Oil Playing Bigger Role In Power Generation
As power generation advances, so must system lubricants
by ian cameron
Continuous subtle changes in gas turbine design and increasing pressure on power generation operations to increase output and slash costs are bearing down on one of the lower profile elements of the energy production equation. In a typical gas turbine application, either on land or offshore, lubricants play a vital yet low-key role in daily operations. The slightest failure to perform by an oil, or a breakdown in its properties and capabilities, can have lengthy and expensive repercussions for operators.
However, as design changes to gas turbines are regularly implemented and users strive to extend intervals between maintenance and reduce downtime, lubricant makers are under pressure to devise new and more reliable oils. Shell operates one of the world’s largest lubricant research and development facilities in Hamburg, Germany. More than 200 staff work at the base, which was established in 1956 and provides technical support and consultancy for fuel and lubrication applications across a range of sectors including power generation, motorsport, off-highway, marine and commercial vehicles.
With sister facilities near Chester, England, and in Houston, Texas, U.S.A., Shell is working closer than ever with OEMs trying to keep pace with the demands placed on them by a string of external factors such as new emission legislation and the use of new types of fuel, including biofuels. “Never before has there been such a pace of technology change for OEMs and ourselves with many new factors to be considered at the same time,” said Cameron Watson, Shell Lubri-cants’ global technology manager OEMs and Direct Sectors. “But fortunately there is an increasing amount of collaboration between lubricant suppliers and OEMs, and we are starting to see a trend whereby OEMs are talking to us at the beginning of their product design work and that is enabling us to provide even greater value and efficiency for them,” he said.
Shell said that the typical trend it is noticing within turbine lubricants is the move away from the use of less refined mineral oils defined by the American Petroleum Institute (API) as Group 1 to the more refined Group 2 and 3, which the company said offers a “more tightly defined composition and additive response.” Previously, around 10 years ago, the oils had a field life of around two to three years in severe use in a gas turbine, although the newer Group 2 and 3 based products could increase the life twofold or threefold, Shell said. “We can only achieve that type of performance by using Group 2 or 3 based oils with a specialized additive system which must have such important properties as oxidative stability, which is critical for a gas turbine,” said Peter Smith, Shell Global Solutions technology manager, Specialty Lubricants.
He said, “Other important properties of the oil are surface properties such as air release, because if the oil entrains air in the bearing it can lead to the air causing cavitation damage. “Rapid air release is another feature of Group 2 and 3 oils, and that is another reason to move from Group 1 oils. “Another key area in the movement toward synthetic-based oil is that we are looking toward achieving system efficiencies and, ultimately, energy efficiencies, through lower friction and better heat transfer.” Smith continued, “It is also important that the oil does not thicken up too significantly under load, otherwise that can result in more friction and heat losses. Synthetic oils churn up much less easily under load than conventional base oils, giving a range of benefits that ultimately creates energy efficiency and operator savings.
“It is difficult to quantify exactly how much extra efficiency is gained because there are so many variables within the gas turbine such as the fluid flow through the bearing, the firing temperature and the design and operation of the turbine all of them impact on efficiency. “The less stress that an oil is subject to, the longer it will last and it will be more reliable. That means there is less chance that equipment will fail or start to deteriorate. In turn, that leads to extended maintenance intervals,” said Smith.
Other design changes to turbines and the way they are operated also heavily influence lubrication development, said Smith. He said, “Gas turbines are running at higher temperatures, so ultimately the temperatures that the oils face are a lot higher, so again, it is vital that the oil has good oxidative stability and resistance to deposits. “Also, oil reservoirs in gas turbines are becoming smaller for design and weight reasons, so it means that there is less volume of oil continually being exposed to high temperatures, whereas previously, in big reservoirs the oil had time to separate and maintain its performance properties better. The smaller sumps and the higher temperatures mean more stress is placed on the oil.”
Combined-cycle systems also provide another challenge for lubrication developers such as Shell. Smith said, “In combined cycle, where steam turbines and gas turbines may be linked, many of the older types of base oils and additives could cope with the steam turbine, which is wet and cooler or with the gas turbine, which is hotter and dryer, but not many additives-based oils can lubricate both without sacrificing some desirable benefits. So we have developed a specific oil to deal with combined-cycle applications.
“Filtration within a turbine can also be an important factor when sourcing an oil,” Smith said. “Gas turbines typically have a 6 micron filtration lubrication system, and there are lubrication additives that can be taken out by such a system. Our additives are designed to remain soluble in the oil, so even with a 6 micron filter the additives won’t be removed.” Another factor that affects oil development and performance is electrostatic discharge, which means that an electric spark can be produced if the turbine system is not properly grounded.
As the rotor spins, it charges up, leading to sparking between the rotor and the bearings. That spark can be enough to melt the white metal babbitt (an alloy used for the bearing surface). Smith said, “You end up with a bearing with a surface that looks like the surface of the moon, and the oil won’t flow correctly over that leading to hot-spots and deposits forming, which can ultimately lead to an unplanned shutdown. However, there are additives we can put in the oil to improve its conductivity.
“Sensitivity to contamination is also critical for all turbine oils dedicated segregated blending is needed because if there is a small ingress of engine oil, hydraulic fluid or gear oil into the turbine oil, the additives designed for the former are not synergistic with a turbine oil. If there is a small amount of any of them in a turbine oil, important turbine oil properties can be lost leading to including very bad air entrainment, foaming and poor water separation.”