Plant Equipment and Calibration Examples
1. Cold Aggregate Bins
The first step in calibrating a proportioning plant is the calibration of the cold aggregate bins. These units determine the final gradation of the mixture.
a. Fixed Speed-Variable Gate Opening Bins
These bins are controlled by gates, which meter the flow volumetrically. They are calibrated by weighing the quantity of material, which passes through a given gate opening during a measured time interval. The interval is determined by counting the number of revolutions that the belt makes while the material is delivered. From the RPM of the belt and the weight of the material, the delivery rate in pounds per minute is calculated (corrected for moisture). The calibration is graphed by plotting the pounds of dry aggregate delivered per minute at the gate openings used in the calibration.
b. Fixed Gate Opening-Variable Speed Bins
With this system, a gate opening is selected for each bin. This gate opening must be maintained throughout the calibration and the job. They are calibrated by weighing the amount of material delivered at several different speeds of the cold-feeder motor over a measured time interval.
The calibration is graphed by plotting the pounds of dry aggregate delivered per minute at the speeds of the belt motor used in the calibration.
These bins are equipped with a master control, which may be used to adjust the production rate. Changing the master control setting changes the speed of all the belts proportionately.
c. With either type of bin, the gate setting is very important and should be checked regularly.
2. Conveyor Scales
The specifications require Dryer Drum Mixing Plants be equipped with continuous weighing central conveyor scales that are interlocked with the asphalt delivery system. These scales are checked for accuracy by two methods as follows:
a. The scale is first zeroed while the conveyor is operating at normal operating speed, but unloaded. It is then adjusted to readout a predetermined total weight using the special
scale beam weights and a standard operating time interval. Both of the foregoing procedures are to be performed in accordance with the scale manufacturer's instructions.
b. The second accuracy check requires the comparison of the weight shown on the totalizing meter, with the weight actually delivered as determined by running material over the conveyor into a tared truck. The truck tare and loaded weights must be obtained by weighing over certified commercial truck scales, or plant scales that have been checked against certified scales and approved by the Engineer. The conveyor scales should be checked at several delivery rates representing the proposed operating range. The contractor shall adjust the weighing system so that when the plant is operating, the final mixture is uniform and consistently within the specified job mix formula tolerances.
3. Asphalt Pump
Some batch plants, and all drum mixing plants deliver asphalt material to the mixer through volumetric pumps. These volumetric systems must be calibrated throughout the proposed operating range at the normal operating temperature. Totalizing meters must be adjusted to readout the quantity delivered within the specified delivery tolerance.
In-line flow meter must be accurate to plus or minus 0.2% as demonstrated through the calibrations process. Use a certified truck scale or a calibration tank in the calibration of flow meters. Perform yield checks.
At least once per year a Certified Scale Company shall verify the load cells on the calibration tank. When the tank is moved certified weights shall be used to test the load cells. Use a minimum of 200 pounds to check for accuracy, sensitivity and that the tank is free floating and level on the load cells.
When troubleshooting asphalt binder measurement:
a. Check capacity of storage tanks.
b. Check tank sticks.
i. Be sure they fit the tanks.
ii. Determine proper use. (touch stick or dipstick, percent of diameter or inches, etc.)
iii. Be sure tanks are level.
c. Check piping and type of pumping system.
d. Check the truck scales.
4. Hot Aggregate Bins on Batch Plants
After the various aggregates have been proportioned and dried, they are fed to the mixer unit.
a. On batch plants the hot aggregates are weighed in batches over calibrated scales as described in the following paragraphs and examples.
The dust collected by the dust collector is fed from a calibrated bin or returned directly to the hot aggregate, depending on the type of plant equipment and the specification
requirements. If the dust is returned separately, the feeder should be calibrated to feed the required quantity of dust in a uniform manner. If the dust is returned directly to the hot aggregate the weight or volume of dust collected and returned is taken into account automatically in the calibration of the hot aggregate delivery system or batch weights.
