Properly inspecting plant piping and components for service damage is an integral part of proper asset management. High energy systems constructed in accordance with ASME codes require appropriate inspections that are based on established industry practices, such as implementation of complimentary and non-destructive examination (NDE) methods that are best suited for detecting the types of damage expected within the system. In any instance where NDE is used to target service damage, it is desirable to perform high quality inspections while at the same time optimizing inspection efficiency in light of the need to return the unit to service. This concept is universally applicable to high energy piping, tubing, headers, valves, turbines, and various other power and industrial systems and components.
Optimized planning and execution of inspections includes one seemingly minor activity that can actually have a very significant impact on the success of NDE inspections – that is surface preparation (“surface prep”). Further, the quality of each NDE examination, which is paramount to safety, is very dependent on proper surface prep. Not only can poor surface prep lead to delays in the outage schedule, it can mask evidence of flaws or service damage, leading to missed indications and increased risk. Therefore, it is critical for workers and examiners to understand both the details and importance of proper surface prep.
In high energy and critical industrial piping systems, locations that require inspection are normally associated with circumferential butt welds (girth welds), longitudinal seam welds, saddle welds, and attachment (fillet) welds. Commonly used NDE methods for these locations are listed in Table 1 in accordance with their application to either the entire weld region or localized areas at or near welds. For purposes of this article, discussion of surface preparation is primarily focused on broad areas associated with the entire weld region.
Typical Baseline Surface Preparation Requirements
Whether a specific inspection method is targeting surface-connected, near-surface, or volumetric damage, proper surface preparation is critical for a successful inspection. As the areas that require surface prep are associated with welds, these areas essentially extend outward from the weld of interest, as described in Table 2. Also note that the required dimensions of surface prep areas are associated with the weld width and the component thickness. This is because angle-beam ultrasonic testing methods require specific angles that can only be maintained by shifting the probe farther from the weld as component thickness increases. This means that thicker components require wider surface prep regions along the welds.
Table 1. Commonly Applied Non-Destructive Examination (NDE) Methods1
|Visual Inspection (VT)||Surface (VT/MT/PT) and Volumetric (LPA-UT/TOFD) Inspections Involving the Entire Weld Region|
|Magnetic Particle Testing (MT)|
|Liquid Dye Penetrant Testing (PT)|
|Linear Phased Array Ultrasonic Testing (LPA-UT)|
|Time-of-Flight Diffraction Ultrasonic Testing (TOFD)|
|Annular Phased Array Ultrasonic Testing (APA-UT)||Surface (Reps, PMI, HT) and Volumetric (APA-UT/UTT) Inspections Performed at Specific Locations|
|Metallographic Replications (Reps)|
|Positive Material Identification (PMI)|
|Hardness Testing (HT)|
|Ultrasonic Thickness Testing (UTT)|
1Although some of the listed NDE methods may be used for new construction (“code”) examinations, this article is specific to “in-service” examinations; for more information on differences between code and in-service examinations, see “Volumetric Ultrasonic Examinations: ASME Code Compliant vs. In-Service Evaluations”, News & Views Issue 38, Spring 2015.
Table 2. Typical Minimum Requirements for Surface Prep Dimensions1
|Weld Type||Surface Prep Area =|
|Circumferential Welds||Weld + (6 x Wall Thickness), Centered on Weld|
|Saddle Welds||2” on Pipe Side + Weld + (3 x Wall Thickness)|
|Large Pipe-to-Fitting Welds||2” on Fitting Side + Weld + (3 x Wall Thickness)|
|Attachment (Fillet) Welds||Weld + 1” on Each Side of Weld|
|Tube Socket Welds||1” on Header + Weld + 4” on Tube Surface|
|Longitudinal Seam Welds||Weld + (4 x Wall Thickness) Each Side of Weld|
1In All Locations, 12” Clearance from Insulation to Surface Prep Area is Typically Required
The preferred method for surface preparation of coated or oxidized components is abrasive grit blasting. Coatings or oxide layers must be removed entirely within the surface prep area (Figure 1). For inspection methods involving surface-connected flaws or damage, coatings or oxide can cover or fill the indications, preventing detection during the inspection. Blasting is more desirable than other means of surface prep due to the consistent surface finish that it produces and for its ability to clean bead-to-bead interfaces and weld toes. Ultimately, the prepared surface should be a white metal finish in accordance with SSPC-SP-5, SA 3, and NACE 1 standards (essentially an exposed, clean metal surface with no significant surface damage, or residual roughness, from the surface prep process).
