Group II oils (hydroprocessed) have replaced Group I oils (solvent refined) as the primary base stock used for lubricants. Group II oils have lower sulfur and aromatic content than their Group I predecessors, which makes them more stable to oxidative degradation. However, Group II oils have a lower solvency than the Group I base oils. Degradation by-products tend to precipitate more easily. This condition leads to varnish formation.
Varnish formation is the primary cause of valve sticking in the hydraulic circuits of gas turbines. This issue is especially critical for peaking and cycling plants, where an unexpected trip can be extremely costly.
Traditional oil analysis methods did not provide a methodology to monitor the varnish potential until recently. Methods such as QSA® and ultracentrifuge now allow turbine users to monitor the tendency of the turbine oil to form varnish deposits.
A number of varnish mitigation technologies have emerged. These include various varnish removal types of systems including electrostatic filtration, balance charged agglomeration, ion exchange resins and cellulose depth filtration. Another approach is the use of a CRV plate1 that provides a small, but continuous, flow of oil around the valves to prevent the varnish from depositing.
One possible approach to reducing the varnish tendency of a Group II base oil would be to increase the solvency by the addition of another fluid.
Oil soluble polyalkene glycols (OSP™) provide a viable option to increase the solvency of mineral oils. Many people are probably familiar with polyalkene glycol (PAG) fluids as being water soluble. However, by increasing the ration of carbon to oxygen, oil soluble polyalkene glycols can be produced. OSP fluids have a number of desirable characteristics. OSP's have been shown to reduce friction and to eliminate deposits when added to Group I base oils. The deposit control was performed with a modified version of ASTM D2893 1.
Would addition of OSP to used Group II base oil reduce the Varnish Potential Rating (VPR) of the fluids? Used turbine fluid with varying levels of VPR as measured by QSA were selected. QSA measurement were performed on the used fluid neat, with 10% OSP and 20% OSP added. The samples were allowed to sit for 24 hours after addition of the OSP.
The addition of 10% OSP produced a measurable reduction in the VPR of the samples. Increasing the addition of 20% OSP to the turbine fluids reduces the VPR significantly. The filter membranes are shown in Figure 1, and the VPR's can be seen in Figure 2.
Addition of OSP's to mineral oils may provide a method to reduce varnish deposits, and subsequently to avoid unexpected trips, in turbine oils. Further testing and field trials will be needed to provide additional data to evaluate the potential of OSP in the power generation market.
Notes: QSA is a registered trademark of Analysts, Inc. OSP is a registered trademark of Dow Chemical.
Gene Wagenseller, is a Technician at Analysts, Inc. Govind Khemchandani works in Technical Services with The Dow Chemical Company. You can find his contact information in our member database.
Other articles from this issue: