This presentation discusses the gravimetric (mass) feeding requirements for polyolefine
extrusion – granulating application. In this application, gravimetric feeders are used to
precisely feed virgin resin powder and additives directly into the extruder. Extruder–granulating
plants from 20 to 50 tons/hr (max. 75 t/hr) are typical.
Loss-in-weight feeders are typically used for additives and a feeder with downstream mass flow
meter is used for the high polyolefine feed rates. The loss-in-weight (LIW) feeder is briefly
discussed. The Coriolis mass flow meter is presented more thoroughly. Paragraphs below
include some important aspects to be considered for the design of such plants.
2) Background
In a modern PE/PP plant compounding/extrusion/pelletizing plays an important role to achieve a
product with desired properties and handling characteristics (Fig1). Most of the pellets produced in the
way described will directly be converted subsequently into a final product (film, yarn, non-wovens etc.),
but some of it will be upgraded in subsequent compounding processes. This could be filling, colouring
or reinforcing. Modern plants (World Scale Plants) have significantly increased in capacity. This means
that also the subsequent process steps have to match this output capacity. Extruder sizes have been
increased thus as a consequence of this and they do match the required rates [1].
Process
Process
Fig. 1:
Feeding
Feeding and
Extrusion
Modern PE/PP Plant, Schematic of a Typical Polyolefine Extrusion–Granulating Line (Figure CWP)
1
The same also applies to the metering of the polymer powder in such plants. In the years after 1980
their performance has increased steadily (Fig.2). Thanks to their high degree of long time and tare
stability loss in weight (LIW) feeders were used for a performance of up to 10 t/hr. Since the feeder
integral hopper (weigh hopper) increases in size as the feed rate increases and corresponding refill
rates increase, a practical limit of 10 tons/hr for continuous loss-in-weight feeders is generally accepted
as discussed further (see also chapter LIW Feeder).
The demand for increasing metering performances in conjunction with the use of weigh belt feeders,
however, results in the problem of a high degree of dust covering by the very fine portions in the
polymer powder (e.g. < 40 µm with copolymer). The metering belt weighers discharge the product from
a hopper. The filling level in the storage hopper above the weighbelt feeder was sensed by a level
sensor or by load cells and kept on the same level by an upstream rotary valve or an upstream
controllable ball cock. That was meant to supply an even product flow to the speed controlled weighbelt feeder. Compared to LIW feeders the cleaning expenditure is considerable due to the open parts.
Following that, alternatively available measuring principles were analysed more closely. Functional
principles including advantages and disadvantages of six potential alternatives are described in the
Attachment. As to polymer powder metering the comparisons presented here show that Centripetal
and Coriolis measuring instruments should preferably be used.
Fig. 2: Increasing of feed rate for higher production (change of feeder design)
2
3) CP FlowMeter
The Brabender CP FlowMeter is a dust-tight compact unit for high-accuracy bulk solids flow metering
by centripetal force measurement. It is suitable for all non-adhesive free flowing bulk ingredients at
temperatures up to +65° C (versions for higher temperatures optionally available).
Working method: The bulk material supplied from above via a supply chute in a special design with
optimum pass-over point is radially diverted on a circularly curved measuring chute. According to
Newton's laws of motion this radial movement results in a centrifugal force in outward direction and an
identical opposed centripetal force towards the centre of the circle as the counter force keeping the
bulk ingredient in its circular motion.
In the measuring arrangement of the CP FlowMeter the resisting force the measuring chute opposes to
the centrifugal force equals the centripetal force, which is in direct proportion to the throughput and is
dynamically measured by a
load cell.
The load cell produces a
continuous
metering
signal
transmitted to an electronic
evaluation
current
unit
flow
where
rate
is
the
both
displayed and issued as a
continuous actual value signal
for recording or control of an
upstream
variable
speed
metering
feeder
(screw
feeder, rotary valve, etc.).
Fig. 3: CP FlowMeter
As the CP FlowMeter works almost independently of the different “impact“ behaviour of the bulk
materials, it was successful in case of higher accuracy demands compared to measuring chutes and
impact meters (for comparison see attachment).