Grinding is an acceptable alternative, and may be the preferred method in locations where blasting media could cause issues within the plant (such as welds near turbines or where valves have been disassembled for service). Grinding is less efficient than grit blasting, but requires less effort in terms of containment, equipment setup, and cleanup. In most instances, it is best to remove coatings or oxide layers using grinding stones as a first pass, and to follow-up with a second pass using an abrasive wheel with a grit size ranging from 40 to 120 grit (Figure 2). However, grinding preparation is impractical for many branch connection and tube-to-header welds.
In comparison to grit blasting and multi-step grinding, other methods of surface preparation are typically inadequate for NDE inspections. Superficial grinding or prepping with wire wheel brush attachments, for example, removes loose material from the component surface, but does not adequately remove tightly adhered coatings or oxide layers (Figure 3). Blasting using non-conventional media, such as dry ice or walnut shell particles, may remove some coatings and oxide, but is often slower, and with tenacious oxides does not perform as well as harder abrasives. While laser blasting has some uses within the power generation industry, the most commonly available services use a green laser (~500 nm wavelength), which has not been adequately efficient in removing all surface oxides from chromium-alloyed steel (the surface finish is comparable to that from wire wheel brushing).
Since most service damage in welds occurs at the weld toes, between weld passes, or within the heat-affected zones (just below the weld toes), removing all non-metallic material along weld toes is imperative. This detail is perhaps the most common requirement that leads to delays, as incomplete surface prep leaves a thin line of oxide in and along the weld toes (in the exact location where service-induced damage is most likely to occur) (Figure 4). Welds prepared in this manner cannot be properly inspected with VT, MT, or PT methods (Figure 5).
In addition to the broader surface prep areas described above, attachment welds often require surface prep for MT or PT inspections for surface-connected flaws or damage. Locations where small bore pipe, thermowells, or support lugs are attached to the piping are often not examined using ultrasonic methods and therefore only require removal of coatings and oxide layers within one to two inches of the weld (e.g., Figure 1c.).
Ultrasonic testing using focused annular phased array requires four to six-inch long areas that must be ground flush with the outer surface of the component (Figure 6). The final surface finish should be white metal (as is the case with the broader area around the weld), but the OD surface must be flat in the area of the examination. This is typically achieved by grinding, even if the broader area is initially prepped using grit blasting.
For tube inspections carried out using VT and MT, grit blasting is efficient and can easily prepare surfaces within six inches of the weld (Figure 7). For examinations involving dissimilar metal welds (DMWs) in boiler tubing, surface preparation is required at accessible surfaces on the low alloy, or ferritic side of the weld (where the ultrasonic transducer will be placed onto the tube or pipe). The prepared area, which should extend six inches along the tube surface from the weld toe, must be clean, white metal. It is important that surface prep is applied to the ferritic side of the weld rather than the austenitic (stainless) side of the weld, and wire brushing is not acceptable as remnant surface oxide can lead to a low quality signal and associated uncertainty in the analysis (Figure 8). In most instances, the surface prep area does not need to include the weld or austenitic base metal. For any tube examinations where internal oxide thickness will be measured using UTT, heating of components should be avoided as it can cause exfoliation of the internal oxides that are being assessed.
In summary, surfaces should be prepared to a white metal finish throughout the specified areas with no remnant debris or oxide along the weld toes. This level of detail is critical to the quality of each NDE process, which, in turn, is the foundation for safe operation of high energy piping systems and long-term asset management. Clear and detailed communication to surface prep personnel, in combination with appropriate surface prep methodology, is the best way to ensure that the preparation process is effective in producing the correct conditions for NDE inspections. A focus on proper and timely surface prep is also an easy way to avoid costly (and unnecessary) outage delays.
For additional information on Surface Preparation, contact Ben Ruchte at BRuchte@structint.com