Industrial Gas Turbines

Main bearing damages are rare. Their appearance often leads to extensive secondary damages. For the analysis and evaluation of bearing failures there are norms/spezifications/directives like DIN 5426- 1 and ISO 10816-4. Where there is a fatigue failure ( "Ill. 3.5-10"), there is the chance of early recognition in the framework of the already mentioned controls (Chapter 3.5.1). The causes are various: unfavorable operation conditions (oil shortage, oil deterioration) , assembly problems, impurities, overload through the loss of other components etc.

Unfavorable operation conditions imply:

• very high or very low bearing loads.
• Lengthy static times can incite corrosion.
• Vibrations and lateral accelerations of the static engine of the standig engine or during tranport ( "Ill. 3.5-13") can damage the bearing race way through wear.

If huge unbalances appear unnoticed, (e.g., after blade fracture), not registered as prohibtive by the vibration sensors, because of the elasticized and damped bearing, respectively, the rotor suspension of modern types of engines, these bearings can be inconspicuously permanently damaged. If such bearings are not exchanged during repair, they are predestined for extensive secondary failure at a later date.

Assembly problems, precisely in engines designed with modular systems can lead to mechanical bearing failures ( "Ill. 4.2-5"). In this phase, the danger of foreign object infiltration and impurities is enormous.

When welding in context of assembly, repairs or local overhaul, one must be careful to see that no electrical continuity through the engine and the bearing follows, for fear of local initial fusing of bearing raceways and rolling objects inducing fatigue failure. Parasitical current flows, from other sources that pass through the casings and rotor, can stimulate similar effects.

Typical problems in this connection will be addressed here. Vibration in inadequately fixed rotors during transport (trucks on bad roads, railway transport with shunted bumps or rail bumps in old tracks, "Ill. 3.5-13") can lead to so called “brinelling” ( "Example 3.5-1"). This is a digging in of the rolling elements in the tracks, as a result of vibrational wear and/or plastic deformation through hammering. Experience shows that after such failures, catastrophic bearing and engine failures can arise very fast, depending from the rotor speed. Therefore small engines can even fail in seconds during the first strart up after a dangerous brinelling occurred. Appropriate cushioned and damped transport devices, e.g., adequate containers and/or tensed rotors have proven as adequate remedy.

A possibility of non permitted enormous accelerations (impacts) from the transport of the engine, to be proved even later, is the application of suitable sensors on an appropriate position of the transport container or the engine. Such an impact sensor says something about the unique excess of an adjustable threshold which shows up permanently. Typical, for instance, are appliances that clamp a ball that will be dislodged through a prohibitive strong impact independent from the direction.

Corrosion is apparent on rolling bearings, experientially, only then, if, during a vast static period, a corrosion medium is effective. Such is the case when an engine is in an inadequately, conserved condition for very long, i.e., after months of static state. Also we have to think about lack of conservation oil in modules and components (e.g., gears) stored as spare parts can suffer corrosion. Normally the corrosion must be seen in connection with condensation water and contaminated air. Corrosive impurities from industry or sea atmosphere are absorbed by condensation water. Keep in mind that the surrounding atmosphere can be very aggressive, depending on the site. For this reason durig stand still condensat and air exchange must be avoided. Therefore changes in temperature and access of air must be as low as possible. A heating (e.g., during the day) causes by the expansion of the air a ‘breathung out’ of the engine. During cooling (e.g., in the night) the engine ‘inhalates’ again. This enables a distinctly air exchange which transports aggressive contaminations to the bearings.

If engines or engine components are stored over longer periods of time, they are to be appropriately (through manufacturer suggestions) conserved. At the begin of the use it is to ascertain if the OEM demands a ‘conservation run’ of the whole engine or a treatment of the components/modules with conservation oil was sufficient. This special sort of oil does not flow so easy from the wetted surfaces as the normal lubricant and so protects over longer non-operating times.

Here, measures towards the avoidance of condensed water should be undertaken. To this belongs as mutch as possible a constant temperature. The storage can be carried out in closed containers or under sufficient dense packing. Those can be flushed with dry, inert gas (e.g. nitrogen). In closed containers or coverings, hygroscopic media can be implemented, e.g., ‘silica gel’. If especially corrosion danger exists (great variations in temperature, sea atmosphere) the engine can be purged continuous with dry air.

Due to the augmented corrosion danger following long static periods, a precise oil analysis should be carried out after commissioning and recommissioning, because the metallic, loosely adhering corrosion products must be detectable due to the higher iron content , which normalizes at subsequent oil analysis. Typical for the corrosion damages, through condensed water during static, are lineal (roller bearing) or circular (ball bearing) corrosion pits around the contact surfaces of the rolling elements. The pits have a characteristic spread at the periphery in the weight loaded regions at the bottom. Cages of superior material, such as bronze and copper or silver plating, can effect an element formation. Corrosion failures on accessories such as gears were observed, after long storage times (e.g., in the workshop), to be insufficiently protected. Catastrophic bearing failures due to corrosion are rare. One can assume that a failure due to a corrosion pit is discovered in time in a routine examination (see previous information, "Ill. 3.5-7").

A special and frequent type of corrosion is initiated through hand sweat. Typical for this form of corrosion are appearances similar to finger impressions, formed by, individual micro areas of single craters, corresponding to percolating drops. Therefore, when handling rolling bearings, it is judicious to wear appropriate cotton gloves. Before the start of such assembly work, such gloves should be kept ready for use.