Keep your steam turbines operating efficiently by knowing these facts about oils.
Luis Rojas Lopez, ExxonMobil Lubricants & Specialties
Steam turbines are widely used in the power industry as prime movers for generators. As a paramount component to a company’s production, these machines generally run on continuous operating schedules. Maintenance professionals are challenged with implementing tactics that enhance equipment performance given the turbine’s extreme operating conditions associated with lengthy periods of time in service, such as high temperatures, water contamination and lengthy periods of time in service.
Lubrication plays a vital role in supporting optimal steam turbine performance. Selecting an inadequate lubricant can have expensive consequences, including unexpected shutdowns and high labor costs associated with frequent cleaning and filtering of lubrication systems and inspections of journal bearings.
Lubrication plays a vital role in supporting optimal steam turbine performance.
This article details key attributes that maintenance managers need to consider when selecting steam turbine oil. By using these insights, plant managers can optimize equipment efficiency, simplify maintenance and generate cost savings.
A steam turbine oil’s most important functions are to:
- Lubricate bearings, both journal and thrust. Depending on the type of installation, this also may include the hydraulic control system, oil shaft seals, gears and flexible couplings.
- Provide efficient cooling.
- Prevent sludge, rust and corrosion while in service.
Maintenance professionals need to evaluate and monitor several integral properties of their steam turbine oil to achieve these optimal performance characteristics. Some of these attributes include viscosity, viscosity index, demulsibility, foam resistance, rust and corrosion prevention and oxidation stability.
Viscosity - Viscosity is the primary requirement for selecting a steam turbine oil. Using a product that has the correct viscosity will provide the necessary film thickness to reduce friction between moving parts. Different types of turbines may require oils with different viscosity ranges to promote optimum film thicknesses. Generally, smaller turbines and marine power propulsion turbines, which rotate at speeds greater than 3,000 rpm, require an oil with a viscosity of ISO VG 22-32. However, their larger counterparts that operate at relatively lower speeds (less than about 3,000 rpm), require an oil whose viscosity ranges from ISO VG 32 up to ISO VG 100.
Viscosity Index - The viscosity index (V.I.) indicates the effects that temperature change can have on a lubricant’s viscosity. The V.I. value is calculated from a fluid’s viscosity at two temperatures; 100 F and 212 F (40 C and 100 C). The higher the V.I. value, the less the oil’s viscosity changes with temperature. Fluids generally become less viscous as temperatures increase; this is almost always the case with oils. Thus, an oil’s formulation is less likely to be compromised under drastic changes in temperature if its V.I. is high enough for the application. Quality turbine oils frequently will have a V.I. of at least 95. Many commercially available turbine oils can have V.I.s higher than 115.
Demulsibility - Demulsibility is an oil’s ability to separate from water. Water can appear in solution, free or emulsified form in oil. All three forms of water are undesirable and must be controlled.
Water contamination promotes oil degradation, chemical corrosion and bearing fatigue. Each condition compromises a lubricant’s capability to perform properly. Many different sources of water contamination exist in a steam turbine. Examples can include condensation of humid air in reservoirs, steam leaks through the turbine gland seals or faulty oil coolers. Good demulsibility is critical to an oil’s success. ASTM D-1401 is used to measure demulsibility. The test requires a mixture of 40 millileters (ml) of distilled water with 40 ml of oil to be stirred for 5 minutes at 54 C. The time for the emulsion to separate to 3 ml of emulsion remaining is recorded. A typical passing result for a new turbine oil is 15 minutes.
The demulsibility of an oil in service can be affected by the presence of contaminants, such as mineral sediments like rust, paint or dust, and by polar organic compounds formed due to oil degradation. Additionally, mixing turbine oils with other lubricants containing high concentrations of detergents and dispersants commonly found in engine oils must be avoided to preserve the oil’s ability to readily separate from water. A small amount of engine oil in some cases can completely destroy a turbine oil’s demulsiblity properties.
An onsite visual inspection is usually the simplest way to test for existing water content in an oil. Take an oil sample in a clear container and hold it up in front of your watch. The oil becomes hazy with the presence of water at approximately 500 parts per million (ppm). If the watch face is visible, the water content is normally less than 300 ppm.
Maintenance professionals can also consult with an expert oil analysis partner to conduct a Karl Fischer Test (ASTM D-6304). This test measures the water content in an oil by titration and is reported either in parts per million or percentage by volume. Ensure that the lab you are using is well versed in testing turbine oils, understands their formulation and has data quality integrity and management systems.
Foam Resistance - The presence of foam entrained in the turbine reservoir is not unusual and is generally of little concern. However, when excessive amounts of entrained air and stable foam accumulate in the oil, foam can overflow on top of the reservoir. And foam introduced into the circulating system can damage pumps and bearings or cause sluggish operation of hydraulic control systems.
