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Asphalt Plant Level II – January 2009 of 36 Module 7 – Drum Plants

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January 2009 7 - 1

ConstructionTrainingQualification Program

Asphalt Plant Level 2Module 7

Drum Plants

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January 2009 7 - 2

Module 7What you will ….

• Review how gradation is controlled in a drum-mixer

• Review how asphalt is controlled in a drum-mixer

• Learn how drum-mixer automation works• Learn about RAP production with a drum-

mixer• Troubleshooting mix problems

In this module we will review how gradation and asphalt content is controlled with a drum-mixer, discuss how drum-mix automation works, explain how recycled mixes are produced in a drum-mixer plant, and troubleshoot common mix problems.

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January 2009 7 - 3

• “Hot Mix Asphalt Production Facilities”(NHI Course 131044)

• Modules 10, pages 10-2 to 10-37• “Hot Mix Paving Handbook 2000”

• Section 9, pages 80-92• Section 10, pages 93-95

The information presented in this section is found …

The information presented in this section is found in Module 10 (pages 10-2 to 10-37) in NHI Course 131044 “Hot Mix Asphalt Production Facilities,” and in Section 9 (pages 80-92) and in Section 10 (pages 93-95) in the “Hot Mix Paving Handbook 2000.”


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January 2009 7 - 4

Drum-Mix Production

Hot Mix production characterized by:

• Final Gradation Blend Control at Cold Feed• Measure aggregate flow with belt scale• Meter asphalt and other ingredients to the

aggregate flow• Produce mix continuously

With a drum-mixer the gradation blend is controlled at the cold feed, the total aggregate flow is measured with a belt scale device, then the asphalt and other mix ingredients are metered or proportioned to that aggregate flow. Mix is produced continuously, rather than a batch or load at a time as characterized by a batch plant. For another basic discussion of this process refer to Module 2. Surge and storage silos are required with drum-mixers so they can have a place to elevate, then store the finished mix prior to dispatch into a truck. Information about silo systems is discussed in Module 8.

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January 2009 7 - 5

BLEND

This figure illustrates the elements of gradation and asphalt control in a drum plant. Notice that because the plant does not have a sizing screen which can be used to scalp off oversized materials, a special screen has to be installed between the cold feed and a dryer for this purpose. The gradation blend is established at the cold feed bins, based on the percentages (speeds) of the individual feeders. The belt scale measures the total aggregate flow, and the control system proportions the asphalt content on a continuous basis to match that aggregate flow. This is brief discussion of the process for reference and continuity. For a more detailed explanation of each step, also refer to Module 2.


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January 2009 7 - 6

RequirementDrum Mixer - Cold Feed

320-2.4

“Equip cold feed bins … for feeding aggregates … in the proportions required for the finished mix”

General plant specification 320-2.4 stipulates that the plant must be equipped with cold feed bins for feeding aggregates in the proportions required for the finished mix.

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January 2009 7 - 7

RequirementScalping Screen

330-5.7.1

Remove oversize with scalping screen and do not re-use without re-crushing and re-screening

There must be provisions for removing oversized material with a scalping screen. Note that department requirements direct that oversized material is not re-mixed with the aggregate stockpile, and that it is crushed and re-screened prior to re-use. If this is not done, the material must be discarded (330-5.7.1).

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January 2009 7 - 8

RequirementDrum Mixer - Weighing Aggregate

320-4.1

Equip each (drum) plant with a “weigh in motion scale” (belt scale) for measuring ...

• Virgin aggregate• RAP (Reclaimed Asphalt Pavement)

Drum-mixers must be equipped with a belt scale or “weigh in motion scale” (FDOT term) to measure the virgin aggregates and the Reclaimed Asphalt Pavement or RAP (320-4.1).


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January 2009 7 - 9

RequirementMix Temperature (from the mix design)

330-6.3

± 30°F “master range from Mix Design ”2 Targets: Mixing temperature at Plant,

Compaction temperature on Roadway± 25°F for any single test± 15°F difference for average of 5 tests

Mix temperature requirements for a drum-mixer and a batch plant are the same. The master tolerance range for the mix designs will be the target mix temperature from the mix design ± 30°F. Tolerance bands for any one test are ± 25°F and tolerance bands for the average of any five consecutive tests is ± 15°F (from the mix design). Exceeding these tolerances requires every load to be monitored until the temperature falls within the specified tolerance.

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January 2009 7 - 10

RequirementMixing Temperature

(from the mix design)330-6.3

Load rejected if outside ± 30°F master band

If a single test varies from established (± 25°F) test …or average of 5 tests varies more than (± 15°F) ... every load is to be tested again until temperatures fall within tolerance

330-6.3 requires any load to be rejected that falls outside the master tolerance band. The truck ticket should be marked, the truck and ticket number recorded, and the plant should be notified as well. If any single test falls outside the ± 25°F tolerance, or the average of five consecutive tests falls outside the ± 15°F tolerance (from the mix design), then each following load must be tested again until tests fall within tolerance. The goal is for consistency of mix temperature. This contributes to consistency of laydown, and consistency of a final finished product.


