Bulletin 88-04: Potential Safety-Related Pump Loss

                                                       OMB No.: 3150-0011
                                                       NRCB 88-04

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                             WASHINGTON, D.C.  20555

                                   May 5, 1988



All holders of operating licenses or construction permits for nuclear power 


The purpose of this bulletin is to request all licensees to investigate and 
correct as applicable two miniflow design concerns.  The first concern in-
volves the potential for the dead-heading of one or more pumps in 
safety-related systems that have a miniflow line common to two or more pumps 
or other piping configurations that do not preclude pump-to-pump interaction 
during miniflow operation.  A second concern is whether or not the installed 
miniflow capacity is adequate for even a single pump in operation.  

Description of Circumstances:

Westinghouse Electric Corporation recently notified all utilities with 
Westinghouse-designed nuclear steam supply systems (NSSS) of the two concerns 
noted above.  NRC Information Notice 87-59 forwarded a summary of these 
concerns to all holders of operating licenses or construction permits for 
nuclear power reactors and indicated that further staff evaluation might 
result in a request for specific licensee actions.  Several licensees have 
confirmed the existence of these concerns in their plants (Turkey Point, H. B. 
Robinson, Vermont Yankee).  This bulletin is the result of the staff's 


When two centrifugal pumps operate in parallel and one of the pumps is 
stronger than the other (i.e., has a higher developed head for the same flow), 
the weaker pump may be dead-headed when the pumps are operating in the minimum 
flow mode.  The phenomenon is manifested at low flow rates because of the 
flatness of the pump characteristic curve in this range.  The head difference 
is not a problem at moderate to high flow conditions because of the shape of 
the pump characteristic curve in these regions.  

Traditionally, the required miniflow for these pumps was established solely on 
the basis of pumped fluid temperature rise.  Today, however, it is generally 

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                                                            May 5, 1988
                                                            Page 2 of 4 

understood that temperature rise is not the only factor influencing safe con-
tinuous minimum flow operation.  Centrifugal pumps will demonstrate a flow 
condition that has been described as hydraulic instability or impeller 
recirculation at some point below the best efficiency point (BEP) on their 
characteristic curve.  These unsteady flow phenomena become progressively more 
pronounced as the flow is further decreased and can result in pump damage from 
pump vibration, excessive forces on the impeller, and cavitation.  It is now 
generally recommended that the limitations associated with these hydraulic 
phenomena be considered when specifying minimum flow capacity.  

The first potential problem involves parallel pump operation with both pumps 
recirculating through a common miniflow recirculation line or with a piping 
configuration that does not preclude pump-to-pump interaction during miniflow 
operation.  The problem was identified on a plant whose licensee requested 
that Westinghouse determine if parallel operation while on miniflow is 
acceptable.  Westinghouse reviewed the plant's residual heat removal (RHR) 
system configuration.  The review determined that the potential exists for the 
stronger pump to dead-head the weaker pump during low flow, parallel pump 
operating conditions while on miniflow only.  In addition, it was determined 
that even without pump interaction the recirculation flow available was not 
adequate to ensure continuous operation of even a single RHR pump on miniflow.  
Although these issues are based on an evaluation of RHR pumps at a particular 
plant, the first concern may exist at other plants configured with a common 
pump recirculation flow path and the second concern may also exist at other 
plants independent of whether or not there is a common recirculation pump flow 

The NRC staff believes that these issues may be relevant to all water-cooled 
reactor designs, regardless of the pump application or the NSSS manufacturer.  
This is based on the belief that miniflow lines have traditionally been de-
signed for only 5% to 15% of pump design flow.  Some pump manufacturers now 
are advising that their pumps should have minimum flow capacities of 25% to 
more than 50% of best efficiency flow for extended operation to protect 
against hydraulic instability or impeller recirculation problems.

Actions Requested:

All addressees are requested to do the following: 

1.   Promptly determine whether or not its facility has any safety-related 
     system with a pump and piping system configuration that does not preclude 
     pump-to-pump interaction during miniflow operation and could therefore 
     result in dead-heading of one or more of the pumps.

