Bulletin 86-02: Static "O" Ring Differential Pressure Switches
SSINS No.: 6820
OMB No.: 3150-0012
IEB 86-02
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, DC 20555
July 18, 1986
IE BULLETIN NO. 86-02: STATIC "O" RING DIFFERENTIAL PRESSURE SWITCHES
Addresses:
All power reactor facilities holding an operating license (OL) or a
construction permit (CP).
Purpose:
The purpose of this bulletin is to request that boiling water reactor (BWR)
and pressurized water reactor (PWR) licensees determine whether or not they
have Series 102 or 103 differential pressure switches supplied by SOR,
Incorporated (formerly Static "O" Ring Pressure Switch Company), installed
as electrical equipment important to safety. Those licensees that have SOR
Series 102 or 103 differential pressure switches installed in systems
subject to Technical Specifications are requested to take certain actions to
assure that system operation is reliable.
Description of Circumstances:
SOR Series 103 differential pressure switches were installed in LaSalle 2 in
mid 1985 as part of an environmental qualification modification which was
performed after initial operation of the unit. Identical switches were also
installed in LaSalle 1. LaSalle 1 and 2 each have about 60 of these switches
in various systems, including the reactor protection system and the
emergency core cooling system.
On June 1, 1986, LaSalle 2 experienced a feedwater transient that resulted
in low water level in the reactor vessel. One of four low level trip
channels actuated, resulting in a half scram. The operator recovered level
and power operation was continued. However, subsequent reviews by the
Licensee's personnel raised concerns that the level apparently had gone
below the scram setpoint and that a malfunction of the reactor scram system
may have occurred. Based on this concern, the Licensee declared an "Alert,"
shut the plant down, notified the NRC, and subsequently informed SOR of
possible switch malfunctions. (This incident is described in greater detail
in IE Information Notice 86-47).
NRC dispatched an augmented inspection team to the site on June 2 to
investigate the root cause and significance of the feedwater transient, the
performance of the differential pressure switches in the low level trip
channels, the response of the reactor protection system, and related
matters.
8607180302
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After recalibrating the level switches on June 1, the Licensee tested the
performance of the level switches by lowering water level (drop test) in the
reactor and reading the levels indicated on level transmitters when each of
the four level switches tripped. The results were erratic with the switches
tripping at levels between 2.4 inches and 12.2 (plus or minus about 1.5
inches, depending on the transmitter read). These measurements are relative
to instrument zero which is at 161.5 inches above the top of active fuel.
The technical specifications require that level channels be declared
inoperable if the actual trippoint is below 11.0 inches.
As of June 9, 1986, the Licensee had tested differential pressure switches
in the residual heat removal systems and the high pressure core spray
system. These switches open valves in minimum flow recirculation lines so
that adequate cooling to pump seals and bearings is provided when system
flow is low. One of the switches actuated within the range permitted by
technical specifications; the others did not. The switch for the high
pressure core spray system was calibrated to actuate at 1300 gpm but did not
actuate until flow decreased to 530 gpm. The switches for the two residual
heat removal systems should have actuated at 1000 gpm but did not actuate
until flow decreased to the 480 to 800 gpm range. On the basis of these
results, the Licensee declared all emergency core cooling systems for Units
l and 2 to be inoperable. Both units remain in cold shutdown.
Information Notice 86-47 was issued by the Office of Inspection and
Enforcement on June 10, 1986 to inform licensees of the erratic behavior of
SOR differential pressure switches during the incident at LaSalle 2 on June
1 and during subsequent testing. An attachment to the information notice
listed licensees to which SOR had supplied Series 103 differential pressure
switches. That list has been revised (Attachment 1) to include Series 102
differential pressure switches which have important similarities to Series
103 switches. It should be noted that the list of affected licensees is not
believed to be fully accurate. The information notice also announced a
public meeting of representatives from NRC, General Electric Company, SOR,
and interested licensees to discuss the application and performance of
Series 102 and 103 switches in safety related systems, which was held on
June 12, 1986.
Testing at LaSalle of other Model 103 SOR differential pressure switches
used to actuate emergency core cooling system, primary containment isolation
system, and other engineered safety feature systems revealed that these
switches displayed the same types of behavior as the switches used for
reactor scram.
