Flow iron recertification is a multi-step process that involves asset tracking, equipment teardown and rebuild, sand blasting and repainting, visual inspection, ultrasonic thickness testing, magnetic particle testing, and hydrotesting. This article focuses on some of the inspection aspects of the recertification process. TKS’ Flow Iron AnalyticsTM is proven process that implements industry best practices across asset management/tracking and high-pressure component asset integrity.
Figure 1: Visual inspection of 2” elbow showing severe erosion on ID.
What is Flow Iron?
Flow iron encompasses a broad spectrum of steel pressure bearing equipment used in the hydraulic fracturing and flowback operations and is generally fabricated to API-6A Wellhead and Tree Equipment and ASME Boiler and Vessel Code. Components range 2” to 5” diameter and are used to support operations in the 2,000 to 20,000 PS range for standard and sour service operations. The different components are commonly manufactured using 4340 steels, a medium-carbon nickel-chromium-molybdenum alloy. There are dozens of unique flow iron used in a standard fracking and/or flowback operation and some more common components including plug valves, swivels, check valve, dart valves, elbows, tees, pup joints, and crosses.
Flow Iron Non-destructive Testing
Flow iron non-destructive testing (NDT) is required periodically to ensure safe operation of the high-pressure equipment on-pad. Frack iron visual inspection is the fastest and most economical way to screen iron for wear and sealing surface damage. Visual inspection should be performed in accordance with American Society of Mechanical Engineer Boiler & Pressure Vessel Code (ASME-BPVC). Wear on the inside diameter (ID) of elbows is commonly detected via visual inspection. Any signs of wear generally required the elbow to be scrapped. This policy differs from other comparable assets in midstream and downstream sectors where some where is acceptable and where corrosion/erosion rates can be established. In contrast, the ID erosion rates in flow iron are generally unpredictable and for this reason it is advisable to scrap components that show early signs of wear. Example elbow erosion is shown below in Figure 1 for a 2” 15,000 PSI scrapped elbow. Sealing surfaces must also be inspected periodically as well. On plug valves, these include the body cap threads pocket walls, plug valve outside diameter (OD), and segment sets. The body cap threads should be checked for damaged threads, especially the lead thread. The pocket walls should be checked for scratches, dings, or pitting, especially in the area immediately surrounding the valve bore and for any sharp edges around the valve bore that can cut the insert O-Rings. The plug valve should be checked for washout, which will render the part unusable. It is advised to hold part in a well illuminated area and inspect for slight scratches in plug. If scratches are visible, a 600-grit sandpaper may be used to remove them. Figure 2 shows advanced erosion on a plug valve side segment.
Figure 2: Visual inspection of side segment showing severe erosion on ID.
Ultrasonic Thickness Testing of Flow Iron
Most flow iron components require periodic ultrasonic thickness testing (UTT) before resuming service. This non-destructive testing procedure is performed with a standard dual element digital or A-scan thickness gage. Thickness measurement locations (TMLs) are specified by the part manufacturer and may differ between manufactured, part size, figure number (6002, 1002, and 1502) and standard or H2S service. An example case study for an American Block 2” 15,000 PSI long sweep radius elbow is shown below in Figure 3. The 2” 1502 elbow, standard service, identifies two ultrasonic thickness testing locations: ‘A’ Male End and ‘B’ Curved Section with new wall of 0.540” and 0.0.685”, respectively. The minimum wall thicknesses in these same areas are 0.400” and 0.440”. The lowest readings in the male end ‘A’ and curved section B determined by ultrasonic thickness were 0.430” and 0.420”, respectively. As a result, this part fails and must be retired from service.
Figure 3: Ultrasonic thickness testing of elbow.
Magnetic Particle Testing of Flow Iron
Fatigue cracks for in at flow iron contact surface like sub ends and ASME threads. The areas are subjected to millions of cycles during a normal fracking operation and subject to crack initiation and propagation. As a result, magnetic particle testing (MT) further supplements the visual inspection to improve the probability of detection of tight fatigue cracks or cracks that are difficult to identify in thread roots. The recommended MT techniques include dry magnetic particle testing, wet visual magnetic particle testing, and wet fluorescent magnetic particle testing. In some cases, dual light magnetic particle suspension may be used. Fatigue cracks generally initiate and propagate in the circumferential direction due to the hoop stress applied across the thickness of the components and the lateral play in the piping system that is subject to high frequency vibration during fracking and flow back operations. Since this is the case, the primary magnetic particle testing is performed in the axial direction of the component as shown in Figure 4. In this scenario, the component is longitudinally magnetized in the axial directions. A fatigue crack will disrupt the induced magnetic field and magnetic flux will leak around the crack opening. The wet bath will be attracted to the magnetic flux leakage (MFL) and show an indication.
Figure 4: Magnetic particle testing of check valve ACME threads.
Hardness Testing of Flow Iron
The suitability of a material for exposure to H2S Service is dependent upon many variables, one of which for steels is the hardness measure of the material. H2S Service equipment shall be designed to meet NACE MR-01-75: Petroleum and Natural Gas Industries — Materials for use in H2S-containing environments in oil and gas production. Pressures containing steel components that may be exposed to sour environments are therefore limited to a hardness value not to exceed 22 on the Rockwell “C” scale. NACE MR-01-75 lists specific grades of steel that meet the hardness equipment.
Figure 5: Hardness measurement on flow iron to determine suitability for HS2 service.