United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 94-25: Failure of Containment Spray Header Valve to Open due to Excessive Pressure from Inertial Effects of Water

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                            WASHINGTON, D.C. 20555

                                March 25, 1994

                               OPEN DUE TO EXCESSIVE PRESSURE FROM INERTIAL    
                               EFFECTS OF WATER


All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission is issuing this information notice to
alert addressees to the potential for valves to fail to open because of
unexpectedly high differential pressures caused by the inertial effects of
water moving in partially filled piping systems.  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 September 13, 1993, at the Waterford Steam Electric Station, Unit 3, the
train A containment spray header isolation valve failed to open when it was
actuated following the successful completion of a test on the containment
spray system.

The containment spray system configuration includes a shutdown cooling heat
exchanger and two check valves located in the piping between a containment
spray pump and the subject isolation valve (Attachment 1).  The isolation
valve is a 25-cm [10-inch] WKM solid gate valve, which is closed by air
pressure and opened by a spring, and is designed to open with a differential
pressure of 2070 kPa [300 psi] across the gate.  The design rating of the
piping near the valve is 2170 kPa [300 psig] and the design shutoff head of
the pump is approximately 2000 kPa [275 psig].

During the test, the operators had successfully cycled the isolation valve
open and closed as required by the inservice surveillance testing program. 
Subsequently, the operators started the containment spray pump.  These two
functions are tested separately to prevent the actual spraying of water into
the containment.  After completing this testing, the operators attempted to
open the isolation valve to refill the containment spray riser, but the valve 

9403210215.                                                            IN 94-25
                                                            March 25, 1994
                                                            Page 2 of 3

would not open.  Therefore, the licensee declared train A of the containment
spray system inoperable.  The header subsequently became depressurized and the
operators were able to open the valve.

The licensee performed a special test on train A of the containment spray
system to determine the pressures that were developed in the piping.  With the
isolation valve closed, the pump was started, run, and shut down while
pressure measurements were made at various locations in the piping.  The
discharge pressure of the pump, which was run with minimum recirculation flow,
was measured at 1900 kPa [260 psig].  The pressure at the inlet of the heat
exchanger, downstream of the first check valve, was measured at 2230 kPa  
[308 psig].  The pressure between the second check valve and the containment
isolation gate valve reached 3330 kPa [469 psig] and then stabilized at   
3210 kPa [450 psig], 1310 kPa [190 psi] higher than the pump discharge
pressure.  Data gathered during this testing also indicated that air had been
allowed to enter the piping system during maintenance activities.


The licensee concluded that excessive differential pressure across the
isolation valve, which had developed during the earlier surveillance test, had
prevented the isolation valve from opening on September 13.  Apparently the
air in the piping had allowed the pump discharge water to accelerate to a
higher-than-normal velocity.  As the air was forced into the space between the
isolation valve and the check valves, it was compacted to a pressure that was
higher than the pump discharge pressure by the inertia (the hydraulic ram
effect) of the water.  This excessive pressure was then trapped against the
isolation valve when the second check valve closed.  

The licensee initially believed that the high pressure would occur only during
the surveillance tests because during an emergency actuation the isolation
valve was expected to open before the pump started.  However, further
investigation revealed that, if offsite power was maintained, the containment
spray pump might start before the isolation valve opened.  This could cause
the valve to remain shut under accident conditions.  Consequently, as an
interim solution, the licensee has requested and received a technical
specification amendment allowing the train A isolation valve to remain open
during normal operation.  In order to prevent the inadvertent spraying of the
containment, the licensee is taking compensatory measures such as disabling
the pump or closing and disabling the isolation valve before performing
maintenance on the system.

The licensee performed an evaluation and determined that the system was not 
degraded due to the overpressure condition.  The licensee also performed
system evaluations on containment spray train B, and concluded that, due to a
shorter piping configuration and a better capability for venting this system,
train B should be considered operable until a permanent solution is .                                                            IN 94-25
                                                            March 25, 1994
                                                            Page 3 of 3

implemented.  This conclusion was supported by testing which showed that,
although somewhat higher-than-expected differential pressures did occur in
this train also, the containment isolation valve would consistently open.

The licensee is exploring various options for a permanent solution.  These
include using a different isolation valve or delaying the pump starting time
to ensure that the isolation valve will open before pressure build-up can

The event illustrates that valve stroke-time inservice testing alone does not
ensure the capability of a valve to operate under all postulated design
conditions.  This event also indicates that the inertia of water (hydraulic
ram effects) may not have been accounted for in the design of certain systems.

This information notice requires no specific action or written response.  If
you have any questions regarding this matter, 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:  Thomas F. Westerman, RIV   Paula A. Goldberg, RIV
                     (817) 860-8145             (817) 860-8168

                     Linda J. Smith, RIV        Patricia L. Campbell, NRR
                     (501) 968-3290             (301) 504-1311

Attachments: (see IN94025.WP1 for figure)
1.  Figure:  Containment Spray System (Typical of Two Trains)

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