Failure analysis of inner pipe leak in spool piece of jacketed pipeline

Failure analysis of inner pipe leak in spool piece of jacketed pipeline

MOC: Inner pipe is 3" Sch.40, ERW, SS316 and jacket pipe is 4" M.S. class CService Life: Unknown

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Description of failure: A leak was observed in the inner pipe of the 2.2 meter long spool pieces forming a 25 meter long jacketed pipeline of plant 2/B manufacturing unit of PMV acid. The fluid on jacket side is HNP. The leak was found out by conducting pressure test on each individual segment of 2.2 meters with pressurized water filling on jacket side.The suspected spool revealed a leak under the jacketed length through a circumferential weld joint made on inner pipe at about 400mm from top flange end. A leak was observed in the inner pipe of the 2.2 meter long spool pieces forming a 25 meter long jacketed pipeline of plant 2/B manufacturing unit of PMV acid. The fluid on jacket side is HNP. The leak was found out by conducting pressure test on each individual segment of 2.2 meters with pressurized water filling on jacket side.The suspected spool revealed a leak under the jacketed length through a circumferential weld joint made on inner pipe at about 400mm from top flange end.

Investigation: Figure 1 shows close-up view at the surface of one of the broken pieces of impeller. Fracture contours with random orientation are seen. At some places, contours are deformed indicating their secondary nature of failure.

Figure 2 shows low magnification view at puncture on inner side weld. The puncture is by way of pit. The pit surface is dark brown in colour. Puncture contours towards OD are uneven.

The butt weld joint between the pipe in longitudinal section shows different section thickness of two pipes. Also the edge of thicker pipe is not properly prepared before welding that left improper fusion (Figure 3).

Figure 4 shows outer edge microstructures of weld and HAZ region which is worked austenite with scattered delta ferrite stringers. Transgranular cracks which are branched in nature are observed origniating from outer edge. The crack is confined to the weld and HAZ.

Location		Micro-Hardness in "VPN" at 100g load
At puncture side tube	ID	260,262,265
	OD	245,250,248
	Core	243,239,235
	OD-weld	259,272,269
	Near puncture ID	227,221,231
At other side tube	ID	158,165,167
	OD	188,187,174
	Core	182,178,164
	OD-weld	176,183,180
	ID-weld	175,181,178

The increased hardness and microhardness of the failed pipe indicates enhanced cold worked condition since manufacturing stage. The cold worked condition has adverse influence on the corrosion resistance. Thus, it is conclusive that pipeline used in PMV acid service was containing incompatible material of 304L spool piece that was susceptible to pitting corrosion. During the operation it failed by way of pitting corrosion and inter-dendritic corrosion at weld led to puncture.

Conclusion: The puncture of the inner pipe of spool is because of use of wrong material, poor fabrication issues, and corrosion as explained below.

• The use of 304L instead of 316L.
• Weld metal indicated carbide precipitation and got susceptible to inter-dendritic corrosion.
• Instead of using single inner pipe, it is fabricated by joining two pipes to meet to the required segment length and the thickness of the supplementary pipe piece was not compatible with the 316L.

The failure of the superheater tube by way of cracking at weld between inlet header and tube is because of improper PWHT.

MOC: SA 210 grade CService Life: about 2½ years

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Description of failure: The window type rupture has taken place as primary failure on parent metal, and there was secondary nature of erosion failure on the adjacent tube.

Investigation:Figure 1 shows the failed portion of water wall tube samples of tube no. 150 and 151. Tube no. 150 is having primary failure which is window opening type. Figure 2 shows rupture lip surface at some other location on one corner.

During failure the metal seems to have been partly caved out from the inner side of the rupture surface. Faint nuances of crack like discontinuities remaining present on the rupture lip which are towards inner side are indicative.

Fractograph of Inner edge of fracture surface rupture lip (Figure 3) indicates inter-granular nature of brittle fracture surface. Discontinuous fissures and grain boundary cracking is also seen (1000X).

Figure 4 shows inner edge microstructure. Surface decarburization is noticed. Also, there is grain boundary fissures filled with scale and preferential attack on pearlite colonies. Inner surface is covered with porous scale (200X).

Conclusion:The failure of water wall tube is essentially due to hydrogen damage leading to formation of fissures and micro-cracks that weakened the tube matrix at failure location. The hydrogen damage occurred due to under-deposit corrosion at localized region.

MOC: Grade 410SService Life: about 4 years

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Description of failure: The heat exchanger was installed in 2009. For the first time it was opened in Feb 2013 for routine maintenance. The leakages were noticed when it was opened for the second time in Nov 2013.

The heat exchanger tube containing crude with identification number ME-RDZ312-S15AB in SEZ Crude sub-section started showing leakage after hydro-testing. More than 400 tubes leaked out of 1054 tubes.

Investigation:
Figure 1 shows half portion of the cut tube having pinhole puncture that was received for investigation. Scattered pitting corrosion damage is noticed in longitudinal direction in line with the pinhole puncture.

Figure 2 shows low magnification close up view on inner surface at puncture location. Puncture is conical in nature with decreasing cross section towards outer surface. The puncture surface is dark brown in color. The puncture contours are uneven but sharp and the surrounding area is free from any corrosion. On surface at away location, there are scattered shallow pits.

Figure 3 shows close up view at puncture location from inner surface. Area surrounding the puncture is heavily corroded. There is thinning at puncture contours.

Figure 4 shows panoramic view of the puncture. Puncture is with large opening at inner edge which tapers down towards outer surface with non-uniform contours. There is slight increased opening on outer edge, this seems to be caused by corrosive fluid oozing out under slight pressure resulting into ventury like shape.

Conclusion:The premature puncture of the exchanger tubes appears to be on account of pitting corrosion damage which was under the influence of oxygen, moisture, sulphur and chloride ions as contaminants in the crude that was being carried.

MOC: ManuritexMService Life: 1,42,000 hours

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Description Of Failur : The primary reformer catalyst tube A20 of primary reformer, A-BA-101 found to have failed by way of rupture during operation. The failure occurred at a distance 3.8 to 4.8 m length from the top and the rupture length was 1 meter.

Investigation:
Figure 1 shows the photograph of reformer tube. The bottom most sample is cut portion having the crack area. In the middle the sample from left end side belongs to away from crack tip end marked with star. The middle one belongs to away from the crack tip marked with arrow. The one on right end belongs to inlet side. The one above is sample 5 from outlet end. The rupture has taken over the longer length up to approximate 1 meter length. Bulging is not observed except at the cracked region.

Figure 2 shows low magnification view of the fracture surface. The columnar nature of grain structure is highlighted on the brittle fracture surface.

Figure 3 shows ID surface fractograph which highlights quasi cleavage nature of fracture surface, which is on the inter-dendritic columnar structure (100X).

Figure 4. The top inlet sample shows primary carbides in the austenite matrix. Very few secondary precipitates are observed around inter-dendritic region. The failed region shows microstructure comprises of primary carbides at inter-dendritic region along with secondary precipitation including carbides within the austenite matrix. Presence of creep at primary carbides along with coarsening and coagulation of secondary precipitation and dissolution in the matrix. Bottom location microstructure shows coarsening of primary as well as secondary carbides at the inter-dendritic region as well as within austenite matrix. This location shows maximum coarse primary carbides along with higher density of secondary precipitations.

Top (inlet) sample	Failed sample	Bottom (outlet) sample
Core   (400X)	Core   (400X)	Core   (400X)

Conclusion:
The failure of reformer tube appears to be on account of localized temperature excursions which resulted in extended creep damage due to microstructural degradations resulted in rupture.