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QUESTION
hyl
In Najbia, power station unit capacity 100MW. The unit was in operation and suddenly shut down because of the net failure and they also lost emergency power. The dearator which is operated at pressure of 5KG/CM^2. The thickness of the dearator and storage tank is 12.5 MM. The design pressure is 8.5KG/CM^2 .
The pressure suddenly increased to 17 KG/CM^2 and the dearator exploded in a catastrophic way with material ejecting destroying the auxiliary piping, near equipment, and the storage tank exploded.
Is this normal? How should we analyze this when it appears the weak and strong points have the same defect?
REPLIES
gr2vessels
I believe that is normal an overpressurized pressure vessel to fail. Perhaps (or most likely) the failure was due to a number of causes, like failure of the overpressure protection system (if any present), failure of safety relief valves, etc…What was the human error component, it’s only you to determine.
You obviously are lacking the experience for a proper investigation, hence I suggest hiring an unbiased expert to conduct the investigation (I assume that you want to get to the bottom of it- most of the people involved would try first to cover their backside). He will list what went wrong and suggest the improvements to the plant to prevent this happening again.
MJCronin
Deaerators can and do fail. Fundamentally, you have a thin walled pressure vessel subject to internal transients and loads which may, or may not be designed to the pressure vessel code.
This particular failure may be due to simple overpressure and possible neglect
There was a spate of US failures in the late 1980s which caused a re-evaluation of the design and rules. There was a spectacular lethal failure that occured in a paper plant that caused the industry regulating agency (TAPPI) to issue warnings and restrictions. In 1999, The Heat Exchange Institute (HEI) issued new guidelines on this matter (Standards and Typical Specifications for Tray Type Deaerators- 7th Edition, latest edition is 2003)
The new recommended guidelines for a more “robust” DA include:
- ASME vessel construction with a minimum design pressure (50 psig…as I recall)
- PWHT of the vessel-head-to-shell joints
Lot of good information here: http://www.heatexchange.org
Several inspection organizations developed special “deaerator inspection programs” and wrote papers about thier methodologies and findings.
One inspection organization with much powerplant DA related experience is:
The DA fabricators also took an active role in this effort. These fine people have always been in the front of technology, IMHO…
metengr
We have a number of DA heaters and storage tanks that are inspected internally every 3-5 years, where 50% of the long and girth welds on the ID surface of the vessel are cleaned and prepared for wet fluorescent MT. As indications are found, the inspection coverage is increased accordingly. The vessels are subjected to a 100% exam between 6-10 years, depending on indications found and damage mechanism.
From my expierence, the two main damage mechanisms related to DA vessels are flow accelerated corrosion (FAC) damage (especially on units that have switched to oxygenated water treatment, and environmentally assisted cracking (corrosion fatigue or to others stress corrosion cracking) or EAC.
The EAC damage can be controlled by post weld heat treatment of weld repairs at or below the water line in these vessels, regardless of original code of construction requirements. For new vessels, PWHT would be mandated.
For flow accelerated corrosion (FAC) damage, this is more complicated and requires a change in material from carbon steel to low alloy steel containing chromium.