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January 2009 7 - 11

Cold FeedProportioning

(This was already discussed in Module 3, and won’t be repeated here. To see how this is done refer to Module 3)

Cold feed proportion information was already presented in Module 3. It will not be repeated here.

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January 2009 7 - 12

Calibrating theBelt Feeders

(This was already discussed in Module 3, and won’t be repeated here. To see how this is done refer to Module 3)

Cold feed bin belt feeder calibration information was already presented in Module 3. It will not be repeated here.

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January 2009 7 - 13

Asphalt Proportioning

Now lets review the asphalt proportioning and calibration process in a drum-mixer. Again, you can see an explanation of this process in Module 2. However, the specification requirements and calibration information specific to the belt scale and asphalt meter is only presented here.


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January 2009 7 - 14

RequirementBituminous Control Unit

320-2.6

Provide satisfactory means of controlling AC content to the tolerance specified in the job mix.

Specification require that a satisfactory means of controlling the asphalt content to the tolerance of the job mix formula is used (320-2.6). Asphalt content is tested with procedures outlined in Module 10. The actual equipment used to control the asphalt content is left to the contractor as long as it meets the specifications outlined in 320-4.2 to follow.

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January 2009 7 - 15

RequirementBituminous Control Unit

320-2.6

Provide heat jacketed lines to maintain the required temperature of the job mix

Drum-mix bituminous control units have the same requirement -- that they be heat jacketed to maintain asphalt temperature at the plant. This specification ties back to the specifications required for all plants (320-2.6).

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January 2009 7 - 16

REQUIREMENT Drum Mixer - Bituminous Feed

Control

320-4.2 Asphalt Pump Meter System (skid) must:

– Be coupled to total aggregate weight device (belt scale) including RAP feed

– Automatically vary asphalt binder feed rate as necessary to maintain the required proportion

The asphalt pump and meter system (skid) must be coupled to the aggregate weighing device (this is typically a belt scale), including the RAP feed which typically has its own belt scale, and must vary automatically to proportion the right amount of asphalt to the final mix (320-4.2).


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January 2009 7 - 17


Asphalt must proportion to aggregate flow.

First step in asphalt proportioning is measuring aggregate flow.

Belt scale is used.

The first step in proportioning asphalt flow is to measure aggregate flow. This is typically done with a belt scale device.

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January 2009 7 - 18

This is a photograph of a belt scale on a drum-mix plant. Note wind guard and gravity take-up pulley.

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January 2009 7 - 19

This diagram shows how a belt scale works, and how each element contributes to the measurement process. As already explained in Module 2, one “idler” or “roller” on the conveyor is not attached to the conveyor frame, but is connected to a “load cell” or “cells.” This provides a continuous real time static weight reading over a span of belt. A speed sensor on the conveyor simultaneously reads the conveyor speed, and the two values together are used to establish “tons per hour.”


The electronic device that processes these signals is either a separate electronic device or is integrated directly into the plant automation. The calibration process is the electronic process required to make this scale read correctly. We will do a sample calibration problem in Section 7.

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January 2009 7 - 20

Belt Scale Displays in DrumMixer Control Systems

This photograph shows an “integrator” or “totalizer” in the control room. The term “totalizer” is often used because these devices also provide a reading of total tons over the scale, much like an odometer in a automobile. Many control systems now perform the belt scale “integration” and “totalization” directly in the plant computer, as seen on the right, so these separate “integrators” or “totalizers” may or may not be found in the control room. The RAP conveyor system would have a separate belt scale.

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January 2009 7 - 21

Calibrating theBelt Scales


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January 2009 7 - 22

Calibrating Belt Scales

• Verify belt scale reads zero per manufacturer’s guidelines after warming up belt.

• Set the cold feed gates at an opening that will allow a good material feed

• Start the collector and belt scale conveyors• Starting with empty belts, run material in a

previously tared truck at the normal plant rate, and stop material flow to end with an empty belt without overflowing truck

• Use as large a sample as possible for accuracy (most technicians use at least 10 tons)

To calibrate a belt scale, first verify that the scale reads “0” or no weight without material on the scale. If the belt scale cannot hold a “0” weight, follow the belt scale manufacturers guidelines to “zero the scale.” Run material into a truck with a known tare (empty) weight. Make sure the belt is empty before the test and after the test. Fill the truck with as large a sample as practically possible. Most technicians use at least 10 tons.

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January 2009 7 - 23

Calibrating Belt Scales

• Record the total tons indicated on the belt scale• Compare weight shown on belt scale with actual

weight on truck• Following manufacturer’s guidelines, adjust belt

scale instrument based on weight difference (some computer controls do this automatically!)

