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"I have a problem with high water content in steam turbine lube oil. Is it applicable to add anti-rust additives in the lube oil tank or do I need to drain all the oil and put in new oil with anti-rust additives?"
It is never a good idea to add additives yourself. This should be left to a lubricant supplier working with a product engineer to decide how much additive is remaining and how much should be added. Oil formulation requires a precise amount of base oil and a certain percentage of additives to protect your equipment.
Without eliminating the source of water contamination, the additives will do very little for you. You might receive a quick fix, but the problem will need to be addressed again. By adding rust and oxidation (R&O) inhibitor additives without solving your ingestion problem, the new additives will be used up and rendered useless.
Your first step should be to prevent the water from getting into the reservoir. Find out how the water is entering the sump. Next, make an action plan to eliminate any possible water entry. This includes any open ports or hatches that might take extra measures to seal properly. If water is entering because of faulty seals, these should be replaced as soon as possible. Water will wreak havoc on any equipment it enters. It will also change the viscosity of the oil, resulting in a film strength failure. With steam turbines that operate at high speeds and temperatures, the situation must be monitored very closely.
If your water ingestion problem can be solved, then you should bring your lubricant supplier onsite to start the testing and evaluation process. To make sure all traces of water are removed from the sump and the equipment, you will need some sort of water-removal tool. For lowly additized turbine oil, gravity separation may be the best option to remove the most water. With this method, the oil will need to stand for a period of time to allow all the water to settle to the bottom. Another solution would be to run the oil through a vacuum dehydrator. Oil analysis should be performed to determine how much water is remaining.
You might also consider replenishing your additive package, especially with larger oil volumes. For smaller sumps, it is often cost-effective to drain and fill, and then involve your lubricant supplier for re-additizing your oil.
Finally, be sure to use high-quality desiccant breathers to prevent water ingestion. Adding sight glasses and bottom sediment and water (BS&W) bowls can also aid in the early detection of water in your oil.