Information Notice No. 91-75: Static Head Corrections Mistakenly Not Included in Pressure Transmitter Calibration Procedures

                               UNITED STATES 
                          WASHINGTON, D.C.  20555 

                             November 25, 1991 



All holders of operating licenses or construction permits for nuclear power 


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information 
notice to alert addressees to recent problems that have been identified with 
the calibration of pressure transmitters.  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 

Recent problems have been identified with the calibration of pressure trans-
mitters that affect safety-related systems.  On July 3, 1991, the licensee 
for the Vogtle Electric Generating Plant discovered that a static head 
correction of approximately 25 psig had not been applied during the 
calibration of the pressurizer pressure transmitters for both units (LER 
50-424/91-05).  All four channels of pressurizer pressure instrumentation on 
each unit were affected.  This correction factor affected the high and low 
pressurizer pressure reactor trip setpoints, the low pressurizer pressure 
safety injection setpoint, and the initial pressure used in the safety 
analyses. The licensee reviewed the effect of this condition on its safety 
analyses for a large break loss-of-coolant accident (LOCA), a small break 
LOCA, a steam generator tube rupture, LOCA events that are not related to 
the departure-from-nucleate boiling (DNB) phenomenon, and other non-LOCA 
events.  It found inadequate margin only for the safety analysis of the 
small break LOCA.  However, the licensee determined that the conservative 
assumptions in the small break LOCA safety analysis would provide sufficient 
margin to accommodate the lack of static head correction.  It corrected the 
situation by recalibrating all eight pressurizer pressure transmitters.  The 
licensee also determined that 58 other instruments had no head correction, 
although none of these were as safety significant.  All of the above errors 
existed since initial startup for each unit. 


                                                       IN 91-75 
                                                       November 25, 1991 
                                                       Page 2 of 3 

On April 18, 1991, an engineer at the William B. McGuire Nuclear Station 
noticed that the three pressure transmitters for the suction piping of the 
turbine-driven auxiliary feedwater (TDAFW) pump were at a different 
elevation than the associated flow element.  The licensee reviewed the 
associated procedures and found that it had made an error in compensating 
for the water leg when calibrating the pressure transmitters.  The three 
pressure transmitters must be calibrated such that when the pressure 
decreases in the piping system, a signal is sent to automatically open the 
isolation valves for the ensured water source (the nuclear service water 
system).  The licensee tested the as-found setpoints and determined that all 
three pressure transmitters would not have automatically opened the 
isolation valves at the required setpoint.  However, other safety-related 
sources to the TDAFW pump were available (NRC Inspection Report 

Other licensees have also found errors in the calibration of pressure 
transmitters that occurred because the effects of static pressure had not 
been considered or had been considered inappropriately.  In January 1990, 
the licensee for Arkansas Nuclear One (ANO), Unit 2, identified an error 
with calibration of water level transmitters for the steam generators (SGs).  
A compensation factor for static pressure effects had not been correctly 
included in the calibration procedure.  This error resulted in an inaccurate 
SG water level measurement.  The ANO licensee determined that, with an 
indicated level of 23.25 percent, the actual SG water level could be 21.8 
percent, which is less than the allowable technical specification (TS) value 
for a low SG water level trip (LER 50-368/90-02).  At the Dresden Nuclear 
Power Station in October of 1987, the licensee found that feedwater flow 
transmitters had not been calibrated to account for static pressure effects.  
Feedwater flow is used in determining core thermal power, and this error 
resulted in the unit operating above the facility's core thermal power limit 
(LER 50-237/87-34).  In September 1986, the licensee for the Davis-Besse 
Nuclear Power Station determined that the startup steam generator level 
transmitters incorrectly compensated for the effects of the static pressure 
span of the instruments.  This resulted in transmitter outputs indicating 
that steam generator level was approximately 7 inches higher than the actual 
level (LER 50-346/86-39).  


The errors in the calibration of pressure transmitters discussed above are 
related in that the effects of static pressure were improperly considered.  
Many were the result of an improper review of pertinent information supplied 
by the vendor or manufacturer when developing calibration procedures.  
Others were a result of inadequate technical review when revising 
procedures.  Fundamental errors in developing calibration procedures could 
result in a common mode problem that could affect various pieces of 
safety-related equipment.  

                                                       IN 91-75 
                                                       November 25, 1991 
                                                       Page 3 of 3 

This information notice requires no specific action or written response.  If 
you have any questions about the information in this notice, please contact 
the technical contact listed below or the appropriate Office of Nuclear 
Reactor Regulation (NRR) project manager. 

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

Technical contact:  Scott Sparks, RII 
                    (404) 331-5619 

Attachment:  List of Recently Issued NRC Information Notices 

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