• Repeat test, adjusting instrument until two consecutive tests are within guideline (± 0.5 %)

• Repeat test sequence at different rates to make sure scale is accurate at all production rates

Record the total amount of material loaded into the truck according to the belt scale device, not the rate. Compare the total amount of material weighed on the belt scale with the net weight in the truck. (Make sure you use a certified truck scale for your test.) Adjust the instrument based on manufacturer’s guidelines, retest, and readjust until two consecutive tests are within guideline. Note that many computerized plant controls do this adjustment process for you automatically! Most technicians use 1/2% or (0.5%) as a guideline. Belt scales should be checked at low, medium, and high rates to make sure they accurately measure a variation in load over the scale device.


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January 2009 7 - 24

Example

• Start belt scale and verify reads zero• Tare empty truck at 22,375#• Record belt scale totalizer at 437897.65• Record the “span” value of instrument at

897654

Going through the steps. Start and make sure belt scale reads zero. Weigh an empty truck, get the tare/empty weight and record. Also record the belt scale totalizer value. The span value is the calibration factor for the scale. This is shown in graphic form on the next slide.

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January 2009 7 - 25

BELT SCALE

TPH____

TONS22,375 Tare (Empty)

437906.87

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January 2009 7 - 26

Example

• Start belt scale and verify reads zero• Tare empty trucak at 22,375#• Record belt scale totalizer at 437897.65• Start feeders at normal production rate• Fill truck, stopping with empty belt• Record belt scale totalizer at 437906.87• Calculate tonnage on instrument at 9.22 tons

(437906.87 - 437897.65 = 9.22 tons)

Examples continues: Fill truck with material, stopping on a empty belt. Record new setting on Totalizer and calculate the difference from the initial reading, netting 9.22 tons. This is shown in graphic form on the next slide.


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January 2009 7 - 27

BELT SCALE

TPH____

TONS 22,375 Tare (Empty)

437906.87 - 437897,65

= 9.22 Tons

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January 2009 7 - 28

Example - cont.

• Weigh truck at 41,401#• Calculate weight in truck at 9.51 ton (41,401-

22,375 = 19,026 ÷ 2,000 = 9.51 ton)

Next step reweighs the full truck and subtracts the tare weight. Converting into tons gives a reading of 9.51 tons. This is shown in graphic form on the next slide.

Slide 29

January 2009 7 - 29

BELT SCALE

TPH____

TONS

41,401 Gross (Full)- 22,375 Tare (Empty)

= 19.026 Net Weight÷ 2,000 (lbs / ton)

= 9.51 Tons

437906.87 - 437897,65

= 9.22 Tons


Slide 30

January 2009 7 - 30

Example - cont.

• Weigh truck at 41,401#• Calculate weight in truck at 9.51 ton (41,401-

22,375 = 19,026 ÷ 2,000 = 9.51 ton) • Recall “span” value of instrument at 897654• Adjust span value (or calibration factor) by

1.031 (9.51 ÷ 9.22 = 1.031)• (Adjust by Actual divided by Indicated)• New span value = 925,481 (897654 x 1.031)

Re-test, re-adjust, until two tests pass

The last step is to adjust the calibration by the ratio of actual weight to indicated weight. Adjust, re-test, until 2 tests pass. This is shown in graphic form on the next slide.

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January 2009 7 - 31

BELT SCALE

TPH____

TONS


= 19.026 Net Weight÷ 2,000 (lbs / ton)

= 9.51 Tons

437906.87 - 437897,65

= 9.22 Tons

897654 “span” value x 1.031 (9.51 ÷ 9.22)

= 925,481 new “span” value(Now test again!)

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January 2009 7 - 32

This is a belt scale calibration screen from a computerized drum control system. Most plants now have this level of automation, and the belt scale “integration” and “totalization” is done as part of the plant process automation. The operator simply follows the prompts and enters the net weight data from the truck. The process automation makes all the calculations.


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January 2009 7 - 33

Best Management PracticesCalibrating Belt Scales

• Larger truck tests (10 ton min.) rather than smaller truck tests decrease probability of error

• Weigh bridges should be checked for wear and binding prior to tests

• If belt scale is out of tolerance on one flow rate, but not another, and adjustments to instrument don’t correct error consider re-aligning weigh bridge

• Wind can affect scale readings - consider installing wind guard over weigh bridge

The following best management practices are useful when considering belt scale calibration: •Larger truck tests (10 ton min.) rather than smaller truck tests decrease probability of error •Weigh bridges should be checked for wear and binding prior to tests •If belt scale is out of tolerance on one flow rate, but not another, and adjustments to instrument don’t correct error consider re-aligning weigh bridge •Wind can affect scale readings - consider installing wind guard over weigh bridge

