Use and Chemical Composition of Turbine Lubricants
Lubricants are materials that reduce friction by increasing the smoothness of moving surfaces in contact. All solids have surfaces with imperfections and roughness. These irregularities lead to increased friction and associated heat generation. By covering these imperfections in a viscous material or slippery liquid, friction is reduced and the moving system’s components are kept cool. The lubricant itself acts as a transmitter of friction-induced heat, carrying it away from the machinery as it circulates and minimizing the threat of overheating. Lubricants can be liquids, solids, or gases—or a combination of these. The amount and type of lubricant applied to working joints, spinning axles, or supporting ball bearings depend on the level of friction reduction required. In addition to aiding movement, lubricants can act as a seal and protective coating to prevent corrosion and rust.
Types of Lubricants and Additives
There are four basic categories of lubricants: oils, grease, dry lubricants, and gases.
Oils include a wide range of lubricant types such as mineral oils derived from petroleum, natural oils made from animal fat, man-made synthetic oils, process fluids produced as byproducts of industrial processes, and emulsions made from two liquids. Oils are freely flowing liquids. This makes them easy to apply, pump, inject, or drain out when they need to be replaced or cleaned. But it also means that they have to be contained within a sealed canister or housing to prevent loss by leakage. Being a liquid makes them excellent conductors of heat, allowing for efficient cooling of the ball bearings.
The most common type of oil lubricants is mineral oils derived from petroleum and petroleum byproducts. Mineral oils are usually the leftovers remaining after the oil’s distilling operations. As such, mineral oils consist of hydrocarbons such as paraffin (with straight polymer chains) and naphthenes (made from ringed polymers). Hydrocarbon compounds containing significant amounts of sulfur, phosphorous, oxygen, and nitrogen are referred to as asphaltenes. Differences in the physical and chemical characteristics of hydrocarbon mineral oils are a result of their individual refining processes and the chemical characteristics of the original crude oil from which they are derived.
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In addition to naturally occurring mineral oils, there are man-made synthetic oils. These are derived from various chemical feedstocks as well as hydrocarbons, utilizing a wide variety of chemical and manufacturing processes. Synthetic hydrocarbons are derived from very long chained molecules similar to paraffin and closely resemble natural mineral oils. There is a group of synthetic oils derived from carboxylic esters (di esters and polyol esters) that is widely used for turbine lubrication since they are very resistant to high temperatures. Silicones are molecules cross-chained with alternating silicon and oxygen atoms that are used as the basis for production of high heat-resistant greases.
Lastly, there are biologically derived natural oils. These are made from animal fats and vegetable oils (such as rapeseed, palm oil, and castor oil). Though inexpensive, these oils are less stable than mineral oils due to their more complicated molecular structure. They also serve as additives to mineral oils to improve surface lubrication.
Although greases are technically oils, they are made with thickening agents that increase their viscosity to the point where they become non-liquid semisolids. Used in a similar way as oils, greases don’t flow like liquids, so leakage losses are not a concern. Their viscous nature makes their application more difficult, but their adhesive properties ensure a seal against contaminants. Greases are made by adding thickening agents (small fibers) to oils to obtain a colloidal, gel-like structure. Being more stable than oil, they need less maintenance and withstand longer intervals between replacement, often lasting for months or years. However, since it is thicker than oil, grease is less effective at carrying away heat through convection, making the systems that utilize grease more susceptible to overheating.
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These thickening agents are added to a base of natural mineral oil almost exclusively (about 99% of greases use mineral oils), with naphthenic oils being the most popular. Typically, thickeners can consist of soaps made from animal or vegetable fats heated with alkali (like caustic soda), metallic powders, fat silica, and bentonite clays. Greases that are expected to function in extreme heat or cold are made from synthetic oils. Very limited, specialized use is made of vegetable oils to make grease. In addition to thickeners, various types of additives (antioxidants, rust inhibitors, anti-corrosion protection, etc.) are also added to guard against long-term wear, and to allow grease to function under extreme pressure.
