Bulletin 83-03: Check Valve Failures in Raw Water Cooling Systems of Diesel Generators
SSINS 6835
OMB No.: 3150-0097
Expiration Date: 11/85
IEB 83-03
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D. C. 20555
March 10, 1983
IE BULLETIN NO. 83-03: CHECK VALVE FAILURES IN RAW WATER COOLING SYSTEMS
OF DIESEL GENERATORS
Addressees:
All nuclear power reactor facilities holding an operating license (OL) for
action and all those holding construction permits for information.
Purpose:
The purpose of this bulletin is to (1) notify licensees and construction
permit holders about numerous incidents of failed check valves in systems
important to safety; (2) inform licensees of a significant generic matter
for which additional NRC action is anticipated; (3) to require appropriate
surveillance and testing of check valves in raw water cooling systems for
diesel generators. A response to this bulletin is required from all nuclear
power reactors holding an OL as discussed below.
Description of Circumstances
A review of available operating experience, data and licensee event reports
(LERs) shows that numerous check valve failures have occurred in systems
important to safety in nuclear power plants. A series of IE generic
communications (listed in Table 1) has been issued which describes a broad
range of check valve failures involving various designs, causes, and
applications. The NRC has evaluated check valve failures in consideration of
the need to request generic action by licensees. The focus of this bulletin
is directed primarily at the failure mode of disassembly or partial
disassembly of check valve internals. For example, the check valve disc
becomes separated from the hinge.
Title 10 of the Code of Federal Regulation, Section 50.55a(g) requires
testing of valves whose function is required for safety. This is implemented
by application of Section XI of the ASME Boiler and Pressure Vessel Code and
Addenda. However, our analysis of operating experience with check valves has
shown that disassembly and partial disassembly of check valve internals is
not effectively found by Section XI testing as it is implemented at this
time. Tests performed for Section XI or Technical Specifications usually
require only forward flow through check valves. These tests may not detect
internal check valve failures unless the disassembled parts move to block
flow during the test.
This bulletin is expected to be part of a generic response to check valve
failures which will result in improved testing to ensure operability and to
improve reliability of check valves. In addition to the generic
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IEB 83-03
March 10, 1983
Page 2 of 5
communications issued by IE, the NRC Office of Nuclear Reactor Regulation
requires that licensees consider all check valves in systems important to
safety for inclusion in the ASME Section XI Pump and Valve Inservice Testing
Programs. Although most check valves in systems important to safety are
included in current IST program reviews, most are not required to be reverse
flow tested or disassembled to detect gross internal failure because
licensees have identified each of these valves as having a single safety
function: the open position. However, forward flow tests to verify the open
position are inadequate for detecting internal disassembly. Effective check
valve testing techniques are necessary to the development of a more
meaningful and productive IST program. Operating experience provides a basis
for determination of what areas of IST check valve surveillance need to be
improved.
The specific requirements of this bulletin stem from analysis of check valve
failures in the raw cooling water supply to the diesel generators at the
Dresden and Quad-Cities nuclear power stations and other events which are
described in Table 1. At Dresden and Quad-Cities, it was found that six of
six check valves in the raw, cooling water systems for the diesel generators
had failed with the disc becoming detached from the pivot arm. Many of the
failures described in t,he generic communications listed in Table 1 also
involved detached discs. The Dresden event is described in detail in IE
Information Notice No. 82-08. Ih summary, the event involved failure of the
check valves in the raw water cooling systems for the diesel generators
which resulted in interruption of cooling water flow to the diesel generator
heat exchangers and subsequent inoperability of the diesel generators. The
Dresden check valve failures rendered two diesels inoperable at the same
time when the valve discs moved to the valve outlets and blocked flow.
However, the true cause of flow blockage was not determined until almost one
month later. All three Diesel Generator Cooling Water Pump (DGCWP) systems
at Dresden Units 2 and 3 involved check valve failures which were discovered
during a short period of time. These failures were not identified by
operator observations and instrument readings during diesel generator
surveillance tests, but were discovered by direct inspection of the
internals of the valves. It is not known how long these check valves were
broken before their condition was detected. The broken valve discs were
found to be free to move within the valve bodies and may have been that way
for some time before coming to rest in a position which restricted flow
enough to cause the diesels to trip on high engine temperature. The subject
check valves are horizontally mounted Crane 8-inch, cast iron swing check
valves, Type 373, and have a pressure rating of 125 psi.
Because of the similarity between the Dresden Units and Quad-Cities Units,
NRC Region III requested that Commonwealth Edison inspect the DGCWP
discharge check valves on the Quad-Cities Units on a schedule consistent
with the availability of valve parts and the availability of the DG units.
The DGCWP systems at Quad-Cities were inspected and all three DGCWP
discharge check valves were found with the discs separated from the pivot
arms.
At Quad-Cities, the failures remained latent; although the discs were lying
free in the valve body, they did not move to the outlet and block flow. The
DGCWP discharge check valves at Quad-Cities were also Crane, 8-inch, swing
check valves, Type 373. However, the Quad-Cities valves and the Dresden
valves were not identical. The Quad-Cities valves had larger, more bulbous
valve bodies and slightly different internals.
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IEB 83-03
March 10, 1983
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For all valves, the most dominant failure mode was due to a combination of
abrasive and corrosive wear of valve internals. In particular, the valve
disc was held to the pivot arm by a stud with washer and nut. Apparently,
flow conditions at the valves were such that the discs vibrated (fluttered)
causing local abrasive wear at the arm bore of the hinge where it joins the
disc. This same action also resulted in severe degradation of the washer
used to retain the disc on the hinge, and once the degree of degradation at
the hinge bore and washer was sufficient, the two components separated. The
stud and nut wore such that the stud and nut assembly pulled through the
enlarged hole in the pivot arm and became detached.