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January 2009 7 - 34

Best Management PracticesCalibrating Belt Scales

• Belts should have gravity take-up to keep belt tension constant

• Belts of different widths and thicknesses are very difficult to calibrate

• Weigh bridge must be “square” with “weigh idler” slightly higher than other belt idlers

• Watch for build up on the belt and “weigh idler”• Calibrate during plant calibration process• Check whenever needed

Other best management practices include: •Belts should have gravity take-up to keep belt tension constant. •Belts of different widths and thicknesses are very difficult to calibrate. •Weigh bridge must be “square” with “weigh idler” slightly higher than other belt idlers. •Watch for buildup on the belt and the “weigh idler.” These elements help improve belt scale accuracy and calibration efficiency in the field.


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January 2009 7 - 35


Asphalt must proportion to aggregate flow.

Second step is regulating asphalt flow.

Asphalt pumping / metering unit is used.(the asphalt skid)

The second step to proportioning asphalt is to control the flow of the asphalt to match the aggregate flow. The asphalt flow is controlled with the drum-mix pumping and metering unit (the asphalt skid).

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January 2009 7 - 36

How the AsphaltProportioning System Works

• Control system reads wet aggregate flow from belt scale.

• Control system accepts moisture content entered by operator to establish dry flow rate of aggregate

• Control system accepts target asphalt content entered by operator.

• Control system drives pumping/metering unit flow control device.

• Control system adjusts asphalt flow control system based on signal received back from meter

The asphalt proportioning system on a drum plant functions by reading the belt scale signal, adjusting it for the moisture the operator enters into the system to establish the dry flow rate of the aggregate, calculating the flow required for the asphalt based on what the operator has entered into the system, then driving the flow control device of the asphalt until the signal coming back from the meter is what the control system expects to see.

Slide 37

January 2009 7 - 37

This diagram shows the process of the plant control system in regulating asphalt flow to aggregate flow, using the flow control device in the asphalt pumping and metering system (often referred to as the “pumping and metering skid”) to vary the flow, and the meter in the asphalt pumping and metering system to measure the flow.


Slide 38

January 2009 7 - 38

This illustration shows an old drum-mix metering system that is often found in the field. The asphalt pump draws asphalt from the tank and a 3-way flow control valve diverts part of the flow back to the tank and part of the flow to the drum-mixer based on the measured amount of asphalt going through the meter. This system is still found in the field, but is no longer manufactured. Fatigue in the actuating device on the flow control valve, and in the valve itself, would cause inaccuracies in flow consistency. The control system would attempt to control the flow but would end up “over shooting” and “under shooting” the target. Most manufacturers now use a variable speed motor on the asphalt pump as the flow control mechanism on the pumping and metering system.

Slide 39

January 2009 7 - 39

This system is typical of the style pumping and metering currently favored by most plant manufacturers. With this system the pump becomes the flow control device by installing a variable speed motor on the pump. If the control system desires more asphalt, it speeds up the pump. If the control system requires less asphalt, it slows down the pump. This system is simpler and has fewer parts to wear and cause maintenance problems that ultimately affect asphalt accuracy.


Slide 40

January 2009 7 - 40

Variable Speed Motor(for flow control)

Feed Line from Tank

PumpAC Meter

(measures flow)

Feed Line To Drum This is a photograph of the same type of asphalt pumping and metering system shown in the previous diagram. The key parts of the system and the flow of the material are identified.

Slide 41

January 2009 7 - 41

Calibrating the Asphalt Meter

Slide 42

January 2009 7 - 42

Calibrating AC Meter

• Tare empty truck or vessel and pump asphalt all the way to end of fill line, suspending line to ensure no asphalt leaks from line (CAUTION - AC HOT!)

• Record totalizer on AC meter, or set totalizer to zero• Pump asphalt into truck or vessel at rate

representing normal production flow• When stopping AC flow, make sure line is not

allowed to drain into truck/vessel after counting AC• Record gallons or tons on AC meter• Weigh truck and calculate net weight in truck

It is important to tare the empty truck or vessel being used to calibrate the meter. Record the totalizer setting on the meter or set the totalizer to zero. Pump asphalt into the truck or vessel. The proper way to do this is to charge the line to the very end of the line, suspend it over the truck, then discharge it into the truck. When the pump is shut off, make sure the line does not drain down into the truck or vessel. This will throw off the weights in the test. Record the new totalizer setting or the gallon counter on the meter. Weigh the truck and calculate the


weight of the asphalt in the truck.