When gradation control is by cold-feed samples, and the batch plant is equipped with hot aggregate screening units, they should be removed or covered so that the gradation is not altered by the balance of the hot bin delivery settings. If the Contractor would like to use the hot aggregate screening process, the approval of the DME is required before beginning. If this process is used, the gradation control will be based on samples obtained from the hot aggregate delivery or on the extracted gradation from samples of the hot mix. When hot aggregate screening is used, the delivery of each hot bin must be calibrated similarly to the cold feed bins.
5. Batch Plant Scales
Calibration of batch plant scales as required by the specifications is performed by incrementally loading the scales with standard test weights and partial batches through the operating range of the scales. As each increment of load is applied, the actual observed weight and the required weight are compared. The differences, plus or minus, are determined and converted to percentages of the required weight. If the percentage deviations are less than the tolerance allowed by the specifications and the scales are sensitive to the test loads, the scales will be considered in calibration. If the scales do not meet the various requirements, the contractor is required to make the necessary repairs or adjustments. Recalibration may be ordered by the engineer if the scale equipment malfunctions or if required material quantities do not agree with actual material quantities. If the batch plant scales are to be used for the determination of pay quantity, a scale calibration to the proper accuracy is required.
6. Initial Plant Settings
Three examples of initial plant settings have been provided: one for a drum mix plant, one for a 3000-pound batch plant with a volumetric asphalt measurement, and one for a continuous plant with a sprocket type asphalt pump.
7. Mixing Rate
The specifications contain requirements regarding the quality and duration of mixing for the various types of mixes and plants. The design, condition, speed and loading of the mixer unit together with the characteristics of the materials being mixed will vary from job to job and need to be taken into account when evaluations are made.
Mixing times are determined in the following manner:
b. Batch Plants. The mixing rate of batch plants is controlled by the batch size and the dry and wet mixing timer settings. The batch size should not exceed the manufacturers rated capacity and the timer should be set to provide the specified mixing time unless more or less time is authorized by the engineer. The accuracy of the timer may be checked with a stopwatch if necessary.
PLANT CALIBRATION EXAMPLE
TYPICAL PLANT COLD-FEED SETTINGS
The following example is based on initial plant output of 70 TPH of mix. See example.
Set for 70 TPH total cold-feed aggregate (Dry Weight):
Material % in Mix lbs./min. Gate Setting
1/2 in. (12.5 mm) Cr. Stone 60% 1400 Approx. 3 5/8
Sand 40% 933 Approx. 2 1/8
DRUM MIXING PLANT
The following example is based on initial plant output of 300 TPH. See example.
Set aggregate delivery controls on plant control console to deliver 300 TPH of dry aggregate.
Master control set on maximum.
300 x 58% 3/4 inch crushed limestone = 174
300)x 30% Sand = 90
300 x12% RAP = 36
BATCH PLANT SETTINGS
After the aggregate scale and asphalt scale has been checked for accuracy, the batch weights are set and mixing operations are begun. The scales are checked by adding weights to the hopper and observing the scale dial indicators (see examples). Some batch plants are equipped with volumetric asphalt pumps rather than scale buckets; these devices are calibrated the same way that asphalt pumps are calibrated on drum-mix plants, but operated on a batch basis:
Assume 3000 lb. batch plant, cold-feeds as cited previously and 5.5% binder content.
Binder: 5.5% x 3000 lbs. = 165 lbs. per batch (see example)
Combined Hot Agg.: 3000 lbs.– 165 lbs.= 2835 lbs. per batch
Asphalt pump (volumetric) = 165 lbs. per batch = 23 counts per batch (see example)
If the dust is returned directly to the hot aggregate, separate computations are not required for the dust being fed since it is automatically included in the hot aggregate delivery.
All gate and scale settings and weighing controls shall be set to target on the required quantities. Offsetting shall not be permitted, except to correct calibration errors.
Example of flow meter calibration intervals:
Production Range = 100-300 TPH
JMF Binder Content = 6.0 % (virgin)
Asphalt Weight/Volume = 8.57 lb/gal
Tank Capacity = 24 Tons (5600 gallons)
Asphalt Binder Range = 6 – 18 TPH (1400 – 4200 gallon/hr)
The lower portion of the range may include an interval starting from 1400 gallons. Intervals should be sufficient to ensure that the tolerance and scale break requirements are met on a platform scale.