|Proceedings of EPRI's sixth Balance of Plant (BOP) Heat Exchanger NDE Conference, Scottsdale, Ariz., June 2000|
Failure of HP feedwater heater tubes is one of the major causes of forced outages in fossil fired power plants. Deterioration of tubes occurs because of either poor design or improper heater operation. The location of damage across the tubesheet and along the tube length depends on the cause of damage. Tube inspections identify damaged tubes and location of damage along tube length.
Tube failures in a heater can be avoided by following a well-planned inspection program. Such a program includes two types of inspections: 1) inspections during planned outages and 2) inspections during forced outages. The inspections during planned outages are performed on a random sample where tubes are carefully selected to cover the entire heater cross-section. This inspection determines the overall condition of the heater and identifies areas with deterioration. Information from this inspection is used to take corrective actions and plug tubes with excessive damage. Inspection during a forced outage is targeted in the area with leaking tubes. Tubes with excessive damage are identified and immediately plugged. Forced outage inspection requires rapid response to complete the inspection. Implementation of these strategies reduces forced outages and makes eddy current inspection of heaters a highly cost effective operation.
Tube failures of high-pressure feedwater heaters are one of the major causes of forced outages in fossil-fired plants. The causes of these failures are related to design problems, fabrication problems and operational problems (1). These basic problems can result in various types of damage to heater tubes. For example, design problems are related to high velocity in tubes, vibration, material selection, and tube to tube sheet joint. Problems related to fabrication include support plate drilling, welds, tube to tubesheet joint fabrication. Operational problems are related to cycling, startup, lay-up, low load operation, operation with excessive flow, flow out of leaking tubes, water chemistry etc. These problems result in a variety of damages to the feedwater heater tubes.
Bell et. al.(2) conducted a detailed survey to rate the problem areas in the feedwater heaters. The top ten problem areas are listed below
Figure 1. Zones in a HP Horizontal Feedwater Heater
Utilities have two options to address heater problems. The first option is to wait until a tube leaks. In this case, the unit is brought down from operation, the leaking tubes identified and plugged. In addition, tubes around the leaking are plugged. The plugging of additional tubes is normally referred to as preventive plugging. However, this calculated guess approach does not guarantee identifying the damaged tubes correctly and results in plugging tubes that may be in good condition. This results both in unnecessary tube overplugging and missing tubes that need plugging. Further tube leaks normally follow this approach.
The second option is to perform tube inspections and determine the condition the heater tubes. A large number of the utilities are discouraged from this option because they fear the cost of inspecting thousands of tubes. A 100 percent inspection of an average size heater with 1000 tubes can definitely be quite expensive to inspect. However, our experience has shown that the heater problems are localized and the main task of the inspection is to identify and evaluate these areas. This approach limits the inspection to a significantly small number of tubes and makes the inspection highly cost effective.
Conventional eddy current and remote field eddy current are the two commonly used techniques for heater tube inspections. Non-ferromagnetic tubes such as stainless steel, copper-nickel alloys, brass are inspected by conventional eddy current method. Ferromagnetic materials are inspected by the remote field eddy current method. Detailed description of the techniques can be found in
Based on our experience with inspecting hundreds of heaters, we present some typical cases. These case studies demonstrate the variety of inspection scenarios the outcome of results on tube plugging, future inspection and heater replacements.
HP Heater 81
Heater Layout: Horizontal
This heater with no plugged tubes was inspected to determine the condition of tubes. Approximately 200 tubes (10 percent sample) of the tubes were selected for inspection in March 1998. The inspection identified two areas with damage on the inlet side. These included the bottom tubes in the drain cooler section and tubes on the top right side. After identifying the damaged areas, the inspection scope was increased to100 percent in the damaged areas. The tubes in these areas were inspected until all the damaged tubes were identified. This resulted in 582 inspected tubes on the inlet side. Of these, 96 tubes had damage greater than 60 percent in depth. Seven of the tubes were in the top right side and remaining 89 in the bottom drain cooler area.
Although the inspection detected two damaged areas with 96 tubes over 60 percent damage, only 10 tubes were plugged. These were all in the in the damaged area at the top right side. No plugging was done in the drain cooler area that had 89 tubes with damage in excess of 60 percent. Since all of the damaged tubes were not plugged, the heater experienced tube leaks when it went back on line. Two tubes leaked between March 1998 and October 1999. In addition, 3 tubes were found to be leaking when the unit was brought down during a planned outage in October, 1999. The five leaking tubes were all at the bottom of the drain cooler zone, identified in the previous inspection with over 60 percent damage. The exchanger was the re-inspected in October 1999. The results were similar to the inspection in 1998. At this point, the utility decided to plug all tubes with damage in excess of 60 percent. A total of 116 tubes were plugged.
The above example clearly shows the benefit of the tube inspections. Had the utility plugged the tubes identified in 1998, none of the tubes would have leaked during operation.
HP Heater 12
Heater Layout: Horizontal
Type of Damage: Tube erosion at inlet side end plate
The heater developed a leak during operation. The unit was brought down on a forced outage to plug the leaking tube. Two leaking tubes were detected on the inlet side. The utility decided to inspect the tubes around the leaking tubes and around the previously plugged 41 tubes. The inspection found significant damage at the end plate. Twenty-one additional tubes were found to have wall loss greater than 60 percent. These tubes were plugged.
