Specifying varnish-resistance in new oils

The ASME Gas Turbine India show in Mumbai showcased a presentation on Mobil DTE 932 GT by James Hannon, industrial lubricants technical advisor for ExxonMobil, in which the company’s new low-varnish turbine oil solution came to the fore.

(Varnish formation on a spool valve and plunger from a hydraulic directional control valve)

The oil has been designed specifically for GE Frame 6, 7, & 9 applications where varnish control in the hydraulic system is most needed. Varnish is most disruptive in hydraulic oils used in control circuits but they also have an impact in the lube oil if bearings clearances are low. If the gas turbine has a combined lube oil hydraulic oil circuit then the varnish produced due to gas turbine operating conditions and static discharge in filters could move to the hydraulic circuit leading to turbine reliability problems.

Turbine reliability problems are created by varnish in hydraulic oil affecting the operation of control valves. A phenomenon called valve sticking in which the valve fails to respond accurately to the control signals is a prime result of varnish. In 2007, ExxonMobil Lubricants & Specialties and COT-Puritech conducted a survey to determine the extent of varnish issues observed within the industry. About 40 percent of the 626 gas turbines (Frame 7) reported varnish problems. All lubricant brands seemed to suffer from varnish.

Hannon argued in his presentation that a new oil’s varnish resistance can be best measured by valve performance besides system cleanliness and so on. He said that the new oil has been rig tested after 670 hours of use to demonstrate effective valve performance.