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Flow MeteringFlow Metering

Bulk Solids Flow Meter and Additive

Feeders in PE and PP Plants

MBS Conference Zurich 2004/2005

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TECHNOLOGIE

Brabender Technologie

0

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: PP, PE, (LDPE, LLDPE, HDPE) - Compounding: 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 hasbeen involved in material handling for 28 years. During this time he has conducted a lot ofpresentations at technical institutes and congresses.

1

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].

Fig. 1: Modern PE/PP Plant, Schematic of a Typical Polyolefine Extrusion–Granulating Line (Figure CWP)

FeedingProcessProcess

Feeding and Extrusion

FeedingProcess FeedingProcessProcess

Feeding and Extrusion

2

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 weigh-

belt 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)

3

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 unit where the

current flow rate is 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.

4

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 of

operation).

Fig. 5: Coriolis feeder principle

5

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

transmitted to a load cell. The

product of torque, speed of

motor and radius is the mass

flow.The speed of the motor

is sensed by a toothed wheel

at the shaft or Motor. A

sensor head sends the signal

to the Congrav (Fig. 7 shows

counter (reaction) force) [5].

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 of the measuring

wheel where 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)

FCE

RA

FCO

FF

M

FF = Friction force

FCE = Centrifugal forceFCO = Coriolis force

FCE

RA

FCO

FF

M

FF = Friction force

FCE = Centrifugal forceFCO = Coriolis force

A C - D r iv e

Im p e l le rW h e e l

M a s s f lo w

A C - D r iv e

Im p e l le rW h e e l

M a s s f lo w

6

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:

- progressively wound blades to

obtain a relaxation of the product

- higher outgoing speed to get an

evenly flow of bulk material

In case of rotary valves-feeders:

- higher number of chambers, e.g. up

to 20 chambers

- outgoing speeds up to approx. 20

min –1 (means maximum for large

rotary valves)

-subdividing into 2 staggered con-

figuration and special chambers with-

out sharp edges to prevent

adherence

Fig. 9: Prefeeders for Coriolis mass flow measuring

Screwfeeder

Speedcontroller

Torqueand

speed

Rotaryvalve

Torqueand

speed

Speedcontroller

Screwfeeder

Speedcontroller

Torqueand

speed

Rotaryvalve

Torqueand

speed

Speedcontroller

7

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

the corresponding expenditures for polyolefine powders, a

bypass-line could be recommended or offered to the customer.

With a standstill of the Mass Flow Scale the product flow does not

have to be interrupted in this case. This separately routed bypass

permits a performance of service works for both kinds of mass

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

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.

Bypass forMassflowmetering

2wayvalve

Rotary-valve

Shut off valve

Bypass forMassflowmetering

Bypass forMassflowmetering

2wayvalve

Rotary-valve

Shut off valve

8

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.

9

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)

10

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 no ingredient

velocity 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.

FFiigg..1122:: WWoorrkkiinngg mmooddee ooff aa lloossss--iinn wweeiigghhtt ffeeeeddeerr ((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.

11

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 change of product,

compact dimensions, etc).

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)

from the back. Item 3 explains the simple

replacement of the 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.

1 2

3 4

1 2

3 4

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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.

13

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. De-

pending 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

extruder (by their special designed

mixing elements, see Fig.18). In

many plants operators can do

without the use of more expensive

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

As to processes in the course of which

premixing is performed via mixers the

latter can also be supplied by the

metering unit supplier. To perform level

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). Fig.19: Polymer plant with mixer

FI

N

Nitrogen locking at sealing

Mixing screw elementsat end of screw FI

N

Nitrogen locking at sealing

Mixing screw elementsat end of screw FI

N

Nitrogen locking at sealing

Mixing screw elementsat end of screw

Intermediate Hopperwith Bin Activator

Down to Mixer / Extruder

Mixers for Polymerand Additives

Conveyor Screws

Weigh Belt Feeder

Intermediate Hopperwith Bin Activator

Down to Mixer / Extruder

Mixers for Polymerand Additives

Conveyor Screws

Weigh Belt Feeder

15

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 (delivery with

completed 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

levels of the feed of force. Fig. 21: Digital load cell

Digital load cellAnalog load cellon requestDigital load cellDigital load cellAnalog load cellon request

16

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.

17

Fig. 22: Feeder Control OP 5 with CB board in cabinet for harzardous areas 11) Test Facility: Coriolis FlowMeters have 3 typical

sizes. For testing of those high feed rates,

Brabender has a test demonstration facility for their

CDW150 using a loss-in-weight feeder and

comparing its feed rate with the mass flow rate

produced by the CDW150 (see Fig. 23/24). 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 )

MAXIMUM FEED RATEBRABENDER

MODEL NO.

INLETDIA. mm

VOLUME m³/HR

PolymerPowder

TONS/HR

CDW 150 150 40 20

CDW 250 250 100 50

CDW 350 350 160 80

18

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)

MEASURED VARIABLES SHORTCOMINGS OF

FLOW METER

VELOCITY

MEASUREMENT MASS

MEASUREMENT

ONE MEASUREMENT COMBINES VELOCITY AND

MASS

IMPACT PLATE

Gravity flow is conditioned in a chute and directed to contact a plate. The horizontal force generated by the impact of the flow on the plate is mea-sured.

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.

CURVED PLATE

Gravity flow is conditioned in a chute to contact a curved plate vertically at its inlet. 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. In theory, changes in friction don’t affect the measure-ment 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.

19

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)

MEASURED VARIABLES SHORTCOMINGS OF

FLOW METER

VELOCITY

MEASUREMENT MASS

MEASUREMENT

ONE MEASUREMENT COMBINES VELOCITY

AND MASS

MASSPLATE/IMPACT

PLATE

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.

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 rotatio-nal speed of the weighed rotary feeder is measu-red.

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 exac-tly duplicates the dis-charging flow. Very susceptible to tare changes resulting in the necessity to retare fre-quently.

20

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)

MEASURED VARIABLES SHORTCOMINGS OF

FLOW METER

VELOCITY

MEASUREMENT MASS

MEASUREMENT

ONE 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 in-gredient 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 measure-ments) 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 move-ment 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 self-regulating tensioning by mass and self-regulating tracking. A larger additional Inlet/ outlet- distance 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 main-tained constant, independent of in-coming flow velo-city.

The rotating measur-ing wheel acceler-ates the flow mass creating a coriolis force tangentially and produces a measurable re-action 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 meas-uring 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.

21

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, Zürichorganized by

MAACK BUSINESS SERVICESMaack & Scheidl Partnership

Plastics Technologie and MarketingZürich, Switzerland

www.MBSpolymer.com

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Programme général:

TECHNOLOGIE

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