Introduction
After recent repeated failures on the feedwater discharge line, a stress analysis was carried out
to determine if the loads on the line are within acceptable limits for the system.
Brief description of the feedwater system
The feedwater station has provision for six horizontal multistage centrifugal pumps. The
feedwater pumps are fed a common suction line and discharge to a common discharge line. All
pumps have isolation and non-return valves fitted. The suction manifold is fed from the deaerator
vis booster pumps. The temperature of the discharge line (as measured by surface temperature
on the pipe) is approximately 100°C. Currently, (as of 12/06/2018), the station is fitted with five
horizontal pumps. The feedwater discharge line forms a ring main configuration, runs the length
of the boiler house and branches to each of the five boilers via feed regulation valves.
Failures
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
The most recent failure has occurred at feed-pump 1 branch into the main discharge
manifold.
The tie-in has been repaired by welding, but suffers failures at the weld joint to main
line.
Stress analysis.
A stress analysis was carried out using Caesar Pipe stress analysis software. The entire feed line
was measured manually to obtain the necessary dimensions to create the pipeline model required
by the software. Due to limited available information on the existing system, assumptions were
made on the component types used in the system, e.g. valve masses. The accuracy obtained by the
manual measurements will be verified once isometrics are generated. Initial analysis will be
carried out to determine the extent of the problem, with focus in the feed pump area - where the
current problems have been observed. The types of supports have been assumed based on those
that were accessible/visible on site.
The line will be assessed to ASME 31.1 Power piping code. With a static line temperature of 100°C,
and operating pressure of 5000 kPa.
Note: the operating parameters can be adjusted to suit exact site conditions after confirmation.
Preliminary results.
After the initial stress analysis model was created, the results obtained were as follows:
The discharge pipeline passes the code allowable stress for the system.
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Discussion of results
Although the line passes the code stress requirements, the highest stress calculated (for the
expansion case) was at the node where the failure has repeatedly occurred. It must be noted that
the analysis carried out represents the ideal steady state situation (all pumps installed and all
lines at equal temperature). The site running conditions need to be evaluated for all possible
configurations and load cases. Furthermore, the simulation assumes that the pipe condition (wall
thickness) is standard – this needs to be confirmed on site. The pump flanges were used as
principle anchors as no other visible fixed anchors were observed on this line.
Further action items
1)
2)
3)
4)
5)
6)
7)
8)
9)
Verify isometric model
Assess line condition
Rerun model with updated site conditions i.e. missing supports/pumps.
Run various operating conditions.
Gather more information on the system and failures.
Gather forensic evidence at the site of previous failures
Investigate other failure mechanisms.
Run alternative piping configuration to minimise high stress at problem areas.
Provide recommendations to minimise/eliminate future failures on the system.
Conclusion
The stress analysis shows that the boiler feedwater line as modelled meets minimum code
allowable stresses. There may be other conditions not accounted for in the simulation that may
misrepresent the analysis, e.g. fixed anchors not observed or broken supports. All other probable
causes of the branch failure should be investigated.
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Appendix A: Layout of Feedwater pump station
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