CP FlowMeters typically have a 5:1 flow range (max. 10:1 possible for best flowable granules). Flow
capacity can be reduced by easily installing a lower capacity load cell.
3
The prefeeder is important to the success of the system since the flow meter feedback signal to the
upstream feeder varies to achieve constant measuring force.
It
is
important
to
select
prefeeders that generate a
pulsation as even as possible
during the feeding operation.
Highest accuracy is achieved
when the flow is consistent.
The points permit to decrease
the pulsation are the same as
they will be described later for
Coriolis feeders.
Fig. 4: Prefeeder for CP FlowMeter
4) Coriolis FlowMeter
The problems of taring faults due to visible product deposits with measuring systems in connection with
chute/impact plates were solved best by the Coriolis FlowMeter for bulk materials.
The reasons for that are due
to the fact that their system
does not allow influences by
tare errors. Tare pollution can
only have a slight indirect
effect
(see
mode
operation).
Fig. 5: Coriolis feeder principle
4
of
4.1) Mode of operation
The Coriolis FlowMeter is a mass flow measuring device that functions according to the Coriolis
principle (Fig.6+7). A rotating impeller wheel runs with constant angular velocity. The bulk material to
be measured is supplied from above to this impeller wheel and diverted radially. After the diversion the
bulk material is intercepted by the guide blades. The centrifugal force permits to accelerate the bulk
material into radial direction. The tangential Coriolis force causes a torque that has to be compensated
by the drive.
This change in torque is
A C -D r iv e
transmitted to a load cell. The
product of torque, speed of
motor and radius is the mass
M
is sensed by a toothed wheel
RA
FF
FCEFCO
Im p e lle r
W heel
flow.The speed of the motor
at the shaft or Motor. A
sensor head sends the signal
to the Congrav (Fig. 7 shows
FF = Friction force
counter (reaction) force) [5].
FCO = Coriolis force
M a s s flo w
FCE = Centrifugal force
Fig. 6: Mass flow inside of Coriolis
Fig. 7: Mode of operation of Coriolis feeder
4.2) Design of Coriolis Mass Flow Feeder
Flow into the Coriolis FlowMeter is via an off center inlet. This “off center” does not affect the measured
result since the velocity and mass measurements are affected by the ingredient as it leaves the
measuring wheel. The inlet
chute directs the flow to the
center
wheel
of
the
where
measuring
the
flow
is
dispersed to the vanes. When
the flow leaves the measuring
wheel, it is directed via the
conical chute to the outlet.
Fig. 8: Mass sensing components
(plan view)
5
The drive shaft from the motor to the measuring wheel is enclosed with a static housing and seals
above the deflection cone ensure no dust migrates to the external drive and mass sensing
components.
Also, the Brabender Coriolis mass flow meter has a simplified construction using a direct drive motor,
easily accessible, since the swivel plate (motor mounted on it) for torque measurement is conveniently
placed below the motor. The drive motor is mounted on a turntable (swivel plate) and the load cell
provides the counter force to maintain the plate stationary. The load cell measures the reaction torque
which is directly proportional to the mass flow on the measuring wheel.
4.3) Prefeeder selection
Also for Coriolis feeder the prefeeder is important to the success of the system since the flow meter
feedback signal to the upstream feeder varies to achieve constant torque. It is important to select
prefeeders that generate a pulsation as low as possible during the feeding operation. Highest accuracy
is achieved when the flow is consistent.
The Prefeeder device can be a screw feeder, vibrating tray, rotary feeder, belt, or any device that
conveys (feeds) ingredient flow from the storage silo to the downstream flow meter. The feed device
must be able to have its volumetric flow varied, for example, the rotor speed of a rotary valve is varied
by varying the speed of the drive motor.
In case of conveyor screws:
Rotary
valve
Screw
feeder
- progressively wound blades to
obtain a relaxation of the product
- higher outgoing speed to get an
evenly flow of bulk material
Speed
controller
Torque
and
speed
Speed
controller
In case of rotary valves-feeders:
- higher number of chambers, e.g. up
Torque
and
speed
to 20 chambers
- outgoing speeds up to approx. 20
min
–1
(means maximum for large
rotary valves)
-subdividing into 2 staggered configuration and special chambers without
sharp
adherence
Fig. 9: Prefeeders for Coriolis mass flow measuring
6
edges
to
prevent
4.4) Adjustment and calibration
A static calibration of the Coriolis mass flow meter is performed with the motor operating, but without
ingredient flow through the meter.
