United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 96-08: Thermally Induced Pressure Locking of a High Pressure Coolant Injection Gate Valve

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                         WASHINGTON, D.C.  20555-0001

                               February 5, 1996


NRC INFORMATION NOTICE 96-08:  THERMALLY INDUCED PRESSURE LOCKING OF A HIGH
                               PRESSURE COOLANT INJECTION GATE VALVE


Addressees

All holders of operating licenses or construction permits for nuclear power
reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to a loss of operational capability and the
recently discovered damage to the internal components of a safety-related
power-operated gate valve, both apparently caused by thermally induced
pressure locking.  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

On November 11, 1995, Susquehanna Steam Electric Station (SSES), Unit 1, was
shut down to repair the main generator.  During this forced outage,
Pennsylvania Power and Light Co, the licensee for SSES Units 1 and 2,
discovered a bent retaining ring in the Unit 1 high pressure coolant injection
(HPCI) valve while performing a modification to eliminate susceptibility to
pressure locking.  This Anchor Darling 14-inch flexible-wedge motor-operated
pressure seal gate valve is installed in the discharge line from the HPCI
pump.  The valve is located about three pipe diameters from the connection to
the feedwater system piping, which the licensee believes is the source of heat
that caused thermally induced pressure locking and the bent retaining ring.

Discussion

On August 17, 1995, the NRC issued Generic Letter (GL) 95-07, "Pressure
Locking and Thermal Binding of Safety-Related Power-Operated Gate Valves," to
request that licensees take actions to ensure that safety-related power-
operated gate valves that are susceptible to pressure locking or thermal
binding are capable of performing their safety functions within the current
licensing bases of the facility.  As stated in GL 95-07, pressure locking
occurs in flexible-wedge and double-disk gate valves when fluid becomes
pressurized within the valve bonnet and the actuator is not capable of
overcoming the additional thrust required because of the differential pressure
created across both valve disks.


9601300092.                                                            IN 96-08
                                                            February 5, 1996
                                                            Page 2 of 4


Various plant operating conditions can introduce pressure locking.  Pressure
in the valve bonnet might be higher than anticipated when (1) the gate valve
is in a line connected to a high-pressure system or (2) the temperature of the
fluid in the valve bonnet increases causing thermal expansion.  Temperature in
the valve bonnet might increase in response to heatup during plant operation,
a rise in ambient air temperature caused by leaking components or postulated
pipe breaks, or thermal conduction or convection through connected piping. 
Over time, bonnet pressure could decrease by leakage past the seating surfaces
or stem packing.  However, during the time to depressurize, the valve may
remain pressure locked, and the system may not be able to perform its safety
function.  Also, valve actuator operation at locked rotor conditions could
degrade the motor torque capability of a motor-operated gate valve.

While evaluating the operational configurations of safety-related power-
operated gate valves in response to the requested actions of GL 95-07, the
licensee found that the HPCI valve, as well as the reactor core isolation
cooling (RCIC) system injection valve, was susceptible to thermally induced
pressure locking.  The HPCI valve is normally closed and is installed about
three pipe diameters from the feedwater system, and the intervening piping is
insulated.  On the basis of its evaluation, the licensee concluded that,
during plant operation, heat transferred from the feedwater line could cause
bonnet heatup of the HPCI injection valve.  This valve had previously been
eliminated from consideration of thermally induced pressure locking because of
a perceived absence of a heat source.

While Unit 1 was shut down to repair the main generator, the licensee
initiated and completed modifications of the HPCI and RCIC valves to eliminate
susceptibility to pressure locking by drilling pressure relief holes in the
downstream disk of each valve of Unit 1.  During disassembly of the HPCI
valve, the pressure seal spacer and pressure seal segmented retaining ring
were found to be damaged.  The inner edge of the retaining ring was bent
approximately 3.4 mm [0.135 inch].  The retaining ring consists of four
segments and has an outer diameter of about 39.6 cm [15.6 inches], an inner
diameter of 34.5 cm [13.6 inches], and a thickness of 22.2 mm [0.875 inch]. 
The retaining ring serves as a support for the valve bonnet pressure seal and
transfers loads from the valve bonnet to the valve body.  This bending
indicates that the retaining ring had experienced a significant force and
supports the licensee's earlier conclusion that this valve was susceptible to
thermally induced pressure locking.  A calculation by the valve vendor shows
that a load of approximately 4.4 Meganewtons [1 million pounds] could cause
the identified bending of the retaining ring and corresponds to an internal
pressure of approximately 21 to 48 Megapascals [3000 to 7000 psi].  The
licensee considers these pressures to be threshold values for physical damage
to the valve.  The licensee's engineering analysis shows that the heatup of
fluid trapped in the valve bonnet could be sufficient to cause a pressure of
this magnitude..                                                            IN 96-08
                                                            February 5, 1996
                                                            Page 3 of 4


The licensee's analysis revealed that the actuator for the HPCI valve did not
have sufficient thrust capability to open the valve if this high differential
pressure were created across both valve disks.  Therefore, the licensee
reported that for an indeterminate period of time between April 1992 (when the
valve was previously disassembled) and November 11, 1995, the HPCI valve was
inoperable, rendering the HPCI system incapable of performing its safety
function.  No inservice testing of this valve was performed during power
operation because the licensee had a cold shutdown justification for this
valve that supported operational testing only when shut down.  However, the
valve was operated numerous times while the reactor was shut down, such as to
repair the main generator.  The licensee believes that the residual mechanical
damage found in the HPCI valve by itself did not affect valve operability.

GL 95-07 does not specifically request that licensees inspect valve internal
components for damage caused by past pressure locking or thermal binding. 
Since pressure locking and thermal binding can occur in varying degrees,
licensees should be aware of the potential for valve damage when there is a
strong indication of past pressure locking or thermal binding.  Licensees
should also be aware that the operational configurations associated with this
event may occur in pressurized water reactor designs as well.  In addition,
this event appears to indicate that sufficient heat transfer can occur through
a static length of fluid-filled piping to cause thermally induced pressure
locking.

Related Generic Communications

.     NRC Generic Letter 95-07, "Pressure Locking and Thermal Binding of
      Safety-Related Power-Operated Gate Valves," dated August 17, 1995  

.     NRC Information Notice 95-30, "Susceptibility of Low-Pressure Coolant
      Injection and Core Spray Injection Valves to Pressure Locking," dated
      August 3, 1995

.     NRC Information Notice 95-18, "Potential Pressure Locking of Safety-
      Related Power-Operated Gate Valves," dated March 15, 1995 and
      Supplement 1, dated March 31, 1995

.     NRC Information Notice 95-14, "Susceptibility of Containment Sump
      Recirculation Gate Valves to Pressure Locking," dated February 28, 1995

.     NRC Information Notice 92-26, "Pressure Locking of Motor-Operated
      Flexible Wedge Gate Valves," dated April 2, 1992

.                                                            IN 96-08
                                                            February 5, 1996
                                                            Page 4 of 4


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.


                                          signed by

                                    Dennis M. Crutchfield, Director
                                    Division of Reactor Program Management
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Thomas G. Scarbrough, NRR
                     (301) 415-2794
                     Internet:tgs@nrc.gov

                     Howard J. Rathbun, NRR
                     (301) 415-2787
                     Internet:hjr1@nrc.gov

                     Jerry Carter, NRR
                     (301) 415-1153
                     Internet:tjc@nrc.gov
Page Last Reviewed/Updated Thursday, November 21, 2013