Bulletin 88-04: Potential Safety-Related Pump Loss
OMB No.: 3150-0011
NRCB 88-04
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
May 5, 1988
NRC BULLETIN NO. 88-04: POTENTIAL SAFETY-RELATED PUMP LOSS
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
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
evaluation.
Discussion:
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
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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
path.
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
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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
needed.
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
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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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Page Last Reviewed/Updated Tuesday, March 09, 2021