To ease the commissioning and the maintenance and to decrease
Rotaryvalve
the corresponding expenditures for polyolefine powders, a
2way
valve
With a standstill of the Mass Flow Scale the product flow does not
bypass-line could be recommended or offered to the customer.
have to be interrupted in this case. This separately routed bypass
permits a performance of service works for both kinds of mass
Bypass for
Massflow
metering
flow meters which is better than in case of a bypass line
integrated into the mass flow meter.
Alternatively the weighing of the storage hopper or of an
intermediate hopper mounted between the silo and the mass flow
meter permits an online calibration. The mode “volumetric
Shut off
valve
feeding“ can be selected with the controller. Here it is possible to
run the product flow through the mass flow meter without
evaluation of the weight signal by revolving rotor.
Fig. 10: Bypass line at CP FlowMeter,
4.5) Special notes regarding installation of mass flow feeders:
High product temperatures:
In most cases the polymer powder flows arriving in the reservoir bin at a temperature of 110 °C decay
to a temperature of just 70 °C to 90 °C. Mass flow meters are designed for such a temperature as a
standard. For product temperatures superior to 70 ° C special bearings and sealings have been
employed. Moreover it is necessary to cool the load cell. Such special executions have to be discussed
with Brabender.
Inert gas blanketing:
The internal flow housing of Coriolis feeder can accept an inert gas at a pressure of 20 mbar (200 mm
W.C.) above atmospheric pressure (100mbar Design pressure). For reasons of measuring the
admissible differential pressure is limited to +5 mbar.
7
The CP FlowMeter is more sensitive against pressure variations of nitrogen blanketing than a Coriolis
feeder because streaming of nitrogen is able to lift the weighing chute of CP FlowMeter. But this
normally could not happen. Naturally during metering the pressure from top is higher (which results
from bulk material stream and its nitrogen blanketing). It should be observed that the rotary valve (the
valve for the evacuation of the storage hopper) is aerated such that no higher process pressure gets
from the existing overlying inert gas to the weighing sector.
The low inert gas pressures of just 2 mbar are reduced in two-stage controllers from a 6 bar inert gas
network to make them show as few pressure variations as possible, (6 to 2 bar and 2 to 0.002 bar).
EEx Applications:
The Coriolis FlowMeter is easily applied for hazardous environments.The execution can be delivered
for the a.m. ex-proof zone 22 because motor is AC and external. CP FlowMeters are also easy to apply
to this Zone 22. Usually, higher protection classes are not demanded, because polypropylene or
polyethylene are flushed with inert gas and in the product chamber itself an explosion proofness like
e.g. in zone 20 does not occur. The load cell can be provided with Zener barriers.
Handling of angel hair
Streamers, fines, angel hair, and husks are all types of residue created when plastic pellets are
pneumatically conveyed. Streamers have flat ribbon-like shapes and can be 20 feet long and 2 inch.
wide. They can all accumulate and form “bird nests” (tangled masses of residue). Feeders/flow meters
do not handle such streamers or bird’s nests too well. First, they block the upstream feed device. If
they get past the feeder, they can block or disrupt the downstream flow meter.
The first choice is to avoid, as much as possible, the formation of these residues. If they are inevitable,
the Coriolis FlowMeter can be modified. The measuring wheel top plate is removed and the flow vanes
are contoured to reduce the possibility of adherence or restriction. Accuracy of the flow meter reduces
since the mass on the wheel is irregular.
Only theoretically CP FlowMeters are advantageous by their free transfer port. Experience shows that
"angel hair" developing under standard operating conditions can also pass a Coriolis mass flow meter.
This applies to all polymer powders which have already passed a rotary valve with an increased
number of chambers or screw feeders with metering screws.
8
5) Loss-in-Weight Feeder for Additives
5.1 Principle of a loss-in-weight feeder
The most reliable and accurate (repeatability better than 1%) gravimetric feeder used in today’s
production is the loss-in-weight feeder. It is the first choice for additive feeders for feed rates ranging
from 0.25kg/hr to some t/hr and more.
