Information Notice No. 93-54: Motor-Operated Valve Actuator Thrust Variations Measured with a Torque Thrust Cell and a Strain Gage
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
July 20, 1993
NRC INFORMATION NOTICE 93-54: MOTOR-OPERATED VALVE ACTUATOR THRUST VARIATIONS
MEASURED WITH A TORQUE THRUST CELL AND A STRAIN
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 a possible error in their diagnostic evaluation
of motor-operated valve (MOV) actuators. It is expected that recipients will
review the information for applicability to their facilities and consider
actions, as appropriate, to avoid similar problems. However, suggestions
contained in this information notice are not NRC requirements; therefore, no
specific action or written response is required.
Description of Circumstances
Diagnostic testing of MOVs is performed to satisfy the commitments made in
response to NRC Generic Letter (GL) 89-10, "Safety-Related Motor-Operated
Valve Testing and Surveillance," and its supplements. Actuator output thrust
is measured by a combination of transducers. The transducers include, but are
not limited to, the ITI MOVATS torque thrust cell (thrust cell) and the
Teledyne quick stem sensor (stem sensor).
On November 9, 1992, ITI MOVATS engineers working at the Turkey Point plant
noticed that thrust measurements taken with the torque thrust cell were
different from the stem-mounted strain gage readings after the thrust cell was
removed. Review showed that all of the readings of the valves that had been
tested using the stem sensor changed somewhat when the thrust cell was removed
from the valve. Data indicated that thrust readings degraded by approximately
20 percent when the thrust cell was removed.
On November 10, 1992, ITI MOVATS management and a licensee engineer began to
investigate the cause.
As discussed above, to meet the commitments made in response to GL 89-10, most
licensees rely on MOV diagnostic equipment to set the thrust delivered by the
actuator in opening or closing its valve. Failure to properly account for
inaccuracies introduced by the use of diagnostic equipment could result in
July 20, 1993
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either the MOV not being able to perform its intended safety function because
of inadequate actuator output or the failure of a component in either the
valve or the actuator because of excessive motor torque.
The devices discussed in the Description of Circumstances operate on the
- The ITI MOVATS torque thrust cell is a radial web calibrated load cell.
The inner ring of the cell is bolted to the top of the valve yoke and
the outer ring to the bottom of the actuator. The rings are connected
by eight webs that have strain gages to measure actuator torque and
thrust simultaneously. Installation of the thrust cell requires that
the actuator be raised a distance equal to the thickness of the thrust
cell. This means that the actuator stem nut is in a new position on the
- The Teledyne quick stem sensor is a strain gage mounted on a strip of
foil with tabs on each end to facilitate installation on the valve stem.
The sensor comes with leads preconnected to the gage. Epoxy is used to
install the stem sensor on the valve stem.
During the test, the valve is stroked and the thrust cell is used as a
calibration reference for the stem sensor. Once the stem sensor is
calibrated, it can be used for future MOV tests without using the thrust cell.
The Limitorque actuator torque switch is then set to produce the required
valve stem thrust. After the thrust cell is removed, a full valve stroke
signature is obtained to evaluate overall actuator performance.
As a result of their review, ITI MOVATS and licensee personnel concluded that
the most significant contributors to the observed changes in thrust when the
thrust cell was removed are stem engagement and stem lubricant.
- Stem engagements of less than one stem diameter tend to increase the
thrust at torque switch trip by improving the stem factor. This effect
is more pronounced when Neolube is used than when EP-O is used as a stem
- Test results indicate that stroke-to-stroke variations in thrust at
torque switch trip are greater with Neolube than with EP-O. For
Neolube, the variations tend to be high on initial lubrication and then
decrease as the valve is stroked.
In January 1993, ITI MOVATS engineers issued the attached report of their
investigation of the discrepancies in thrust measurements which confirms that
the primary causes are stem lubrication and stem nut wear..
July 20, 1993
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Related Generic Communications
NRC has issued a number of information notices pertaining to the accuracy of
MOV diagnostic equipment. Most recently, these have included NRC Information
Notice (IN) 93-01, "Accuracy of Motor-Operated Valve Diagnostic Equipment
Manufactured by Liberty Technologies;" IN 92-23, "Results of Validation
Testing of Motor-Operated Diagnostic Equipment;" and IN 91-61, "Preliminary
Results of Validation Testing of Motor-Operated Valve Diagnostic Equipment."
NRC also issued Supplement 5 to GL 89-10, "Inaccuracy of Motor-Operated Valve
Diagnostic Equipment," on June 28, 1993.
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 project manager.
/s/'d by BKGrimes
Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation
Technical contacts: Francis Jape, RII
Thomas G. Scarbrough, NRR
1. ITI MOVATS Report dated January 1993
2. List of Recently Issued NRC Information Notices
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