In the case of the Dresden Unit 2 valve, the valve disc remained barely
attached to the pivot arm and the arm had broken at its hinge to the valve
hinge pin. In this case, abrasive/corrosive wear was found to have occurred
at the hinge pin bore. All of the failed valves showed wear at this
location. The degree of degradation varied from very slight in the
Quad-Cities 1/2 hinge to extreme in the Dresden Unit 2 hinge which had
fractured at this location. As plants age, failure modes of these types may
be expected to become more prevalent.
The bulletin focuses on check valve failures in the raw water cooling system
of diesel generators for the following reasons:
(1) Six of six check valves in these systems at Dresden and
Quad-Cities failed with the potential for interrupting flow of
cooling water to the diesel generators.
(2) Both diesels on Dresden Unit 3 tripped on overtemperature due to
lack of cooling water flow and the unit was without emergency
power.
(3) The cause of the lack of raw cooling water flow to the Dresden
Unit 3 dedicated diesel (failed check valves) was not discovered
for almost one month following the event despite numerous test and
surveillance procedures and was attributed to other causes. The
condition of the swing diesel and the unit 2 dedicated diesel
check valves at Dresden was not discovered until more than one
month after the initial event.
(4) None of these check valves was included in the plant IST program
at the time of the event. Since the event, the NRC Office of
Nuclear Reactor Regulation has added these valves to IST
requirements for plants currently being reviewed.
(5) Even if the valves had been in the IST program, it is doubtful if
the normal forward flow test would have discovered these latent
failures except by chance, as occurred at Dresden. The failures
were finally discovered by direct inspection of valve internals.
It should be noted that the popular use of swing check valves in safety
related plant fluid systems considerably expands the scope of concern for
check valve maintenance and testing beyond diesel cooling systems. Licensee
event reports indicate that other systems important to safety have
experienced failures of check valves which are not included in the IST
program and have not been
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IEB 83-03
March 10, 1983
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discovered during testing. Other licensee event reports indicate that for
those valves which are not leak tested, both the type and frequency of
testing may not be adequate to detect valve failure. Maintenance and IST
programs should be reconsidered in light of detecting and preventing gross
and multiple check valve failures that can defeat functions of systems
important to safety. This includes concerns both for check valve opening and
closure.
Required Actions For Holders Of Operating Licenses:
1. Licensees are requested to review the plant Pump and Valve In-Service
Test (IST) program required by Section XI of the ASME Boiler and
Pressure Vessel Code and modify it if necessary to include check valves
in the flow path of cooling water for the diesel generators from the
intake to the discharge. Those portions of the cooling water system
which do not directly supply the diesel may be excluded from this
review. For example, if the cooling water to the diesel is supplied by
the normally operating service water system, the loop of piping to the
diesel from the service water piping and back must be considered, but
not the complete service water system. For those cooling water systems
which come into operation only upon demand for diesel cooling, all
portions of the system which are required to change state must be
reviewed.
2. For the valves described in (1) above, licensees are requested to
examine the IST program and modify it if necessary to include
verification procedures that confirm the integrity of the valve
internals. This may be accomplished by using both a forward flow and a
back flow test or by valve disassembly and inspection. Other equally
effective means of assuring integrity of the valves may be used. A
reasonable schedule for the test of these valves shall also be included
in the IST program.
3. Licensees are requested to perform initial valve integrity verification
procedures for the valves identified in (1) above using the methods
described in (2) above, to be completed by the end of the next
refueling outage commencing after April 1, 1983.
4. Licensees are requested to submit a report to the NRC within 90 days of
the date of this bulletin, which lists the valves identified in (1)
above and describes the valve integrity verification procedure methods
and schedule identified in (2) above. This report should include the
history of any known previous failures of these valves at your plant.
5. Licensees are requested to submit a report to the NRC within 90 days of
completion of the results of the initial valve integrity, verification
procedure performed in accordance with (3) above. For those valves
which are found to have undergone either partial or complete
disassembly of valve internals, a description of the failure mode
should be included.
6. The written reports required shall be submitted to the appropriate
Regional Administrator under oath or affirmation under provisions of
Section 182a, Atomic Energy Act of 1954, as amended. The original copy
of the cover
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IEB 83-03
March 10, 1983
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letters and a copy of the reports shall be transmitted to the U. S.
Nuclear Regulatory Commission, Document Control Desk, Washington, D. C.
20555 for reproduction and distribution.
Required Actions for Holders of Construction Permits:
None. This information is provided to holders of Construction Permits to
provide guidance in preparing their proposed IST programs. These programs
will be reviewed during the licensing process.
Although no specific request or requirement is intended, the following
information would help the NRC evaluate the cost of implementing this
bulletin:
o Staff time to perform requested valve integrity verification
procedures
o Staff time to prepare written responses
This request for information was approved by the Office of Management and
Budget under clearance number 3150-0097 which expires on November 1985.
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 regarding this matter, please contact the Regional
Administrator of the NRC Regional Office or the technical contact listed
below.
Richard C. DeYoung, Director
Office of Inspection and Enforcement
Technical Contact: George F. Lanik, IE
(301) 492-9636
Attachment:
1. Table 1 - IE Generic Communications
2. List of Recently Issued IE Bulletins
Page Last Reviewed/Updated Tuesday, March 09, 2021