Rebuilding aged feedwater heaters can make better economic sense than buying new ones, because rebuilding reduces capital cost and plant outage expenses. At one Texas generating station, rebuilding saved one-third the cost of a new feedwater heater while improving the plant’s heat rate.
Generally, plant operators believe that rebuilding a feedwater heater takes longer than installing a new replacement. Even if rebuilding were an option, they think that it can’t be done while the unit remains in operation. But today, feedwater heaters can be rebuilt in a matter of weeks — while the plant continues to operate — keeping plant downtime to a minimum.
For generating stations that need a new feedwater heater immediately, the option of on-site rebuilding may make good economic sense, because new replacement units can take a year or two to manufacture. Rebuilding can also mean avoiding long periods of derated operation and perhaps a forced outage or two.
Compelling Case Study
Rebuilding heaters can save in the range of 25% to 50% of the cost of a new replacement unit, making the rebuilding option tough to ignore. "It required four weeks to completely rebuild two heaters, and by rebuilding over buying new feedwater heaters, we saved about one-third the cost," says Nick Samford, the engineering supervisor at NRG’s Limestone Generating Station, located in central Texas.
"For about half of the heaters we rebuild, we isolate the heater and re-tube it on-site while the unit is online, by isolating the defective heater," says David Grimes, an engineer at American Power Services (APS), a provider of heat transfer equipment services that include troubleshooting, repairing, and complete rebuilding projects. "Bypass lines can be installed so that the rebuilding can proceed without the plant having to wait for an outage. This, by itself, saves money."
Contrary to the opinion held by some heater manufacturers that on-site rebuilding isn’t practical, it’s a complete replacement that often proves impossible.
"When you buy a new heater from a manufacturer, it will typically come already assembled and ready to be installed," continues Grimes. "But we’ve seen cases where there was not sufficient room to physically get a new heater into position. As a plant expands over the years around the original installation, extra equipment gets installed, and, before you know it, the heater that was once in the wide open is now behind three walls. In contrast to installing a replacement heater, a rebuilding project can still take place within those same space constraints."
Any doubts that rebuilt units can’t match the thermal efficiency of new replacement units are quickly dispelled when you realize that the original tube material can be upgraded during a rebuild to actually improve the thermal performance of the original design.
Additionally, a rebuild or retubing can extend the life of an existing heater by 20 to 30 years — just as long as a total replacement, but at far less capital expense. Additionally, many components that once were considered obsolete and/or not reusable — such as the tubesheet, channel, and shell — can be returned to as-new condition.
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