Advanced Sleeving Revives Tubes
“At Tanner’s Creek the tube defects were roughly up to 6 feet back behind the tube sheet,” says APS Sales and Services Engineer David Grimes. “Instead of plugging the problem tubes, we went in with sleeves that were roughly 7 feet long and installed the sleeves in the inside diameter [ID] of the tubes and the sleeves extended past the area where the stress cracking or excessive wall loss was located. So, if the tubes continued to weaken and fail, then we have those liners installed that should prevent the tubes with through wall cracks from leaking.”
In installing the sleeves, APS employed an advanced hydraulic expansion method that utilized a flexible hydraulic expansion mandrel (Figure 3). The use of a hydraulic expander provides more uniform expansion and superior contact of the sleeve outside diameter with the ID of the parent tube. Additionally, the sleeves were strength-welded to the parent tubes at the face of the outlet tubesheet.
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| 3. Sleeve the heater tubes. Instead of plugging the tubes, APS went in with sleeves that were roughly 7 feet long and installed in the tube inside diameter. The sleeves extended past the area where the stress cracking or excessive wall loss was located. Courtesy: American Power Services |
Grimes adds that with projects that utilize the advanced tube testing and sleeving technologies, there are benefits that are somewhat immeasurable. “In most cases, sleeving can be performed with the heater isolated while the unit remains online, which allows the utility to continue to generate electricity.”
Grimes notes that APS recommended installing the sleeves because plugging such a large percentage of tubes would have led to reduced thermal performance and increased feedwater velocity in the inlet of the unplugged tubes. Additionally, the plant may have had to cut a bypass orifice in the pass partition plate in order to prevent higher tube inlet velocities that would have led to further heat transfer degradation.
“I would say that, using this technology, our customers can realize savings of over 80% of the cost of a new heat exchanger if they elect to install sleeves rather than replace their problem heat exchangers,” says Grimes. After the sleeving project at Tanner’s Creek unit was finished, Jay King was surprised to see that heater performance was substantially maintained (Figure 4).
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| 4. Weld the tube ends. The hydraulic expander provides more uniform expansion and superior contact of the sleeve outside diameter with the inside diameter of the parent tube. Additionally, the sleeves were welded to the parent tubes at the face of the outlet tubesheet for added strength. Courtesy: American Power Services |
“We would have expected to see a slight performance decay, since you have the sleeves in place in the desuperheating zone, because it’s now a heavier-walled tube,” he says. “But that didn’t happen. We maintained the same performance levels. The TTD [terminal temperature difference] and the saturation temperature of the heater (based on the pressure) versus the feedwater outlet, have not decayed at all.” Quite possibly that resulted from the selective, limited use of sleeving.
“The thing I liked best about going this route is that we didn’t have to abandon the heaters,” adds King. “That would have presented us with a very difficult situation on how we would start and shut down the unit without some serious pipe modifications and major changes in operating procedures. These heaters are used during start-up and are equipped with alternate drains that drain back to the condenser that must be utilized during start-up. The piping modifications would have meant a large added O&M expense and time lost for engineering to assess needed operational changes. The installation of the sleeves allowed us to maintain efficiency and return the unit to service maintaining a design basis.”
—Contributed by American Power Services.
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