Cross Relief Valve (CRV Plate) A New Advance in the War on Varnish

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1 Cross Relief Valve (CRV Plate) A New Advance in the War on Varnish By: Cary Forgeron, CLS Analysts, Inc. Since 2004, many articles have been published on the subject of varnish and related problems in gas turbine operations. These articles have outlined what causes varnish, tests used to measure varnish levels, and available technologies to solve the problem. However, here we are five years later and still fighting the war on varnish. Varnish What we know now Simply defined, varnish is a thin insoluble film that is usually formed on bearings and servo-valves. It is a high molecular weight substance that is insoluble in lubricating oils. Oil insolubles are made up of over 75% of soft contaminants that are less than 1 micron in size. Insoluble compounds have polar affinities and begin to migrate from the body of the lubricant to machine surfaces over time and based on conditions of the system and lubricating oil. The question then becomes, what are these conditions? The million dollar answer is Lubricant Temperature & Flow. In a presentation at the 2008 STLE 1 Annual Meeting, ExxonMobil presented a paper detailing the effects of lube temperature on varnish solubility 2. ExxonMobil s research identified that varnish becomes resolubilized by increasing the oil temperature and conversely becomes insoluble when temperature is decreased. Additional research conducted in the Netherlands by Ansaldo Thomassen 3, identified that while temperature was a major factor in the plating out of varnish on critical system components, insufficient lube oil flow rate is an equally significant aspect. A study conducted by Ansaldo Thomassen demonstrated that when oil was stagnant (even at normal operating temperatures); conditions existed for varnish to agglomerate and plate out onto system components and surfaces. Example: If a lubricating oil is at operating temperature and has been circulating through the system at the time of sampling, a QSA 4 varnish test performed on-site will report a low varnish potential rating 1 Society of Tribologist and Lubrication Engineers 2 STLE Annual Meeting 2008, ExxonMobil - James Hannon Sensitivities in Turbine Oil Varnish Prediction Sensitivities Prediction 3 Global Service Engineering 2009, AnsaldoThomassen Hans Overgaag Up to Date Turbine Oil System Management 4 QSA is a registered trademark of Analysts, Inc.

2 (VPR). If the same sample is submitted to a laboratory for QSA testing, where it is tested at ambient temperature (+/- 70 o F) and has been in a static state for 72+ hours, the results could be at alert levels. The differences in the results are due to the temperature and flow characteristics. Laboratory testing is important in that it clearly identifies the systems that have high varnish potential and require intervention and corrective action practices such as filtration, bleed & feed or total fluid replacement. References to terms such as auto-degradation or thermokinetics, imply that a change to the chemistry of the fluid takes place. This is not correct. Onsite testing reflects the solubility of varnish particles in operating conditions and QSA testing in the laboratory environment identifies the actual varnish potential. Addressing the Symptoms The major affects of varnish in gas turbine systems has routinely been found in the hydraulic control systems. Inspections of these systems have found plugged last chance filters and pencil filters and lacquered servo valve components. The net results of this were costly unit trips (inability to start the unit) and system flushes. Focusing on the components most affected by varnish and the conditions that lead to varnish formation, AnsaldoThomassen developed the Cross Relief Valve (commonly referred to as the CRV Plate). The operating principle of the CRV Plate is based on the creation of a controllable oil flow through the servo valve independent from control system settings and commands but without interfering with the principle control functions.

3 IGV Servo Servo-Valve CRV Plate Figure 1 - CRV Plate installed on IGV of GE Frame 7FA Turbine Since the nominal flow capacity of the servo valves is rarely attained, most of the time the servo will only face a steady state or static condition with little to no oil flow. Once installed, the CRV Plate generates an oil flow through the servo servo-valve up to the actuator. This oil flow results in normal lube oil operating temperature in the normally stagnant parts of the electro hydraulic control system. This innovative solution ensures that the control system is continually exposed to the proper conditions necessary to prevent the formation of varnish on critical system components. The CRV Plate does not alter the working and operation of the servo valve. The control system does not have to be modified physically. However, in n some cases minor modifications might be required on the servo valve conduit. The pictures below are of a last chance filter for a gas splitter valve on a Frame 6 turbine. Figure 2 shows varnish material covering both the inside and outside of the filter. The operating time on the oil is approximately 4, hours, and QSA testing confirmed a VPR of 68 (an alert level). The valve and an filter were replaced, and a CRV Plate was installed (the lube oil was not changed out).. Figure 3 shows the filter after approximately 3,000,000 hours of operation. Note that there is no agglomeration of varnish on the filter media, and that the QSA test has indicated an increase in VPR of the oil. Since installation of the CRV Plate there have been no operating issues relating to varnish formation in the corresponding servo valve.

4 Figure 2 - Fuel Splitter Valve Filter (Prior to Installation of CRV Plate). Figure 3 - Fuel Splitter Valve Filter (After Installation of CRV Plate).

5 Conclusion Continuing the Fight The CRV Plate is currently designed for and applied on MOOG type G771 and G772 servo valves as used on GE type heavy duty gas turbines. To date, the CRV Plate has operated over 50,000 hours at five sites successfully and without problems. After extensive field testing over the past three years, no sludge or varnish build-up in servo-valves or last chance filters was observed in units where the CRV Plate is installed. It is important to note, the CRV Plate does not remove varnish from the system or reduce QSA varnish potential ratings (VPR) levels. However, the CRV is an excellent tool to be used as part of a comprehensive strategy to control turbine lube oils and improve unit reliability. To date, there is still no silver bullet fix for solving the varnish issue. The best strategy to controlling varnish is a multifaceted approach that incorporates the CRV Plate, fluid condition monitoring, and appropriate mitigation technologies (filtration, top-offs, etc.). The CRV Plate provides a major advancement in the fight on varnish. It is designed to immediately mitigate the effects varnish has within the hydraulic control system of gas turbines. Implementing the CRV Plate solution allows you the time needed to properly evaluate the condition of your lube oil (without risking an expensive unit trip) and make the right decision on how best to address varnish in your system.