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THE PRESENCE OF WATER, RUST AND CARBON IN OIL TANKS CAN CAUSE DAMAGE AND FRICTION IN BEARINGS, TILTING PADS AND JOURNAL PARTS
Fouling of the main oil tanks can cause serious problems in high-speed machinery as these impurities can flow inside the bearings. This can cause rupture and friction within internal bearing parts such as the tilting pads or the journal parts of the shaft, as well as damaging the internal components of the lube oil pump. Oil filters are mainly used to remove these impurities and particulate.
One indication of the presence of impurities is a rise in pressure difference on the filter, which indicates that a large amount of particulate can be found on it. However, this is not the main reason for a rise in the pressure difference; increasing water content in oil may also cause this phenomenon.
The source of water content is mainly condensate leakage from steam sealing to the end shaft labyrinth and from there to the oil retaining ring of the bearing. Steam flows inside bearings and then is returned to the tank with returned oil.
But as the density of water is greater than oil, water stands at the bottom of the tank at the end of slope (the tank bottom contains a slope but the suction pumps are situated on the opposite side at the upper region of the slope) and has to be removed from the tank by means of a centrifugal pump.
Prolonged high-temperature operation can cause carbon and rust to precipitate on the inner walls of the tank. Additionally, above the level of oil inside the tank, oil mist vapors can form rust and corrosion due to water content within the vapor. Such vapors, depending upon the design of the tank, may be removed by means of an oil mist fan to keep the tank pressure under vacuum in order to maintain oil flow speed and cool the oil.
Alternatively, these vapors can be released via a vent as can be done with an ambient pressure tank design. An oil mist fan also makes it possible to reduce the size of oil tanks in cases where limited space is available.
When the oil level rises suddenly, this causes rust to flake off into the oil. It then flows into the oil filters. The filter cartridge traps particulate. However, when it is saturated with impurities, the pressure difference on the filter cartridge increases to the point where the oil pressure decreases. Consequently, an alarm set point should be provided to ensure system safety.
The rate of change for oil filters in severely rusted environments will be high. Filters should be changed every week until all rust impurities are cleaned. In this scenario, it is not wise to add new oil as this may further raise the level of the oil within the tank and create even more rust issues. Some oil can be added, but no more than the amount of heavily contaminated oil that is removed.
There are many reasons for a change in oil level. Adding too much oil is the obvious cause. Therefore, care should be taken to maintain oil at a normal level. The overall level decreases over time, of course, due to leakage from gaskets, seals and flanges. Further, changes in oil pressure must be watched carefully as this may cause a shift in the oil level.
The source of changing oil pressure can be found in any of several areas:
• Shifting operation between oil coolers or a malfunction in the three-way valves of these coolers can cause a drop in header pressure. Therefore, check that the oil cooler is filled and pressurized, as well as the cooler vent to the tank
• Shifting operation between two oil pumps
• Pump suction strainer contaminated by water or impurities; or the pump suction or discharge valve is partially open
• Reduced discharge pressure of the lube oil pump or increased oil temperature
• A defect in the pressure control valve or internal leakage due to tearing of the diaphragm
• Malfunction of accumulators within the lube oil circuit
• Blockage of the flow orifice can decrease oil passage leading to a decrease in oil pressure
• Shifting in operation between two oil filters as the filter casing may not be filled or pressurized.
Another problem can be generated by a tear in the oil filter cartridge which may then allow the passage of impurities which can enter bearings and gearboxes. This can lead to friction and damage of internal parts. It is wise to monitor the oil filter pressure difference. If no change occurs over a long period of time, an inspection should be done to look for any tears.
There is an inverse relationship between oil temperature and oil filter pressure difference. If no change occurs even after an instrumentation check, a cartridge inspection should be conducted.
Amr Hatem Rashed is Turbomachinery Manager at Abu-Qir Fertilizers Company in Alexandria, Egypt. He has a Masters Degree in Mechanical Engineering.