Main causes leading to excessive air entrainment and foam include:
- Air intake in suction side of the pump
- Low oil level in reservoir
- Excessive splashing of oil returning to the main reservoir
- Insufficient size of oil return lines
- High temperature differences between the oil that is replaced and the one that is in service
- Excessive pressure changes that allow dissolved air to release from the oil.
In a well-formulated oil, the foam should dissipate or remain at minimum stable levels while residing in the main reservoir. Turbine oils typically have an anti-foam additive package that assists with the breakdown of foam. However, excessive amounts of anti-foam additives can actually lead to an increased foaming tendency and increased air separation times. A lubricant with the right balance of base stocks and additives helps avoid these types of problems. Consulting a lubrication specialist with application-specific expertise can help maintenance professionals gain insight into making a selection with the appropriate air release and foaming characteristics.
Rust and Corrosion - Chemical corrosion and rust formation are mentioned together, but they actually represent two different mechanisms of metal degradation. Chemical corrosion occurs when strong acids or bases attack metal surfaces. Rust is a metallic oxide formation that appears when oxygen, usually in the presence of water, comes into contact with a metal for prolonged periods. To prevent both from forming, rust preventive and metal passivating additives are typically added to properly formulated turbine oils. These are the “R” in the R&O additive system, for rust and oxidation. These agents act by preferentially attaching themselves to the metal surface, forming a protective coating.
As with antifoamants, a balanced formulation approach is equally important. Excessive amounts of rust and corrosion inhibitors can interact unfavorably with other lubricant additives and properties that can then affect resistance to oxidation, demulsibility and air release. It is important to understand that most lubricant additives are surface acting, competing for the metal surface of the steam turbine’s bearings.
Oxidation Stability - Steam turbine oil resides in the machine’s reservoir for extended periods where it is exposed to oxygen, which can have a deleterious effect on a lubricant’s performance capabilities. Thus, oxidation resistance is a vital property to look for when selecting a steam turbine oil.
Oxidation is the reaction of hydrocarbon molecules that form when oxygen is introduced to the base fluid of an oil. The rate of oxidation increases exponentially as temperature rises and with the presence of metallic contaminants. An increase of around 10 C in the oil’s temperature effectively doubles the oxidation rate. Copper, bronze, brass and iron contaminants are typical materials that catalyze the oxidation reaction.
From a practical standpoint, poor oxidation resistance shortens the oil’s service life. Additionally, as the oil oxidizes, foam control, demulsibility and air release will likely be compromised. Sludge and deposits can form in more severe cases, impeding proper lubrication and hydraulic control of the turbine.
A wristwatch can help determine the water content in lubrication oil. If the watch face remains visible when viewed through a clear container of oil, then the oil’s water content is likely less than 300 ppm.
The rotating pressure vessel oxidation test (RPVOT) is one commonly used method to determine an oil’s remaining oxidation resistance by comparing a new to a used sample. The RPVOT test is not intended as a means to compare two new oils with different formulations. Rather, it is designed to evaluate the remaining useful oxidation life of an in-service turbine oil. Remember that a high RPVOT value for new oil does not mean the oil will have a long service life or remain free of deposits while in use. Some oils with relatively low new oil RPVOT values will last longer than those that have a relatively high new oil RPVOT value. The important aspect of RPVOT is the rate at which it changes in service rather than the new oil value.
Many turbine manufacturers recommend that oil should be changed when the RPVOT value decreases to 25 percent of the new oil value. This is a rule of thumb, and decisions made must be considered carefully. It is important to work with your lubricant provider when performing these sorts of assessments. Some manufacturers have a series of tests for determining a turbine oil’s suitability for continued use.
Like other attributes and properties of turbine oils, oxidation is also affected by the oil’s formulation. By selecting an oil with highly refined base oils and a proper balance of anti-oxidant additives, a product’s formulation is less likely be compromised by exposure to oxygen during long-term service.
Steam turbines require lubricant solutions that are able to meet demanding operational needs. Maintenance professionals can successfully select a steam turbine oil by evaluating a product’s performance properties such as viscosity, viscosity index, demulsibility, foam resistance, rust and corrosion prevention and oxidation stability. Selecting a well balanced turbine oil formulation involves working with a reputable lubricant provider and understanding the oil’s complete performance profile.
However, achieving long-term production success requires testing in-service oil on regular intervals to detect degradation issues early enough so they do not lead to costly or catastrophic consequences. These tests should be done by an experienced lab and monitored by the turbine professional. By taking this proactive approach, maintenance professionals will help promote optimized equipment efficiency and generate valuable cost savings.
Luis Rojas Lopez is a lubrication engineer for ExxonMobil Lubricants & Specialties (Mexico) In this role, he provides technical support to customers and OEM’s on all areas of lubrication technology. A graduate of the Instituto Tecnologico de Chihuahua (Chihuahua, México), he has been an STLE member since 2001.