Asphalt metering
Two methods are used to add the specified amount of asphalt cement to aggregate for making hot mix asphalt. Batch plants use a weigh pot to weigh liquid asphalt cement in the batch tower before the asphalt cement is dropped into a pugmill for mixing. The pot is mounted on load cells. It is essentially a batch metering system. Drum-mix plants use a continuous flow asphalt metering system.
The standard continuous metering system used by Heatec has two pumps (Figure 53). The system is commonly known as “a pump pushing a pump.” The system is very accurate and has proved to be very reliable, durable, and cost effective. The system can be calibrated to meet the most stringent accuracy requirements of plus or minus one-tenth of one percent (0.1 percent), as required by the state of Illinois and other states.
One pump is active and the other is passive. The active one is motor-driven and functions as a conventional supply pump. The passive one is forced to rotate by the liquid asphalt being pumped through it and functions solely as a flow meter. The asphalt flowing through the passive pump causes its drive shaft to turn at a speed proportional to the flow rate. An encoder on its shaft generates an electrical signal representing its revolutions. This signal is used by the computer in the control house to display the amount of asphalt cement used and its rate of flow. The system incorporates a temperature measuring system that compensates for changes in the volume of asphalt cement due to temperature variations.
Another continuous metering system sometimes used at drum mix plants is known as a mass flow meter. This is an electronic measuring system. Mass flow meters have a high degree of accuracy (plus or minus 0.15 percent) and virtually no moving parts. However, they are significantly more expensive than other systems used for asphalt metering and are hard to justify solely on the basis of their increased accuracy. The system has an electronic sensor and transmitter that measure flow rate, density, pressure, viscosity and temperature of the asphalt. It compensates for temperature variations in the material.
Still another type of continuous metering system used for drum mix plants is the Brodie meter. It uses a rotor assembly, an output register, and a signal generator that produces pulses in proportion to the rate of flow. It is compensated for temperature variations. The Brodie meter is more expensive than the “pump pushing a pump” system and is less durable. Consequently, Heatec does not use the Brodie meter unless specifically requested by the customer.
Calibration
Calibration tanks are recommended for calibrating the liquid asphalt metering system of a drum mix asphalt plant. They save considerable time and money. The tanks are available in two styles, a vertical (Figure 55) and a horizontal (Figure 56). The choice between the two styles is mostly a matter of personal preference.
Figure 55. Vertical Calibration Tank.
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Figure 56. Horizontal Calibration Tank.
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Using a calibration tank is far more efficient than using a distributor truck, a common practice in the industry. Efficiency becomes significant when calibration must be done frequently. It wasn’t so bad when you only had to calibrate every month or two. But now some states now require it every week.
Heatec calibration tanks provide an efficient way to calibrate the metering system. The tank has built-in load cells and a digital readout. The load cells are highly accurate. They are factory calibrated and furnished with a calibration certificate. The system provides a higher degree of accuracy than using a truck scale. Once the calibration tank is installed, there is no need to connect and disconnect asphalt lines each time the system is calibrated. Here’s all you have to do using a Heatec calibration tank:
- Open the valves to the calibration tank and pump about 1000 gallons of AC into the calibration tank as indicated by the asphalt metering system.
- Note the weight shown on the readout of the calibration tank.
- Adjust the metering system to agree with the weight shown on the readout. Switch the valves, then pump the AC back into the AC tank.
- Repeat as many times as necessary.
You can run three cycles with the calibration tank in less time than it takes for one cycle with a distributor truck. You may save two or three hours every time you calibrate. Maybe more. And don’t forget about improved accuracy. In any case, the savings in time and money is apt to be more than worthwhile.
Just compare using a truck with using calibration tanks. Here’s what you have to do when using a truck to calibrate your metering system:
- Get an empty asphalt distributor truck. Drive it across a truck scale, record its weight, and then drive it to your plant.
- Connect the truck to your asphalt tank.
- Pump one or two thousand gallons of asphalt into the truck as indicated by the metering system. Disconnect the lines.
- Drive the truck back across the weigh scale and record its weight.
- Subtract the truck’s empty weight from its loaded weight. Adjust your metering system readouts to agree with the calculated weight.
- Reconnect the truck to the asphalt tank and pump the asphalt back into the tank.
- Repeat the whole process again. And again.
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