This gravimetric (mass) feeder incorporates:
•
a feed device (picture shows screw feeder)
•
a feed rate measuring device (picture shows digital load cell)
•
a feedback controller (picture shows Brabender single feeder controller Congrav )
®
Feed device: - can be a screw feeder (vibrating tray, rotary feeder), or any device that conveys (feeds)
ingredient flow from the integral hopper to the discharge outlet. The feed device must be able to have
its feed flow varied, for example, the screw speed is varied by varying the speed of the drive motor.
Feed rate measuring device:
The principle of operation of a
continuous
loss-in-weight
feeder is based on a varying
weight where the feed rate is
directly related to the change
in
weight.
The
feed
rate
measurement is simply the
change in weight divided by
the
change
in
time
as
ingredient is fed out of the
feeder.
Fig.11: LIW feeder and control
Feedback controller: Typically the mass flow feedback calculation in the controller is performed at least
every 30 ms. For example, for a desired additive feed rate of 360 kg/hr, the mass change calculation is
as follows:
Mass Flow Rate = 360 kg/hr = 6 kg/min = 100g/sec = 3 g/30ms measuring time (should not be longer)
9
This calculated weight flow rate is compared to the desired feed rate (set point) and the controller
adjusts the screw speed to achieve set point (min. every 0.5 seconds).
There
is
velocity
no
ingredient
measurement
required as for belt feeders.
This is because the weight
loss is measured and the
weight
changes
in direct,
perfect relationship with the
feed
rate
during
the
gravimetric cycle.
Fig.12: Working mode of a loss-in weight feeder (Feed Rate = Weight/time)
Since the weighed mass of ingredient in the feeder is continuously reducing (since the ingredient is
being fed out), the feeder eventually reaches a low weight and must be refilled. During refill, the weight
flow rate cannot be calculated. As a result, during refill, the feeder is not under gravimetric control. The
controller has programmed smarts to enable it to turn the screw at correct speeds to achieve the
desired set point during refill.
Even so, properly designed loss-in-weight feeder systems are designed to achieve refill within
approximately 15 seconds. Modern scales stabilize quickly (2 seconds) after refill (a considerable upset
to the scale) and gravimetric control resumes. Gravimetric cycles range from 60 seconds (plastic
pellets) to 4 minutes (for poor flowing ingredients). Usually the volume refilled is approximately 60% of
the total feeder storage volume.
5.2 Limitation of LIW Feeders
Reason for 10 tons/hr Limit for LIW feeders: At 10 tons/hr feeding virgin resin powder at 0,5kg/l,
using a gravimetric cycle of 4 minutes and a 60% refill. The integral feeder weigh hopper is approx.
2200 Liter. The refill rate to refill the hopper is approx. 1350l/15sec (refill should not be longer than
approx. 15 seconds).This means a refilling flow of 324 m³/hr! At this point cost for the refilling units
increases considerably.
The increase in cost, however, is even more considerable due to the structural height of such LIW
scales provided with a 2.5-m³ weighing container. Last but not least the production of huge amounts of
dust during the filling of the LIW feeder must not be forgotten.
10
Therefore, LIWs are usually used for refeed bulk material of some 1000 kg/hr and metering of
additives. Additives are necessary for improvement of hardness, electrical insulation, extensibility,
antibloc, light protection, antioxidant agent, chemical resistance processibility. Those additives can be
metered without problems in polypropylene and polyethylene plants.
®
5.3 FlexWall Feeder Advantages
®
When outside paddle activation is applied with the FlexWall metering unit, this leads to an especially
safe and increased mass flow (= improved accuracy!) for those bridge-building bulk materials. In the
®
past years those metering units which became known as FlexWall metering units pushed back the
classical agitating metering units thanks to their easy-to-service handling of additives (easy cleaning,
fast
1
change
of
product,
compact dimensions, etc).
2
Feeders of that type can be
seen in fig. 13.
Item 1 shows the position of the
outside paddles.
Item 2 shows the possibility to
replace screws (incl. twin-screws)
3
from the back.
4
Item
3
replacement
explains
of
the
the
simple
flexible
polyurethane hoppers.
