Operation, Maintenance and Reliability of Steam Turbines

Steam turbines prove to be very convenient drivers for wide ranges of applications. They can be configured to operate at the speeds of driven equipment. Therefore, direct-drive is possible. They are more compact, lighter, better, and more economical than many other drivers such as reciprocating engines, gas turbines, electric motors, etc.

Steam turbines have been substantially developed as a result of more than a century of extensive use. The efficiency has been significantly improved, output capacity has also been increased, and specialized steam turbines were designed and manufactured for various applications.

This column discusses operation, maintenance, and reliability of steam turbines, focusing on critical topics for operation and reliability including corrosion, erosion, and lubrication.

Corrosion & Erosion

Corrosion is the most common damage mechanism in steam turbines. Too often, this results from deposits. Increased surface roughness acts to increase deposition. Corrosion fatigue (CF) and stress corrosion cracking (SCC) of steam turbine components have been consistently identified among the main causes of reported problems. Both phenomena are characterized by two stages: initiation and propagation. In steam turbines, initiation most frequently occurs at micro-cracks that emanate from pits that form when deposits become corrosive too often during unprotected shutdowns. Although other reasons for such micro-cracks exist, cracks can also initiate at locations of fretting, manufacturing defects, inclusions, microscopic imperfections, and other features. These locations are where deposition is preferential. Propagation of CF and SCC is usually driven by cyclic or steady stress situations in regions where dynamic liquid films are present.

Pitting and localized corrosion are important precursors to more extensive damage from SCC and CF, although extensive pitting of blades can cause significant loss of stage efficiency or, in extreme cases, weaken component integrity to the point of failure. Pitting and localized corrosion are unlikely to originate during steam turbine operation due to the absence of oxygen in the liquid films on the steam turbine surfaces during operation. Rather, pitting results from corrosive deposits absorbing moist air during steam turbine shutdown.

During non-protected shutdowns where the blade and disk surfaces are actually open to the atmosphere, any deposits, particularly chloride or sulfate, which have formed on steam-path surfaces during operation, can lead to local, conductive, aqueous environments that contain ppm levels of oxygen. These local environments initially lead to breakdown of the blade metal passivity, then to metastable pit formation, and finally to stable pits after repeated shutdown cycles.

For many steam turbines, each shutdown period is followed by operation where the dynamic situation of droplet formation, liquid films, and deposition occurs. Once the steam turbine has resumed operation, liquid films can repassivate areas where passivity was lost during shutdown and metastable pits had formed. However, deposition continues to occur during operation. Deposits associated with a loss of passivity that caused a metastable pit during one unprotected shutdown will lead to further growth of that pit during the next extended unprotected shutdown. Repetition of this process will eventually lead to a stable pit. Too often, these pits are not visible, but because they have resulted from an active corrosion mechanism during shutdown, the internal surfaces will be rather irregular. Therefore, the different environments that exist during the repetitive operation and shutdown periods eventually lead to the initiation and growth of a number of pits on the surface.

Steam turbine components may also be attacked by flow-accelerated corrosion (FAC) when liquid films form on steam turbine components in the presence of two-phase wet steam. Poor steam purity can cause low pH in such films, and thus trigger or enhance FAC. Suitable alloys (such as advanced Cr-alloyed steels or others) can mitigate and even prevent FAC.

Particle erosion is another significant problem in steam turbines. The liquid erosion is commonly reported, particularly for saturated steam and condensing steam turbines. Solid particle erosion is caused by different particles, mainly iron oxide particles, which scour the surface of blades, mainly in the initial stages of each steam turbine casing. The source of such particles is oxide on super-heater and re-heater tubes and piping that exfoliates during transient operation such as startup and shutdown.

Steam Turbine Lubrication

Steam turbine lubrication oils are subjected to a wide range of adverse conditions such as extreme heat, entrained air, moisture, contamination by dirt and debris, inadvertent mixing with different oil/fluids, and others. All these degrade the integrity of the oil base stock and deplete the additive chemistries, causing irreversible molecular changes. There are two primary degradation mechanisms in steam turbine oils: oxidation and thermal degradation.

The oxidation is a chemical process where the oxygen reacts with the oil molecules. The rate at which this occurs depends on several factors. The temperature is perhaps the most critical one. As a rough indication, the rate of oxidation doubles for every temperature rise of 10°C. The temperature effect on the oxidation of the oil is also related to the presence of catalysts and pro-oxidant conditions such as water, air, certain metals, fluid agitation and pressure.

Thermal degradation is the breakdown of the oil molecules by heat (high temperature), forming insoluble compounds that frequently are referred to as soft contaminants. Over time, it has become clear that the oxidation performances of the different lubrication base stock classes are quite different. The high natural oxidative resistance of some superior turbine oils combined with specific anti-oxidants employed (usually based on phenol and amine compounds) provides a nonlinear behavior in terms of their molecular degradation over time. As a result, most standard oil analysis tests offer little to no warning as the lubrication oil starts to degrade and generate system deposits. Instead of degradation occurring in a linear and predictable fashion, many of the modern turbine oils fail rapidly.

Changes in the oil’s molecular structure due to additive depletion and the development of insoluble particulates are among the first oil degradation conditions that affect steam turbine performance. The sequential result is the formation of sludge and varnish, which are common occurrences in steam turbines. Besides these, oxidation and thermal degradation by-products are the main contributors to the development of varnish and deposit problems in steam turbines.

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