(1) Background to a steam drum in a power plant and reasons for cracking.
(2) Reasons for the selection of the material (normally) used for the drum.
(3) Clear description of each step in FAD approach with a justification of the level used for flaw evaluation.
(4) Any calculations (or approximations) used for temperature and stress distributions.
(5) Any assumptions made in the assessment.
(6) Conclusions on the life assessment of the drum, safety margins, and any immediate remedial actions or future NDE schedule.
Design and Structural Integrity Assessment of a Steam Drum
Design Brief:
A steam drum is a part of a boiler system in a steam power plant with a maximum of 30 start-up/shutdown pressure cycles annually.
These cylindrical drums (typical OD = 1800mm. ID = 1500 mm) have been found to experience internal in-service cracking on both sides of the feedwater nozzle and oriented parallel to the drum axis.
The test conditions and the operating conditions for the pressurised vessel (drum) are as follows:
Test Conditions: Internal Pressure = 27.9 MPa
Metal Temperature =43°C
Operating Conditions:Internal Pressure 17.6 MPa
Metal Temperature = 330°C
During the operating conditions, the metal temperature undergoes a drop of 160°C at the inside surface.
A typical NDE examination of the drum after a few years in service suggests tliat many small Maws are located in close proximity to each other. From this NDE data, the effective flaw Size for the drum is approximately 178 rum long and 4.6 mm deep.
Required:
Follow BS 7910:2005 to carry out a fracture-mechanics-based structural integrity assessment of the steam drum based on the current NDE data, and hence establish the lower-bound margins of safety under test and operating conditions.