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 Reference 3.
 

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.
 

Heater Layout: Horizontal
Total Tubes: 1931
Plugged tubes: None in March 1998
Tubes Inspected: 582 (Inlet), 307 (Outlet) in March 1998
Tubes Inspected: 480 (Inlet), 191 (Outlet) in October 1999
Tube Material: Carbon Steel
Tube Size: ¾ inch OD x 0.095
Inspection Technique: Remote Field Eddy Current Testing (RFECT)
Type of Damage: Tube erosion at inlet side End Plate and bottom of drain cooler

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.

Heater Layout: Horizontal
Total Tubes: 739
Plugged tubes: 41
Tubes Inspected: 163
Tube Material: Carbon Steel
Tube Size: 5/8 inch OD x 0.060
Inspection Technique: Remote Field Eddy Current Testing (RFECT)

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.

Heater Layout: Horizontal
Total Tubes: 1526
Plugged tubes: 20
Tubes Inspected: 153 (Inlet), 150 (Outlet)
Tube Material: Carbon Steel
Tube Size: 3/4 inch OD x 0.102
Inspection Technique: Remote Field Eddy Current Testing (RFECT)
Type of Damage: Steam impingement in the desupeheater zone

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.

Heater Layout: Horizontal
Total Tubes: 994
Plugged tubes: 19
Tubes Inspected: 745 (Inlet)
Tube Material: Carbon Steel
Tube Size: 3/4 inch OD x 0.085
Inspection Technique: Remote Field Eddy Current Testing (RFECT)
Type of Damage: Tube erosion at inlet side end plate

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.

Heater Layout: Horizontal
Total Tubes: 1134
Plugged tubes: 21
Tubes Inspected: 135 (Inlet), 205 (Outlet)
Tube Material: Stainless Steel
Tube Size: 5/8 inch OD x 0.035
Inspection Technique: Conventional Eddy Current Testing (ECT)
Type of Damage: Support wear under the steam inlet

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.

Heater Layout: Vertical
Total Tubes: 307
Plugged tubes: 7
Tubes Inspected: All tubes
Tube Material: Cu-Ni 7030
Tube Size: 3/4 inch OD x 0.095
Inspection Technique: Conventional Eddy Current Testing (ECT)
Type of Damage: Support wear on the inlet side and OD cracking on the outlet side

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.
 

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.

• Inspection during a planned outage. This inspection is performed on tubes around plugged tubes, peripheral tubes and a random sample to cover the tube sheet cross-section. When damaged tubes are detected, the inspection scope is increased until all the damaged tubes have been checked. This inspection is completed in one shift.
• Inspection during a forced outage. This inspection is localized around the leaking tubes. An inspection during a forced outage should be completed in less than four hours.

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.

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.
 

1. Feedwater Heater Survey. Palo Alto, Calif: Electric Power Research Institute, August 1991. GS 7417.
2. High-Reliability Feedwater Heater Study, Palo Alto, Calif: Electric Power Research Institute, June 1988. CS-5856.
3. A. S. Birring, "Selection of NDT Techniques for Inspection of Heat Exchangers," Presented at the ASNT International Conference on Petroleum Industry Inspection Conference, Houston, TX. June 1999.