"What is the allowable water content for turbine oil, and what standard governs this issue?"
There is no established standard governing allowable water content for turbine oil. However, it is briefly mentioned in ASTM D4378-08 (Standard Practice for In-Service Monitoring of Mineral Turbine Oils for Steam and Gas Turbines). This standard states that 0.1 percent or 1,000 parts per million (ppm) of water is considered a significant volume and should be condemned.
Indeed, 0.1 percent or 1,000 ppm is a large amount of water for a turbine system. Most turbine manufacturers would say this is unacceptable. Turbines may continue to run at these elevated moisture levels, but damage to the lubricant and machine will take place.
In oil, water exists in three forms: dissolved, free and emulsified. The dissolved state occurs naturally and is not noticeable to the naked eye. Once the oil reaches its saturation point, the dissolved state of the water will become free or emulsified. Some free water will fall to the bottom of the sump due to gravity, while other water will be introduced to internal components. Emulsified water will appear cloudy or white and will impair film strength.
Some turbine manufacturers have defined 500 ppm or 0.05 percent as warning limits. A good strategy is to establish cautionary and critical moisture limits using the Karl Fischer test method. This method doesn't distinguish between the three states of water.
Machine criticality will also need to be evaluated. When reviewing criticality, consider the safety risks upon failure, the cost and length of downtime, the material and labor costs to repair, and the early warning systems. The higher the liability, the lower the target dryness should be set.
A better approach to selecting moisture limits based on machine criticality would be to employ the tactic of as low as reasonably achievable (ALARA). This takes the logic of measuring the moisture content a step further to exclude, remove or monitor water contamination.
Simpler devices like desiccant breathers do an excellent job of preventing humid air from entering the reservoir. Bottom sediment and water bowls can show free water that has settled to the bottom of the reservoir. If your budget allows or if your needs are critical, purchasing a vacuum dehydrator may even be warranted.
The best option is to keep the moisture content below the saturation point of the in-service temperature. The higher the operating temperature, the lower the oil's saturation point. Keep in mind that the fluid's age and the contaminants in the system will also have an impact on the saturation point.