New Tests Needed for Gas Turbine Oils?
Gas turbines represent the most severe operating conditions for turbine oils. They operate at much higher speeds and temperature than water and steam turbines. As a result, oil service life is one-half to one-third of that in the other two applications.
According to Ameneh Schneider of Lukoil Lubricants Austria, sludge and varnish have become the most serious oil-related problems for gas turbines. These deposits can arise even in so-called high-performance oils using high-quality synthetic base stocks.
Sludge and varnish in servovalves has caused valve sticking, said Schneider. In other cases, varnish and deposits have caused reduced oil flow, increased wear and potentially a change from hydrodynamic to boundary lubrication. Other problems cited by Schneider in a presentation at the OilDoc Conference in January include reduced cooler performance, increased bulk oil temperature, prematurely plugged filters and strainers, and plugging of small oil orifices.
She explained that formulating gas turbine oil with high stability and low or no tendency to form sludge and varnish requires careful selection of both base oil and additives. In general, the base stock defines a lubricants thermo-oxidation stability and aging characteristics. Therefore, formulations based on nonconventional base stocks provide higher oxidation and thermo-oxidation stability than API Group I oils, along with lower varnish potential because of the absence of aromatics and excellent additive response, Schneider said. However, these oils also have lower solvency, which results earlier sludge formation and additive dropout.
The main bench tests used to evaluate turbine oil thermo-oxidation stability are the Rotating Pressure Vessel Oxidation Test (RPVOT) and the Turbine Oil Stability Test (TOST). The problem with these tests, said Schneider, is that they dont reflect real-life operating conditions for gas turbine oils. As a result, the high antioxidant levels commonly used to generate higher TOST and RPVOT values can also induce higher sludge formation in service.
Lukoil testing concluded that the critical properties of gas turbine oils cannot be simulated with existing bench tests. To solve this problem, the company is investigating three methods to evaluate the sludge and varnish formation tendency of gas turbine oils:
Modified MAN Turbo rapid screening test for thermal stability, solvency and antioxidant dropout.
Modified Baader aging test (DIN 51554) to evaluate sludge formation potential in the presence of a copper coil.
High-temperature oxidation stability test, using CEC-L-48-A-00 method B apparatus.
The parameters of Lukoils proposed tests are summarized in the table.