Development of NDT Inspection Techniques For Heavy Wall. Stainless Steel Piping.

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Brandon Hood
6 years ago
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1 NDE Pressure Lab Analysis Development of Inspection Techniques For Heavy Wall Presented by: Stainless Steel Piping. Brian K. Beresford Canspec Group Inc. Field Service Supervisor Tubular Services Presented at: 8 th EPRI Balance-of of-plant Heat Exchanger NDE Symposium Key West, Florida June 21-23, 23, 2004

2 Acknowledgements Wayne Smith. Oxy Vinyl's, Canada. Hang Zheng. Canspec Group Inc. Gary Kroner. Carbon Steel Inspection.

3 Overview Introduction Detection method Probe development Test standard Inspection of the test pipes Analysis technique Conclusions

4 Introduction Canspec developed a nondestructive detection test method for inspection of NPS 3 in. Diameter (0.121 in. Wall thickness) and NPS 6 in. Diameter (0.146 in. Wall thickness) SA-312 TP 316L stainless steel pipes while in operations. The pipes were known to have corrosion pits on the internal surface. The pipe, which was in a horizontal position, had been used to carry liquid vinyl chloride monomer (VCM) at 150 psig and approximately 17 C since 1996.

5 Introduction (Cont d) Water condensates tended to gather at the bottom of the pipe; and the chloride containing water resulted in pitting on the pipe internal surface. After a corrosion pit was created, water was collected in the pit and thus high concentration of chloride ions was created inside the pit. This increased the corrosion rate inside the pit, eventually caused a large cavity inside the pipe steel.

6 Introduction (Cont d) NPS 3 in. diameter test piece

7 Detection Method With considerations of the material and the size of the pipes as well as the type of the defects to be inspected, an eddy current inspection technique was considered to be the best choice. Eddy current should be able to defect corrosion pits on the pipe internal surface with a reasonably good sensitivity. This technique should allow an inspection of the pipes while the plant is in operations with a reasonably fast inspection speed.

Probe Development The probe consisted of a differential coil that was embedded in a hand-held housing and two stainless steel bars that kept the clearance between

8 Probe Development The probe consisted of a differential coil that was embedded in a hand-held housing and two stainless steel bars that kept the clearance between the coils and the pipe surface while providing good wear resistance to the probe. The probe could easily be held in one hand and slid easily along the pipe surface

9 Test Standard A section of the test pipe (NPS 3 in.) Was longitudinally split in half and one half was made to be a test standard piece. The test standard piece had four 3 / 64 in. (0.047 in.) Diameter drilled holes with depths representing 100%, 75%, 50% and 20% wall losses respectively. Tests showed that the detection system could detect all the drilled holes including the shallowest hole (20% wall loss).

Test Standard (Cont d) The test standard piece also contained an approximately 0.05 in. Diameter, 0.032 in.

10 Test Standard (Cont d) The test standard piece also contained an approximately 0.05 in. Diameter, in. Deep (26% wall loss) natural corrosion pit the calibrated test system could detect this pit with an accurate estimation of the wall loss.

11 Test Standard (Cont d) A permeability variation replication was also added to the test standard piece in order to set the system to be able to differentiate false indications due to permeability variations.

12 Inspection of the Test Pipes Inspection revealed a through-wall thickness pinhole and two inside diameter indications. The first inside diameter indication appeared to be a large pit that has caused an approximately 90% wall loss; And the second was a small pit with an approximately 20% wall loss. Both the inside diameter indications and the through-wall pinhole were located at the bottom of the pipe.

13 Inspection of the Test Pipes (Cont d) The pipe was then longitudinally split in half and the internal surface cleaned using a wire brush for examination of the internal surface. This revealed small diameter pits at locations where the inside diameter indications were detected. Subsequent metallographic examination of the first indication revealed that the pit had a large cavity that covered 3.5 mm X 5.0 mm area inside the tube wall and an 1 mm diameter opening on the pipe internal surface wall loss was measured to be 90%

14 Inspection of the Test Pipes (Cont d) Close up of the large pit (view from the internal surface of the pipe)

15 Inspection of the Test Pipes (Cont d) Scanning electron microscope (SEM) image of the cross section of the large pit

Inspection of the Test Pipes (Cont d) The second indication was a 0.04 in diameter, 0.020 in.

16 Inspection of the Test Pipes (Cont d) The second indication was a 0.04 in diameter, in. deep pit which accounted for a 17% wall loss. More importantly, the system could detect a pit where the wall loss was as small as 17%, which met the required 25% wall loss criteria.

17 Data Analysis Analysis Technique An inspection and analysis procedure has been developed specifically for the pipes to be inspected. This procedure detailed the equipment required, the calibration standard, the operation parameters and procedure. More importantly, methods of differentiation of false indications were specifically addressed.

18 Analysis Technique (Cont d) ASME standard 100% 60% 20%

19 Analysis Technique (Cont d) Permeability Mix Standard 100% Perm Test Pipe

20 Analysis Technique (Cont d) Defect in test pipe 17% wall loss

21 Analysis Technique (Cont d) Inspection on another NPS 3 in. diameter pipe section in the plant detected an indication different from a pit-like indication. Subsequent metallographic examination of this area revealed longitudinally oriented crack-like defects associated with corrosion. The defects were up in. deep inside the pipe, which accounted for a 20% wall loss.

22 Analysis Technique (Cont d)

23 Conclusions The pit was caused by chloride corrosion as a result of moisture in the liquid VCM in the pipe. The wall loss measured from the cross section of the pit was 90%, which confirmed the eddy current inspection result. The inspection system can detect pits larger than 20% wall loss, which meets the 25% wall loss criteria required by the customer.

24 Conclusions The system can give a reasonably good estimation of the amount of wall loss and the size of a pit within its resolution ranges. The system can also detect crack-like defects associated with corrosion. The system has the ability to eliminate false indications caused by permeability variations.

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