1. Feedwater Heater Shell Corrosion
Feedwater heaters are used in the power generation industry to preheat feedwater delivered to a steam generator. This component improves the thermodynamic efficiency of the system and reduces plant operating costs.
The carbon steel shell of the heat exchanger is prone to corrosion. Since the shell is typically insulated to prevent heat loss and further improve thermal efficiency, the structure is susceptible to Corrosion Under Insulation (CUI). Typical inspections require shutting down the component in addition to the costly and time-consuming stripping of the insulation to gain access to the surface for visual or Ultrasonic Testing (UT) inspections. Re-insulation is also required at the end of the process. All these steps are completed even if the absence of corrosion in the structure.
PEC has shown to be a suitable technology to address this challenge! The ability of the system to measure remaining wall thickness through insulation layers and weather jackets enables a cost-effective screening of the feedwater heater shell for corrosion under insulation. On this specific example, erosion was detected on the top area of the feedwater, the insulation was removed to perform local measurement, and ultrasonic testing was used to confirm the value of exact remaining wall thickness. When areas of interests are found, removing the insulation only at specific locations for prove-ups can help maximize efficiency of the inspection with no compromise on safety and confidence.
2. Elbow Inspection for Flow Accelerated Corrosion (FAC)
Steam and water carrying piping systems are common in the power generation industry. As with feedwater heaters, the lines are insulated for operator safety and to maximize process efficiency. A common damage mechanism found in pipe elbows is known as Flow Accelerated Corrosion (FAC). Water and wet steam carbon steel piping carrying deoxygenated water and wet steam are prone to this damage mechanism. The protective oxide layer dissolves in the constantly flowing fluid, exposing the underlying metal to continuous corrosion and wall loss over time.
Unlike CUI, FAC occurs inside the pipes. FAC is more common in the outer curve region of elbows known as the extrados. The capability of the Lyft PEC system to detect both near side (external) and far side (internal) wall loss through insulation is well suited to address the challenge of FAC.
Figure 2: Insulated elbows can be inspected in-service for CUI with PEC
Lyft software offers a special scan pattern to ensure 100% coverage of the bend. The result is a diamond-shaped C-scan image where the extrados is shown as the longer vertical section in the center of the scan. It is in this region were FAC, erosion, and corrosion are visible as shown in the following example.
Figure 3: Insulated elbow inspection with the Lyft and CUI of elbow with FAC damage
3. Cooling Water Discharge Pipe Inspection
Unlike the previously discussed applications, cooling water discharge pipes are not necessarily insulated, but they may be protected with thick coatings such as epoxy which renders UT thickness measurements impossible. Surface preparation required to carry UT thickness readings can be impractical or extremely expensive, leading to costly and time-consuming inspections.
Figure 4: Lyft PEC system and PECA-HR probe
Although PEC is known for its ability to inspect through liftoff in the order of inches, it can also be used to assess wall thickness through coatings that prevent the utilization of UT for screening. More sensitive probes such as the PECA-HR can be used in low liftoff applications to assess more accurately the remaining wall thickness. Furthermore, PEC works through corrosion product and scab/blistering corrosion which may be detected visually by the bulging and cracking of the coating.
Figure 5: 3D model of inspected component generated with the SurfacePro 3D reporting software
If you're in the power generation sector, you've likely encountered the three inspection challenges addressed here. We invite you to discuss your next inspection with our experts to leverage leading edge pulsed eddy current technology for reliable results. Get in touch with our team today!