Utilization of Environmentally Acceptable Lubricants (EALs) in Kaplan Runners

Transcription

1 Utilization of Environmentally Acceptable Lubricants (EALs) in Kaplan Runners Seth Smith, P.E. Lead Mechanical Engineer, Engineering Supervisor, Kaplan Turbine Engineering Department Voith Hydro Inc. 760 East Berlin Rd. York, PA Thomas Ort Kaplan Runner Module Owner and Lead Mechanical Engineer, Turbine Engineering Department Voith Hydro Inc. 760 East Berlin Rd. York, PA Abstract One of the most critical design challenges facing Kaplan runners is maintaining a reliable seal between the oil-filled hub and the water passage. An aging fleet of hydroelectric powerhouses, combined with more stringent environmental regulation, has further exacerbated the need for a reliable solution. The most reliable solution is to eliminate the source of potential contamination by removing the oil from the hub side of the seal. Voith offers a water-filled hub solution so that, if a seal fails, only water will be released into the water passage. With advancements made in Environmentally Acceptable Lubricants (EALs), a second option may be available to replace the traditional mineral oil with an EAL. Basic attributes that define the environmental acceptability of an EAL are biodegradability, toxicity, and bioaccumulation. A property that may also be of interest to hydroelectric operators includes formulations that do not sheen when introduced to water. The use of an EAL in a hydroelectric power unit could provide several benefits, including minimizing the environmental impact, potentially decreasing fines in case of a spill, and furthering an energy company s green image. Many EAL options exist for use in Kaplan runners. Synthetic esters are the best option for new Kaplan turbines, as these lubricants are compatible with the mineral oils used to shop test the runner and provide performance equal to mineral oils. Additionally, these lubricants have been applied in real-world operations in the marine industry. For existing Kaplan runners, vegetablebased oils are the best option since they are compatible with most paint systems, are compatible with mineral oils, and are also compatible with most seal materials. Keywords Environmentally Acceptable Lubricant, EAL, Kaplan Runner, Lubricant, Water-filled

2 1. Introduction One of the most critical design challenges facing Kaplan runners is maintaining a reliable seal between the oil-filled hub and the water passage. An aging fleet of hydroelectric powerhouses, combined with more stringent environmental regulation, has only further exacerbated the need for a reliable solution. The most reliable solution is to remove the oil from the hub side of the seal. Voith has a water-filled hub solution so that, if a seal fails, only water will be released into the water passage. With advancements made in Environmentally Acceptable Lubricants (EALs), a second option may be to replace the traditional mineral oil with an EAL. An EAL, at its most basic level, is a lubricant that, in the event of a spill, is less harmful to the environment. Basic attributes that define the environmental acceptability of a lubricant are biodegradability, toxicity, and bioaccumulation. A property that may also be of interest to hydroelectric operators includes formulations that do not sheen when introduced in water. The use of an EAL in a hydroelectric power unit could have several benefits, including minimizing environmental, legal, and financial risks in case of a spill and also furthering an energy company s green image. 2. Kaplan Design Considerations Several performance and design concerns need to be considered when replacing a mineral oil with an EAL. First, compatibility with mineral oil is of great importance, even for new Kaplan runners, as new Kaplan runners may be shop tested with mineral oil. Changing the existing shop infrastructure to support EALs would likely be unpractical and expensive. Due to the complexity of the shapes of the internal runner components, it is unlikely that the mineral oil can be sufficiently flushed from the runner after assembly and test. Additionally, outside of the Kaplan runner, other components such as the oil head, blade servomotor, and gate servomotors may also prove difficult to adequately flush in situ to prevent compatibility issues with some EALs. Seal compatibility is also a concern as some materials, such as natural rubbers and silicone, are susceptible to damage when exposed to certain EALs. Some EALs can also cause excessive swell in the sealing material, leading to premature seal failure. Seals throughout the power unit, as well as throughout the balance of plant equipment (Governor Hydraulic Power Unit (HPU), accumulator, piping, instrumentation, etc.), also need to be considered. Seal failure, especially in oil-to-water interfaces or difficult to access locations, could result in massive oil leaks or costly plant outages. In existing runners, it could be unclear what protective coating may exist, if any, on the interior surfaces of the hub. For this reason, when filling an existing runner, it is not recommended to fill with an EAL that is known to be reactive with any coating system. However, with new and rehabilitated runners, specific coatings can be chosen to mitigate this risk. How an EAL reacts when exposed to water also needs to be considered as water ingress into the hub is likely at some point. Solubility in water and hydrolytic stability define the performance of the EAL in the presence of water. Solubility is defined as the ability of a fluid to dissolve in a solvent in this case, water. Hydrolytic stability is the ability of a lubricant and its additives to resist chemical decomposition in the presence of water. Some lubricant