Dry (solid) lubricants are usually loose powders like carbon graphite. In effect, they act like grease, except these solid lubricants don’t flow at all. Therefore, they have more of the advantages of grease but also more of its drawbacks. Many types of materials can act as dry lubricants, but the three most common are graphite, molybdenum disulfide, and polytetrafluoroethylene (commercially referred to as Teflon). The carbon ring structure of the crystalline graphite molecule is arranged in layers that result in a very slippery interface at the molecular level. Its naturally strong surface-adherence properties can be enhanced by the addition of epoxy adhesive binders. Molybdenum disulfide occurs naturally as molybdenite ore. Like graphite, it has a lattice and layer structure that is inherently slippery. The long-chain molecules of Teflon adhere to metal while allowing for very low-friction surfaces.
Lastly, gas bearings are typically pockets of air or some inert gas that will not chemically attack the bearing components. Again, gas bearings have more of the advantages of oil but also more of its difficulties. For example, if there is a penetration in the containment walls of the ball bearing container, the pressurized air will rapidly escape instead of slowly dripping out.
In addition to the basic formulations described above, lubricants can be modified by the inclusion of performance additives (1–3% by weight) which can improve the performance and longevity of the lubricant. Performance enhancers allow a lubricant to function when subject to extreme heat and pressure by preserving the lubricant’s viscosity. Longevity increasers inhibit corrosion, oxidation, foam, and rust. Additives can improve protection against wear and friction while improving adsorption (the ability to stick to surfaces). Anti-wear additives act by reacting to surfaces via chemisorption (involving a chemical reaction between the surface and the adsorbed substance).
Additives designed to protect against extreme pressure utilize reactive nonmetal elements, such as sulfur, antimony, iodine, or chlorine, that react with exposed metallic surfaces to form surface films, which further reduce wear and tear. Oxidation inhibitors prevent the oxidation of the lubricant oils, which can result in lowered viscosity and increased friction. Corrosion inhibitors and rust inhibitors include non-ferrous metals like zinc and calcium or barium, respectively. Other additives include those used to control contaminants and acidity, dispersants to prevent flocculation and lacquer formation, and increase viscosity.
Lubricating Oil Systems
Turbines and engines are equipped with pumping systems that apply lubricants where needed. These systems are designed to maintain proper lubricant oil pressure, reduce abnormally high lube temperatures, remove and filter out impurities, and prevent oil reservoir tank level upsets.
Lubricant oil enters the turbine bearing housing via the bearing inlet. Flows going into the bearing housing are controlled by an oil pressure relief valve. Under high pressure, the relief valve allows oil to exit, bleeding off the excess pressure. This discharged oil then drains back into the main oil storage tank for reuse. Similarly, oil pumps are often equipped with their own individual regulators that perform the same function as the main oil relief valve. These mainly serve as backup pressure relief systems and are set at lower pressures than the main relief valve, so they normally remain closed during operations. Backup oil pumps are also set at lower pressure values (typically 40–60% of standard operating pressure) to keep the main pressure relief valve closed. The goal is to maintain sufficient circulation of lubricant to prevent overheating in an emergency situation, and maintain proper oil pressure at the bearing inlets.
The following diagram is a simplified schematic of a basic turbine lubrication system:
If the turbine lubricant oil is subject to excessive pressure heads, significant damage to the turbine can occur. Bearings can overheat and the entire turbine can receive damaging shocks from the resultant excessive vibration. The turbine may even catch fire. Even when avoiding this damage, turbines can be subject to reduced pressure, which can also damage bearings by reducing the amount of available lubricant or simply the loss of operational production. Excessive pressure can create new leakage points or force more lubricant out of existing leaks. As such, the best-designed systems have a pressure relief valve in each oil pipeline as well as the main relief valve.
Most oil pumps are centrifugal pumps or positive displacement pumps. Use of a positive displacement pump results in the lifting of the rotor axle off its bearings. In effect, the rotor is not floating on pressurized oil, so pressure at the bearing is not directly controlled. Rather, it is allowed to build up until the bearing’s resistance to lubricant flow is overcome and the oil can freely circulate.
In addition to pressure extremes, lubricant oil can be subject to excessively high temperatures, and it can also be affected by temperatures that are too low. Low operating temperature decreases the oil’s viscosity and its ability to flow freely. This can also result in the potential structural damage. Counterintuitively, cold lubricant oil can cause bearings to overheat. More obviously, oil that is too hot can cause the metal bearings to overheat as well—once again leading to vibratory damage.