Item 4: The circular arrangement
shows the potential saving of space,
e.g. above the premixers.
Fig. 13: Advantage of FlexWall® Feeder
For further information see our internet presentation
http://www.brabender-technologie.com/
5.4 Use of NDB
Processes including the use of NDBs (No Dust Blends) can do without one of the premixers for the
production of a fluff (polymer incl. additives) acc. to fig. 19, because those NDBs comprise all additive
components defined.
11
From the point of view of metering it can be said that - on the basis of the performances for additives in
PP plants demanded and because of the additional further mixing in screw conveyors – LIWs always
meet accuracy requirements with powder. Hardly any difference to the metering of the NDB-granules
should be expected. The customer can profit more from the potential advantages by the dust-free feed
from NDBs thanks to the easier handling (e.g. pneumatic conveying, big bag discharge). Here it is
more reasonable to compare the prices of both methods.
6) Liquid feeding (Peroxyde)
If peroxides are added into the extruder, such adding can be done by means of a mass flow meter in
compliance with the Coriolis force measuring principle or by an LIW feeder for liquids. For explanation
of the Coriolis mass flow meter see the relevant literature [6].
A “controlled reology“ for example requires peroxide even in case of low additions of peroxides in the
per mil range in polymer plants with a production of more than 50 t/hr.; the amount required is 2 to 20
kg/hr. That metering performance is mastered safely by both feeding systems (LIW and Coriolis feeder
for liquids). As manual adding is often performed due to that low metering performance a storage tank
can be filled without interruption of weighing operations at any time in connection with a Coriolis mass
flow meter. Therefore, it is preferably used.
Fig. 14 + 15 show an arrangement. Here, all units required for the peroxide incl. the cooling system are
implemented (frame cooled enclosed pump, pressure gauge, pulsation damper, pressure valve etc.).
The Coriolis mass flow meter is installed in its preferred vertical position. Fig. 16 shows a storage tank
including agitator as double shell tank for cooling. Optionally, it can also be installed into a load cell for
level monitoring (instead of level indicator).
Fig.14: Coriolis mass flow station
Fig.15: Coriolis mass flow station
Fig.16: Hopper with agitator
The housings presented are closed by a cover comprising an air conditioning unit to guarantee the cooling
temperature.
12
7) Plant design for polyolefine plants
7.1) Example of plant design
From the papers it can be seen that BRABENDER TECHNOLOGIE does not only deliver the
components mentioned above but also supplies a partial engineering for our scope of supply including
the planning of mounting on site.
The loss-in-weight feeders are refilled by a big bag discharge station with intermediate hopper and bin
activator. The intermediate hoppers are required to guarantee the safe and reproducible LIW filling.
The discharge from those intermediate hoppers is effected by discharging oscillating bottoms, e.g. type
BAV, which can also be supplied by the company of Brabender Technologie KG.
To receive the required premix the polymer powder and the additives are fed into a screw conveyor
and then supplied to the mixer or to the extruder.
Refeed material (not shown)
is stored in silos (refilling by
pneumatic
conveyors)
and
fed by a CP- FlowMeter and
rotary valve/screw prefeeder
(in case of feedrate > 5t/hr) .
If the throughput is lower than
approx. 5t/hr, refeed material
is fed by LIW feeders. Depending on the volume of the
process the refeed is metered
safely with performances of
up to 10% of the overall
output.
Fig.17: Plant design with big bag discharge (Refeed line not shown)
The above plant configuration shows that it is advantageous for the engineering companies to work
with partners manufacturing the major part of unit systems themselves to avoid unnecessary order
interferences. This applies to day bins (alternatively big bag discharge stations), piping, scales, mixers,
collecting spiral conveyors, rotary valves and even for steel constructions in the surroundings of the
scales. Thus, the engineer for extruder and plant constructions is supported competently by the
company of Brabender Technologie.
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7.2) Conveying screw
A process section of twin screw extruder for compounding of polypropylene or polyethylene is approx.
20 to 30 L/D in length. Therefore, there would be sufficient space available for the erection of all
metering
units
involved,
and,
actually, spiral conveyors for the
collection of PE/PP powder and
additives
are
not
needed.