Slide 43

January 2009 7 - 43

Calibrating AC Meter

• Convert gallons to weight if meter reads in gallons• Compare registered meter weight with actual weight

on truck• Following manufacturer’s guidelines, adjust meter

based on weight differences• Repeat test, adjusting instrument, until two

consecutive tests are within guideline (most technicians use 0.5%)

• Repeat test at high flow rate, then low flow rate to make sure meter is accurate for all production rates

If the meter reads in gallons, then it is important to convert the gallons to weight by using the temperature calibration factor based being used by the plant control system. Now compare the registered weight to the actual weight of the truck. Following the meter manufacturer’s guidelines, adjust the meter based on the weight difference and repeat until the meter registers the correct amount of asphalt. Most technicians use 1/2% or 0.5% as the acceptable tolerance. Repeat the test both high and low flow rates to make sure the meter is operating within tolerance across the entire production range.

Slide 44

January 2009 7 - 44

Example - AC Meter Calibration

• Tare empty truck at 22,375#• Record AC gallon counter at 546,089• Record AC weight/gallon at 8.54 # • “span” value of instrument at 88798

Following the example in the illustration, let’s run through the calibration procedure on a generic flow meter.


Slide 45

January 2009 7 - 45

ASPHALT METER

GPM____

GALS22,375 Tare (Empty)

AC TankM

546,089 gallons

8.54 lb / gallon

Slide 46

January 2009 7 - 46


• Tare empty truck at 22,375#• Record AC gallon counter at 546,089• Record AC weight/gallon at 8.54 # • “span” value of instrument at 88798• Start pump at approximately 50% flow • Fill truck and stop• Record AC gallon counter at 547,307• Calculate weight of AC pumped at 10,402#

(547,307 - 546,089 = 1218 gallons x 8.54#/gal = 10,402#)


Slide 47

January 2009 7 - 47

ASPHALT METER

GPM____

GALS22,375 Tare (Empty)

547,307 gallons- 546,089 gallons= 1,218 gallonsx 8.54 lb / gallon= 10,402 lbs

AC TankM


Slide 48

January 2009 7 - 48


• Weigh truck at 33,175#• Calculate weight in truck at 10,800 # (33,175 -

22,375 = 10,800#)

The example shows that an adjustment is required to the meter. Typically the meters are very close to being properly adjusted and calibrated in the field.

Slide 49

January 2009 7 - 49

ASPHALT METER

GPM____

GALS


= 10,800 Net Weight


AC TankM

Slide 50

January 2009 7 - 50


• Weigh truck at 33,175#• Calculate weight in truck at 10,800 # (33,175 -

22,375 = 10,800#) • Recall “span” value of instrument at 88798• Adjust span value by 1.038 (10,800÷10,402 =1.038)• New span value (calibration factor)

= 92,172 (88798 x1.038 = 92,172)• (Adjust by Actual divided by Indicated)• Re-test, re-adjust, until two tests pass• Re-test at low and high flow rates



Slide 51

January 2009 7 - 51

ASPHALT METER

GPM____

GALS


= 10,800 Net Weight


88798 “span” value x 1.038 (10,800 ÷ 10,402)

= 92,172 new “span” value(Now test again!)

AC TankM

Slide 52

January 2009 7 - 52

This is an asphalt meter calibration screen from a computerized drum control system. Most plants now have this level of automation, and the asphalt meter calibration is done as part of the plant process automation. The operator simply follows the prompts and enters the net weight data from the truck. The process automation makes all the required calculations.

Slide 53

January 2009 7 - 53

AC Calibration Tanks Photo shown here shows a stationary AC calibration tank and the shear beam that weighs the tank. This would replace having to arrange for a tanker to do a manual calibration. Using an AC calibration tank improves the accuracy and is readily available.


Slide 54

January 2009 7 - 54

Best Management PracticesCalibrating AC Meters

• Larger tests rather than smaller truck tests decrease probability of error (1000 gallon minimum typically used in industry)

• Thermocouples or RTD’s used to measure temp of AC at meter for temperature calibration should be checked against known calibrated thermometers to make sure proper AC temp is being taken

• Charged AC lines must not be allowed to drain at start or stop or AC quantities in tests will be off

The following best management practices are useful when considering asphalt meter calibration: •Larger tests rather than smaller truck tests decrease probability of error (1000 gallon minimum typically used in industry) •Thermocouples or RTD’s used to measure temp of AC at meter for temperature calibration should be checked against known calibrated thermometers to make sure proper AC temp is being taken •Charged AC lines must not be allowed to drain at start or stop or AC quantities in tests will be off

Slide 55

January 2009 7 - 55

Best Management PracticesCalibrating AC Meters

• Verify proper specific gravity or AC weight/gallon is entered in system (true also when running!)

• Truck scales are calibrated ± 20 pounds. Small AC test sample sizes should be avoided! 40 pound tolerance on a truck scale is 4.7 gallons of AC!

