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42 Inch Vac transfer line developed leak at 6 clock position at the 20 inch branch connecting location which was connected at 12 clock position of the pipeline. Total 2 no of branch in which one no of branch location leak observed in 42 in header due to internal erosion. In other branch location at 42 inch header the thickness was found to be 4mm over a length of 500mm at 6 clock position straight bottom to the 20 inch branch. The line is of P5 metallurgy with SS 316 clad. original thickness 12.7mm. The line was in service for 19 years. Remaining locations thickness was found to be above MAT. Only in 2 branch locations at localized location severe erosion observed.
In this instance, the leak was found to be due to turbulence and velocity which caused erosion but are there any other factors which cause this kind of failure?

 
Answers
11/08/2017 A: NS Murthy, GE, murthy.ns@ge.com
You may like to look into couple of points.
1. Check the level of shear in the failed location as excessive shear can lead to erosion.
2. Trust the feed long residue has not picked up nap acid above typical tolerable limit of 0.6mgKOH/gm in crude mix to the unit.
3. Carry out SEM analysis of failed location to get any 'tell tale' indication on type of corrosion witnessed.
If needed ask nearest GE Water and Process Technology team to interact with your operations and inspection team to assess the root cause and eliminate recurrence. Good luck.
10/08/2017 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Three comments: (1) 19 years is better than 10 years, but when we design for a “20 year life”, we usually experience 30 years with good maintenance. (2) Designing the piping layout exiting the vacuum heater requires extensive experience to achieve operability and durability. Without seeing your layout, we cannot judge the quality of it. (3) Velocity is not the cause. Critical velocity cannot be exceeded due to physics, and critical velocity at some point in the system is not unusual. Most engineers design for 80% of critical as a target.