March 15, 2008

Extend EOH tracking to the entire plant

Peter S. Jackson, PE, and David S. Moelling, PE, Tetra Engineering Group Inc.

Beyond EOH

As mentioned, gas turbine vendors have long based their maintenance recommendations on formulas that relate cumulative component wear and tear to proxies such as EOH, starts, trips, and fuel switches. Indeed, most suppliers now include these formulas in their maintenance and service support agreements to help their power plant customers plan inspection and overhaul outages.

The earliest of these calculations produced results in terms of EOH only. Using a particular formula, an end user could equate the negative impact on reliability of a turbine start or trip to that of running the unit for a specific number of hours. Today, however, many gas turbine suppliers use formulas that produce more than just EOH numbers. Newer formulas also state the impact in terms such as equivalent starts (ES) and equivalent hours (EH), and some even calculate maintenance intervals. The first limit reached determines when maintenance will first be needed.

Rolling your own program

Unlike turbines, most other combined-cycle plant systems—including HRSGs—lack manufacturer-supplied EOH formulas or detailed PM recommendations for reliable service. If any maintenance guidelines are provided, they are rough, such as “overhaul every five years.” This lack of guidance requires the plant's maintenance staff to determine HRSG PM intervals based on their personal experience or the experience of peer plants.

Tetra Engineering Group recommends taking the following steps to incorporate more complex EOH-type scheduling into a plant's overall maintenance program:

• Identify the systems and components to be included in EOH calculations.
• Identify major service-related failure modes by component using Failure Mode and Effects Analysis (FMEA).
• Relate operational parameters such as starts, trips, high/low load, shutdown time, high/low temperatures, etc. to each failure mode.
• Match PM requirements to the failure modes.
• Determine the relationship of a system's operational parameters to its longevity based on experience, analysis, or guidance from the supplier.
• Integrate manufacturer-supplied EOH values for the gas turbine, steam turbine, and HRSG(s) to produce a comprehensive and consistent EOH-based maintenance and inspection program for the entire plant.
• Implement EOH tracking either using an on-line system such as OSI PI data historian (www.osisoft.com), a link to the plant's maintenance management system, or another maintenance scheduling tool.