• Calibration as part of plant calibration process • Check whenever needed

Other best management practices include: •Verifying the proper specific gravity or AC weight/gallon is entered in meter. This obviously affects asphalt content accuracy in the mix. This is necessary not only during calibration, but also while the plant is running. The control system will convert the temperature corrected gallon reading to weight by multiplying it with this value entered by the operator. •Truck scales are certified to only ± 20 pounds. Small AC test sample sizes should be avoided, because a 40 pound tolerance on a truck scale is 4.7 gallons of AC! This means the asphalt meter could be exactly correct, but the scale you are testing it against has the possibility of advising you that it is off by up to 4.7 gallons. •These elements help improve asphalt meter calibration accuracy and calibration efficiency in the field.


Slide 56

January 2009 7 - 56

Using Hydrated Lime Additive

Required when GRANITE is used in:

337 (FC-5) Friction Course Mixes

Hydrated lime is used on some Florida mixes to control the potential for stripping. It is required in 337 Friction Course mixes (FC-5) when granite is used as an aggregate.

Slide 57

January 2009 7 - 57

RequirementHydrated lime for 337 (FC-5) mixes

337-9.2 All Plants

• Separate feed system• Accurately proportioned• Coat the aggregate before the AC is injected• Must not be entrained in the air stream• Interlock the proportioning device• ± 10 % accuracy• No-flow = plant shutdown

There are several requirements for adding hydrating lime in the plant process for all plants (337-9.2). •Separate feed system •Accurately proportioned •Coat the aggregate before the AC is injected •Must not be entrained in the air stream •Interlock the proportioning device •± 10 % accuracy •No-flow = plant shutdown

Slide 58

January 2009 7 - 58


337-9.2 Two equipment methods

• Method (A) - dry

• Method (B) - wet

(Wet method covered in Module 3)

There are two equipment alternatives: the dry method A and the wet B method. The wet method was discussed in Module 3 and is used when aggregate is going to be treated with hydrated lime prior to drying the aggregate. In this module we will discuss the dry method A.


Slide 59

January 2009 7 - 59


337-9.2.1 Method A - dry- drum plant

• Uniformly disperse before the AC is injected• Must not be entrained in the air stream

Several equipment approaches are acceptable when adding hydrated lime in the drum-mix operation. With whatever method is chosen, it is important that the lime is dispersed uniformly prior to the asphalt injection and that the lime is not entrained in the air stream (337-10.2.1).

Slide 60

January 2009 7 - 60

Lime Silo for Dry Addition Optional Baghouse Fines Silo This photograph shows a lime additive system on a drum plant. With this method, lime is being introduced in the mixing area with the baghouse fines, but prior to the asphalt being injected. This plant is a counter-flow drum-mixer so the lime, baghouse fines, and asphalt are all injected outside of the air stream for the dryer.

Slide 61

January 2009 7 - 61

Using Fiber Additives

Fibers are used in ALL337 Open Graded Friction Course mixes (FC-5)

Fibers are also used in all open-graded 337 Friction Coarse mixes (FC-5). Fibers are used to help keep the asphalt in suspension between the aggregate particles in these coarse mixes to enhance their field performance.


Slide 62

January 2009 7 - 62

RequirementFiber systems for 337 (FC-5) mixes

337-9.1 All plants• Separate feed system• Accurately proportion• Interlock to aggregate quantity• ± 10 % accuracy• No-flow = plant shutdown

The fiber systems on all plants are required to be separate from other feed ingredients, be interlocked to the aggregate quantity, adequately proportion to the aggregate quantity, be interlocked for plant shutdown on no-flow conditions, and be accurate to ± 10% (337-9.1).

Slide 63

January 2009 7 - 63

RequirementFiber systems for 337 (FC-5) mixes

337-9.1 Drum plants

• Uniformly distribute before AC is injected• Must not be entrained in air stream

In addition to this general spec, as with hydrated lime, the fibers must be distributed before the AC is injected and not be entrained in the air stream (337-9.1).

Slide 64

January 2009 7 - 64

Fiber Injection System on Drum-Plant This photograph shows a fiber injection system on a drum-mix plant. With this system, fibers are weighed continuously, and introduced with their own pneumatic system into the mixing area of the plant.


Slide 65

January 2009 7 - 65

Plant Automation

Many types of systems and levels of sophistication found in the field.

There are many types of automation systems found on drum-mix plants. Whether the plant has simple or complicated automation, mix of equal quality can be produced at the facility.

Slide 66

January 2009 7 - 66

This photograph shows the main control screen on an automated drum plant. Note the mix formula including the cold feed ingredients, the two RAP products, and the asphalt content. Flow rates of the individual materials are shown. The moisture is automatically computed with this system based on the moistures entered by the operator of the individual materials and the mix formula called for. Composite moisture is shown at 5.3%. Most plant control systems have a main screen similar to this where the operator can see the mix formula, see critical plant production rate data, and watch the flow rates of the individual materials.


