A pressure vessel is a container designed to hold gases or liquids at a pressure substantially different from the ambient pressure. The pressure differential is dangerous, and fatal accidents have occurred in the history of pressure vessel development and operation.
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The number of fatalities as a direct result of boiler and pressure vessel accidents has been recorded as 127 between 2001-2008in the USA. [Referance: Pressure Vessel Accidents: Safety Approach, Sumit Dubal , Hemantkumar kadam, International Research Journal of Engineering and Technology]. There are 5 key factors to decrease the safety risks
- Design, fabrication and construction of pressure vessels
- Ensuring that repairs of vessels are only done by qualified and experienced authorized person
- Keeping the inspection reports
Components of pressure vessels
An inspection shall not only conducted on the main parts but also the accesories of the pressure vessels.
- Vessel shell
- Vessel Support
- Vessel Mountings
- Vessel accessories
Reasons of failure
Thr paper of Sumit Dubal and Hemantkumar kadam showed that 6 main resons of failure. However only one of the consist the 61% of the total risk which is maintanance. Because another valuable paper claimed that many of the pressure vessels are over 40 years old (Inspection of Unfired Pressure Vessels, Bill McStraw, Facilities Instructions, Standards, and Techniques Volume 2-9, Aug 2001).
- Improper design
- Construction and Installation
Frequency of Inspections
Unfired pressure vessels must be inspected and tested before being used for service and after any modification or major repair. The next inspection shall be conducted within 2 years, and the interval of following inspections must not exceed 5 years. However, local regulations might be different and it is you responsiblilty follow latest regulatory amendements both for your safety and compliance.
Bureau of Reclamation employees who perform the inspections must have the following education and experience (one of the following 3 options are valid for experience and education requirements)
1) A degree in engineering from an accredited school plus 1 year of experience in design, construction, repair, or inspection of pressure vessels. The inspector must also be an operator trained and certified to perform ultrasonic thickness examinations.
2) An associate degree in mechanical technology plus 2 years of experience in design, construction, repair, or inspection of pressure vessels. The inspector shall also be an operator trained and certified to perform ultrasonic thickness examinations
3) A high school education or the equivalent plus 3 years of experience in design, construction, repair, or inspection of pressure vessels. The inspector shall also be an operator trained and certified to perform ultrasonic thickness examinations.
The main security measures for the high pressure vessels inspection it to review the previous inspection report and records. This review should include the following as minimum:
- Operating conditions
- Normal contents of the vessel
- Date of last inspection
- ASME Code Symbol stamping or mark of code of construction
- The type of connections used during fabrication of the vessel
- Serial number and materials of construction
- Records of wall thickness surveys
In addition, you should remove inspection plugs and covers and clean vessel sufficiently to allow for visual inspection if required.
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The inspection may be external, internal, or both and use a variety of non-destructive examination techniques. Following inspection methods are essential for the main parts.
- External inspection
- Thickness Survey
- Stress Analysis
- Internal Inspection
- Non Destructive Testing
- Pressure Testing
Each of the above inspection methods (either used alone or in combination with others) has its own detaied control list. Additionally, following safety devices and accesories should also be inspected:
- Safety Device
- Rupture Disks
- Piping Systems
- Pressure Gages
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