Editor’s Note: Beginning in 2018, AsphaltPro magazine is offering readers a new column to help when training employees at the plant or in the field. This back-to-basics series will give rudimentary—yet useful—tips and tricks for working in the lab, running the plant and paving a quality mat successfully. For this month’s production-related theme, we’re starting with how to manage a heater in the tank farm.
One of the basic truths at the asphalt tank farm is that temperature rules the day. You want to monitor and control temperatures to keep the material in tanks from experiencing wide fluctuations that might degrade molecular bonds or cause other materials to solidify. Here’s an example of something you might experience.
Let’s say the dial thermometer on your asphalt tank reads 306 degrees F. All other controls should read the same. But what if they don’t? What if the temperature controller connected to the thermocouple nearby reads 296 degrees? And the temperature controller connected to a thermocouple in the asphalt piping reads 325 degrees while the computer screen in the control house shows the asphalt temperature in the tank is 290 degrees. You want to get those temperatures in sync.
Start with the hot oil. Make sure there is oil in the wells of the tanks where the thermometers and thermocouples are installed. This oil should have no bubbles in it.
Lack of oil in these wells can cause temperature errors, which means your material may be just fine, but your measuring devices are starved for the material they need.
The wells should be partially filled with thermal fluid—or a heat transfer oil—and no water. Make sure there’s room for expansion, but no bubbles.
If regulating the heat transfer oil in the wells of the tanks didn’t solve the problem of varying temperature readings, look to the output signal the controllers are sending. It might not be properly scaled to the input of the PLC and its computer screen.
Make sure the PLC is set up so its 4 to 20mA input is properly scaled to the temperatures represented by the 4 to 20mA signal from the temperature controller.
If you still see varying temperature readings, it’s time to do some testing. Get a high-end thermometer with a manufacturer who boasts it’s “more accurate than required tolerances” (such as the Fluke 1552A EX Thermometer). The Fluke mentioned here is reported to have an accuracy of + or – 0.09 degrees F.
First: Use the instrument you prefer to check the temperatures at the locations where your thermometers, thermocouples and capillary bulbs are installed.
Second: Compare the instrument’s readings with the readings from each of your devices.
If the devices are reading true, yet are all giving different measurements for your asphalt temperature, you have variations within your tank and piping, etc. If one or more devices are showing readings that are incorrect, then they are the items that need to be repaired or replaced—not elements in the tank or heating system.
New employees are probably not the ones appointed to repair or replace elements within a tank or heating system.
Once a problem has been identified and its source found out, the team can make recommendations for a resolution.
Source: Heatec Inc., an Astec Industries Company, Chattanooga.