Troubleshooting of heating system faults in asphalt concrete mixing plants

The MARINI asphalt mixing plant in Italy is an asphalt concrete mixing equipment centrally controlled by a control room. The entire set of equipment requires electricity during operation, which is provided by 1-2 generator sets. The combustion heating system of the drying cylinder of this equipment consists of a combustion automatic controller R7241, a gas igniter, a fuel atomization burner and a weathering ratio regulator, etc. Controller R7241 is a microprocessor located under the operation console in the control room. It can automatically control the entire process of ignition and combustion and has self-diagnostic capabilities. When a fault occurs in the heating system during the control process, controller R7241 will display the fault code in the form of flashing indicator lights. The device is equipped with a fault code checklist on both the manual and the R7241 controller housing. The checklist lists the cause of each code's failure and the inspection items. Based on this, the operator can quickly eliminate the fault.

Fault phenomenon

During a normal operation, the combustion heater of the drying cylinder went out. After resetting the controller R7241 and re-igniting, it went out again after about 2 hours of normal operation. Since then, the intervals between each flameout have become shorter and shorter until it could no longer work normally.

Fault diagnosis

First, check the fuel supply system. It can be known from the indication of the fuel pressure gauge that the fuel pressure is normal. When the engine stalled for the first few times, the fault code displayed by the controller R7241 was 6EF. By looking up the table, it was found that the cause of the fault was high-frequency interference. So, the shielded wires of the photoelectric sensor, fan and solenoid valve, etc. were inspected according to the items listed in the checklist, but no abnormalities were found. Then, all the electrical control parts were inspected, and no abnormalities were found. So, the equipment was put back into load operation, and at the same time, the changes in the voltage and frequency of the output power supply of the generator set were closely observed. At this point, it was found that under the maximum load of the generator set, the drying cylinder heater would go out whenever the frequency dropped to 46Hz. Therefore, it was determined that the fault might be caused by the generator set. Later, another normal generator set was started, and everything turned out to be fine. It can be seen that the fault was caused by the low rotational speed of the generator set when it was operating under load.

Upon inspection, it was found that the low speed of the generator was caused by the wear of the high-pressure oil pump governor of the diesel engine. As a result, when the generator was operating under load, its speed decreased. When it was at the maximum load, its frequency dropped outside the normal working range of the controller R7241, eventually leading to the shutdown of the heating system.