3 manufacturers tout their lubricant s solubility in water, while others claim that high solubility is a detriment because it reduces the ability to remove the water from the working fluid, causing the operator to replace the oil earlier. Effects of higher water content in the oil include lower lubricity and lower viscosities. A decrease in these properties could increase wear in components. Additionally, lower lubricity would result in higher friction in the oiled bushings in the Kaplan runner, potentially increasing the force required by the servomotor to operate the blades. It would be beneficial to use an EAL with a lower solubility in a Kaplan runner since EALs with a higher solubility would require replacement more frequently than those with a lower solubility. Due to the extra cost of EALs (in some cases 2 3 times the cost of mineral oils) and since most Kaplan runners in the US market have built-in methods to extract leakage water, EALs with a lower solubility would allow for water removal, and thereby extend the useful life of the oil. To that end, some EALs have a specific gravity greater than 1, meaning that water would float on the EAL rather than sink. In order for these extraction methods to be effective without a unique and unproven design, the specific gravity of the EAL would need to be less than 1. Some plant owners may be interested in an EAL that does not sheen when introduced to water. Certain EAL base stocks may sheen less than others; however, this is generally because the EAL has a lower specific gravity, causing it to sink in water instead of sheen. Taking this into consideration, the desire for an EAL to not sheen may not be compatible with the existing equipment used to remove water from the bottom of the Kaplan runner. Hydrolytic stability is important as water infiltration into the lubricant is inevitable and, with poor hydrolytic stability, reduced lubricant service life is likely. Certain EALs have poor hydrolytic stability and become acidic as they breakdown when exposed to excessive water. This can cause damage to unprotected metal surfaces. The hydrolytic stability of most EAL bases can be improved upon with the proper additives. Other considerations include increased oil weight. A quick review of several stock EALs show a wide variance in densities. Standard ISO VG 46 mineral oils have a density of approximately 860 kg/m 3 (53.7 lb/ft 3 ). Comparatively, Mobil EAL 224H, a vegetable-oil-based lubricant, has a density of 921 kg/m 3 (57.5 lb/ft 3 ), an increase of 7% in oil weight. Neptune AW-68, a polyalklene-glycol-based lubricant, has a density of kg/m 3 (64 lb/ft 3 ), an increase of 19% in oil weight. Mobil SHC Aware H68, a synthetic-ester-based lubricant has a density of 915 kg/m 3 (57.12 lb/ft 3 ), an increase of 6% in oil weight. Consequently, the owner should take care to check the axial load capacity of the thrust bearing to ensure the increased load is acceptable as some thrust bearings are designed at their functional limits. Reference Fluid (lb/ft 3 ) (kg/m 3 ) gallons (lb) litres (kg) ISO VG ISO VG ISO VG ISO VG

