Information Notice No. 87-40: Backseating Valves Routinely to Prevent Packing Leakage
SSINS No.: 6835
IN 87-40
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
August 31, 1987
Information Notice No. 87-40: BACKSEATING VALVES ROUTINELY TO
PREVENT PACKING LEAKAGE
Addressees:
All nuclear power reactor facilities holding an operating license or a con-
struction permit.
Purpose:
This information notice is provided to alert recipients to potentially sig-
nificant safety problems that could be caused by backseating valves routinely
to prevent packing leakage. It is expected that recipients will review the
information for applicability to their facilities and consider actions, if
appropriate, to preclude a similar problem. However, suggestions contained in
this notice do not constitute NRC requirements; therefore, no specific action
or written response is required.
Description of Circumstances:
On June 12, 1987, Virginia Electric and Power Company reported (Licensee Event
Report [LER] 87-011-00) that on May 16, 1987, a low flow reactor trip occurred
at its Surry Power Station, Unit 1. The cause of the low flow was the failure
of the stem of the A hot leg loop stop valve. The stem failure permitted the
disk to drop and partially block flow in the A loop. The licensee is
performing a detailed metallurgical examination to determine the failure mode
and mechanism of the valve stem. The preliminary report indicated that
failure was due to stress or fatigue.
Before this event, the licensee had routinely backseated the loop stop valves
as part of its Containment Checklist Procedure before startup. In accordance
with this procedure, the valves were manually torqued onto their backseats to
1/16-inch deflection while the unit was in cold shutdown. This value was
reverified when the unit reached a hot shutdown condition. To reduce the
stress on the valve stem, the licensee is revising the operating procedure so
that the valves are normally operated off the backseat.
A similar event had occurred with the B hot leg loop stop valve on December 1,
1973. This failure was evaluated by Westinghouse, and a failure report,
"Surry Unit No. 1 Reactor Coolant Isolation Valve Stem Failure Report," was
issued on March 7, 1974. In the report, the failure mechanism was identified
as a high
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strain, low-cycle failure with little deformation. The high strain was at-
tributed to the licensee's practice of routinely electrically backseating the
valves on torque during plant startup. In this method, the valve's motor
operator is used to drive the valve open until the forces resulting from the
disk pressing against the backseat are high enough to cause the open torque
switch to open. This practice caused the valve stem to be subjected to high
stresses each time the valve was opened. Depending on the gearing within the
motor operator, these stresses may have remained until the valve was closed
during the next plant shutdown.
The report recommended that the valves not be electrically backseated on
torque. It further recommended that if backseating became necessary during
maintenance, it should be done manually by the use of the handwheel with
minimum applied load and without exceeding the compensating spring pack
deflection specified in the manufacturer's revised instruction manual. This
manual, "Instruction Manual Motor Operated Reactor Coolant 30" Loop Stop
Valves for Reactor Coolant System Westinghouse WNES 546-CAK-70497B Darling
Valve S.O. E-5004," states in a caution that manual backseating is
permissible only if the open deflector indicator reading does not exceed
1/16-inch maximum and that manual backseating may be used only when the
packing needs replacement. As noted above, contrary to this recommendation,
it had been the licensee's recent policy to routinely manually backseat these
valves during plant startup.
Before the 1973 valve stem failure, it had been the licensee's practice to
routinely electrically backseat the valves during plant startup. Although
Virginia Electric and Power Company had stopped this practice in 1974, the NRC
staff is aware that other licensees routinely electrically backseat valves.
The most common reason for this is to stop valve leakage from around the stem
on valves that are not readily accessible during plant operations.
The following inspection reports reflect current practices and problems
related to electrical backseating.
(1) NRC Inspection Report 50-321/85-34 provides the results of an inspection
at Edwin I. Hatch Nuclear Plant, Unit 1, performed between November 10
and December 20, 1985. In the report, the inspectors noted observing
information tags that indicated that the reactor core isolation cooling
(RCIC) system inboard steam line isolation and the reactor water cleanup
(RWCU) system inboard isolation valves had been electrically backseated.
The operating personnel indicated that they were not aware that any
testing had been done to verify that the valves would close within the
required time limits. A violation was issued when subsequent testing of
the RWCU valve demonstrated that it could not close within the required
time limits. Additional testing of the RWCU valve showed that it did
meet the closure time limits when it was not starting from a backseated
condition.
NRC Inspection Report 50-321/86-22 and 50-366/86-22 provides the results
of an inspection performed at Hatch Units 1 and 2 between July 28 and
August 1, 1986. In the report, the inspectors noted that the licensee
routinely electrically backseated containment isolation valves in the
high pressure coolant injection, RCIC, RWCU, and recirculating pump
systems.
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Review of maintenance records indicated that the valves had been electri-
cally backseated as many as 18 times during the previous 2 years. The
procedure used by the licensee to electrically backseat the valves con-
sisted of bypassing the open limit switch and then driving the valve disk
onto the backseat until the locked motor current of the motor was ap-
proached.
(2) NRC Inspection Report 50-277/86-25 provides the results of an inspection
at Peach Bottom Atomic Power Station, Unit 2, performed between December
8 and 19, 1986. In the report, the inspectors noted the licensee's
practice of electrically backseating valves suspected of having excessive
packing leakage. The procedure used by the licensee involved an operator
manually closing the motor contacts at the motor control center. The
contacts were held closed until the operator noticed an increase in the
motor current shown on a clamp-on amp meter.
Discussion:
General Electric Company's Service Information Letter (SIL) 385 issued
November 1982 discusses potential valve damage and provides recommendations
for motor-operated valves that are normally backseated or are subjected to
excessive backseating torque. This describes the types of damage that could
be the result of backseating as valve stem failure, valve stem elongation,
backseat damage, cracking of the stem nut, and other related component
distress. It also notes that most damage progresses slowly to the point where
valve operability is uncertain or valve failure occurs, before the damage is
apparent. Among the recommendations were:
(1) identification of the valves that are normally backseated or that may
have had excessive backseating torque applied and establishment of a
program for evaluation, inspection, and repair of these valves
(2) consultation with valve and motor operator vendors to establish any
procedures or modifications that could minimize damage, including the use
of torque switches or other vendor-recommended alternatives to
backseating motor-operated valves.
The inspectors also indicated that the licensees for the Hatch and Peach
Bottom plants are taking actions to prevent recurring packing leaks. Georgia
Power Company is instituting a program at the Hatch units to replace the
packing on these valves at regular intervals and also plans to live load the
packing to help prevent packing leakage. At Peach Bottom, Philadelphia
Electric Company is modifying the packing gland area, replacing the existing
asbestos-based packing with graphite packing, installing carbon bushings, and
live loading the packing.
The information herein is being provided as an early notification of a
possibly significant matter that is still under consideration by the NRC
staff. If NRC evaluation so indicates, further licensee action may be
requested.
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No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the Regional
Administrator of the appropriate regional office or this office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: George A. Schnebli, Region II
(404) 331-5582
Larry E. Nicholson, RI, Surry Power Station
(804) 357-2102
Keith Poertner, RI, Edwin E. Hatch Nuclear Plant
(615) 842-8001
Richard J. Kiessel, NRR
(301) 492-9605
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