HIGH-PERFORMANCE LUBRICANTS

Dr. Peter Smith, Global Technology Manager for Shell Global Solutions in the UK, is responsible for all turbine and transformer oil development, testing and commercial support.

How can Shell lubricants benefit the turbomachinery field? Turbomachinery is placing more severe demands on lubricants than ever before. Shell power sector lubricants offer significantly higher performance compared to earlier products. They support asset life extension, reliability and efficiency optimization, and reduce the risk of unplanned shutdowns. This includes turbine, gear, hydraulic, compressor and transformer oils, and greases to help assets operate longer. Shell also provides services and technical advice such as oil selection, oil condition monitoring, oil storage, and change-over procedures, such as flushing, draining and filling. Turbo T, based upon conventional API group II mineral oils, has a long track record in steam and light-duty gas turbines (GTs). Our latest generation of turbine lubricants are branded: Shell Turbo S4 X for turbines, and turbomachinery that does not need higher load carrying performance; and Turbo S4 GX for geared turbines and turbomachinery requiring higher load carrying performance. They are based upon Shell gas-to-liquids (GTL) base oil technology, which is a high-performance API group III hydrocarbon. They work in combination with a proprietary additive system to provide superior performance in steam turbines (STs) and GTs.

What are the challenges for turbomachinery? Turbomachinery lubricant demands are increasing, due to: Power generation turbines operating under frequent stop-start conditions rather than continuous operation; higher operating temperatures; smaller equipment designs and lubricant reservoirs with similar or higher loads and power outputs; shock loads; fewer spared assets; increased need for asset reliability; and oil and asset life extension. These factors can place increased oxidative and thermal stress on the oil which can result in premature ageing and degradation. Further, the production of oil soluble and oil insoluble degradation products such as varnishes and deposits can block or damage filters and bearings. This can cause turbomachinery failure. Faster oil recirculation times can lead to increased air entrainment and cavitation issues in bearings, or foaming problems that increase bearing wear. Accelerated oil ageing is another symptom. ST oils, too, can suffer from water ingress due to poor steam seals. This can generate stable emulsions and poor bearing lubrication along with increased wear and corrosion.

How are these issues addressed? All lubricating oils will degrade in service over time. But products can be developed with much slower rates of degradation even under harsh conditions. Shell Turbo S4 oils have a longer lifespan as they resist degradation, thereby preventing the build-up of harmful deposits and corrosion products. Their surface properties are another important factor. Rapid air release minimizes air entrainment even in systems with rapid recirculation. Low foaming reduces wear in bearings, as does accelerated oil oxidation. And rapid water separation is of particular value in STs due to the potential for water ingress.

What trends relate to turbomachinery lubrication? More than 10 years ago, most turbomachinery was lubricated with lightly refined hydrocarbon mineral oils designated by the American Petroleum Institute (API) as group I oils. These had performance and oil life limitations. In the last decade or so, more highly refined group II oils were devised with better performance and longer life. The latest trend is highly refined group III hydrocarbons. Shell group III GTL oils are amongst the highest performing products in laboratory and field testing. They support asset life extension, reliability and efficiency optimization, and reduce turbomachinery operational costs (Figure).

How oils have changed over time using the turbine oxidation stability test (TOST) designed to mimic oil degradation in service

What factors should users consider when choosing oil? What does oil need to do in their application; what reliability and equipment life is needed; what is the oil performance specification; what performance data is available from the supplier; and can the supplier provide OEM and utility approvals, field performance data, or customer testimonials?