Monitoring Turbine Oils in Power Plants
Steam, Gas, Hydro, and Aero Derivative

A turbine oil is designed to provide effective lubrication to all parts of a stationary turbine. A typical turbine oil consists of a highly refined paraffinic mineral oil which has been formulated with one or more of the following additives:

a. oxidation inhibitors
b. rust inhibitors
c. metal deactivators
d. pour point depressants
e. extreme pressure additives
f. foam suppressants

The use of a high quality oil is the best assurance for long service life and the recommendations of the turbine manufacturer should be followed. A turbine oil should be selected based on typical test data from the oil supplier. However, it is always advisable to take an oil sample from the first fill in order to develop baseline data that can serve as an index for future comparisons. A one-gallon retain sample is recommended.

The sum capacity for a turbine oil system can range from 1000 to 20,000 gallons. These oils are not exposed to fuel oil or soot, therefore they do not require frequent draining and replacement. However, the effective life span of a turbine oil depends on a good maintenance program.

In-service monitoring of a turbine oil ensures long, trouble-free operation of a power plant and it is the responsibility of the power plant operator to have a program of sampling and testing that will guard against the onset of problems associated with oil degradation and contamination.

There are many operational factors that affect the service life of a turbine oil. These factors determine the severity level for a given unit and how the lubricant will deteriorate with time. The more severe a turbine system, the shorter the service life for a given oil.

Steam and hydro turbines operate at a lower level of severity, therefore longer service life can be expected. Well-maintained steam turbine oil, with moderate makeup rates, should last between twenty and thirty years.

Gas turbines and aero derivative systems operate at higher levels of severity, therefore these turbine oils require more frequent monitoring and testing.

Turbine oils are formulated to provide low-temperature fluidity as well as excellent resistance to thermal degradation. In addition, the lubricant must provide excellent wear properties, protection against rust and corrosion, good air release characteristics, and resistance to foaming. In order to assure that theses characteristics are being maintained for the in-service oil, a rigid sampling and testing program is recommended.

Frequency of samples:

The schedule for monitoring turbine oils depends upon conditions and severity of the turbines. In-service testing for oils in gas turbines operating under normal conditions are:

Property	Interval
Appearance	100 hours
Color	200 hours
Viscosity	500 hours
Acid Number	500 to 1000 hours
Cleanliness	1000 hours
RPVOT	1500 to 2000 hours

In-coming supplies of make-up turbine oil should also be monitored.

How to sample:

a. Preferred sampling:

- oil samples should be taken by dipping from the tank
- the lubricant should be circulated before taking a sample
- make simple comparisons to assure that the sample is representative of the bulk
- look at color, clarity, visible particulates.

b. Sampling from a line:

- line should contain lubricating oil which is free-flowing.
- avoid stagnant or dead-headed samples.
- inactive filters or heat exchanger sites are not suitable.

c. Sampling from a reservoir:

- lubricating oil must be thoroughly agitated.
- tap line must be flushed before sampling.

d. Special samples:

- samples may be requested that are not representative of the bulk oil.
- common example is top, middle, and bottom of a storage tank to check for contaminants.
- identify the sample point in these cases

Sample containers:

- Ultra clean bottles are required for particle count.
- Otherwise, flush the container with the oil to be sampled.

Disposal of spent turbine oils:

Lubricating oils that have reached the end of their useful life should be handled so as to protect the environment. Consult your oil supplier for options available in your area. These include:

- collect and re-refine as a base oil
- feed stock to catalytic cracker
- fuel for certain types of industrial boilers.

References:

ASTM D 4378 is the standard practice for in-service monitoring of turbine oils. It should be consulted for background information on how turbine oils perform and how they degrade in service. In addition, the individual turbine manufacturers issue specifications and recommended testing intervals for their lubricant products. The following test package is based on ASTM D 4378.