A power plant in Pakistan recently experienced an abrupt drop in the base number of its diesel engine oil. This decrease was discovered during routine oil analysis while the engine was on stand-by. When the sump was inspected through the side doors, water was observed in the oil.
Onsite oil analysis revealed the water content was higher than 0.5 percent. According to the engine manufacturer, the maximum allowable water content was 0.3 percent. Thus, the oil was severely contaminated, and the base number had fallen to a very low level.
The oil was then drained, since it was not fit for further use, as the presence of water had depleted the additives. Plant personnel searched for the source of the water, checking for water leaking from the cylinder heads, the oil cooler and separator heater, as well as oil drum contamination from external water, rain, etc.
The plant contacted the engine manufacturer, who inspected the engine but not the separator heater, since it had been checked during recent maintenance. After the oil was drained, the maintenance team cleaned the sump. They observed the cylinder heads through the side doors of the crankcase, but no water leaks were found. The oil cooler was also inspected but didn’t provide any clues of water mixing in the oil.
When the entire oil circuit was examined, no areas of concern were identified. Plant personnel were satisfied and confident there would be no more water mixing in the oil, so they added fresh oil to the sump.
The oil separator was operated for one day with the fresh oil. The engine wasn’t run, as the team wasn’t sure if water would again be mixed with the oil. When a sample was taken from the sump, the water content of the fresh oil was found to be 0.5 percent.
A sample was also collected after the separator, which revealed water content of 0.55 percent. This indicated that either the separator was working at a low efficiency or water was actually mixing in the separator. However, if the separator’s efficiency was low, there should not be much water present in the oil.
The maintenance team immediately dismantled the pre-heater in the separator to see if steam was mixing with the oil. A pre-heater is used in the separator to heat the lubricant for efficient cleaning. This particular pre-heater incorporates a brazed plate heat exchanger, with the oil heated by steam. It was suspected that the steam was mixing with the oil in the heat exchanger.
After the heater was dismantled, water was added to the steam line. The working pressure in the steam line was 7 bar but only 1 bar in the oil line. The test pressure was 50 bar. When pressure was gradually increased in the steam line, water began coming out of the oil line soon after 3 bar. This revealed that the plates between the oil and steam lines were damaged.
The pre-heater was then replaced by the maintenance team. After operating the separator for two days with the new pre-heater, test results showed only 0.1 percent water content.
An immediate investigation of the water ingression in the oil system prevented damage to the engine’s internal parts. The plant would have lost thousands of dollars if it had not promptly investigated the source of the water mixing in the oil.