Slide 67

January 2009 7 - 67

This photograph shows the entire control console of a drum-mix plant. The silo controls are on the extreme left. The draft and baghouse controls are on the forward left above the start/stop buttons. The main control screen is directly in the middle of the console. The eight individual feeder controls are located to the front right. The burner control is on the extreme right.

Slide 68

January 2009 7 - 68

Some plants are now being manufactured with a “touch-pad” or “touch-screen” operator interface. All plant controls are available from this one terminal. The manual backup controls on this plant are located to the left of where the operator is sitting.

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This is a picture of the main screen from a computerized plant control system. Notice that the same key information discussed above is on the screen. This is the screen the operator views when he runs the plant. Other brands of plants have similar information, although the format will vary.


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This screen is where the operator enters the feeder moistures daily. Other brands of plants have similar information, although the format will vary.

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And this is the screen where the operator selects his mix formula. Other brands of plants have similar information, although the format will vary.

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Again, the main screen is used to monitor the ongoing operation. The operator can access required burner control, baghouse control, and calibration screens from this “main screen.”


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Recycling with a Drum-Mixer

Regardless of drum-mix type, RAP is introduced similar to any other aggregate in

the plant process

Regardless of the style of drum-mixer, RAP is introduced very similar to any other aggregate in the plant process. From an operator’s and observer’s standpoint, it doesn’t seem to be any different than feeding any other aggregate material.

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Recycling with a Drum-MixerParallel-flow drum-mixers heat RAP convectively with hot gases (50% possible)

Counter-flow drum-mixers heat RAP conductively with super-heated virgin aggregate (25-50% based on style/conditions)

RAP Enters Here, or Here

Although RAP is introduced in the drum similar to the virgin aggregates, different styles of drum-mixers actually heat the RAP differently. Parallel flow drums heat RAP convectively, like a regular aggregate. High quantities of RAP can be used easily. The newer counter-flow drum-mix designs heat RAP conductively, by first superheating the virgin aggregates, then introducing the RAP together with the superheated stone. This style of heat transfer is more environmentally friendly, but there can be limitations to the RAP percentage that can be adequately dried and heated based on field variables.


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Recycling with a Drum-MixerImportant not to exceed capabilities of equipment for moisture/ RAP type

(moisture and particle size are two key issues)(smaller RAP particles dry and heat faster)(drier RAP particles dry and heat faster)(these variables affect drying efficiency)

RAP Enters Here, or Here

It is important not to exceed the RAP heat transfer capabilities of the equipment. Moisture and particle size are two limitations that affect RAP heat transfer. Small and/or dry RAP particles heat transfer more rapidly. Large and/or wet RAP particles heat transfer more slowly. Both of these variables can affect drying and heating efficiency, just like virgin aggregates.

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• Retained moisture in mix (inadequate drying) can be checked easily … (put mix sample in large sealed container, weigh, remove cover, dry at 300°F, weigh again, calculate weight loss)

(corrective action =reduce RAP %, size,or total tph – then seeif flighting change cancorrect problem)

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Best Management PracticesChecking for Adequate Drying

Retained moisture in the mix resulting from inadequate drying of the RAP is easy to check, just as it is for virgin mixes. Take a sample of the mix, put it in a sealed container, weigh it, uncover and dry it at no more than 300°F, weigh it again, and calculate the weigh loss to determine moisture retention, just like you would virgin mixes. The corrective action if the RAP is not being thoroughly dried and affecting the mix quality is to either reduce the RAP percentage, the size, or the total tph. This should allow the plant to reach a proper equilibrium. The “long term” solution might be to re-flight the dryer as you would with virgin mixes.


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Recycling with a Drum-MixerThe plant automation automatically adjusts for the asphalt content and moisture in the RAP

When recycling, the plant automation adjusts the flow of the new liquid asphalt to compensate for the asphalt being introduced and being reused from the RAP. The operator enters the AC percentage in the RAP as part of the mix recipe, and the automation will automatically adjust the flow of the new liquid AC to reach to final target asphalt percentage.

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InspectionDrum-Mix Gradation Control

• Calibrate-able feeders? (320-2.4)• Scalping screen prior to dryer? (330-5.7.1)

Specs require that the feeders are calibrated (320-2.4) for proper aggregate blending, and a scalping screen is installed between the cold feed and the dryer (330-5.7.1).

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InspectionDrum-Mix Asphalt Control

• Belt scale measures virgin aggregate and RAP? (320-4.1)

• AC content within tolerance? (320-2.6)• Heat jacketed lines on AC pipes? (320-2.6)• AC pump coupled to aggregate and RAP belt

scales? (320-4.2)

Asphalt control has several specific inspection points: •Belt scales must be able to adequately measure virgin aggregate and RAP material flow (320-4.1). •The asphalt content must be within tolerance (320-2.6). •Asphalt lines must be heated between the tanks and the drum-mixer (320-2.6). •The asphalt pump must be coupled to the aggregate and RAP belt scales (320-4.2).


