The 54,000 bbl and 26,000 bbl tanks in the center of the first photo (made before the terminal was completed) are regarded as reserve tanks. The 5,000 bbl tanks on the left are known as day tanks. The day tanks are designed to heat asphalt faster than the reserve tanks and are used primarily to store mix that will be loaded out daily.
The heater raises the temperature of the asphalt from 285 degrees F to 360 degrees F (a total of 75 degrees F) as it flows through the heater at 300 gpm.
The different tank sizes facilitate storing the asphalt at different temperatures to conserve heating costs. Asphalt in the reserve tanks is usually stored at 280 degrees F, but may be stored at higher temperatures when usage rates are very high. Asphalt in the day tanks is stored at 350 degrees F, the usual temperature for load-out and for making polymer mixes.
Asphalt can be transferred from any storage tank to another and to the loading rack. However, only the day tanks can transfer asphalt to the polymer blending system. Likewise, the polymer system can transfer polymer mix only to the day tanks and loading rack.
In addition to heating coils in the bottom of each tank, a special heating coil runs up the sidewall of the tank. It is known as a “crust buster” and is heated with hot oil. It is used during heat up after the asphalt has been allowed to cool and a solid crust has formed on its top surface. This coil quickly liquefies a portion of the crust so that its expansion will not damage the tank before the crust becomes entirely liquid.
All asphalt storage tanks have many components in addition to their heating coils. The reserve tanks have a single mixer, which blends incoming streams of cooler asphalt with existing zones of hotter asphalt to quickly achieve a uniform temperature throughout the tank. The day tanks have two mixers that ensure uniform temperature and also ensure that ingredients of polymer mixes do not separate.
Each tank has a gauge board and a pressure transmitter to indicate the level of liquid inside the tank. The gauge board provides direct visual indication of levels. The pressure transmitter provides a readout at the transmitter. It also provides level signals for a controller (and its display) located on the control panel at the tank (see photo). The controller provides level signals for computer controls.
Each tank has two thermometers mounted in oil-filled wells in the side of the tank for direct readings of temperature. Each tank also has a thermocouple that provides signals for the temperature controller (and its display) located on the control panel at the tank. The controller provides signals for computer controls.
The temperatures of the liquid in each tank is controlled by a motorized three way valve that modulates the flow of hot oil in the circuit in response to the 4 to 20 mA signals (see photo).
Each tank has a sampling valve that facilitates taking small samples of liquid from the tank.
Each tank has an overflow pipe to keep the tank from being overfilled. The overflow pipe also serves as an air vent. An emergency air vent is also provided. The emergency vent is a safety feature that prevents pressure build-up inside the tank in case the overflow pipe becomes blocked. Excess pressure could rupture the tank.
Each tank also has a spiral stairway from ground level to the top of the tank.
The photo shows hot oil heating coils in a large asphalt storage tank. Asphalt in a single reserve tank can be heated at a rate of one-half degree F an hour. Asphalt in a single day tank can be heated at one degree an hour. If more than one tank is being heated at the same time the rate will be proportionally slower.
When the asphalt in a tank has been allowed to cool to a temperature less than 200 degrees F, it will likely have a crust on its top surface. The tank has a crust buster coil in addition to the heating coils in the bottom of the tank.
This coil should be activated when heating the asphalt to restore its temperature. This coil has an inlet and outlet valve that has to be opened so that hot oil will heat the coil.