by Anzar Hasan, Chief Inspector
As you know, the standard guidelines for internal inspection and NDT of the welds is every five years. However, the frequency of internal inspections and NDT should be increased based on the findings and extent of repairs. The following NB Bulletin provides the inspection guidelines based on the findings:
The guidelines to perform internal inspection and NDT of the welds originated from history of cracking and catastrophic failures of deaerators.
The following article is part of National Board Classic Series and was published in the National Board BULLETIN in 1988.
Catastrophic failure of deaerator pressure vessel welds has included incidents that resulted in plant personnel fatalities. Failures originated as cracks caused by residual, thermal, static, and dynamic stresses, with growth of the cracks accelerated by corrosion fatigue. Weld deformities and hardened material in the heat-affected zone of welds further contributed to these failures. In some instances shell cracking has resulted in small leaks; in others, complete failure has occurred. In response to the life-taking and life-threatening failures reported in 1982 and 1983, technical advisories and guidelines were prepared outlining the necessity of internal weld inspection and recommending methods for inspection and repair. Advisory statements of this type were issued by TAPPI, the National Association of Corrosion Engineers (NACE), and The National Board of Boiler and Pressure Vessel Inspectors. TAPPI and NACE formed advisory groups to address the cracking problem. The TAPPI group is an ad hoc committee of the Steam and Power Committee. NACE established Task Group T-7H-7, Deaerator Cracking, to study and compile information on the subject. The summary of the TAPPI committee ‘s findings is presented here.
Pressure vessel weld cracks
A large number of deaerators have now been inspected. Cracking has been more prevalent than first suspected, appearing in 30 – 50 percent of all industrial deaerators that have been inspected. Cracks have appeared in circumferential and longitudinal weld seams of both the heater and storage vessels. Circumferential seams are susceptible to cracking more than longitudinal seams, with the highest incidence occurring in the ‘T’ junction of the two seams. Cracks have also appeared randomly at different service nozzle welds affected by thermal expansion and thermal shock.
Cracks are thought to be caused by residual stresses imposed during manufacture, along with thermal and dynamic shock and stress loads imposed during uneven deaerator operation. New evidence suggests that some cracks may originate in the area of pits caused by corrosion. Once cracks begin, they are stressed open during periods of operation, allowing iron oxide to collect on newly fractured surfaces, which causes corrosion fatigue. The slightly hardened heat-affected zone of the weld is where most cracks occur. Therein lies the whole problem corrosion and weld discontinuities followed by cracking and corrosion fatigue along the heat-affected zone of the welded joint.
Inspection and repair
Inspection and repair is the tool for the control of the growth of cracks. All deaerators, including those vessels designated as ASME constructed and those designated as non-ASME constructed, should be periodically inspected. Inspection, or the inspection supervision, should be conducted by a certified ASNT SNT-TC-1A Level II inspector. After a deaerator is first put into service, it should be inspected within one to two years. After that, the following frequency of inspectionsa was suggested at the NACE 1986 conference:
- REPAIRED CRACKS, 1 YEAR OR LESS
- NONREPAIRED CRACKS, 1 – 2 YEARS
- NO CRACKS/NO REPAIRS, 3 – 5 YEARS
Repaired vessels, at the same operating conditions, tend to recrack faster than the original vessel. Inspections therefore should be more frequent after repairs.