Varnish formation in lubricating oil and hydraulic systems has been present for many years in the power plant industry. Historically, varnish formation has been attributed to a singular root cause. For example, there was a #2 bearing drain line of a gas turbine was touching the inside of the exhaust strut, which caused thermal degradation of the oil and varnish formation.
Varnish can be reddish brown to black in appearance, depending on the mechanism that caused the oil molecule to break and varnish to form.Recent studies have revealed that oil varnishing is usually the result of a complex string of events. To start this chain of events, oil molecules must be broken. The mechanisms that break oil molecules fall into these general categories: chemical, mechanical, and thermal.
Chemical: Many chemical reactions occur as the oil ages. Oxidation of the oil leads to numerous decomposition products, including acids and insoluble particulates. Heat and the presence of metal particulates such as iron or copper accelerate the process. Additionally, highly aerated oils are far more susceptible to oxidation. Ensure that oils are compatible before adding or mixing them, as different oil additives may react adversely, further degrading the oil.
Mechanical: “Shearing” occurs when oil molecules are torn apart as they pass between moving mechanical surfaces.
Thermal: When air bubbles become entrained in the oil, severe failure of the oil may occur due to conditions known as Pressure-Induced Dieseling (PID) or Pressureinduced Thermal Degradation (PTG). These phenomenons are enabled in areas of high pressure within the hydraulic systems. Pressure Induced Dieseling, also known as micro-dieseling, occurs when air bubbles are collapsed under high pressure. This yields localized temperatures in excess of 1000 deg F (538 deg C), which in turn leads to thermal degradation and oxidation.
Methods for Detecting Varnish
An oil condition-monitoring program should be part of normal maintenance including a combination of inspections and oil analysis screening tests. Inspections include viewing sight glasses for varnish and fouling, examining used filters for end-cap varnish and sludge, inspection of servo inlet ports and lastchance filters, and periodic inspection of tank bottom sediment.
While there is no direct way to measure (quantify) varnish formation on servo valve surfaces, the active use of screening tests may provide an effective early warning. The patch colorimetric test can be used to trend the varnish potential of oil. Lower numbers indicate a lower risk of varnish formation. For general reference, a varnish potential rating between 0 and 40 would be considered acceptable. The range 41-60 would be a reportable condition, indicating the need to
monitor the oil more frequently. Readings above 60 are considered actionable and should trigger a work plan to quickly remediate the condition. Monitoring of the sub micron particles in the oil along with the results from patch colorimetric testing can help in determining the effectiveness of removal of varnish particles. The test used to measure the sub micron particles is ASTM F 312-97 (Standard Test Method for Microscopical Sizing and Counting Particles from Aerospace Fluids on Membrane Filters) It is recommended that both of these tests be used to monitor the performance of oil conditioning equipment.
Mitigation and Prevention
Customers currently utilizing electrostatic oil purifier, or Balanced charge oil purifier and Varnish removal unit, have reported very good results in reducing the varnish potential of their oil. These results show that trips caused by sticking servo valves have been drastically reduced or eliminated. Unlike conventional mechanical filters, these technologies induce electrical charges on suspended particles (oxides, carbon fines, etc.) that facilitate their transfer out of the oil, either by filtration or simply by electrostatic precipitation onto a collection device. It should be noted that an initial downward trend is realized during the clean up phase followed by and
upward trend as varnish that had been plated out on the system surfaces becomes reabsorbed into the oil. Over time, this varnish bloom will drop back down to desirable levels as the reclamation unit remains in service, leaving the oil system’s surfaces and turbine oil clean. This technology can be used either to mitigate a current varnishing issue or to prevent the occurrence of it.
Varnish formation in lubricating oil and hydraulic systems has been present for many years in the power plant industry. Historically, varnish formation has been attributed to a singular root cause. For example, there was a #2 bearing drain line of a gas turbine was touching the inside of the exhaust strut, which caused thermal degradation of the oil and varnish formation. Varnish can be reddish brown to black in appearance, depending on the mechanism that caused the oil molecule to break and varnish to form.
Recent studies have revealed that oil varnishing is usually the result of a complex string of events. To start this chain of events, oil molecules must be broken. The mechanisms that break oil molecules fall into these general categories: chemical, mechanical, and thermal.