Nevertheless, they are provided, as
they are mainly intended to work as
a mixing screw upstream of the
Mixing screw elements
at end of screw
FI
extruder (by their special designed
mixing elements, see Fig.18). In
many
Nitrogen locking at sealing
plants
operators
can
do
without the use of more expensive
N
fast mixers. Screws may have a
length of eight (8) meters and more.
Fig.18: conveying screw design
7.3) Measuring production results during the production of granules
Certainly also downstream equipment such as underwater pelletizers are components of equal
importance. In the case of producing pure granules some plants are provided with further granule mass
flow meters mounted downstream. Both, the Coriolis and the Centripetal measuring principles are
suitable without restrictions.
7.4 Mixer installation
Mixers for Polymer
and Additives
As to processes in the course of which
premixing is performed via mixers the
Conveyor Screws
latter can also be supplied by the
metering unit supplier. To perform level
Intermediate Hopper
with Bin Activator
monitoring the mixer is supported by load
cells,
and
the
mixing
program
is
implemented via a PLC control system
(Fig. 19 shows a process for which the
licenser demanded a belt weigher).
Weigh Belt Feeder
Down to Mixer / Extruder
Fig.19: Polymer plant with mixer
14
In those plants a very close
arrangement of the scales for
the additives is required. This
becomes clear in fig. 20.
There, seven (7) LIWs were
installed ready for operation
in explosion-proof design on
a
platform
completed
(delivery
with
wiring
and
including the platform!).
Fig. 20: Delivery of 7 LIW’s on one
platform for installation above a
discontinuous mixer
8) New: Fully digital design for Coriolis mass flow metering available!
One major important point of large extruders in polymer plants is that it will either provide two different
screw speeds or it will offer a variable screw speed in the operating range. [1]. A gradual start-up of the
extruder is meant to avoid production scrap in this start phase. Therefore it is required that polymer
metering units cover a high adjustment range. Further to that, the metering unit control system must
implement a fast synchronous modification of the different material flows (polymer and additives). For
that purpose, it needs fast access to the weighing signals which means that directly digital from the
digital cells (without analog/ digital converter) is the best solution.
Immunity
to
interfering
is
increased by the fully digital
technique and serial weight
transmission.
An
analog
calibration is not required. A
linear
signal
is
still
de-
termined in the start phase at
just 10 % of the performance
and the relevant lower signal
Digital load cell
Analog load cell
on request
levels of the feed of force.
Fig. 21: Digital load cell
15
Therefore, even the standard versions of the LIW and mass flow meter are equipped with digital cells
(Fig. 21). If configurations exploit analog cells for any other reasons as a standard, those cells can be
replaced with digital cells without problems. The same applies to the Coriolis scales which are the only
unit of their kind worldwide which can be obtained in a fully digital version!
9) Comparison of LIW and Coriolis feeder
Loss-in-weight feeders are the “benchmark” for accurate, reliable continuous gravimetric feeding of
some 1000 kg/hour with a practical limit of 10 tons/hr. Coriolis mass flow meter systems provide high
long-term accuracy for the following reasons:
•
Changes to ingredient characteristics do not affect the measurement. Such characteristics
include bulk density, particle density, impact coefficient, head, coefficient of friction, and
humidity.
•
The systems are almost unaffected by caking which would result in tare errors in other mass
flow metering units. But the Coriolis FlowMeter is not designed for very sticky ingredients.
However, all ingredients (particularly fines) have a tendency to adhere. Since the Coriolis
accelerates to flow on the measuring wheel, the wheel is virtually self-cleaning. As a result,
once the flow meter is calibrated, the “ tare torque ” remains constant.
•
Torque to mass relationship is easily checked with a test weight.
Based on all of the above, the Coriolis FlowMeter as installed produces a mass flow signal very close
to the actual mass flow.
10) Controls
The throughput measured by the mass flow meter provides the master signal for the additive feeders.
The LIW feed the ingredients in preset ratios based on the master signal (master/slave).The Brabender
Field Bus System allows the integration of the intelligent additive feeders without the need of control
cabinets. Interconnected by a field bus, those “intelligent” feeders can directly be connected to
Host/PLC systems. Modern connection to customer will be done by Ethernet interface. The Brabender
Remote Multi-Feeder Controller RC4 can operate up to 16 feeders. Customers mainly need it just for
start-up and later if they operate the feeders via Ethernet from Host / PLC.
