Coal, O&M

Rebuilding aging feedwater heaters

Today’s feedwater heater rebuilding companies can rebuild aging units in a matter of weeks, while new replacement units can take a year or two to manufacture. In many cases, rebuilding can proceed with the plant online, so even plants with only one string of heaters can continue to generate electricity. And rebuilt units save anywhere from 25 percent to 50 percent of the cost of a replacement unit.

“It required four weeks to completely rebuild two heaters and by rebuilding over buying new feedwater heaters, we saved about one-third the cost,” said Nick Samford, engineering supervisor at NRG’s Limestone Generating Station in central Texas.

Rebuilding cost-savings
Located between Houston and Dallas, NRG’s Limestone station is a coal-fired plant with two units—one 836 MW unit and one 864 MW unit. Both of the original equipment manufacturers (OEM) heaters had been in service for more than 20 years. However, the desuperheater was a little too large, resulting in saturated water exiting at excessive velocities that caused tube erosion.

“We initially considered buying completely new heaters, but ultimately decided not to because of the additional cost,” said Samford.

American Power Services (APS) in Erlanger, Ky., a provider of heat transfer equipment services, undertook the rebuilding job. The process started with a failure analysis that identified vibration in the desuperheating zone. APS then redesigned the desuperheater to eliminate the wet wall condition of the tubes and to reduce the velocity of the steam entering the desuperheater while maintaining original design thermal and operational parameters.

Work began in late February 2009. The internal structure was rebuilt while changing the steam inlet location and the desuperheater flow path was rerouted during the rebuilding process. To speed up the process, a specialized motor-driven plasma arc cutter was used to remove the hemi head to facilitate better access to the tubesheet during the rebuilding process. This additional space led to a more efficient rebuilding project by providing full access to the tubesheet, rather than working inside the hemi head. This allowed the use of efficient tools to quickly pull the tube stubs, weld the new tubes to the tubesheet and mechanically expand the tube ends into the tubesheet.

The job was finished within 25 days, completed per code and insurance regulations within the time frame of a scheduled four week inspection outage. The rebuilt feedwater heater allows the Limestone plant to facilitate superheat at exit of the desuperheater, thereby eliminating the wet wall condition. As a result, either one of the two units can be taken offline and the plant can still generate 100 percent power output. Since the condensing zone steam velocity was increased, thermal efficiency improved slightly.

Advanced rebuilding techniques
Engineering staff at plants without built-in redundancy traditionally thought they would have to go offline to repair a feedwater heater problem. Their only option was to order a new heater, which can take more than a year to build. In the meantime, the plant was operating in derated status, burning more coal and driving down efficiency. Now, rebuilding companies can now overcome any lack of redundancy within the facility.

“For about half of the heaters we rebuild, we isolate the heater and re-tube it onsite while the unit is online, by isolating the defective heater,” said David Grimes, an engineer at APS. “Bypass lines can be installed so that the rebuilding can proceed without the plant having to wait for an outage.”

Rebuilt units can match the thermal efficiency of new replacement units since the original tube material can be changed during a rebuild to actually improve the thermal performance of the original design. A rebuild or retubing can extend the life of an existing heater by 20 to 30 years, just as long as the extension from a total replacement but at less capital expense. Additionally, many components that were once considered obsolete and/or not reusable such as the tubesheet, channel and shell can be returned to as-new condition.

The rebuilding process begins with testing and troubleshooting analysis. Equipment determines the exact location, nature and severity of a problem with eddy current, ultrasonic, dye penetrate, magnetic particle, hydrostatic and boroscopic testing procedures. With testing data in hand, a rebuilding company can make recommendations on cost-effective options for remedial correction, retubing to original specification or rebuilding to improve performance.

Hydraulic tube pullers, power tube strippers, tube joint milling tools and plasma arc ID tube cutting facilitate the rebuilding process. New technology allows for defective or leaking tubes to be explosively plugged with the unit online by utilizing a robotic arm to install the required tube plugs, a safer method of installing tube plugs.

Sometimes, a complete replacement is difficult because there may not be sufficient room to physically get a new heater into position. Over the years as a plant expands around the original installation, extra equipment is installed and the heater that was once in the wide open is now behind three walls. A rebuilding project can still take place within the same space constraints.