"Abstract

Steam turbines today are required to run for well beyond their intended lifetimes. Opening up machines for inspection is expensive, and owners need to consider all relevant information in making the decision. Problems in steam turbines which reduce machine efficiency and output, such as deposits on blades and erosion of internal clearances, can be detected and monitored using condition monitoring by performance analysis. The paper outlines with some examples some condition monitoring techniques which have contributed to running some large machines for up to 17 years without opening high pressure sections.

In machines with an HP-IP opposed-flow casing, increased N2 packing internal steam leakage can occur from the high pressure turbine section into the intermediate pressure section has a large effect on output and efficiency. The application of a simple test method for estimating this leakage explained observed poor performance on two sets.

Key Words: condition monitoring; optimisation; plant performance; predictive maintenance; steam turbines; testing; N2 packing
INTRODUCTION
Steam turbine generators are reliable machines, and often operate continuously for many months. Such operation at steady outputs can lead to deposition from the steam on the fixed and moving blades. Deposits cause output and efficiency to drop, by reducing the efficiency of energy transfer and eventually restricting steam flow. This occurs less on sets which vary in load, as they undergo a regular bladewashing effect.

Where a machine is taken from service, coastdown and running up through shaft bending critical speeds can allow momentary rubbing at the internal seals. The resulting enlarged flow area can reduce the internal efficiency, such that less energy is extracted from the steam. This also results from internal leakage within a casing which allows steam to bypass blading stages. These effects are particularly evident on the turbine design with both High Pressure and Intermediate Pressure sections in the one casing, with flow in opposite directions.

Retractable packings have been developed by manufacturers and after-market suppliers. These avoid shaft rubbing as they do not close into their normal clearances until the machine is near operating speed, having passed through the bending critical speed or speeds.

Vibration analysis can detect the occurrence of such shaft rubbing and other conditions of the rotor line, but cannot detect the extent of internal wear or deposition. It is well suited for other quite different failure modes, such as when blades or parts of them come off and cause consequential damage. As with the application of all condition monitoring, the rule is to choose techniques to match the likely failure/wearout modes. As steam turbines are critical machines, all the main techniques have their place.

Performance analysis can be applied to most machines, rotating and stationary. It is the one condition monitoring technique which allows the optimum time for restorative maintenance to be calculated, where the deterioration results in increased fuel consumption, or in reduced output, or both. (Beebe 1998)

For some plant items, it is possible to use the normal plant instruments and data processing system to determine condition parameters. (Beebe 1998a). In the case of steam turbines, a more refined method using test quality instruments is needed to give warning well in advance of changes evident from permanent instrumentation systems. (Groves 1996).

This paper describes some performance tests used for monitoring turbine condition and their application.
MAINTENANCE OF LARGE STEAM TURBINE GENERATORS
Large steam turbines have usually between two and five individual casings. Manufacturers vary in their recommendations for opening up a casing for inspection and refurbishment. (Tezel, 1989). Manufacturers have a vested interest in supply of spare parts, and therefore cannot be expected to be unbiased in their recommendations. The author well remembers walking through the works of one manufacturer and seeing several HP casings in for overhaul. The accompanying engineer said "off the record" that most of them would not justify the work!

An outage for such work on one casing may take several weeks, and cost millions of dollars. In making this decision, plant owners need all the relevant information. Condition monitoring by performance testing has been used to extend time between opening of casings to up to 17 years, making its cost/benefits very favourable. (Beebe 1995, Vetter et al 1989). The overhaul decision should not be made unless there is a compelling technical or economic reason for opening a casing. A current EPRI project is aimed at extending the accepted interval between overhauls (McCloskey et al 1995).

It should be accepted that an outage after such a long time in service will probably take longer than if scheduled more frequently, as distortion is likely to have occurred, and parts such as casing studs will probably need replacement (Coade 1993). Once a casing is opened and clearance measurements made, it is possible to estimate the performance improvements achievable by refurbishment and so justify the expenditure (Kuehn 1993; Sanders 1989). However, it is clearly preferable to try and determine the internal condition by testing, and use this information to help make the decision as to the extent of overhaul."

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