United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 94-49: Failure of Torque Switch Roll Pins


July 6, 1994



All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to the potential inservice failure of the torque
switch roll pin in Limitorque Corporation (Limitorque) actuators.  It is
expected that recipients will review the information for applicability to
their facilities and consider actions, as appropriate, to avoid similar
failures.  However, suggestions contained in this information notice are not
NRC requirements; therefore, no specific action or written response is

Description of Circumstances

Pilgrim Nuclear Power Station, Unit 1.

On March 9, 1994, after a surveillance test of a motor-operated valve (MOV) in
the high-pressure coolant injection (HPCI) system, the MOV could not be opened
after being closed.  The failed valve is a normally open test and maintenance
valve on the pump discharge line.  When closed, the valve maintains the
pressure isolation boundary during inservice stroke time testing of the
normally closed HPCI system injection valve; it receives an open signal upon
initiation of the HPCI automatic start logic.  The MOV breaker tripped open on
overload and prevented the MOV from being opened remotely as designed.  The
valve is operated by a Limitorque SMB-0 actuator (Licensee Event Report
(LER) 50-293/94-002).

Evaluation of this failure revealed that the direct cause of the inability of
the MOV to open was the failure of the MOV torque switch drive pinion gear
roll pin.  With the roll pin broken, the torque switch did not deenergize the
motor upon completing the close stroke and the MOV electrical breaker tripped
open.  The licensee, Boston Edison Company, declared the HPCI system
inoperable and determined that an increase in the impact load on the roll pin
apparently resulted from high pullout thrust and increased forces due to
hardened grease in the torque switch spring pack.  The licensee learned about
a hammering of the torque switch on valve unseating observed at another
nuclear power plant (described below for Washington Nuclear Project, Unit 2)
and considered that this dynamic effect might also have contributed to the
roll pin failure at the Pilgrim plant.  The licensee concluded that the roll

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pin design, a small diameter pin (2.4 mm [3/32 in.]) made of relatively
brittle high carbon steel (American Iron and Steel Institute (AISI) 1070), was
the root cause of the failure.

The licensee opened the failed MOV to its safety position, returned the HPCI
system to an operable status, and established corrective action to replace the
failed torque switch model with one that has a larger diameter roll pin of
improved material.  The licensee evaluated the potential for this problem to
affect those MOVs that must close during an accident scenario and subsequently
reopen.  The licensee determined that such MOVs are operable with the existing
roll pin design.  As MOVs are disassembled for diagnostic testing, the
licensee plans to install spring packs with internal relief devices to prevent
grease hardening in the spring pack.

Hope Creek Nuclear Station, Unit 1.

On August 14, 1993, during inservice testing of valves, a HPCI pump discharge
valve failed to fully open during an open stroke test.  The valve, a 14-inch,
horizontally mounted, flexible wedge gate valve, provides coolant from the
HPCI system to the core spray system during actuation of the emergency core
cooling system.  The motor overload contacts for this same valve tripped
during the subsequent close stroke.  The valve is operated by a Limitorque
SB-3 actuator (LER 50-354/93-005).

Evaluation of this failure revealed that the torque switch roll pin was
broken, probably because of shearing forces.  The failure of the roll pin
allowed premature torque switch actuation on the open stroke and failure of
torque switch actuation on the close stroke.  The licensee, Public Service
Electric and Gas Company, replaced the torque switch and is administratively
controlling the number of valve strokes for this valve as part of an ongoing
root-cause evaluation.  The licensee plans to complete the root-cause
evaluation and perform further diagnostic testing during the next planned
outage in the spring of 1994.

Four previous torque switch roll pin failures have occurred at the Hope Creek
Station.  Three of these failures (March 1988, November 1989, and September
1991) involved the same HPCI pump discharge valve actuator that was involved
in the failure that occurred in August 1993.  The fourth failure, in November
1990, involved an SMB-1 actuator installed vertically in the HPCI steam
supply line.  Each of the valves is stroke-tested quarterly in accordance
with the American Society of Mechanical Engineers (ASME) Boiler and Pressure
Vessel Code, Section XI, "Inservice Testing."

