August 15, 2007

Detecting and solving lube oil varnish problems

Greg J. Livingstone and Jon Prescott, EPT Inc. and Dave Wooton, Wooton Consulting

Solving the problem

There are three steps that gas turbine operators can take to minimize autodegradation.

Minimize spark discharge. Most gas turbine lube oil filters show some evidence of spark discharge damage (Figure 6). The damage is caused by the buildup of static electricity, both in the oil and on the filters, as high-velocity oil flows through the tight clearances in the filter media. The damage is usually visible only at the microscopic level.

6. Blame it on the heat. The cumulative damage to a last-chance filter core caused by multiple spark events. Courtesy: EPT Inc.

There are some proactive measures that plants can take to minimize spark discharge damage. On some turbines, it's very easy to switch to using both sides of duplex lube oil filters, which has the effect of cutting the velocity of the oil through them in half. On other units, doing so requires removing a mono-block diverter valve and installing a spool piece and butterfly valves.

Another possibility is to increase the micron ratings of hydraulic and last-chance filters to the maximum allowable by the turbine manufacturer. Along these lines, yet another option is to investigate alternative filtration media. Researchers are developing filtration media that are less prone to maintaining static charges and therefore less likely to produce spark events.

Some plants have removed certain last-chance filters that showed evidence of spark discharge. Before you follow suit, be sure to discuss with your turbine vendor the possible operational impact of doing so. The good news is that minimizing spark discharge will do more than eliminate a source of autodegradation. It also will extend the life of the antioxidants in your turbine oil.

Keep your lube oil warm and moving. Heat tracing and insulating hydraulic piping and valve manifolds can help minimize autodegradation and instances of sticking valves in this part of the lube oil system. When the oil cools, varnish can form where it would not if temperatures were higher. EPT has measured drops of 75 degrees F in the IGV piping of some systems while the turbine was on turning gear/ratcheting. It is advisable to maintain the temperature of the slow-flowing and static oil in the IGV, speed, fuel, and other servo systems as close to the reservoir temperature as possible. Plants that have heat-traced and insulated their IGV lines all the way to and from the IGV valve report dramatic reductions in servo failures and visible varnish. At these plants, oil analysis has shown a reduction in the rate of additive depletion as well.

Another possibility is to install a so-called crossport-relief valve plate, a product co-developed by the GE turbine aftermarket service provider Thomassen Turbine Systems (www.thomassenturbinesystems.com) and Moog Inc. (www.moog.com). This retrofit to Moog servo valves (available in Europe and soon available in North America) allows continual flow through key valve blocks. It has reportedly reduced the impact of autodegradation and varnish deposits. Yet another option is changing the software in the turbine's control system to allow for regular stroking of the unit's valves while it is on turning gear.

Reduce levels of soluble contaminants. Minimizing spark discharge and keeping turbine oil warm and moving target the root causes of autodegradation, but they don't completely eliminate its effects in current turbines. One way to do so is to lower the concentration of soluble contaminants in lube oil systems.

EPT uses its patent-pending ion-charge bonding (ICB) technology to remove soluble contaminants, like acids, from lubricants. Over two decades, ICB has been retrofitted to hundreds of gas turbines and electro-hydraulic control systems. The technology uses carefully chosen and treated ion-exchange resins to remove specific soluble products from lube oil. EPT's unique blend of resins and delivery methods can selectively remove soluble soft contaminants from turbine oil without disturbing other critical soluble components and additives. Returning to the bucket analogy (Figure 7), ICB treatment removes soluble contaminants from the oil and holds them in a filter, while electrostatic contamination control does the same for insolubles.

7. Double-barreled approach. Working together, two technologies—ion-charge bonding, and electrostatic contamination control—can remove both types of contaminants in lube oil that cause it to autodegrade. Source: EPT Inc.

Dozens of laboratory experiments have studied ICB technology in this application. They are now being correlated with several field trials on Frame 7FA gas turbines and other systems. The patches in Figure 8 represent some typical results. A sample of oil was drawn from the reservoir and split into two equal measures. One was immediately treated with ICB, and the other was left untreated. Both samples were then aged for 14 days under identical conditions and tested using MPC. The whiteness of the ICB-treated patch ("Hot ICB") indicates that all soluble contaminants in it were removed, stopping its autodegradation.

8. Treated vs. untreated. The positive impact of ICB treatment on autodegradation. Courtesy: EPT Inc.

EPT has incorporated ICB technology into its electrostatic contaminant removal system, the ECR 8000 (Figure 9). The system enables removal of both soluble and insoluble materials from turbine lube oil using the same equipment.

9. Varnish buster. EPT's electrostatic lube oil cleaning system for large gas turbines. Courtesy: EPT Inc.

—Greg J. Livingstone (glivingstone@cleanoil.com) and Jon Prescott (jprescott@cleanoil.com) are certified lubrication specialists and work for EPT Inc. Dave Wooton (davewooton@wooton-consulting.com) has a PhD in chemistry and operates Wooton Consulting.