Technical Bulletin
Tri Nuclear Corp.
Re: Tri Nuclear Pump failure at Point Beach Nuclear Power Plant
This Technical Bulletin is to inform you of Tri Nuclear’s response to the failure of a PP-260 pump at the Point Beach Nuclear Power Plant in October 2002.
Since 1990, Tri-Nuclear has used Grundfos pumps in our Underwater Filter/Vacuum units Models UFV-100, UFV-260 & UF-600. Grundfos has made several modifications to their pumps over the years, improving their reliability and overall efficiency.
Grundfos pumps built prior to 1995 did not include an upper thrust bearing in its design because in well water applications (which they were originally designed for), up thrusts generally do not occur. In Tri Nuclear’s applications, an up thrust condition will occur by running the pump in reverse when the pump motor has been phased incorrectly.
All Tri Nuclear Operating Instructions contain detailed instructions about conducting phase rotation checks to ensure the pump is phased correctly.
Grundfos introduced many design changes in 1995 to their submersible pump lines, one being the addition of an upper thrust bearing to protect the pump impeller from an up thrust condition.
Tri Nuclear knows of no pump failures (of pumps built after 1995) similar to the type noted by Point Beach.
We have confirmed the pump used by Point Beach was built prior to 1995, and did not contain the newly modified upper thrust bearing. The pump failure most likely was caused by an up thrust condition placed on the impeller of the submersible Grundfos pump that may have been phased incorrectly at some time. An up thrust condition on the impeller mostly likely led to failure on the upper pump bearing not designed for this type of load. An upper bearing failure could cause material damage to some of the impeller & diffuser vanes in the pump.
Recommended Action
Verify when the Tri Nuclear pump was first purchased. If pumps were purchased after 1995, Tri Nuclear does not recommend that corrective action to be taken.
Recommended Action (Cont.)
If pumps were purchased during the transition period in 1995, a verification of the Grundfos manufacturer pump number (See Figure #1).
Figure 1
If pumps were purchased before 1995, Tri Nuclear recommends one of two options:
Replace the older pump with the upgraded Tri Nuclear pumps that have the included upper thrust bearing.
Install Discharge Screens. Tri Nuclear has designed and recommends installing discharge screens on the discharge of any pumps that have been in service prior to 1995. These strainers will prevent any material greater than .375” from exiting the pump.
The discharge strainer assemblies with piping will thread to the discharge tee of the pump (PP-260) or discharge hose (PP-600). Part numbers for ordering are listed below.
P/N: ST-260/SS: PP-260 Discharge Strainers (2 req. per pump), Strainer assembly with piping, .375 perforated holes
P/N: ST-600/SS: PP-600 Discharge Strainers (2 req. per hose), Strainer assembly with hose coupling, .375” perforated holes
Conclusion
The Tri Nuclear Underwater Filter/Vacuum systems have been an industry standard for many years, and Tri Nuclear understands the severity of the problem that occurred at Point Beach. We have the highest confidence with our pumps manufactured after 1995 that a similar incident is unlikely to occur.
If you have technical questions or if we can be of any further service, please contact us.
Best regards,
James L. Warden
President
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