332530 - Pine Hill Tank, Blountville, Tennessee Weld Seam Failure Forensic Investigation
Saturday, April 21
8:00 AM - 9:30 AM
On Saturday, August 22nd, 2015 Utility Service Company, Inc. (USCI) received a call from Blountville Utility staff indicating a split in one vertical weld seam in the first ring of the Pine Hill Tank wall shell. The tank was full at the time the split was discovered and there was noticeable outward lateral deformation (bulging) at the split as well as visible leakage.
On Wednesday, September 2nd, 2015, a team led by Donnell Duncan, P.E., S.E., Professional Structural Engineer of USCI completed a visual structural assessment of the tank. The assessment included a forensic investigation of two main issues; the weld split and the reported deformation (bulging) in the first wall shell of the tank wall when full.
Inspection guidelines were provided by Chapter 9 – “Professional Examination and Renovation” of AWWA Manual of Water Supply Practices - M42: “Steel Water-Storage Tanks” and Chapter 4 – “Suitability for Service” of API 653: “Tank Inspection, Repair, Alteration, and Reconstruction”. The onsite investigation found that there were other broken vertical weld seams on the first two rings, some of which were clearly observable. They were all at different stages of deterioration and future weld fractures were possible if not imminent.
In addition to the weld issues, there was visible evidence of extensive metal loss due to pitting corrosion along the interior surface of the tank wall. Thus, an ultrasonic thickness gauge was used to measure the thickness of the tank shell. The required cylindrical shell plate thickness based on Equation 3-40 in AWWA D100-11 Section 3.7 was then established as the baseline for comparison to the measured thickness of the tank shell.
Further investigation revealed that the resulting weld failure could have been prevented. Maintenance workers who previously worked on the tank provided insight based on their observations of the tank’s condition over time. In 2009, the tank underwent major renovations including abrasive blast cleaning of the interior surfaces, interior and exterior coating, installation of a bolted shell manhole and other miscellaneous repairs.
Routine maintenance and inspections since that time did not reveal any major structural deficiencies. However, a closer look at the maintenance and inspection process exposed one minor oversight which allowed this failure to take place. What appeared to be a cosmetic issue at the time of observation was the initial sign of a structural failure.
The investigation and resulting repair consisted of the following general steps;
Step One: Identifying the Problem
Step Two: Diagnosing the Cause of the Problem
Step Three: Locating the Source of the Problem
Step Four: Emergency Repairs
Step Five: Long Term Repair