During the vessel water level drop tests at LaSalle 1 on June 2, one of two
Series 103 switches used to provide a confirmatory water level input signal
to the automatic depressurization system failed to function. On June 17,
1986, testing showed that the trippoint had shifted nonconservatively by 25
inches. In this application, the relative locations of the instrument taps
are such that the system could not produce sufficient differential pressure
to actuate the switch. Therefore, this amount of shift constitutes a
functional failure of the switch. On June 25, the switch was disassembled
and inspected. Rust
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July 18, 1986
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(severe corrosion) was found inside the switch assembly and probably caused
a cross shaft bearing, which is outboard of the O-rings, to seize.
A similar event (Licensee Event Report 86-001-00) occurred at Oyster Creek 1
on January 17, 1986, during monthly surveillance of four SOR differential
pressure switches which detect low water level in the reactor vessel. The
"as-found" setpoints for three of the switches had drifted downward as much
as 6 inches. During the subsequent 11 weeks, the level switches continued to
perform erratically, each switch was replaced one or more times, and
modified switches were installed. On April 7, after a modified switch had
nonconservative setpoint drift, the Licensee performed daily surveillance
until about April 12 when the reactor was shutdown for a six month outage.
Increased surveillance frequency did not resolve the problem.
Earlier concern for mechanical level indication equipment was expressed in
NRC Generic Letter No. 84-23 which addressed water level instrumentation for
BWR reactor vessels. The generic letter was based on NRC's evaluation of a
report by S. Levy, Incorporated, which had been commissioned by a BWR
Owner's Group. The generic letter addressed the need for BWR licensees to
review plant experience related to mechanical level indication equipment,
indicated that analog trip units have better reliability and greater
accuracy than mechanical level indication equipment, and stated that BWR
licensees should replace such equipment with analog transmitters unless
operating experience indicates otherwise.
Responses to Generic Letter No. 84-23 show that 80% of BWR licensees have
replaced or plan to replace their mechanical level instrumentation with
analog level transmitters. Recipients of this bulletin should recognize that
while this bulletin focuses on more immediate problems with two similar
models of mechanical differential pressure switches manufactured by SOR,
Incorporated, the reliability of other mechanical instrumentation is also in
question because it may be vulnerable to similar problems. Because the same
urgency has not been demonstrated for other mechanical differential pressure
switches, the NRC plans to address that matter separately.
Discussion:
DESCRIPTION OF SERIES 102 AND 103 DIFFERENTIAL PRESSURE SWITCHES
The Series 102 and 103 differential pressure switches consist of a piston
(Series 102) or a diaphragm (Series 103) which moves a lever that rotates a
cross shaft. These components are contained in a steel case designed to
withstand system pressure. Both ends of the cross shaft extend out of the
wetted volume and O-ring seals are provided to form the pressure boundary
and prevent leakage along the cross shaft. The condition of these surfaces
and the O-rings will determine the extent to which frictional forces cause a
torque which opposes rotation of the cross shaft. A lever is attached to
each end of,the cross shaft. When the cross shaft rotates, one lever moves
to actuate a microswitch. The other lever bears on a helical spring. An
adjusting screw is used to change the compression of the spring and thus
change the setpoint of the differential pressure switch.
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July 18, 1986
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The case contains two ports on either side of the piston or diaphragm. The
lower port on one side is connected to the system reference leg, and the
lower port on the other side is connected to the lower instrument tap (i.e.
variable leg). The upper ports on both sides are used as vents and are
plugged when the switch is in service.
The design of the cavity containing the diaphragm (or piston) is such that
motion of the diaphragm is limited to 0.015 inch. Most of the time, the
diaphragm is against one or the other of the mechanical stops which limit
motion of the diaphragm. Thus the sum of the unbalanced hydraulic forces
across the diaphragm is supported by one stop or the other except when the
microswitch is forced to change position. This occurs when the absolute
value of the torque caused by the unbalanced hydraulic forces changes from a
value less than to a value greater than the torque caused by the helical
spring. This movement causes the cross shaft and the levers to rotate 1.8
degrees.
PROBLEM AREAS
Differential pressure switches are often calibrated in situ after isolating
them from the reactor system. A test rig consisting essentially of two
bottles each containing water and air or nitrogen are connected to the
differential pressure switch with one bottle on either side of the
diaphragm. The differential pressure for calibration is established by
adjusting the gas pressures in the bottles. Often, the lower pressure is at
or near atmospheric pressure.