The inspection was beneficial because it identified the damaged tubes for plugging. There were no unnecessary plugged tubes.
HP Heater 32
Heater Layout: Horizontal
Heater inspection was performed during a planned outage. The heater had 20 plugged tubes. The inspection included tubes around the plugged tubes, peripheral tubes and a five percent random sample across the tube sheet cross-section. Three peripheral tubes on the outlet side with damage in excess of 60 percent were detected. The tubes were symmetrically opposite to each other (one on far-left and two on far-right). The damage to the tubes was caused by the steam impingement in the desupeheater zone. All three tubes were plugged. No damage was detected on the inlet tubes. No damage was detected around the plugged tubes.
The inspection was beneficial as it identified the three specific tubes with damage. The inspection showed that overall the heater was in good shape and the damage was limited to three peripheral tubes on the outlet side.
HP Heater 51
Heater Layout: Horizontal
This heater with 19 (2 percent) plugged tubes developed leaks during operation. Inspection was performed during the forced outage. Tubes around the leaking tubes were inspected. RFECT inspection found several tubes with damage. The damage was at the end plate in the drain cooler zone. Some tubes were plugged during the forced outage but a more detailed inspection was planned during the next outage. During the planned outage, all tubes in the drain cooler were inspected. The inspection found that almost all the inspected tubes had some level of damage. Of these damaged tubes, 147 tubes (15 percent) had wall loss in excess of 60 percent. In order to avoid further tube leaks, 15 percent of the heater tubes would have to be plugged. Even after plugging these tubes, there will still be a large number of tubes with some level of damage. A plug rate of over 10 percent would start affecting the heat rate of the heater. The utility decided to plug 54 tubes that had damage in excess of 65 percent and replace the heater at the next outage.
The inspection was instrumental in determining the overall condition of the heater. Even though the heater had only 19 plugged tubes, a large number of tubes were reaching the end of life. Plugging all the 147 tubes could operate the heater without further leaks but this would have reduced the heat rate. The inspection helped in justifying timely replacement of heater tubes.
HP Heater 22
Heater Layout: Horizontal
The heater had a history of tube leaks in the peripheral tubes on the outlet side. The damage was caused by steam impingement in the desuperheating zone. An inspection was performed during a planned outage. The inspection plan included inspecting all the tubes around the plugged tubes, peripheral tubes and a 5 percent random sample. Eddy current inspection showed several additional damaged tubes around the plugged tubes and peripheral tubes. The tubes showed high degree of support plate wear and wall loss. The support wear was localized to the first two supports next to the tubesheet and directly under the steam inlet. Fifteen tubes were detected with damage over 60 percent. These tubes were plugged.
The inspection was beneficial as it identified the fifteen specific tubes with damage. The utility engineers were also got the information that the damage was due to support plate wear. Corrective action could be taken to avoid future failures.
HP Heater 31
Heater Layout: Vertical
The heater had only seven plugged tubes but experienced leaks. Inspection quickly showed that there was extensive wear at the first baffle on the inlet side. The utility decided to conduct a 100 percent inspection to determine overall condition of the heater. The inspection showed almost a third of the tubes experienced baffle wear. Of these tubes, 21 tubes had baffle wear in excess of 50 percent. In addition, OD cracking was detected on the outlet side. Thirteen peripheral tubes showed OD cracking.
There were two options. Plug the tubes and operate at a low heat rate. The other option was to keep the plugging below 10 percent but risk more leaks. The utility determined that the heater was at the end of its life so decided to replace it at the next available opportunity.
The inspection was beneficial as it established that the heater was at the end of its life. The utility used the eddy current data to justify heater replacement although the heater only had seven plugged tubes.
Discussion of Case Studies
The above examples provide a spectrum of heater cases that demonstrate how the inspections can be effectively used for taking maintenance decisions (see Table 1). The inspection data locates and identifies the type of damage. This information can be used to plug the tubes and take corrective actions to further avoid additional tube damage.
Since the damage is localized, not all the tubes have to be inspected in the heater.
There are essentially two types of inspections to determine the condition of heater tubes.
Table 1 provides a summary of the five case studies. The table demonstrates how the inspection results are used to take repair and replacement decisions.
Table 1. Summary of Case Studies
Damage to heater tubes is caused during operation. This may be because of poor heater design or improper operating conditions. In any case, leaks in HP feedwater heaters should be avoided to reduce forced outages.
Heater operating experience has shown that the damage is usually localized. Therefore, the strategy during an inspection is to first find the damaged area and then perform a detailed inspection of the damaged area. This approach reduces both the total number of inspected tubes and the inspection cost. Six case studies on feedwater heater heaters were presented. The studies demonstrate the effectiveness of a carefully planned inspection program. The heater inspections are carried out by performing the following: 1) general inspections of a carefully selected tube sample during a planned outage and 2) inspections around the leaking tubes during a forced outage. These plans keep the inspection costs to a minimum while locating the damaged tubes quickly.
NDE Associates, Inc.