Corrosion on welded joints of buried head race pipe A case study of Daraudi Khola A Hydropower Project

Abstract

Engineering joints are done by welding processes and shielded metal arc welding is the found most common. Many hydro powers in Nepal have buried-type pipelines as a water conveyance system. These water conveyance systems pipelines suffer corrosion externally or internally also if protective coatings are done properly if scouring either on pipelines or tunnels which require timely maintenance for its increased service life. However, no any catastrophic failures have happened on pipelines due to corrosion of hydropower except massive earthquake knock in Nepal. A study was carried out study on external welded regions of pipelines of Daraudi Khola “A” hydropower regarding corrosion. Most projects developed by NEA have buried pipelines. To carry out corrosion development on this project may be insignificant due to less years of operation. Two test specimen pipes of I.D. 300mm length, 1.25m, 6mm thickness, were developed using metal plates E250 and laid on two separate pits being laid with two different backfill materials without being painted. The study suggests that soil sample containing sample A as loose soil with clay is more corrosive in comparison to sample B as sand mix gravels. Secondly, metal degradation buried by sample A is 0.2mm thickness whereas insignificant metal loss is found on sample B. On calculation, it is found that metal loss is 0.2mm on average when buried for almost 3 months. It considers that the remaining life of the test specimen pipe shell when buried by sample A lasts for 7.50 years without considering corrosion allowance. Location A on weld reinforcement are the most prone area for corrosion development. Considering 5% direct cost and neglecting indirect cost for utility/hydropower sectors then 26.71% i.e. NRs. 1,787.08 million of the total imported cost of materials must be allocated for the safety hazards and interruptions in plant operations.