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InspectionDrum-Mix Hydrated Lime

337-9.2• Separate feed system• Accurately proportioned• Coat the aggregate before the AC is injected• Must not be entrained in the air stream• Interlock the proportioning device• ± 10 % accuracy• No-flow = plant shutdown• (if wet see requirement Module 3)

If hydrated lime is being introduced to the system dry, then the following requirements need to be met (337-9.2): •A separate feed system must be present. •The hydrated lime must be accurately proportioned. •The lime must coat the aggregate before the AC is injected. •The lime must not be entrained in the air stream. •The lime proportioning device must be interlocked to the aggregate flow within ± 10 % accuracy, and interrupt the plant process in no-flow conditions. •If lime is being added wet to the aggregate prior to the dryer, see Module 3 for regulations and inspection guidelines.

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InspectionDrum-Mix Fiber Systems

337-9.1 • Separate feed system• Accurately proportion• Interlock to aggregate quantity• ± 10 % accuracy• No-flow = plant shutdown• Uniformly distribute before AC is injected• Must not be entrained in air stream

The same type of requirements for feeding hydrated lime into a drum-mix process are called for when introducing fibers in the mix (337-9.1).


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Quality Mgt in DM ProductionIf test results show…

Let’s look at some results of the testing and make decisions on what to do with production.

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Gradation Shift / Coarser (Aggregate), check…Wrong mix formula (human error)

Cold feed bin flow restriction / changeCold feed bin calibration shiftGradation change in feed aggregates


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Gradation Shift / Finer (Aggregate), check …Wrong mix formula (human error)

Cold feed bin flow restriction / changeCold feed bin calibration shift Gradation change in feed aggregates



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Gradation Shift / Coarser (Low Dust), check …Wrong mix formula (human error)

Drastic change in production rate (low to high takes time for BH fines to catch up to cold feed change)

Over-drafting dryer (less fines in mix from damper being more open relative to previous or typical production rate, together with BH fines being wasted or partial return)

BH dust return or MF additive malfunction (causing reduced BH dust return or MF additive)

Gradation change in feed aggregates

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Gradation Shift / Finer (High Dust), check …Wrong mix formula (human error)

Drastic change in production rate (high to low takes time for BH fines to catch up to cold feed change)

Under-drafting dryer/Puffing (more fines in mix from damper being less open relative to previous or typical production rate, together with BH fines being wasted or partial return)

BH dust return or MF additive malfunction (causing increased BH dust return or MF additive)

Gradation change in feed aggregates

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Low Voids, check …Wrong mix formula (human error)

Dust return or dust additive malfunctionProduction rate change (high to low with immediate sampling)

Lack of draft in dryer/Puffing – retained fines in dryer/primary vs. to baghouse (and BH fines are being wasted or used partially)

Gradation change in feed aggregatesAC content high



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High Voids, check …Wrong mix formula (human error)

Dust return or dust additive malfunctionProduction rate change (low to high with immediate sampling)

Over-drafting dryer – more fines to baghouse vs. in dryer/primary (and BH fines are being wasted or used partially)

Gradation change feed aggregatesAC content low

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AC Content High, check …Wrong mix formula (human error)

Calibration of belt scale (reading high)

Binding belt scale (reading high)

Wind effect on belt scale (reading high)

Calibration of AC meter (reading low)

Improper moisture setting (too low)

Wrong AC weight/gallon entry (too low)

Inadequate drying of aggregate (drying efficiency low & ignition oven AC test method)


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AC Content Low, check …Wrong mix formula (human error)

Calibration of belt scale (reading low)

Binding belt scale (reading low)

Wind effect on belt scale (reading low)

Calibration of AC meter (reading high)

Improper moisture setting (too high)

Wrong AC weight/gallon entry (too high)


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AC Content Varies, check …Malfunction in flow control mechanism (difficulty in regulating / mechanical issue)

Improper moisture settings (not changing entry with moisture changes)

Inadequate drying of aggregate (drying efficiency varies & ignition oven AC test method)

Inaccurate AC weight/gallon entry (not changing entry with AC changes)

Drain Down of ACFlighting issues

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Inadequate Coating, check …AC injection point too close to discharge (inadequate mixing time)

Mixing flights inadequate (not tall enough, caked, not blending mix)

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Module 7What we covered...

• Review how gradation is controlled in a drum-mixer

• Review how asphalt is controlled in a drum-mixer

• Learn how drum-mixer automation works• Learn about RAP production with a drum-

mixer• Troubleshooting mix problems


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QUESTIONS ?

Any Questions?

asphalt plant level ii january of module drum plants s ... plant 2/07 - module 7 - drum...asphalt...

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