In case of explosion hazardous areas Zone 22, the alternative feed controller OP 5 with CB boards
mounted in a control cabinet is installed as shown in Fig. 22. The CB board includes the intelligent
control of a LIW feeder, and installation in explosion hazardous areas is avoided.
16
Fig. 22: Feeder Control OP 5 with CB board in cabinet for harzardous areas
MAXIMUM FEED RATE
11) Test Facility: Coriolis FlowMeters have 3 typical
BRABENDER
INLET
sizes. For testing of those high feed rates,
MODEL
DIA.
VOLUME
Brabender has a test demonstration facility for their
NO.
mm
m³/HR
and
CDW 150
150
40
20
comparing its feed rate with the mass flow rate
CDW 250
250
100
50
produced by the CDW150 (see Fig. 23/24).
CDW 350
350
160
80
CDW150
using
a
loss-in-weight
feeder
Polymer
Powder
TONS/HR
Fig. 23 u. 24: Pilot plant station
(Fig 23: feeding with LIW and refilling by
big bags, Fig 24: Test of Coriolis feeder
below LIW feeder arangement).
In those plants it is possible to really test
the effect of finest particles on mass flow
meters during high performance rates in
recirculation.
Technical data of pilot plant station
Max. feed rate: 20 t/hr (limitation of bucket
elevator14000 m³/hr )
17
Attachment
Table 1-2: A Comparison Of Some Common Mass Flow Meters
(This is a summary only, more information is available from manufacturers of this type of flow meter)
SHORTCOMINGS OF
MEASURED VARIABLES
VELOCITY
FLOW METER
MASS
ONE MEASUREMENT
COMBINES VELOCITY AND
MEASUREMENT
MEASUREMENT
MASS
Gravity
flow
is
conditioned in a chute
and
directed
to
contact a plate.
IMPACT PLATE
The horizontal force
generated
by
the
impact of the flow on
the plate is measured.
Gravity
flow
is
conditioned in a chute
to contact a curved
plate vertically at its
inlet.
CURVED PLATE
The flow slides along
the curved plate. The
force generated by the
change in direction of
the flow on the plate is
measured.
There is one variable to
measure. The flow velocity is
assumed to remain constant.
However, it is sensitive to
changes in friction between
ingredient and chute. This
changes the flow velocity,
and
hence
the
force
measured. Also, since the
force measured is due to
impact, particle hardness,
even density and particle
size
(that
means
the
ingredient
characteristic)
affect the short duration
impact force.
There is one variable to
measure
The flow velocity is assumed
to remain constant. In
theory, changes in friction
don’t affect the measurement since there is a positive
and negative resulting force
that is balanced out.
The mass measurement is
affected by tare changes. It
should be tared regularly.
In practice, the flow meter is
better than the impact scale,
but has the same problems
integrating 2 variables in one
measurement.
18
Attachment
Table 3-4: A Comparison Of Some Common Mass Flow Meters
(This is a summary only, more information is available from manufacturers of this type of flow meter)
SHORTCOMINGS OF
MEASURED VARIABLES
VELOCITY
FLOW METER
MASS
ONE MEASUREMENT
COMBINES VELOCITY
MASSPLATE/IMPACT
PLATE
MEASUREMENT
MEASUREMENT
The flow from the
inclined mass plate
is
directed
to
contact a vertical
chute. The force
generated by the
flow on the chute is
measured.
Gravity
flow
is
conditioned in a
chute to contact an
inclined plate. The
flow mass flows
along the inclined
plate. The vertical
component of the
force generated is
measured.
AND MASS
Highly sensitive to tare
changes on the mass
measurement chute. Must
be tared regularly.
Also, highly sensitive to
changes in friction between
ingredient
and
mass
measurement chute
This changes the flow
velocity and the force
measured on the impact
chute.
WEIGHED FEEDER
The fixed rotational speed of the
weighed
rotary
feeder is measured.
The weight of the
weighed
feeder
and ingredient are
maintained const.
