Steam Turbine Oils
What You Need to Know About Steam Turbine Oils
Steam turbines are widely used in the power industry as prime movers for generators. As a paramount component of a company’s production, these sophisticated 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, such as high temperatures, water contamination, and lengthy periods of time in service.
Lubrication plays a vital role in supporting optimal steam turbine performance. Selection of an inadequate lubricant can have expensive consequences; including unexpected shutdowns, high labor costs associated with frequent cleaning and filtering of lubrication systems and inspections of journal bearings.
This article details key attributes that maintenance managers need to consider when selecting steam turbine oil. By leveraging these insights, plant managers can optimize equipment efficiency, simplify maintenance and generate significant cost savings.
The main functions of a steam turbine oil include:
- Lubricating bearings, both journal and thrust. Depending on the type of installation, this list may also include the hydraulic control system, oil shaft seals, gears and flexible couplings.
- Providing efficient cooling.
- Preventing sludge, rust and corrosion while in service.
Maintenance professionals need to evaluate and monitor several integral properties of steam turbine oil to achieve these optimal performance characteristics. Some of these key attributes include viscosity, viscosity index, demulsibility, foam resistance, rust and corrosion prevention, and oxidation stability.
Viscosity is the primary requirement for selecting a steam turbine oil. Employing a product that has the correct viscosity will provide the necessary film thickness to reduce friction between the 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 with a viscosity that ranges from ISO VG 32 up to ISO VG 100.
Viscosity index, or V.I., is a measure that indicates the effect on a lubricant’s viscosity with respect to changes in temperature. The V.I. value is calculated by measuring a fluid’s viscosity at two temperatures; 100°F and 212°F (40°C and 100°C), and the higher the V.I. value, the less the oil viscosity changes with temperature. Fluids generally become less viscous as temperatures increase, and for oils this is almost always the case. Thus, an oil’s formulation is less likely to become compromised under drastic changes in temperature if its V.I. is high enough for the application. Quality turbine oils frequently have a V.I. of at least 95 and many commercially available turbine oils can have V.I.s higher than 115.
Demulsibility is defined as oil’s ability to separate from water. Water can appear in solution, free or in 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 of these conditions compromises a lubricant’s capability to perform properly. There are many different sources of water contamination 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 for measuring demulsibility. The test requires a mixture of 40 ml of distilled water with 40 ml of oil to be stirred for five 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, including rust, paint, or dust, and by polar organic compounds formed through oil degradation. Additionally, mixing of 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 separate readily from water. A small amount of engine oil in some cases can completely destroy the demulsiblity properties of a turbine oil.
Usually, the simplest way to test for existing water content in an oil is by conducting onsite visual inspections. 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 roughly 500 parts per million (ppm). So 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 in either 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.
The presence of foam entrained in the turbine reservoir is not abnormal and is generally of little concern. However, when excessive amounts of entrained air and stable foam accumulate in the oil, the foam can overflow the top of the reservoir. And if the foam is introduced into the circulating system, this can lead to pump and bearing damage or cause sluggish operation of hydraulic control systems.
The main causes that lead to excessive air entrainment and foam include:
- air intake in the suction side of the pump.
- low oil level in the 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 that which 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 who has application-specific expertise can help maintenance professionals gain insight in selecting the appropriate air release and foaming characteristics.
Rust and Corrosion
Chemical corrosion and rust formation are mentioned together, but they are actually 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 of time. To prevent both from forming, rust preventive and metal passivating additives are typically added to properly formulated turbine oils. These are the “Rs” in the R&O additive system, for “rust and oxidation,” and these agents act by preferentially attaching themselves to the metal surface to form a protective coating.
As with antifoamants, a balanced formulation approach is important. Excessive amounts of rust and corrosion inhibitors can interact unfavorably with other lubricant additives and can then impact lubricant properties, such as 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.
Steam turbine oil resides in the machine’s reservoir for extended periods of time where it is exposed to oxygen. Oxygen 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 oxygen with the hydrocarbon molecules in the base fluid of turbine oil. The rate of oxidation increases exponentially as temperature rises and with the presence of metallic contaminants. An increase of 10°C in the temperature of the oil effectively doubles the rate of oxidation. Copper, bronze, brass, and iron contaminants are typical materials that catalyze the oxidation reaction.
Practically the result of poor oxidation resistance is seen as a shortened service life of the oil. Additionally, as the oil oxidizes, foam control, demulsibility and air release likely will be compromised. Sludge and deposits can form in more severe cases, impeding proper lubrication and hydraulic control of the turbine.
The rotating pressure vessel oxidation test, or 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 to be used as a means of comparing two new oils with different formulations. Rather, the test is designed to provide an indication of the remaining useful oxidation life of an in-service turbine oil. It is also important to remember that a high new oil RPVOT value does not mean that 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 last longer than those with very high new oil RPVOT values. So, 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 of the oil decreases to 25 percent of the new oil’s value. This is a rule of thumb, and decisions must be thought through carefully. It is important to work with your lubricant provider when performing these sorts of assessments. Some manufacturers have a panel of tests for determining a turbine oil’s suitability for continued use.
As with other attributes and properties of turbine oils, oxidation is 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 to be compromised during long-term service from exposure to oxygen.
Steam turbines are sophisticated pieces of machinery that warrant a lubricant solution that meets their 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 complete performance profile of the oil.
However, selecting the turbine oil is only the first step. To achieve long-term production success, testing in-service oil at regular intervals is important to detect degradation issues early enough so they do not lead to costly or catastrophic consequences. These tests should be performed by a qualified oil analysis lab and should be monitored by the turbine professional. By taking this proactive approach, maintenance professionals will promote optimized equipment efficiency, simplified maintenance and valuable cost savings.