Analyzing the poor repairs on a boiler feed pumps and how they affect the pump's performance and reliability.
It is often said that a picture is worth a thousand words. This old adage could not have been truer when a coal-fired power station received pictures from the inspection team at a service center. The plant had pulled a previously rebuilt Worthington boiler feed pump from storage and sent it to the service center for disassembly, inspection and reverse engineering. What the inspection team found was a number of repair defects uncovered from the previous repair process.
Because many repaired pumps initially go into storage as this one did, the consequences of a poorly rebuilt pump may not be revealed until several years later. The unfortunate results can range from reduced pump efficiency and shorter mean time between repair to catastrophic failure and unplanned outage. In this particular case, the repaired pump had been in storage and not been run since the repair was completed. It was therefore in its "as built" condition when it arrived at the service center. The photographs that follow illustrate a number of the defects that were uncovered and how they affect the performance and reliability of the pump.
The bearing journal surfaces were not chrome-plated. Lack of chrome plating decreases the shaft and sleeve bearing life. Chrome plating reduces surface friction, reduces wear and helps to reduce contaminants in the oil from the shaft base material.
There was also lack of smooth radii at diameter transitions. Radii help reduce stress concentrations and help prevent shaft failures.
A newly manufactured shaft with the proper radius.
Bearing journals had not been chrome-plated in previous low quality repair.
A through crack existed at the periphery of two impellers. Defects near the periphery of an impeller can lead to catastrophic failure from centrifugal forces. Moreover, suction vane edges were ground flat, which results in a loss of performance and NPSH. As much as five to seven additional feet of NPSH required by poor practices could result in cavitation and reduced operating life.
NDE inspection revealed a 1/2 in. linear indication on the back shroud of impeller #5.
This defect could lead to catastrophic failure of the impeller and pump.
Properly dressed impeller vane
Suction vanes had been ground down and not properly dressed.
Poor machining practices left internal thread chatter. The machine tool chatter marks increase the fluid
bypass flow past the balancing disc, which reduces pressure and ultimately increases the axial thrust on the thrust bearing. The thrust bearing operating temperature will increase and result in reduced life.
Thread chatter on balance disc inner threads is evidence of poor previous machining practices.
Improperly repaired stage piece shows obvious widening of the diffuser vane due to numerous skim cuts without restoration to original diffuser dimensions.
Tongue indentation on stage piece shows leakage under the volute tongue
Properly repaired stage piece
Close up of stage piece after proper repair