The tare load is very high
compared to the flow load.
The flow load is constantly
moving. There is an impact
load. The meter works best
if the incoming flow exactly duplicates the discharging flow.
Very susceptible to tare
changes resulting in the
necessity to retare frequently.
19
Attachment
Table 5-6: A Comparison Of Some Common Mass Flow Meters
(This is a summary only, more information is available from manufacturers of this type of flow meter)
SHORTCOMINGS OF
MEASURED VARIABLES
FLOW METER
ONE
VELOCITY
MASS
MEASUREMENT
MEASUREMENT
MEASUREMENT
COMBINES
VELOCITY AND
MASS
WEIGH BELT
The flow velocity
is determined by
the belt speed
which is easily
measured either
off the belt drive
motor or off the
tail pulley.
A section of the belt
carrying
the
ingredient is weighed.
The belt section
being weighed and
the weigh bar are
tared out.
The ingredient load
is normally large
compared to the
tared dead load
resulting
in
an
accurate weighment.
The weigh belt feeder are still in
wide use due to the easy feed
rate
calculation
(separate
velocity and weight measurements) and their low headroom
even at high feed rates. A
feedrange of 1:20 is possible.
It requires a high amount of
maintenance. The belt movement components and the load
cell are enclosed within the
ingredient flow housing and are
subjected to dust build up.
The belt is prone to ingredient
build up and this results in a
necessity to tare the belt. The
belt requires constant tension
and proper tracking.
Modern belt feeders offer selfregulating tensioning by mass
and self-regulating tracking.
A larger additional Inlet/ outletdistance is a disadvantage.
There is a limitation of max
temperature approx. 90 degree
and customer have to pay
attention that housing is pressure
tight after each maintenance at
feeder.
Prehopper with level sensor is
necessary to ensure bulk
material on feeder
CORIOLIS
The flow from the
feed device falls
through a chute
onto a rotating
measuring wheel
and is accelerated
to the peripheral
velocity of the
wheel. This radial
velocity is maintained constant,
independent of incoming flow velocity.
The rotating measuring wheel accelerates the flow mass
creating a coriolis
force
tangentially
and produces a
measurable
reaction torque that is
directly proportional
to the mass flow.
This meter was designed to
ensure the two measured
variables
are
distinctly
measured.
To achieve this, the flow is
accelerated. This is created by a
motor driven rotor and measuring wheel. Most other flow
meters rely on gravity flow to
generate the mass/velocity force,
and hence have no rotating
parts.
The Coriolis FlowMeter has a
moving part. As a result,
abrasive ingredients necessitate
abrasive resistant materials of
construction on some parts.
20
Acknowledgements
[1] Kapfer CWP, Polypropylene: From the Reactor to the Final Part
Maak Conference 2004, Zürich
[2] Terry Fahlenbook, Andy Kovats, Brabender, Gravimetric Feeding For Polyolefine Extrusion,
Polyolefine Conference, Houston 2003
[3] Bernd Hüppmeier, Brabender, Feeding For Polyolefine Extrusion, Rep Conference BT 2000
[4] Rolf Welsch, Brabender, Minerals in Compound, AMI Conference 2002, Cologne
[5] Heinrici, Schwedes und Schulze, Dosing Handbook, Elsevier 1998, Page 463 ff.
[6] Vetter, Dosing Handbook, Elsevier Advanced Technology, 461 ff.
PP 2004/PE 2005 Congress, Zürich
organized by
MAACK BUSINESS SERVICES
Maack & Scheidl Partnership
Plastics Technologie and Marketing
Zürich, Switzerland
www.MBSpolymer.com
21
Gesamtprogramm:
Total Program:
Programme général:
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Gravimetrische Dosiergeräte
Volumetrische Dosiergeräte
Chargenverwiegesysteme
Austragshilfen für Schüttgüter
Durchflussmessgeräte für Schüttgüter
Gravimetric Feeders
Volumetric Feeders
Batching Systems
Discharge Aids for Bulk Solids
Flow Meters for Bulk Solids
Doseurs gravimétriques
Doseurs volumétriques
Systèmes de préparation de charges
Extracteurs pour des produits en vrac
Débitmètres pour produits pulvérulents
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