February 15, 2007

Focus on O&M (February 2007)

Plant details

The Termocentro plant, rated at 300 MW, is powered by two Siemens Westinghouse W501D combustion turbines (burning low-sulfur natural gas without supplementary firing) and one GE steam turbine-generator, in a 2 x 1 configuration. The combustion turbines are served by two identical, horizontal, double-pressure HRSGs (with no reheat) designed and manufactured by Hanjung (now part of Doosan Heavy Industries & Construction Co., Ltd.). The plant is not equipped with a selective catalytic reduction system.

Each HRSG extracts energy from its combustion turbine's exhaust gas (at 1,004F) to produce 317,000 lb/hr of 1,350-psig steam and 71,400 lb/hr of 185-psig steam. A desuperheating spray system between the high-pressure/high-temperature superheater and the high-pressure/intermediate-pressure superheater stages controls temperature in the high-pressure steam loops.

Termocentro Puerto Berrio was built as a simple-cycle plant but converted to combined-cycle operation in November 2000. Since then, it has been cycled lightly (daily, every few days, or weekly) for no more than a few months per year. Under the terms of Termocentro's contract with one of Colombia's two transmission grid operators, it must be ready to start up on short notice from lay-up any time during the rest of the year.

Assessing asset longevity

As Isagen converts its assessments of Termocentro's HRSGs into a predictive maintenance plan, Gensa is focusing on determining whether to repair, refurbish, or replace the assets at its aging coal-fired stations. The utility has just completed a "remaining life" assessment, inspection, and analysis of the boiler at its Termopaipa plant.

The coal-fired boiler powering the 165-MW Unit 1 of the Termopaipa facility (Figure 2) was designed by Stein & Roubaix in 1957 using the design techniques of Combustion Engineering and (presumably) the latest ASME Boiler & Pressure Vessel Code.
As inputs, the recent remaining-life analysis used data from a detailed boiler inspection of August 2006, plant design details, and historical plant operating records. Unlike Termocentro, Termopaipa operates every month, but only for a few hours, so most start-ups are cold.

2. Live long and prosper. Gensa recently assessed the remaining life of pressure parts in the coal-fired boiler powering Unit 1 of its Termopaipa plant. Courtesy: Tetra Engineering Inc.

The first part of the analysis concentrated on identifying the key damage mechanisms that might affect boiler components under pressure: waterwall tubing, economizer tubing, superheater tubing, and high- and low-temperature headers. Probabilities of these mechanisms occurring were then calculated and extrapolated into predictions of the components' remaining useful life. With these equipment "actuarial tables" in hand, plant staff are now making the transition to a more predictive mode of boiler maintenance.

—Contributed by Dr. Frank J. Berté, president of Tetra Engineering Inc.; José Sandoval of Tekna S.A. (Tetra's local agent); and Alejandro Carrillo of Gensa S.A. Dr. Berté can be reached at 978-589-0992 or frank.berte@tetra-eng.com.