When the SOR Model 103 differential pressure switch is calibrated to a
setpoint at atmospheric pressure and then connected to a system operating at
a static pressure of about 1000 psig, the actual setpoint shifts in most
cases in the conservative direction toward less differential pressure
required to trip. In other cases, the offset of setpoint due to calibration
at atmospheric pressure has been found to be in the opposite direction. The
manufacturer has stated that each switch has unique characteristics and that
switches with the same model number do not all behave in the same way. It
has been postulated that this may be caused by deformation or movement of
the O-rings on the cross shaft when system pressure is applied. For water
level applications and depending on the location of the lower instrument tap
relative to the required setpoint, offset may be so large that the switch
will not actuate before the level drops below the tap. In this case, the
switch would not actuate no matter how low the level dropped. The vendor has
indicated to the staff that factory tests showed an offset between behavior
at atmospheric pressure and behavior at system pressure and that this
information is provided to all customers. It has been the practice at
LaSalle to calibrate at atmospheric pressure without compensating for errors
due to static pressure effects.
For minimum flow applications where it is necessary to open a valve in a
recirculation line to protect a pump in an emergency core cooling system,
assurance is needed that offset will not delay that action and result in
pump damage.
Testing of Series 103 differential pressure switches at LaSalle showed that
application of a static pressure to the switch for a period of time also
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July 18, 1986
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resulted in a significant shift in the setpoint of the switch, and that the
shift due to prolonged pressure was generally in the opposite direction from
the shift due to the initial application of static pressure. After being
calibrated at atmospheric pressure, a static pressure of 1000 psig was
maintained. A recheck of the setpoint of one switch at the end of 24 hours
showed that the setpoint had shifted by a net amount that was
nonconservative by about 10 inches. Subsequent rechecks continued to show
shifting but in lesser amounts. To be valid, it appears that calibration and
tests would need to be rechecked after static pressure has been maintained
for at least 48 hours.
Recent testing at LaSalle has also shown that the point at which trip occurs
depends on whether the switch setpoint is being approached from low
differential pressure or high differential pressure. This is particularly
important for automated blocking valves in the recirculation lines which
protect emergency core cooling pumps from damage when system flow is low.
When flow decreases to a value below the setpoint, the switches should
actuate to open the valves. Conversely, when flow increases, the switches
should deactuate to provide maximum flow to the core.
In addition to showing offset problems, some of the Series 103 switches
evidence sticky behavior, i.e. a larger change in differential pressure is
required to actuate the switch on the first demand than on subsequent
operations and on subsequent tests actuation may be erratic. It is believed
that starting friction and the condition of the cross shaft surfaces may
cause these problems. If the O-rings stick, then the torque that they apply
is added to the torque applied by the calibration spring. SOR is conducting
a long range test with switches that have more highly polished finishes on
those parts of the cross shafts that are in contact with O-rings.
It has been common practice at LaSalle to actuate the switches several times
and then to record the differential pressures required for the third or
fourth actuation. It appears that the Licensee has not emphasized that the
"as-found" condition of the switch is the value of differential pressure
required to actuate the switch during the first demand. It is this value
that must be used to determine whether the switch and its system would have
performed their intended functions if called upon to do so.
The life of Series 103 switches has been said to be 20 to 40 years. However,
the shelf life of the elastomeric material used in the O-rings is
considerably less than 40 years. The O-rings may need to be changed several
times during the life of the plant. Further, there is some concern for the
effect of reactor water on the O-rings, cross shaft surfaces bearing on the
O-rings, and on the diaphragm material, and possible corrosion of the cross
shaft bearings.
OBJECTIVES OF REQUIRED ACTIONS
General Design Criterion 21 "Protection System Reliability and Testability"
requires that the protection system be highly reliable. It is clear that the
SOR differential pressure switches that have been tested carefully to date
have not performed reliably.
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July 18, 1986
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A significant uncertainty exists as to where SOR differential pressure
switches are currently installed or planned to be installed. A list provided
by SOR, Inc. included one utility that had ordered the switches but later
decided not to install the switches. The NRC later learned that another
utility that was not on the SOR list had installed SOR switches. It is
important to assessing the safety impact to know with certainty which plants
have SOR switches installed and in what plant systems. Since these switches
were installed predominately as environmentally qualified electrical
equipment important to safety, as described in 10 CFR 50.49(b), this
Bulletin requests all licensees to identify each such installation. The NRC
intends to evaluate this information, in combination with the results of
other actions required by this Bulletin, to determine if further actions
should be required.
