Turbines Put Pressure on Bearings and Seals

While developments in turbine technology have resulted in improvements in performance and efficiency, they have also led to increasingly aggressive demands being placed on components such as bearings and seals. The faster shaft speeds and higher outputs which the new equipment is capable of, together with today's requirements for near continuous operation, require bearings which can offer consistently high levels of performance, with long operating lives and low maintenance requirements. Likewise, the growing use of variable speed drives as control devices has introduced new problems which need to be overcome, such as the insulating of components from resulting electrical currents that can cause bearing damage and, in some cases, failure. The latest generation of bearings and seals have been specially designed to respond to these demands, enabling manufacturers and end users of diesel and gas turbines to achieve greater levels of reliability, and longer operating lives, than previously attainable. These new components are able to work efficiently under the extreme temperatures present in turbine engines and also to withstand the high pressures present. The need to minimise friction so that performance is optimised has been at the forefront of the new designs, resulting in bearings with better than ever performance. Included in this new breed of components are bearings that have rings of steel and rolling elements of bearing grade silicon nitride (Si3N4). These Hybrid bearings display all of the properties required by turbines, with a higher speed capability than conventional bearings, robust construction resulting in longer service life than all-steel bearings, and also excellent levels of electrical insulation. As the density of silicon nitride is only 40% of the density of bearing steel, the rolling elements in this new kind of bearing weigh less and have lower inertia than conventional designs. The result is significantly less cage stress and also much lower friction at the high rotational speeds which modern turbines operate at, with lower friction enabling cooler running and longer lubricant service life. Silicon nitride rolling elements have a lower thermal expansion than steel rolling elements of a similar size, resulting in less sensitivity to temperature gradients within the bearing and more accurate preload control, making the bearings particularly suited to the high temperatures which can be experienced in diesel and gas turbines. Due to silicon nitride having a higher hardness and a higher modulus of elasticity than steel, the bearings have increased stiffness and offer longer service life in contaminated environments. Hybrid bearings are designed to last much longer in applications operating under severe dynamic conditions or lubrication conditions with low operating viscosity; under insufficient lubrication conditions there is no smearing between silicon nitride and steel. Hybrid bearings can now even be specified with oil-free lubrication for even longer service life without maintenance. Finally, the electrical insulating property of silicon nitride is one of the essential features of the bearings, protecting the rings from electric current damage known as washboarding, where the bearing surfaces are subject to premature wear and potential failure. These problems are particularly prevalent where variable speed drives are used as control devices for turbines. The protective properties of the silicon nitride lead to increased uptime of equipment, with lower maintenance requirements and therefore increased bearing service life. In addition to specialised bearings, seals are also required which can withstand the extreme operating temperatures inside turbines, and which are able to protect bearings from contamination. A new generation of seals has been developed that use the latest engineering plastics, such as PTFE (polytetrafluoroethylene) and FKM (fluoro-rubber), to address these needs.