Chemical: Many chemical reactions occur as the oil ages. Oxidation of the oil leads to numerous decomposition products, including acids and insoluble particulates. Heat and the presence of metal particulates such as iron or copper accelerate the process. Additionally, highly aerated oils are far more susceptible to oxidation. Ensure that oils are compatible before adding or mixing them, as different oil additives may react adversely, further degrading the oil.
Mechanical: “Shearing” occurs when oil molecules are torn apart as they pass between moving mechanical surfaces.
Thermal: When air bubbles become entrained in the oil, severe failure of the oil may occur due to conditions known as Pressure-Induced Dieseling (PID) or Pressureinduced Thermal Degradation (PTG). These phenomenons are enabled in areas of high pressure within the hydraulic systems. Pressure Induced Dieseling, also known as micro-dieseling, occurs when air bubbles are collapsed under high pressure. This yields localized temperatures in excess of 1000 deg F (538 deg C), which in turn leads to thermal degradation and oxidation.
Methods for Detecting Varnish
An oil condition-monitoring program should be part of normal maintenance including a combination of inspections and oil analysis screening tests. Inspections include viewing sight glasses for varnish and fouling, examining used filters for end-cap varnish and sludge, inspection of servo inlet ports and lastchance filters, and periodic inspection of tank bottom sediment.
While there is no direct way to measure (quantify) varnish formation on servo valve surfaces, the active use of screening tests may provide an effective early warning. The patch colorimetric test can be used to trend the varnish potential of oil. Lower numbers indicate a lower risk of varnish formation. For general reference, a varnish potential rating between 0 and 40 would be considered acceptable. The range 41-60 would be a reportable condition, indicating the need to monitor the oil more frequently. Readings above 60 are considered actionable and should trigger a work plan to quickly remediate the condition. Monitoring of the sub micron particles in the oil along with the results from patch colorimetric testing can help in determining the effectiveness of removal of varnish particles. The test used to measure the sub micron particles is ASTM F 312-97 (Standard Test Method for Microscopical Sizing and Counting Particles from Aerospace Fluids on Membrane Filters) It is recommended that both of these tests be used to monitor the performance of oil conditioning equipment.
Mitigation and Prevention
Customers currently utilizing electrostatic oil purifier, or Balanced charge oil purifier and Varnish removal unit, have reported very good results in reducing the varnish potential of their oil. These results show that trips caused by sticking servo valves have been drastically reduced or eliminated. Unlike conventional mechanical filters, these technologies induce electrical charges on suspended particles (oxides, carbon fines, etc.) that facilitate their transfer out of the oil, either by filtration or simply by electrostatic precipitation onto a collection device. It should be noted that an initial downward trend is realized during the clean up phase followed by and
upward trend as varnish that had been plated out on the system surfaces becomes reabsorbed into the oil. Over time, this varnish bloom will drop back down to desirable levels as the reclamation unit remains in service, leaving the oil system’s surfaces and turbine oil clean. This technology can be used either to mitigate a current varnishing issue or to prevent the occurrence of it.
RECOMMENDATIONS
Failure to eliminate all possible causes may result in a repeat occurrence. Fleet information has shown that electrostatic absorption filtration technology and resin technology have been successful in mitigating, as well as preventing, the effects of varnishing. These systems are typically set up as a side-stream configuration to the existing lube oil system. They can operate continuously while the turbine is online or off-line. For those customers who have not experienced trips associated with varnish formation, it is recommended that varnish removal unit be used as a preventive measure. The formation of varnish is partly depend on the oil’s age, and it is believed that all customers may experience this issue over time. Please note that the systems referenced are considered a mitigation strategy that addresses the symptoms of oil degradation and not the root cause. There are ongoing studies with oil manufacturers aimed at developing methods of prevention of oil varnishing
RECOMMENDATIONS
Failure to eliminate all possible causes may result in a repeat occurrence. Fleet information has shown that electrostatic absorption filtration technology and resin technology have been successful in mitigating, as well as preventing, the effects of varnishing. These systems are typically set up as a side-stream configuration to the existing lube oil system. They can operate continuously while the turbine is online or off-line. For those customers who have not experienced trips associated with varnish formation, it is recommended that varnish removal unit be used as a preventive measure. The formation of varnish is partly depend on the oil’s age, and it is believed that all customers may experience this issue over time. Please note that the systems referenced are considered a mitigation strategy that addresses the symptoms of oil degradation and not the root cause. There are ongoing studies with oil manufacturers aimed at developing methods of prevention of oil varnishing.
Post time: Jul-14-2022