Licensees, who have SOR switches installed, are requested to determine which
of those switches are installed in systems which are subject to Limiting
Conditions for Operations of the plant Technical Specifications. For SOR
differential switches that are not in systems subject to Technical
Specifications, licensees are expected to review the information in this
Bulletin and consider actions, if appropriate, to preclude problems similar
to those discussed in this Bulletin from occurring. For SOR differential
pressure switches that are installed in systems subject to Technical
Specifications, the Bulletin requests licensees to take certain actions to
assure that these switches and systems will be capable of performing
acceptably, if called upon during an actual plant transient or accident.
First, each licensed reactor operator (and senior reactor operator) on duty
should be made aware of the potential problem that may occur at his/her
plant. This information should include a knowledge of the incident at
Lasalle, where SOR differential pressure switches are installed in his/her
plant, how to detect a malfunction or failure of any of these switches, and
the remedial actions that he/she should be prepared to take if a malfunction
were to occur.
Second, the Bulletin requests licensees to conduct special operability tests
of each system that is subject to Technical Specifications that involve SOR
differential pressure switches. Special tests are necessary to determine the
actual trippoint of the switches and the operability of the systems since
tests of the type typically conducted may not be adequate to reveal the type
of problems that have been revealed at the LaSalle station.
It is important that the tests simulate the conditions of the operation of
the system. Further, the test results from LaSalle suggest that the system
operating conditions should be maintained for at least 48 hours before
attempting to measure the performance of the SOR switches. For those systems
that are not testable during plant power operations, it is anticipated that
licensees will use test rigs in order to simulate operating conditions and
not impact plant operations. It is also expected that licensees may take
credit for the 48 hours that the switch was at system operating conditions
prior to connecting the test rig, in order to minimize the time the
switch/system is bypassed or tripped. If the test rig can be connected to
the switch so as to make a virtually "bumpless" transfer from the system to
the test rig without
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July 18, 1986
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tripping the switch, such credit may be appropriate. A primary objective of
the special tests is to determine how the switch will respond to its first
demand after being at system operating conditions for a period of time.
Special care may be necessary to assure that the first actuation is
measured.
If one channel of a system of redundant channels (or similar equipment in
redundant safety systems) is found to have an actual trippoint that is
outside the Technical Specifications or otherwise unacceptable for
adequately reliable system operation, then the redundant channels (or
similar equipment) should be tested as soon thereafter as practical. The
short term corrective actions to be taken to return the set of channels to
operable status should be based on an analysis that conservatively considers
the performance of the set of redundant channels (or similar equipment). In
view of the generic safety concerns and the possibility of common mode
failures, unacceptable performance of an SOR differential pressure switch
should be reported to the NRC in accordance with 10 CFR 50.72 and 10 CFR
50.73.
Since the conduct of any special test could have potential adverse affects,
the requirements for follow-up tests to verify continuing proper functioning
of the switches and systems have been minimized to the extent possible
consistent with the safety objective. The Bulletin requests that licensees
propose an interim performance monitoring program that would cover the time
between the special tests and full implementation of long term corrective
actions. The objectives of the program are to detect any instance of
unacceptable performance, to provide for timely initiation of additional
corrective action, and to gather additional switch performance data.
The Bulletin requests licensees to determine what long term corrective
actions may be appropriate and will be taken. Part of this determination
would include considering the potential effects of common mode failures.
This determination should be based upon an analysis using the worst observed
shift of the actual trippoint from the calibration setpoint for SOR switches
in each general type of application, e.g., water level measurement or main
steam flow measurement. The purpose of the analysis is to determine if
improvements in calibration and testing methods, improvements in setpoint
methodology, additional safety analysis to establish a revised licensing
basis for the plant, change in the Technical Specifications, repair,
modifications, or replacement, or other improvements are needed in order to
meet existing regulatory requirements (e.g., General Design Criterion 21 or
plant technical specifications). The analysis should demonstrate that the
long term corrective action will provide an adequate margin for safety so as
to assure high functional reliability.
Actions Required of All Licensees:
1. Within 7 days, submit a report on the extent to which SOR Model 102 or
103 differential pressure switches are installed (or planned) as
electrical equipment important to safety, as defined in 10 CFR
50.49(b). Include in the report: the model number of the switch, the
system in which it is installed (e.g., low pressure safety injection),
the application of the switch (e.g., water level measurement, system
flow measurement), and the
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July 18, 1986
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function of the switch (e.g. , control of minimum flow recirculation
valve). A negative report, if appropriate, is required.
