When heat exchanger tubes—sometimes numbering a thousand or more per unit—begin to crack or wear, the effects can lead to a cascade of subsequent failures in adjacent tubes. If too many tubes are plugged, heat exchanger effectiveness is compromised, and power generation may be curtailed. If conventional mechanical plugs are used, they can break loose, leak, and fail. At that point, the replacement of a very costly heat exchanger is imminent.
However, if the latest sleeve installation technologies and techniques are used, the results can be lower materials and installation costs plus improved heat exchanger performance, without the need to take generation units offline while completing repairs. Perhaps most importantly, the sleeved heat exchanger can operate reliably for many years, saving the operator’s capital until a planned rebuilding or a replacement unit is installed.
Impending Death of Feedwater Outlet Tubes
At American Electric Power’s (AEP’s) 995-MW Tanner’s Creek station, located on the Ohio River near Lawrenceburg, Ind., thermal stress cracking and wall loss in fall 2009 indicated impending failures of 60% of the feedwater outlet tubes in the heat exchanger on the plant’s 500-MW supercritical Unit 4.
“We had done some testing and were hopeful that the heater could be repaired,” explains Jay King, process supervisor. “It was not feasible to replace the heaters at that time. We had to have a certain amount of time to put together a replacement package to present to the board and show that it was justifiable to replace the heaters in the future.”
“We were at the point where we had to discuss abandoning the heaters until we could replace them. Unfortunately, this would possibly cut down our output because it would not be desirable to operate the heater in each string. This would greatly accelerate the end life of these heaters. To operate without any heaters in service would have meant as much as a 50-MW curtailment,” King adds.
The two high-pressure feedwater heaters were eddy current–tested and found to have severe stress cracks from the back face of the tube sheet in the desuperheating zone extending approximately 6 feet to the back of the zone (Figure 1).
 |
| 1. Test the heater. The pressure feedwater heaters were tested and found to have severe stress cracks from the back face of the tube sheet in the desuperheating zone extending approximately 6 feet to the back of the zone. Courtesy: American Power Services |
In order to verify the severity of the stress cracking, a tube sample was taken for laboratory analysis that proved the eddy current test was accurate. It would have been extremely difficult, if not impossible, to cut and pull a tube sample from these heaters via conventional methods due to the very heavy tube wall thicknesses. However, American Power Services (APS) was able to extract a tube sample using its advanced plasma arc tube cutter (PATC). APS’s PATC enables the cutting of heavy wall tubes at any length up to the tangent point of the U-bend in order to facilitate tube sample removal. Because the thickness of the tube walls ranged between 0.083 inch and 0.115 inch, cutting and removing (or even accessing) a tube sample would be very tough otherwise (Figure 2).
 |
| 2. Remove the heater. American Power Services (APS) was able to extract a tube sample using its proprietary and patented advanced plasma arc tube cutter (PATC). APS’s PATC enables heavy wall tubes to be cut at any length up to the tangent point of the U-bend to facilitate tube sample removal. Courtesy: American Power Services |
“They came up with the idea of sleeving the outlet tubes to increase the longevity of the heaters and return the heaters to service,” says King. “They sleeved all the feedwater outlet tubes that had 70% or greater wall loss and had signs of stress cracking.”
Email
Comments
Print
Reuse
