Aug 19, 2021 Unit Introduction
The Styrene unit 4M16-60.6/0.4-27.3 exhaust gas driven compressor is a four-row symmetrical balanced and four-stage reciprocating water-cooled compressor. It is used to compress the hydrogen from 0.038-0.048MPa(G) to 2.73MPa(G) through four stages.
For the reciprocating compressor, there is oil-free lubrication between the cylinder and piston ring, support ring, between the packing case sealing ring and piston rod. The piston ring, support ring, and sealing ring of the packing case are made of graphite-filled PTFE that has good self-lubricating performance.
This compressor mechanism is of oil lubrication. Its crank connecting rod adopts the circulating oil lubrication, and the units are configured with an independent lubricating oil station. Before starting up the compressor with a gear oil pump on the bearing end, the auxiliary oil pump is started. When the start-up oil pressure of the main unit reaches the specified value, the auxiliary oil pump will be manually shut down by the operator.
The circulating lubricating oil enters the oil cooler through the crankcase and the oil filter then flows into lubrication points through the pipe and the internal channel. The oil scrapers are arranged on the side of the crosshead slipper at all levels to scrape the oil on the cylinder surface of the piston rod. The oil flows from the return hole of the oil scraper assembly to that hole of the oil scraper seat and then returns to the crankcase.
The reciprocating compressor unit includes the crankshaft bearing bush, middle crankshaft bearing bush, connecting rod big end bush, and connecting rod small end bush. Among them, the big end bush and crankshaft bearing use split structure whose bearing surface’s average thickness of Babbitt alloy is 1mm, which belongs to the thin wall bush. It has the advantages of high elasticity, small pressure ratio, quick heat conduction, and high accuracy.
Bearing bush using forced lubrication, and crankshaft bearing installation clearance is 0.09-0.12mm. Big end bush installation clearance is 0.09-0.12mm, and small end bush 0.08-0.12mm.
Reciprocating Compressor Bearing Failure Causes
Failure Phenomenon
The reciprocating compressor oil filter differential pressure was high, and operation department workers switched the oil filter. After that, the oil pressure difference still maintained rising. At the same time, the vibration and abnormal noise of the compressor were severe. The lubricating oil pressure difference between the inlet and outlet gave a low-voltage alarm. After dissembling and inspecting the compressor, they found high content of metal particles and Babbitt alloy particles.
The crankshaft bearing in the driving position was seriously worn, and slight wear in the non-driving position. In addition, some part of the Babbitt alloy layer has been peeling.
Cause Analysis
In the operation process of the reciprocating compressor, the main oil pump of the bearing end coupled with the crankshaft provides the lubrication effect of each point. The corresponding pressure difference(ΔP) of the lubricating oil directly indicates its flow condition and indirectly reflects the lubrication condition. When ΔP is lower than 0.25Mpa, there should be an alarm. If ΔP is lower than 0.15Mpa, the interlock stop should be displayed.
If the oil pressure difference on the DCS is low, resulting in oil absorption difficulty after the main oil pump is opened, which will directly affect the lubrication condition of the bearing bush. The decrease of the oil pressure will cause the bush to burn out, and the Babbitt alloy in the burnt and fused state will stick to the shaft. This will accelerate the wear of the bearing bush.
The worker found that the low-pressure difference interlock alarm and the high-pressure difference interlock alarm signal had been connected in inverse, which made the reciprocating compressor unit unable to interlock when the oil pressure difference was low in the process of operation. And when the oil pressure difference was low, the auxiliary oil pump could not be opened to supplement pressure for the unit, thus losing the safety protection for the reciprocating compressor unit.
Analyzing the abnormal phenomenon of the reciprocating compressor before the shutdown and after the disassembly, the worker concluded that it was mainly due to poor lubrication. There was serious wear in the axial direction of crankshaft bearings and connecting rods at the driving position, and iron filings and Babbitt alloy particles in the lubrication system.
In addition to the alarm signals connected in reverse condition, the hand winding was insufficient before the reciprocating compressor was started. Before the start of the compressor, an oil film could be formed on each friction surface for automatic lubrication between the bearing and crankshaft.
But the lubrication between the big end bush and connecting rod required manual lubrication. The insufficient hand winding would cause the oil film to be incomplete, thus leading to the wear of the edge contact. The high bearing temperature would melt the Babbitt alloy on the surface of the bearing bush, and it could be pealing. The high viscosity of lubricating oil made the peeling Babbitt alloy particles drift between friction surfaces, thus accelerating the wear of the bearing bush.
Reciprocating Compressor Bearing Bush Troubleshooting
Inspection Measures
- Replace the crankshaft bearing bush, middle crankshaft bearing bush, and connecting rod big end bush with serious wear. Focus on the classification control of the installation clearance, and the repair and grinding of the crankshaft journal. Clean the crankshaft oil hole and bottom filter.
- Check the crosshead, repair the sliding shoes. Clean the slide, and inspect the crosshead pin and the connecting rod small end bush.
- Examine the isolation room. Replace the oil scraping ring, oil retaining ring, guide bearing sleeve, packing case, and assemble them qualified.
- Inspect and clean the piston, cylinder, and air valve. You should ensure that the clearance at internal and external dead spots is qualified, and the piston rod is connected with the pressure body.
- Check the lubrication system and shaft seal.
Improvement Measures
The main cause for the reciprocating compressor accident is that the wrong alarm connection for the oil pressure difference of the lubricating oil. The instrument PLC is connected in reverse, causing the oil pressure difference to be low and the interlock to fail, so the compressor loses the protection. In addition, the operator was so careless that hand winding was not in place.
To summarize it, we can strengthen the DCS monitor of the key operating parameters of the large unit, add specific operation records, pay attention to the DCS alarm, and analyze its reasons and possible consequences. Besides, the starting and stopping steps of the device should be revised and improved. You should make and place cards explaining the starting conditions of the field equipment, the starting and stopping steps, and the troubleshooting. Train the operators for the start and stop of key equipment.
Conclusion
To solve the failure of reciprocating compressor bearing bush is so important that it ensures long, full, optimal, and stable operation of the compressor. Therefore, we must intensify the daily inspection. Learning from practical experience is helpful to improve the professional and technical level of personnel. So they can figure out problems effectively.
