Bulletin 86-02: Static "O" Ring Differential Pressure Switches

                                                       SSINS No.: 6820
                                                       OMB No.: 3150-0012
                                                       IEB 86-02 

                                UNITED STATES
                            WASHINGTON, DC 20555

                                July 18, 1986



All power reactor facilities holding an operating license (OL) or a 
construction permit (CP). 


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 


                                                            IEB 86-02
                                                            July 18, 1986
                                                            Page 2 of 9

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 


                                                            IEB 86-02
                                                            July 18, 1986
                                                            Page 3 of 9

(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. 



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. 


                                                            IEB 86-02 
                                                            July 18, 1986
                                                            Page 4 of 9

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 

                              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 


                                                             IEB 86-02
                                                             July 18, 1986
                                                             Page 5 of 9

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. 


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. 


                                                             IEB 86-02
                                                             July 18, 1986
                                                             Page 6 of 9

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 


                                                             IEB 86-02
                                                             July 18, 1986
                                                             Page 7 of 9

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 

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 

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 

                                                            IEB 86-02
                                                            July 18, 1986
                                                            Page 8 of 9

     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 

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 


                                                            IEB 86-02
                                                            July 18, 1986
                                                            Page 9 of 9

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 

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

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

                                                            Attachment 1  
                                                            IEB 86-02
                                                            July 18, 1986


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 


Page Last Reviewed/Updated Friday, May 22, 2015