Actions Required of Licensees That Have SOR Model 102 or 103 Differential
Pressure Switches Installed in Systems That Are Subject to Limiting
Conditions for Operation in Technical Specifications:
2. Within 7 days, take positive action to assure that licensed reactor
operators on duty are prepared for potential malfunctions of SOR
switches.
3. Within 30 days, conduct a special test of each SOR switch to determine
if the switch and system function properly or if short term corrective
actions are necessary. The tests are to determine if the
switches/systems will respond acceptably on the first demand after
being at system operating conditions for a period of time. The tests
should be planned and conducted so as to minimize any potential adverse
affects of the testing. If any corrective action includes the
replacement of SOR switches with mechanical differential pressure
switches by another manufacturer, the licensee should , submit a
technical justification, including a reliability demonstration. Repeat
the special tests on a monthly basis until two consecutive successful
tests are attained.
4. Report failures in accordance with 10 CFR 50.72 and 10 CFR 50.73.
5. Within 60 days, develop, implement and submit a written report
describing your interim performance monitoring program to provide
continuing assurance that the performance of the switches and plant
systems remains acceptably reliable until long term corrective actions
are fully implemented.
6. Within 60 days, submit a written report which describes the margin and
basis for switch actuation. The report should also describe the long
term corrective actions to be taken, including the implementation
schedule, the impacts of potential common mode failures, and an
analysis to demonstrate that the system involved will meet regulatory
requirements and function reliably. The report should include specific
information on the installed SOR switches: the manufacturer's specified
range for the switch, the nominal and allowable values for the
calibration setpoint in the Technical Specifications in the same terms
as the manufacturer's specified range for the switch, the relative
locations of the instrument taps for water level monitoring
applications, sources of systematic errors such as the differences in
elevations of the installation of condensing pots, and "as found" and
any subsequent test data for any switch that does not conform to the
Technical Specifications or is otherwise unacceptable.
Recipients of this Bulletin who hold construction permits and licensees of
plants that are shutdown for an extended period (e.g., Browns Ferry) are not
required to complete the actions of this Bulletin on the schedule shown. In
each case, compliance with this Bulletin should be addressed prior to the
next critical operation of the plant or within 1 year, whichever occurs
first.
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If, because of plant unique conditions, a licensee should determine that any
action requested by this Bulletin jeopardizes plant safety, the action
should not be initiated and the NRC should be notified as soon as practical.
This notification should include the basis for the determination. Further,
if a licensee determines that, even with "best efforts," an action requested
by this Bulletin can not reasonably be completed within the prescribed
schedule, the NRC should be notified within 7 days of receipt of the
Bulletin.
The written reports shall be submitted to the appropriate Regional
Administrator under oath or affirmation under the provisions of Section 182a
of the Atomic Energy Act of 1954, as amended. Also, the original copy of the
cover letters and a copy of the reports shall be transmitted to the U.S.
Nuclear Regulatory Commission, Document Control Desk, Washington, DC, 20555
for reproduction and distribution.
The request for information was approved by the Office of Management and
Budget under blanket clearance number 3150-0012. Comments on burden and
duplication may be directed to the Office of Management and Budget, Reports
Management, Room 3208, New Executive Office Building, Washington, DC, 20503.
If you have any questions regarding this matter, please contact the Regional
Administrator of the appropriate NRC Regional Office or one of the technical
contacts listed below.
James M. Taylor, Director
Office of Inspection and Enforcement
Attachments:
1. Plants with Similar SOR Switches
2. List of Recently IE Bulletins
Technical Contacts: J. T. Beard, NRR
(301) 492-4415
Roger W. Woodruff, IE
(301) 492-7205
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Attachment 1
IEB 86-02
July 18, 1986
PLANTS WITH SERIES 102 OR 103 DIFFERENTIAL PRESSURE SWITCHES
Series 102:
Florida Power and Light
Series 103:
Commonwealth Edison
General Public Utilities - Nuclear Corporation
Houston Lighting & Power Company
Northeast Utilities
Pennsylvania Power & Light
Southern California Edison
Tennessee Valley Authority
Washington Public Power Supply System
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