Proper maintenance and use of turbine lubricants represent significant cost-savings for power generators, as shown in a study performed by Shell Lubricants. The company’s studies have demonstrated that in general, power companies greatly underestimate the significant potential cost-savings and electrical generation productivity gains that can be achieved with the simple use of effective lubrication. As a result of failing to utilize proper lubricants, 20% of power companies incur over $1,000,000 in unnecessary costs annually. (Source: Survey commissioned by Shell Lubricants and conducted by research firm Edelman Intelligence, based on 212 interviews with power sector staff who purchase, influence the purchase of, or use lubricants or greases as part of their job across 8 countries from November to December 2015).
According to Michelle Gibb-Taylor, Shell’s Lubricants Global PR Manager, and Marcelo Goldberg, Shell Global Sector Manager for Power:
Power companies are incurring significant costs from errors in equipment lubrication, according to a study by Shell Lubricants. Companies admit that around six in ten incidences of unplanned downtime in the last three years were likely due to their incorrect selection or management of lubricants. This is having a financial impact–26% of companies estimate that these shutdowns cost their business at least $250,000 and one in five (18%) state that costs have exceeded $1 million.
The international study of power companies across Asia, Europe, and the Americas commissioned by Shell Lubricants reveals that companies are underestimating the potential cost savings and productivity gains from effective equipment lubrication. Many companies do not realize that lubrication can significantly influence equipment reliability, with 60% of companies stating they wouldn’t expect higher quality lubricants to result in a reduction in unplanned downtime.
Goldberg explains, “In power, every day is critical. When people flip a switch, they expect the power to be there instantly. This places power companies under immense pressure to ensure reliable, efficient operations and avoid costly, unplanned downtime. However, companies tend to underestimate the potential impact of equipment lubrication, often due to a lack of understanding. Shell Lubricants has a team of technical experts who work closely with customers to help ensure that they select the right lubricants and, importantly, manage them correctly. In doing so, we have delivered millions of dollars of savings.”
Two of the major barriers preventing companies from maximizing the cost-saving potential of their lubrication procedures are insufficient employee expertise and a lack of process. 59% of those surveyed think they don’t conduct staff training on lubricants as regularly as they should, and only 43% of companies have all the correct procedures in place to manage lubricants effectively. Misunderstandings around lubricants are also evident. Only half of companies (52%) understand how lubrication management can help reduce maintenance costs, and 41% understand how lubricants can deliver cost savings through improved wear protection.
Goldberg comments, “Companies do recognize the potential for savings but underestimate the opportunity. While 58% of companies recognize that selecting the right lubricant could reduce costs by 5% or more, only one in four think that savings could exceed 10%. But with a gap in staff expertise on lubrication and only 25% of businesses making use of regular visits from their lubricant supplier’s technical staff, most are not well-equipped to take action. One of the services our technical experts offer is to work with customers to help coach their staff in effective lubrication management procedures.”
Shell Lubricants is a leader in the development and manufacturing of a wide range of lubricants (including consumer motoring, heavy-duty transport, mining, power generation, and general engineering), with its turbine applications being just one item of its extensive product line. The term “Shell Lubricants” collectively refers to Shell Group companies engaged in the lubricants business. Shell’s portfolio of lubricants includes Pennzoil, Quaker State, Shell Helix, Shell Rotella, Shell Tellus, and Shell Rimula. They are active across the full lubricant supply chain with product manufacturing based on oils from seven plants. Shell blends them with additives to make lubricants in over 40 plants, and distributes, markets, and sells lubricants in more than 100 countries.
They also provide technical and business support to customers by offering lubricant-related services in addition to their product range. These include: Shell LubeMatch, the market-leading online recommendation tool; Shell LubeAdvisor, which helps customers select the right lubricant through highly trained Shell technical staff; and Shell LubeAnalyst, an early warning system that enables customers to monitor the condition of their equipment and lubricant, helping to save money on maintenance and avoid potential lost business from equipment failure.
