SEAL RING FAILURE IN MAIN AIR COMPRESSORS
13th July 2006
This incident highlights a problem faced by myself recently on a newly built oil tanker with main air compressors.
The ship was built in Oct 2003 and the guarantee period was just ending when I joined the vessel. The vessel has a latest Hyundai B&W Engine with plenty of PLCs and automation & pneumatics.
Soon after joining, the inert gas generator one of the pneumatic IP converters was malfunctioning. When opened up, it was found to be having lot of carryover oil from the air system. I was informed by the EL/O that this was a common feature on the ship. But I decided to investigate the problem.
The vessel is fitted with three Sperre main air compressors with average running time of 12hours per day. The compressors are one of the latest series from Sperre, which have no cylinder liner lubrication. The LP rings are self lubricating cast iron composite types. HP rings are normal rings lubricated from the crankcase. Further, the c/case breather system is a closed one, with a pipe connecting the c/case to the suction side of the LP side, just after the silencer. The oil filling cap is a closed one, just for filling oil.
All three compressors were checked and found having slight traces of oil carryover, though in MAC#2, the oil carryover seemed excessive. These compressors are tandem piston types with LP valve assembly fitted on the cylinder head and the HP valve assembly by the side of the liner, at about 1/3 distance from the top.
Suspected that oil scraper ring must me leaking or worn out. Running hours only 1800. Recommended overhaul time 7500Hrs. The pumping rate was calculated and found to be poor too. Decided to withdraw and inspect the piston. On withdrawal found LP piston and all the rings of the HP stage okay, even the oil scraper ring. Since the vessel was still new with only one set of rings supplied by makers, only the broken rings were renewed and compressor boxed back. The reasons for such early breakage of rings was not yet clear.
Tried out and found pumping rate normal. However, oil carryover though reduced, was persisting. The c/case breather pipe to the LP side was removed at the filter end and compressor run. Found traces of oil coming out thru this pipe from the crankcase. The crankcase also seemed to be pressurized. The reason for pressurizing of the crankcase was not understood as the rings were all new/ in good condition.
After many trials & errors we landed upon the real culprit, i.e. the attached L.O pump. The L.O. pressure seemed lesser than prescribed by the makers, and so this pump was opened up. Found the oil lip seal ring in damaged condition. Lost all its properties, brittle and breaking. This now explained lot of things.
The fitting of the attached L.O. pump is such that if the oil seal leaks, the pressurized oil built up by the pump sprays inside the crankcase and this spray falls upon the breather pipe connected to the LP suction side. This causes the spraying oil to be sucked up by the LP suction side causing oil carryover.
The problem gets compounded if the suction filter is even partially dirty. More c/case oil gets sucked in. Our feeling was that this oil carryover causes the piston rings to get jammed and subsequently broken. The L.O. attached pump oil seal was renewed. MAC#2 air suction filter was dirty.
Since the ship had a history of oil traces and carryovers in the various pneumatics, decided to check the other two compressors also for the oil carry over and found the other two also having oil being sucked in from the breather pipe. The attached L.O pumps were opened up and found both lip seals damaged.
Thus, all three compressors, with just about 2000 running hours had oil seal failures. It was also felt that the other two compressor piston rings should be inspected. Both opened up subsequently after sufficient spares indented and received. Found these two compressors also LP first and second rings broken. Renewed.
In less than 2000 hours running, all three compressor LP rings and L.O. oil seal rings had failed.
The matter was taken up with the office and Sperre and though the guarantee period was over Sperre sent over their engineer who was shown all the evidence. Sperre agreed that there was a problem. It was also felt by the ship staff that the grade of Lube oil used (FAMM) was not suiting the lip seal material. Sperre agreed with ship's conclusion.
New seal rings of different material were sent and the three seal rings changed during the L.O. pump overhaul renewed. When examined, these removed rings found already getting brittle, thus proving ship's contention.
Sperre later sent five complete sets of piston rings and pistons for the compressors as guarantee claim compensation.