United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 89-90, Supplement 2: Pressurizer Safety Valve Lift Setpoint Shift

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

                             September 5, 1991 


NRC INFORMATION NOTICE 89-90, SUPPLEMENT 2:  PRESSURIZER SAFETY VALVE 
                                             LIFT SETPOINT SHIFT 


Addressees 

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

Purpose 

The U.S. Nuclear Regulatory Commission (NRC) is issuing this supplement to 
Information Notice (IN) 89-90 to alert addressees to information about more 
recent pressurizer safety valve setpoint tests using steam, loop seal water, 
and nitrogen as test fluids.  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 supplement do not constitute NRC requirements; 
therefore, no specific action or written response is required. 

Background 

The staff issued IN 89-90 to inform addressees about problems that could 
result from operating pressurizer safety valves in environments that are 
different from those used to establish the lift setpoints for the valves.  
The information notice focused on problems encountered when valves were 
operated with a loop seal containing water after setting the valve lift 
setpoints using steam under the seat.  The staff issued IN 89-90, Supplement 
1, to inform addressees about the possible effects of temperature variations 
on safety valve setpoints. 

Description of Circumstances 

In IN 89-90, the staff noted that the Westinghouse Electric Corporation 
tested the setpoints on several plant pressurizer safety valves (PSVs) and 
found that when the valve setpoints are set with steam as the test fluid, 
the setpoints could shift upward 4 to 8 percent if the PSVs are installed on 
water-filled loop seals.  Westinghouse discovered the shift while attempting 
to set PSVs more correctly by using the same fluid against the valve seats 
as is found during normal operation (subcooled loop seal water).  
Westinghouse tested and adjusted the setpoint of several plant PSVs by 
mounting the valves on a test facility loop seal piping configuration, 
filling the loop seal piping with water, and increasing the inlet pressure 
to the observed lift setpoint of the PSVs.  After Westinghouse performed 
these tests, three events occurred in which PSVs, which had been set by this 
method, actuated at normal plant operating pressure, about 200 psi below the 
required setpoint.  These three events 

9108290122 
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                                                  IN 89-90, Supplement 2 
                                                  September 5, 1991 
                                                  Page 2 of 3 


occurred at the Virgil C. Summer Nuclear Station in May and August of 1989 
and at the Surry Power Station, Unit 2, in October 1989.  As discussed in IN 
89-90, a loss of the loop seal water appeared to cause the valve setpoints 
to shift downward. 

In late 1990, the Westinghouse Owners Group had Westinghouse perform 
additional diagnostic tests on several PSV models to address concerns 
regarding the three plant events and findings from the preliminary test 
results that indicated a 4 to 8 percent difference between the resultant 
setpoints for the steam and for the subcooled loop seal water.  The results 
of these diagnostic tests showed that by making very precise measurements of 
valve spindle motion (i.e. disk lift), Westinghouse found a much smaller 
corresponding shift in setpoint between the steam and the loop seal liquid 
inlet conditions.  These tests indicated that what had earlier appeared in 
the above three events as downward shifts in setpoint caused by a loss of 
loop seal water probably occurred because the actual PSV setpoints were too 
near the system operating pressure.  By not measuring the disk lift very 
precisely at the loop seal test facility, Westinghouse had apparently 
adjusted the PSV setpoints too low.  Westinghouse also found that the time 
needed to purge the loop seal was longer than had been previously measured 
in the Electric Power Research Institute (EPRI) test program for safety 
valves in 1981. 

Discussion 

In performing the recent diagnostic tests of PSV setpoints with saturated 
steam and subcooled loop seal water, Westinghouse did not observe the large 
shift in setpoint that it had observed earlier and that the staff had 
reported in IN 89-90.  When the disk lift was measured very precisely, 
testing with subcooled loop seal liquid provided setpoint results nearly the 
same as for steam, even with a large difference in temperature between the 
two fluids.  Therefore, these test results indicate that valves set with 
steam should begin to open at approximately the same pressure when installed 
with steam or subcooled loop seal water against the seats.  However, in 
performing the most recent tests, Westinghouse also found that more time was 
needed to purge the loop seal than had been earlier determined from the EPRI 
test program data.  Safety valves will not relieve the system steam pressure 
until the loop seal has purged at which time the valves will fully open.  
Therefore, the longer time needed to purge the loop seal could cause the 
maximum pressure in the primary system to increase.  If the system design 
margin is small, a rapid pressurization rate could cause the pressure to 
exceed the allowable limit. 

Responding to concerns regarding setpoint testing with water-filled loop 
seals, Westinghouse also submitted inquiries regarding the issue of setpoint 
testing of PSVs to the American Society of Mechanical Engineers (ASME) 
Boiler and Pressure Vessel Code committees that establish setpoint testing 
standards.  The ASME Section III and Operation and Maintenance Code 
committees addressed this issue by clarifying the testing requirements for 
the Code setpoint to specify that safety valves for saturated steam service, 
which are installed on water-filled loop seals, should be tested with steam.  
In addition, the Section III Code committee stated that the effects of the 
water-filled loop seals should be considered in evaluating the maximum 
system pressure. 

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                                                  IN 89-90, Supplement 2 
                                                  September 5, 1991 
                                                  Page 3 of 3 


In conducting the recent safety valve tests, Westinghouse also conducted 
tests using nitrogen and compared the setpoints with those obtained for 
saturated steam.  Westinghouse found that the setpoint varies more between 
nitrogen and steam than it does between loop seal water and steam.  
Westinghouse also found that the setpoints for nitrogen and steam may need 
to be correlated for specific valves individually. 

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. 




                                   Charles E. Rossi, Director Division of 
                                   Operational Events Assessment Office of 
                                   Nuclear Reactor Regulation

Technical contacts:  Charles G. Hammer, NRR
                     (301) 492-0791 
               
                     Mary S. Wegner, AEOD
                     (301) 492-7818 

                     Frank C. Cherny, RES 
                     (301) 492-3945

Attachment:  List of Recently Issued NRC Information Notices

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