4 Reference Fluid (lb/ft 3 ) (kg/m 3 ) gallons (lb) litres (kg) Water Table 1: Various Densities and Associated Weights 3. Selecting an EAL When considering an EAL, special attention must be paid to the base stock. While the performance of the EAL can be modified with additives, the particular base stock, or base chemistry, primarily defines the performance of each EAL. Each base stock (vegetable oil, polyalkylene glycol (PAG), saturated synthetic ester, and polyalphaolefin (PAO)) has specific benefits and drawbacks. Additionally, some less environmentally-friendly lubricants claim to be an EAL, as this term does not carry an official global definition. In the US, the EPA released the Vessel General Permit (VGP), which regulates discharges in US national waters. In 2013, this was updated to include requirements for EALs to be used in all oil-to-sea interfaces, unless technically unfeasible. It also defined what lubricant properties were required in order for a lubricant to be considered an EAL. For hydroelectric power equipment, the definition set by VGP 2013 is sufficient for selecting an EAL; however, some European standards (e.g., Nordic Swan, European Ecolabel) include renewability requirements (i.e., the ability for the material to be manufactured from renewable resources), as well as technical performance requirements. Generally, despite the environmental benefits associated with EALs, there are few financial or legal benefits to incorporate these lubricants in hydroelectric power plants. One exception is the state of Alaska, which offers a reduction in fines based on the toxicity, degradability, and dispersibility of the oil [2]. Additionally, any EAL spill would still require the same cleanup efforts as traditional oils to minimize the environmental impact. Due to PAGs generally poor compatibility with mineral oils and coatings, these are not recommended for use in Kaplan runners for reasons noted earlier. Additionally, many of the PAGs are water soluble, which would prevent removing the water from the oil if infiltration were to occur. PAGs are made from petroleum-based materials (ethylene or propylene oxide), so these also will not meet requirements for renewability. PAOs exhibit very good performance characteristics and are seal, paint, and mineral oil compatible, but are also made from petroleum-based materials (ethylene). These PAO base oils are not discussed in the EPA s white paper on EALs [1]. PAOs also have a high initial cost and a slower rate of biodegradability as viscosities increase. Saturated synthetic esters are the best option for new Kaplan turbines, as these lubricants are compatible with the mineral oils used to shop test the runner, provide performance on par with mineral oils, and satisfy the definition of an EAL. Additionally, these lubricants have been used in the marine industry for some time with proven success. For existing Kaplan runners, vegetable-based oils are the best option as they are compatible with most paint systems, are compatible with mineral oils, and also compatible with most seal materials. When using synthetic esters and vegetable oils, aggressive oil monitoring programs need to be maintained

5 to check for water infiltration, as the hydrolytic stability of these oils are not ideal. As a result of water infiltration, these oils may breakdown, resulting in acidic water, which can cause corrosion. 4. Conclusion Incorporating EALs into Kaplan runners offers few economic and legal benefits; however, it is important to recognize that EALs greatly increase the environmental friendliness of hydroelectric energy. This benefit, when weighed against the risks of EALs, could make implementation of an EAL worthwhile. The best solution is to completely eliminate the risk of oil leaking into the water by completely removing oil from the hub. Voith Hydro has a wide experience in water-filled Kaplan runners, which use self-lubricating bushings for the trunnion and guide bushings instead of oiled bronze bushings. This technology has been used in the US market on several projects and also extensively in other countries with reliable results. Increased emphasis on environmental stewardship will continue to grow in the future. Several options exist to mitigate risks associated with continued use of lubricants in Kaplan runners. The usage of EALs must be weighed in comparison to the risks presented and in light of other technologies available. References [1] U.S. Environmental Protection Agency, EPA 800-R White paper on Environmentally Acceptable Lubricants [2] Etkin, Dagmar Schmidt, Ph.D. A Worldwide Review of Marine Oil Spill Fines and Penalties [3] Pirro, D., Webster, M. and Daschner, E. (2016). Lubrication fundamentals. Boca Raton: CRC Press, Taylor & Francis Group. The Authors Seth Smith, P.E., Lead mechanical engineer at Voith Hydro in York, PA. He is responsible for leading the mechanical design of new and rehabilitated turbine components for existing powerhouses. Additionally, Mr. Smith is the supervisor for the Kaplan runner engineering team. He has 6 years of experience in hydro turbine design. Mr. Smith has a bachelor s degree in mechanical engineering from Pennsylvania State University. Thomas Ort, Kaplan Runner Module Owner at Voith Hydro in York, PA. He is responsible for developing and maintaining global standards for Kaplan runners and oil heads. In addition, he is a lead design engineer for new and rehabilitated Kaplan runner projects. Mr. Ort has 6 years of hydropower experience. He holds a bachelor s degree in Mechanical Engineering and is pursuing a Master of Engineering degree in Engineering Science, both at Pennsylvania State University.

6 Contact: Voith Hydro, Inc. 760 East Berlin Road York, PA USA Phone Fax Voith Hydro Holding GmbH & Co. KG Alexanderstr Heidenheim/Germany Phone Fax info.voithhydro@voith.com A Voith and Siemens Company