Jul-2009

Improving turbocompressor availability

Modular replacement maintenance can reduce major overhaul downtime by nearly 50%, and time between combustion and major overhauls can also be extended

Filippo Cinelli, Riccardo Valorosi, Gianni Mochi, Antonio Pumo and Dante Sabella
GE Oil & Gas

Article Summary

In recent years, market demand for plant uptime has progressively increased. Turbocompressor technology usually represents the core equipment of a plant, and its availability is one of the key factors in optimising plant uptime. Original equipment manufacturers (OEMs) have concentrated significant engineering efforts on developing new hardware to extend the mean time between maintenance (MTBM) and to reduce the need for downtime inspection activities.

The new generation of material technology for hot gas path parts is focused on enhancing the durability of machines. The combination of proven materials and advanced technology from other industries, such as the aircraft engine industry, accelerates the introduction of improved materials, coatings and design solutions into new machines as well as into retrofit kits to uprate installed base units. Uprate kits are designed to be fully interchangeable with original parts. Hence, they can be installed during planned shutdowns without any additional impact on plant downtime and, in some cases, can even significantly reduce downtime.

In addition, substantial improve-ments have been made in specialised software to manage and analyse data on operating equipment, which provides faster and more accurate troubleshooting. Predictive capabilities have also been embedded into software tools and, in the near future, it is expected that they will be able to cover residual life assessment of hot gas path components as well. These advances are all key to extending equipment availability, which translates into additional days of production and reduced life cycle cost.

Increased uptime
Uptime is one area where oil and gas OEMs are concentrating their efforts, investing in new technology for products that address this key need. Maximising uptime means reducing outages, extending the mean time between maintenance, shortening site activities and reducing unplanned shutdowns due to failures.

Reliability and availability are the metrics used to measure uptime and are specified by the ISO 3977-9 definition:1

Reliability    R = [(PH-FOH)/PH]x100
Availability    A = [(PH-(FOH+POH))/PH]x100

Where:
F = Number of failures
PH = Period hours: cumulative hours of monitoring
FOH = Forced outage hours: period of time during which the maintenance team is actually working during forced outages
POH = Planned outage hours: period of time during which the maintenance team is actually working during planned outages
Key factors critical to enhancing uptime include:
• Reduction of site activities at turnaround time
• Reduction of planned maintenance
• Reduction of unplanned maintenance.

The cases addressed by this article concentrate on the FR5-2D model gas turbine. The FR5-2 is a heavy-duty, double-shaft gas turbine. The 17-stage axial compressor is a high efficiency model. The 12 combustion chambers, which have individual burners, are set parallel to the machine’s centreline. Expansion is achieved in two turbine stages with impulse rotor buckets: the first stage drives the axial compressor and the two together comprise the high-pressure shaft; the second stage drives the low-pressure shaft and load. The partition airfoils of the first-stage fixed nozzles are cooled with air from compressor bleed, while the partition airfoils of the second-stage fixed nozzles have an adjustable angle that enables broad speed range and load flexibility. Owing to their flexibility, high capacity, simplicity and low maintenance requirements, these units are ideal drivers for pipeline centrifugal compressors. The FR5-2D is also used in natural gas re-injection plants to drive high-pressure re-injection centrifugal compressors. An overview of the case study presented is shown in Table 1.

Maintenance
Operation of a gas turbine, like any other rotating machine generating mechanical power, must include a maintenance programme for repair and/or replacement of parts when needed to ensure maximum availability and reliability of the unit. Naturally, this maintenance programme will begin with minor operations, which will increase in scope over time and according to the severity of operation up to the point of a major inspection. This maintenance pattern is repeated cyclically.
A gas turbine maintenance programme can be split into:
•  Preventive maintenance inspection during operation
•  Standby inspection.

Preventive maintenance inspection during operation consists of logging the key operational parameter during startup and while the unit is in service. Subsequent analysis of this data establishes the general conditions of the unit and auxiliary systems. Standby inspection is conducted when units are out of operation due to the plant schedule and therefore does not interfere with availability but increases reliability. Activities may include changing filters, checking the lube oil level, and checking and regulating instrumentation.

Scheduled maintenance
Scheduled maintenance is performed with machines out of service, as the unit has to be disassembled so that internal components can be inspected. The scope of the area to be disassembled depends on the type of inspection. Inspections start from the most critical zone, which is the point where the temperature is highest, and continue to cover the entire machine at subsequent increased inspection levels:
•  Combustion inspection = CI
•  Liners and transition pieces inspection = LTPI
•  Major inspection = MI.