Remaining Life and NDT Interval for Carbon Steel Deaerator Tanks and Other Pressure Vessels

Corrosion cracking of welds is a generic problem affecting industrial and utility steam generation.  There have been catastrophic failures of deaerator storage tanks, steam accumulators, and other vessels.  Over 30% of inspected vessels are found to have cracks.  The same problem exists in other hot water and wet steam tanks and piping.  Inspection and repair of these vessels is a maintenance burden leading to extension of outages and delay of production. Depending on the local regulatory environment, deaerators are inspected every one to four years.

In most instances, there is no rational evaluation of NDT results, and there often are unnecessarily frequent inspections and repairs (too conservative), while in other instances these vessels operate with large undetected defects.  This procedure introduces an engineering evaluation of inspection results leading to an assessment of the remaining life and the inspection internal.    

A prediction of the time required for existing or potential cracks (described below) growing to a critical size is determined from separate considerations of corrosion fatigue and stress corrosion crack propagation, and service experience.  This way, twelve different results (times to failure) are obtained. The shortest time is selected as the critical time.  The critical (maximum allowed) size is never allowed to be deeper than 40% of the wall thickness of the shell.   

The purpose of this manual calculation procedure is to provide a fracture mechanics evaluation of NDT results.  Remaining life from the time of inspection to a leak or break is predicted and repair or next NDT interval is recommended.  A block diagram of the procedure is shown in Figure 1.  

Four crack situations (location vs. orientation) are considered and the deepest longitudinal and circumferential (tangential) cracks are evaluated (Figure 2).  Other situations with complex geometry should be treated separately.  

The four treated situations are:   

LL - Longitudinal weld with a longitudinal crack

LC - Longitudinal weld with circumferential crack

CC - Circumferential weld with a circumferential crack

CL - Circumferential weld with a longitudinal crack

The most important NDT information is crack depth.  It should be determined as accurately as possible by grinding, ultrasonics, or other reliable methods.    

Figure 1.  Life Assessment of Deaerators

Figure 2.  Weld Crack Orientation

References

"Recommended Practice for Prevention, Detection, and Correction of Deaerator Cracking", NACE Standard RP0590.

O. Jonas. "Deaerators, An Overview of Design, Operation, Experience, and R&D", Proc. of the Amer. Power Conf., Vol. 49, A. 979, Ill. Inst. of Tech., 1987.