2.   If the situation described in Item 1 exists, evaluate the system for flow 
     division taking into consideration (a) the actual line and component re-
     sistances for the as-built configuration of the identified system; (b) 
     the head versus flow characteristics of the installed pumps, including 
     actual test data for "strong" and "weak" pump flows; (c) the effect of 
     test instrument error and reading error; and (d) the worst case 
     allowances for deviation of pump test parameters as allowed by the 
     American Society of Mechanical Engineers Boiler and Pressure Vessel Code 
     (ASME Code) Section XI, Paragraph IWP-3100.

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                                                            May 5, 1988
                                                            Page 3 of 4 

3.   Evaluate the adequacy of the minimum flow bypass lines for safety-related 
     centrifugal pumps with respect to damage resulting from operation and 
     testing in the minimum flow mode.  This evaluation should include 
     consideration of the effects of cumulative operating hours in the minimum 
     flow mode over the lifetime of the plant and during the postulated 
     accident scenario involving the largest time spent in this mode.  The 
     evaluation should be based on best current estimates of potential pump 
     damage from operation of the specific pump models involved, derived from 
     pertinent test data and field experience on pump damage.  The evaluation 
     should also include verification from the pump suppliers that current 
     miniflow rates (or any proposed modifications to miniflow systems) are 
     sufficient to ensure that there will be no pump damage from low flow 
     operation.  If the test data do not justify the existing capacity of the 
     bypass lines (e.g., if the data do not come from flows comparable to the 
     current capacity) or if the pump supplier does not verify the adequacy of 
     the current miniflow capacity, the licensee should provide a plan to 
     obtain additional test data and/or modify the miniflow capacity as 

4.   Within 60 days of receipt of this bulletin, provide a written response 
     that (a) summarizes the problems and the systems affected, (b) identifies 
     the short-term and long-term modifications to plant operating procedures 
     or hardware that have been or are being implemented to ensure safe plant 
     operations, (c) identifies an appropriate schedule for long-term resolu-
     tion of this and/or other significant problems that are identified as a 
     result of this bulletin, and (d) provides justification for continued 
     operation particularly with regard to General Design Criterion 35 of 
     Appendix A to Title 10 of the Code of Federal Regulations (10 CFR 50), 
     "Emergency Core Cooling" and 10 CFR 50.46, "Acceptance Criteria for 
     Emergency Core Cooling System for Light Water Nuclear Power Reactors." 

5.   Within 30 days of completion of the long-term resolution actions, provide 
     a written response describing the actions taken.

6.   An evaluation of your actions in response to this bulletin should be 
     documented and maintained at the plant site for a minimum of two (2) 
     years.  That evaluation should, as a minimum, address the piping system 
     configuration in accordance with Item 1 above, each of the four factors 
     discussed in Item 2, pertinent test data and field experience on minimum 
     flow operation, and verification of the adequacy of current miniflow 
     capacity by the pump manufacturer.  

The written reports, required above, shall be addressed to the U.S. Nuclear 
Regulatory Commission, ATTN:  Document Control Desk, Washington, D.C.  20555, 
under oath or affirmation under the provisions of Section 182a, Atomic Energy 
Act of 1954, as amended.  In addition, a copy shall be submitted to the 
appropriate Regional Administrator.  

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                                                            May 5, 1988
                                                            Page 4 of 4 

This requirement for information was approved by the Office of Management and 
Budget under clearance number 3150-0011.  Comments on burden and duplication 
should be directed to the Office of Management and Budget, Reports Management, 
Room 3208, New Executive Office Building, Washington, D.C. 20503.

If you have any questions about this matter, please contact the technical 
contact listed below or the appropriate NRR project manager.  

                              Charles E. Rossi, Director
                              Division of Operational Events Assessment
                              Office of Nuclear Reactor Regulation

Technical Contact:  T. Collins, NRR
                    (301) 492-0897

Attachment:  List of Recently Issued NRC Bulletins


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