"Can you explain how to find the source of water contamination in a steam turbine boiler feed pump? We have problems with high water contamination in the pump’s oil tank. Oil analysis tests showed water at approximately 15,000 parts per million (ppm) in less than a month after changing the potential water leak area (gland seal, cooler oil seal and seal to the pump)."
Water contamination in the oil of a boiler feed pump for a steam turbine can come from a number of different sources. With water being part of the process, it can easily be condensed from the steam in the surrounding environment. The lube oil system is forced to interact in close proximity with these water sources, so moisture contamination is a common concern.
Proactive maintenance practices with oil analysis as the primary indicator should be used to determine if and how much water is present in the oil. Water concentrations of 15,000 ppm are alarming. To help identify the source, the following questions should be asked:
It is assumed that the water was mostly removed from the oil after the potential leak area was discovered and corrected. If not, the water levels should not be expected to decrease after the potential ingression source was corrected. Several water-removal technologies are available, including vacuum dehydration and dialysis.
In a circulating oil system, there are many places where water can enter. For instance, the reservoir headspace typically breathes into the environment. If a proper breather arrangement isn’t implemented or there are other breathing locations, humid air will condense in the headspace and water will drain into the oil.
Pipe junctions and flanges can also allow ingression of water and other contaminants. Internal and external seals should be considered as well. These may include nearby steam joint leaks, leakage past seals and gland seams, heat exchangers, seal water zones, seals being washed down by high-pressure hoses, etc.
Above the water saturation point in oil, water does not naturally disperse into the oil homogenously. The longer the settling time, the more likely water is to fall to any bottom areas in the system. If oil is sampled from the bottom drains, you may receive a false positive. If oil is sampled from the top surface of the oil level, you may get a false negative.
If samples are only taken from the reservoir, there’s a chance the water contamination results will not represent the moisture levels in the system’s most crucial areas. Obtaining secondary samples from other locations in the system will provide clues as to where an ingression source might be.
The return-line elbow after the bearing is a good option for collecting a sample that better represents the oil after it has flowed through the bearing cavity. Taking samples at both the thrust bearings and sleeve bearings can distinguish if either bearing is ingesting water.
In addition, sampling before the bearing, such as at the lube oil supply line, will allow a comparison to the other samples after the bearing.
If the system is not circulating oil, then the sample options will be limited. In any case, it is best to avoid drain sampling or drop-tube sampling. Instead, use a pilot-tube minimess valve or minimess catch-pipe to obtain samples from a live zone.