Washington Nuclear Project, Unit 2.

In August 1993, the licensee, Washington Public Power Supply System, notified
Limitorque about a problem with torque switch roll pins in MOV actuators with
model numbers SB/SMB 0 through 5.  The valves were apparently sticking in
their seats when closed and causing increased stress on the torque switch roll
pin during valve unseating (see Pilgrim LER 50-293/94-002).
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Palo Verde Nuclear Station, Units 1, 2, 3.

On May 26, 1992, the licensee, Arizona Public Service Company, requested
technical assistance in a letter to Limitorque to address failures of torque
switch roll pins.  The licensee had experienced at least 14 failures of pins
(roll and groove pins) in torque switches on 10 different MOV actuators.  Two
failures occurred in actuators with model number SMB-00, two for SB-0, eight
for SMB-1, and two for SB-3.  (NRC Inspection Reports 50-528/94-11,
50-529/94-11, 50-530/94-11).

Using strain gages, the licensee found the dynamic torque during operation to
be excessive during rapid acceleration (snapback) of the spring pack
immediately following unseating of the valve.  The licensee found that for
actuators with model SB that have a lost motion drive sleeve (hammerblow
feature), the compensator spring increased the snapback acceleration.

The licensee observed another effect of impact loading of the torque switch.
During unseating of the valve, the torque switch contact bar flipped onto its
side, interrupting electrical continuity and motor operation as though the
torque switch had actuated.  The licensee replaced the contact bar compression
springs with stiffer springs and attributed the root cause to the recoil force
that impacts the contact bar when the valve is unseating.


On March 23, 1994, Limitorque submitted a report to the NRC in response to the
recent failures of torque switch roll pins, in accordance with Part 21 of
Title 10 of the Code of Federal Regulations  (Part 21 report).  This report
discusses potential failures of the torque switch roll pin in Limitorque MOV
actuators with model numbers SMB/SB/SBD-0 through -4, -4T, -5, -5T, and -5XT.
Limitorque stated that (1) it did not have the required expertise relative to
valve and system design to perform a root cause analysis of these potential
failures, (2) an affected isolation valve that must move from its open
position to its closed position may not reopen reliably, (3) an affected
injection valve would open and close, provided that no abnormal conditions
were present during the previous valve closure, such as thermal overload trip
or excessive motor current, (4) the licensee corrective action is to acquire
and analyze site data relative to this issue, (5) Limitorque modified the roll
pin design by replacing the 2.4 mm [3/32-in.] diameter pins with 3.2 mm
[1/8-in.] diameter shear-proof pins and (6) Limitorque will replace the
303 stainless steel shaft material for new torque switches with 416 stainless
steel material.

On December 11, 1990, Limitorque submitted a Part 21 report of the potential
failure of the torque switch roll pin in SMB-00 actuators.  This report
limited the scope of concern to MOV actuators with model numbers SMB-, SB-,
and SBD-00 that have heavy spring packs that are declutched under maximum
rated load.

The recent torque switch roll pin failures occurred during motor-driven
operation of the MOVs.  In contrast, the Limitorque Part 21 report dated.
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December 11, 1990, discusses failures during declutching of the actuator under
loaded conditions to allow manual operation.

Related Generic Communications.

Potential overthrusting of motor-operated valves is discussed in NRC
Information Notice 92-83, "Thrust Limits for Limitorque Actuators and
Potential Overstressing of Motor-Operated Valves," issued March 27, 1992.

Information Notice 92-83 discusses several industry qualification testing
programs that are being used by licensees to increase motor-operated valve
thrust allowable limits.  These higher thrust limits may cause various
actuator subcomponents, including torque switch roll pins, to be subjected to
higher stress levels and thus to incur higher failure rates.

This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.

/s/'d by BKGrimes

                            Brian K. Grimes, Director
                            Division of Operating Reactor
Support                             Office of Nuclear Reactor Regulation

Technical contacts:  James S. Stewart, RI
                 (215) 337-5240

                 Thomas G. Scarbrough, NRR
                 (301) 504-2794

                 Christopher Myers, RIV
                 (510) 975-0260

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