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When a global leader in the design, manufacturing, and maintenance of binary organic Rankine Cycle (ORC) power systems entered the geothermal energy market, the company’s engineers knew that its existing turbines would be tested.
The working principle of ORC is similar to the thermodynamic Clausius-Rankine cycle (CRC), which is the most widely used power generation process. CRC converts heat into work through a closed circuit that usually uses water as the working fluid. In contrast, ORC systems evaporate organic fluids, which are characterized by a higher molecular mass than that of water.
This organic fluid has a lower boiling point and higher vapor pressure than water. Therefore, it can use low-temperature heat sources to generate electricity without emitting harmful substances. At the same time, due to turbine mandrel space constraints, corrosive media and increased shaft speeds, ORC applications have recently become more and more challenging.
In order to compete for market share in this increasingly competitive field, a European company knew it needed to focus on cost-effective turbine design, each with a power output of up to 40 MW. As part of the new design, ORC introduced a new silicon-based fluid in the process. Even so, the new fluid not only brings temperature problems, but also challenges corrosive conditions to the existing dual-tube mechanical seals of turbines.
In order to adapt to silicon-based fluids, the company worked with John Crane to provide reliable solutions for new and demanding operational challenges. Since 2009, John Crane has been working with ORC to provide engineering products and services, including mechanical seals, couplings, fluid dynamic bearings, and seal support systems.
John Crane engineers worked closely with ORC turbine manufacturers to determine that the original turbine included traditional heavy-duty cartridge seals that were not suitable for the ORC environment. Engineers then evaluated various sealing designs to cope with the increasing heat and corrosion conditions. These designs include traditional heavy-duty double O-ring push rod seals. The main ring is made of a flat wear-resistant insert in a metal carrier, combined with different elastomers, including polytetrafluoroethylene (PTFE), hydrogenated acrylonitrile Butadiene rubber (HNBR) and silicone rubber.
Based on the experience of double O-ring push rod seals, John Crane engineers tried a variety of solutions to withstand high temperatures, namely 230F to 644F (110C to 340C). Different seal main ring designs were also tested. However, none of them met John Crane's heat transfer, leakage, and other performance standards. In addition, there are concerns about maintaining the proper liquid film level required to lubricate the sealing surface and the amount of process fluid leakage.
As a result of the test, the metal carrier press-fit carbon insert was discarded, and a graphite-supported silicon carbide (SiC) main ring was used instead, which provided deformation resistance and significantly better heat exchange coefficient.
The requirement of fluid film formation is met by adding a water cushion to the sealing design. As the name implies, a water cushion is a groove on the sealing surface that generates a hydrodynamic separation force between the sealing rings. For many years, this technology has been successfully used in the presence of easily vaporized fluids (including propylene, butane, etc.).
Due to the low vapor pressure of the oil, the leakage will not evaporate and disappear. Users often require that the oil leakage rate is much lower than the generally acceptable oil leakage rate. Therefore, although traditional water cushion technology is effective in many high-pressure applications, it does not meet John Crane's standards for new ORC turbine conditions.
1. Embedded water cushion technology, specially designed for double pressurized O-ring push rod seal, reduces the risk of uneven contact on the seal interface, and provides better and stronger lubrication. It provides the advantages of non-contact sealing, but with less leakage. Courtesy: John Crane
Drawing on the experience of the company's engineers in Germany, John Crane developed a new and effective embedded water cushion technology (Figure 1), which has been proven in other turbine applications. In order to adapt to leaks, an innovative embedded water cushion technology is introduced-specifically designed for double pressurized O-ring pusher seals.
Usually used with low vapor barrier fluids such as mineral oil, the recessed water cushion technology optimizes the formation of liquid film thickness. The thickness of the fluid film affects all aspects of sealing performance. A thickness that is too thin will cause contact between the sealing rings, resulting in high wear and power absorption. On the other hand, leakage is related to the film thickness with a cubic factor. The traditional water cushion technology operates in a completely non-contact working mode, but the amount of oil leakage is unacceptable.
The recessed variant reduces the risk of uneven contact on the sealing interface, while providing better and stronger lubrication, making the sealing interface less sensitive to seal ring deformation. It provides the advantages of non-contact sealing, but the leakage is much less.
The ORC turbine manufacturer was impressed that the proposed upgrade to a double O-ring push seal with a grooved water cushion would provide all the advantages of a traditional seal. Nevertheless, before implementing the recommendations, verification is required.
Every 1,500 hours, seals are disassembled and inspected in the John Crane service facility. During each inspection, the sealing ring was found to be intact, with no signs of wear or damage. Testing confirmed that the embedded water cushion technology can prevent the risk of uneven contact, even in the case of low film thickness. The absence of thermal damage (thermal inspection) is also a clear result of effective heat dissipation.
Partly due to seal upgrades, ORC turbine manufacturers can confidently provide their turbine technology as a reliable source of income to the geothermal energy market. Seal leakage and overheating issues have been resolved, and reliability is now a reality.
In direct comparison with the previous design, leakage has been reduced by 65%, and power requirements have been reduced by 32% (average calculated under three different pressure values). Finally, the turbine consistently meets performance targets, including having a five-year time between maintenance (MTBM) track record. ■
—Mario Severino is John Crane's drawing and application team coordinator.
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