Shell has lubricant research centers in China, Germany, Japan (in a joint venture with Showa Shell), and the US. The company has invested significantly in technology while working closely with customers to develop innovative products. The result is a patent portfolio with more than 150 patent series for lubricants, base oils, and greases developed by a research staff of more than 200 scientists and lubricants engineers dedicated to lubricants research and development.
One of the ways Shell pushes the boundaries of lubricant technology is by working closely with top motor racing teams such as Scuderia Ferrari and BMW Motorsport. These technical partnerships enable Shell to expand their knowledge of lubrication science and transfer cutting-edge technology from the racetrack to commercial products.
MAN Diesel & Turbo is one of the world’s leading suppliers of turbomachinery. Their compressors, turbines, and expanders are designed for power, flexibility, and reliability in a wide range of industrial processes. They offer turnkey machinery trains, including drive and expanders, along with reactors for the chemical and petrochemical industries. MAN turbomachinery can be applied to nearly every industrial gas processing and production process. They offer reliable turbomachinery service both on and offsite through their MAN PrimeServ brand. This service is provided by several workshops specializing in repairs, rebuilds, and even manufacturing specifically for turbomachinery, which offer comprehensive service portfolios that include rotating equipment repair, machine shop services, field service, parts manufacturing, technical support, revamps and overhauls, and customer training.
Their complete product line provides tailored operations, maintenance contracts, and personal support. Their upgrades performed by their service experts can modernize client equipment and improve its performance. For those client businesses that require a change in production site, they will serve as a partner for each stage of the machinery relocation. As their field service team assists clients onsite, their service and O&M agreements provide ongoing tailored service and maintenance solutions. They perform almost all rotating equipment maintenance repairs on routine work and emergencies with capabilities that include milling and drilling, honing, equipment base fabrication, hole boring, and alignment—including 50,000-pound balancing capabilities.
The latest addition to their range of lubricants, PrimeServGrease, combines many effective benefits for its users. To prevent potential errors on turbochargers, PrimeServ introduced PrimeServGrease, a screw lubrication paste. One benefit is the paste’s constitution, which allows it to substitute up to five greases. As a lubricant or a high-temperature screw paste, this high-performance grease operates in a wide temperature range from -4°F (-20°C) to 2,200°F (1,200°C) without losing the main attributes such as adhesive power, pressure resistance, and corrosion protection. This lubricant is delivered in ready-to-use 3.5-ounce (100-gram) tubes and is included as an integrated part of an overhaul kit for major turbocharger maintenance.
Sulzer Management and its subsidiary Sulzer Turbo Services have been providing quality lube system technicians, parts, shop repairs, and field services for lube systems since the 1980s. The company provides “value-added” services including in-house design of lube systems (upgrade retrofits), fully warranted repairs, and remanufacturing of units, with comprehensive inventory held under climate-controlled conditions in its five strategically located facilities. They also deal in rebuilt and repaired units that are thoroughly tested on their in-house designed test stand prior to shipping. Sulzer Turbo Services provides users of rotating and reciprocating machinery with experienced and safety-trained mechanical field service teams on a round-the-clock basis. The company performs onsite inspections, overhauls, and repairs on gas and steam turbines, axial and centrifugal compressors, hot gas expanders, reciprocating engines and compressors, gearboxes, screw compressors, industrial blowers, and pumps. The company also supplies ancillary parts (both new and exchanged) for reciprocating and rotating machinery.
Their lube oil systems are custom-engineered to meet the industry’s lubrication needs. Manufactured and tested as per Aramco specification 32-SAMSS-013, API 614, API 611, API 610, API 618 as well as manufacturer’s standards. In doing so, they provide a reliable operating system, reduce expensive downtime of the rotating equipment, and utilize a highly efficient lubrication system. The main applications of their lube oil systems include high-pressure pumps, turbines, centrifugal compressors, oil engines, and electric motors for the oil and gas industries and power industries. Its lube oil systems are designed with a compact reservoir design, optimal instrumentation to ensure reliable operation of equipment, high-efficiency coolers, 1.33–525 GPM (5-2,000 LPM) capacities, filtration down to five microns, and carbon steel or stainless steel construction.