Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
February 2, 1989
Information Notice No. 89-11: FAILURE OF DC MOTOR-OPERATED VALVES TO
DEVELOP RATED TORQUE BECAUSE OF IMPROPER
CABLE SIZING
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This information notice is being provided to alert addressees to potential
problems resulting from motor-operated valves (MOVs) not developing rated
torque because of the failure of the original cable sizing calculations to
include the proper current values and cable resistances. It is expected
that recipients will review the information for applicability to their faci-
lities and consider actions, as appropriate, to avoid similar problems. How-
ever, suggestions contained in this information notice do not constitute NRC
requirements; therefore, no specific action or written response is required.
Description of Circumstances:
As a part of the Equipment Qualification and Nuclear Plant Aging Studies, the
NRC obtained a motor-operated wedge-gate valve from the Shippingport Atomic
Power Station following its decommissioning. The valve had been installed
in a secondary system for the 25-year life of the plant. A review of the
maintenance records for the valve and the operator indicated that there were
no reportable incidents associated with either the valve or the operator.
Following removal, the valve and the operator were refurbished by an NRC
contractor and tested to demonstrate the valve's ability to operate at the
conditions expected during the forthcoming testing. The valve, the operator,
and the controller were then transported to Germany where they were installed
in the Heissdampfreactor (HDR) decommissioned reactor test facility as part of
a jointly sponsored seismic test research program. With the exception of the
use of equivalent metric-sized cable, the electrical installation of the
operator was identical to that at Shippingport.
Each test of the valve consisted of a closure cycle, an opening cycle, and
a final closure cycle. When the data from these tests was analyzed, it was
determined that the valve had routinely failed to fully seat during the second
closing cycle of each test which had subjected the valve to system pressure
and flow and ambient temperature conditions. In addition, the valve had
routinely
8901270329
.. IN 89-11
February 2, 1989
Page 2 of 3
failed to fully seat during both closing cycles of each test which had sub-
jected the valve to system pressure, flow, and elevated temperature
conditions. Further evaluation of the data indicated that (1) although the
motor had stalled, the output torque had not been sufficient to open the
torque switch; (2) the maximum recorded current appeared to be significantly
less than that expected for a stalled motor; and (3) the motor voltage at
stall was only slightly less than the running voltage. In response to a
request from NRC, the contractor conducted a second series of tests at HDR.
Because of the concern that degraded voltage might have been a contributor to
the failures, the electrical instrumentation inputs were moved from the motor
control center to the junction strip in the switch compartment of the motor
operator.
The second test series produced the same valve failure pattern that had been
obtained in the initial test series. As the second test series was thought
to have eliminated degraded voltage as a cause, it appeared that the problem
might be improper motor operator sizing. The NRC requested the contractor to
conduct additional testing to determine whether the motor and the operator had
been properly sized initially or whether they had degraded during their
service at Shippingport. To accomplish this testing, the motor operator and
the control-ler were removed from the HDR decommissioned reactor test facility
and shipped to the Limitorque manufacturing facilities for dynamometer
testing.
The dynamometer testing of the motor operator by Limitorque demonstrated that
the performance of the motor operator was better than that at HDR, but that
the motor performance was less than that originally specified. It was
therefore decided to perform motor dynamometer testing at the motor
manufacturer's facilities. To accomplish this testing, the motor was removed
from the motor operator and shipped to the motor manufacturer. The motor
dynamometer testing by the motor manufacturer demonstrated that the motor's
performance did not differ appreciably from that of a new motor.
However, before any additional evaluations could be performed, the NRC contrac-
tor became aware of the recent MOV failures at Brunswick Steam Electric Plant,
Unit 1, which had been described in NRC Information Notice 88-72, "Inadequacies
in the Design of DC Motor-Operated Valves." Because of the similarity of
conditions noted in the HDR and Brunswick failures, the NRC contractor,
working with Limitorque, focused on the analysis of the cable sizing for the
MOV. This analysis pointed out the following two factors:
(1) The first factor concerned the current values that should be used to
establish the cable size. As a motor operator is not a continuous-duty
device, the normal operating voltage and current data contained on the
nameplate are of only limited applicability to cable sizing. Rather, it
is the voltage and current required to develop the rated output torque of
the motor operator that is important. As the current required for this
rated torque must be developed during locked rotor conditions, the cable
should be sized on the locked rotor current instead of on the full load
current. The ratio of these currents is quite large as the locked rotor
current is typically in excess of five times the full load current stated
on the nameplate.
.. IN 89-11
February 2, 1989
Page 3 of 3
(2) The second factor concerned the cable resistance that should be
considered in the analysis. The wiring diagram for a typical motor
operator is shown in Attachment 1. Significant cable resistances and the
location of the voltage measurement during the second HDR tests are also
shown. Attach-ment 2 presents a simplified schematic of the same circuit.
As can be seen, any attempt to measure the voltage drop at the power
supply bus (i.e., the sum of the voltage drops across the armature and
the series field) includes the resistance contribution from four cables.
Even taking the voltage measurement at the motor (as was done during the
second HDR tests) will result in including the voltage drop across three
cables.
From the above information, it can be seen that the required current for the
MOV to develop the design torque should be based on the motor resistance and
the cable resistance (i.e., four times that of a single cable) under a minimum
postulated bus voltage.
As indicated in their August 17, 1988, maintenance update, Limitorque recom-
mends that the cable be sized to allow for five times full load current flow
at the minimum voltage condition. This maintenance update also contained a
method for calculating the maximum current draw at locked rotor conditions.
No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact one of the techni-
cal contacts listed below or the Regional Administrator of the appropriate
regional office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: Yun-Seng Huang, NRR
(301) 492-0921
Peter J. Kang, NRR
(301) 492-0812
Richard J. Kiessel, NRR
(301) 492-1154
Attachments:
1. Cable Resistances and Voltage Measurements Identified
2. Motor Functional Schematic
3. List of Recently Issued NRC Information Notices
.. Attachment 3
IN 89-11
February 2, 1989
Page 1 of 1
LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________
Information Date of
Notice No._____Subject_______________________Issuance_______Issued to________
89-10 Undetected Installation 1/27/89 All holders of OLs
Errors In Main Steam Line or CPs for BWRs.
Pipe Tunnel Differential
Temperature-Sensing Elements
at Boiling Water Reactors.
89-09 Credit for Control Rods 1/26/89 All holders of OLs
Without Scram Capability or CPs for test and
in the Calculation of the research reactors.
Shutdown Margin
89-08 Pump Damage Caused by 1/26/89 All holders of OLs
Low-Flow Operation or CPs for nuclear
power reactors.
89-07 Failures of Small-Diameter 1/25/89 All holders of OLs
Tubing in Control Air, Fuel or CPs for nuclear
Oil, and Lube Oil Systems power reactors.
Which Render Emergency Diesel
Generators Inoperable
89-06 Bent Anchor Bolts in 1/24/89 All holders of OLs
Boiling Water Reactor or CPs for BWRs
Torus Supports with Mark I steel
torus shells.
89-05 Use of Deadly Force by 1/19/89 All holders of OLs
Guards Protecting Nuclear for nuclear power
Power Reactors Against reactors.
Radiological Sabotage
89-04 Potential Problems from 1/17/89 All holders of OLs
the Use of Space Heaters or CPs for nuclear
power reactors and
test and research
reactors.
89-03 Potential Electrical 1/11/89 All fuel cycle and
Equipment Problems major nuclear
materials
licensees.
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit
..
