A.3.1 Equipment description
A power plant has 2 condensing steam turbine generator sets installed, with a single unit capacity of 135MW. The boiler ash discharge method is pneumatic ash conveying. The ash removal system is equipped with 4 screw air compressors, with air compressor specifications of 110kW, water-cooled units, exhaust volume: 20m³/min, and motor speed 1480r/min.
A.3.2 Fault phenomenon
#4 During the operation of the ash conveying air compressor, an electrical serious fault alarm occurred and the air compressor was shut down. After eliminating the alarm signal, the operating current was measured to be 238A (design full load current 200.8A), and the maximum vibration value of the air compressor was measured to be 0.12mm. Because the operating current and vibration values exceeded the standard, the air compressor was shut down. When the air inlet was opened, a large amount of light-colored colloid was found to be adhered to the air compressor rotor, and a large amount of light-colored colloid was also adhered to the air compressor housing. After confirming that the air compressor had carbon deposits, the maintenance records were reviewed and it was found that the air compressor had been running continuously for 8000 hours without maintenance. After cleaning and maintaining the air compressor and its system, the air compressor was started again, and the operating current was measured at 205A. The maximum vibration value of the air compressor was measured at about 0.12mm. Since the vibration value exceeded the standard, it was decided to return the air compressor to the factory for inspection. After the manufacturer dismantled and inspected, it was found that there were obvious wear grooves on multiple bearing raceways and rollers of the air compressor, and the gap between the main rotor and the auxiliary rotor of the air compressor was less than the design value. Both the main and auxiliary rotors had obvious signs of wear. After replacing the air compressor bearings and grinding the worn main and auxiliary rotors, the air compressor was assembled and reinstalled by the power plant and tested normally.
A.3.3 Cause Analysis
(1) As the oil level mirror showed that the lubricating oil was clear and had no obvious color change, it was mistakenly believed that the lubricating oil could continue to be used. In fact, since the lubricating oil had been used for 8000 hours, its viscosity, impurity particles, and carbon deposition tendency no longer met the use standards. Continuing to use it caused the compressor bearing to wear and carbon deposition.
(2) The cause of the electrical serious fault alarm was that the rotor carbon deposition increased the shaft power of the motor load. At the same time, the pressure difference before and after the oil fine separator was not replaced for a long time increased, which also increased the shaft power of the motor accordingly and caused the overcurrent protection to operate.
A.3.4 Preventive Measures
The service life of the commonly used air compressor synthetic lubricating oil is about 6000 hours. In actual application, if the lubricating oil has been used for 6000 hours and the color is clear, it can still be used for a certain period of time. According to experience, it is safer to control this time within 6000 hours. Therefore, on the basis of improving economic benefits, we must also pay attention to safety and not lose the big picture for the small.
