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United States Nuclear Regulatory Commission - Protecting People and the Environment




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

                               April 30, 1992 


NRC INFORMATION NOTICE 92-33:  INCREASED INSTRUMENT RESPONSE TIME WHEN 
                               PRESSURE DAMPENING DEVICES ARE INSTALLED 
     

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 increased response times for pressure sensing 
instruments that occur when pressure dampening devices are installed in the 
instrument sensing lines.  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 25, 1991, the GPU Nuclear Corporation, licensee for the Oyster 
Creek Nuclear Generating Station, initiated a plant shutdown after 
determining that seven of eight isolation condenser line break pressure 
sensors did not meet the plant technical specification requirements for 
instrument response times.  The licensee's trouble shooting determined that 
pressure dampening devices in the sensing lines for these differential 
pressure sensors had caused an increase in the response times.  Furthermore, 
the licensee found that the time delay caused by these devices is 
significant at both low and high pressures.

Discussion

The pressure dampening devices (snubbers) which utilize sintered stainless 
steel elements are generally installed in instrument sensing lines to dampen 
pressure oscillations or to protect the instruments from particulate 
contamination.  

When the sensors failed to meet the required response times during 
surveillance tests, the licensee reviewed the pressure sensing system and 
found that the snubbers were causing unacceptable time delays.  The licensee 
later removed the snubbers because they are not needed when using the 
upgraded Barton pressure sensors that were installed.


9204240146 
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                                                            IN 92-33 
                                                            April 30, 1992 
                                                            Page 2 of 2 


Snubber time delay will not be detected in response time testing conducted 
directly at the sensing instrument (see Figure 1.)  The effect of a snubber 
will differ from sensor to sensor because of differences in the volumetric 
displacement of fluid within the pressure sensing mechanisms.  System 
response time can also be degraded by the accumulation of foreign material 
in sensing line snubbers.

The NRC's Office of Nuclear Regulatory Research is conducting a generic 
study on the performance of pressure instrumentation at nuclear power 
plants.  The staff plans to publish the results in NUREG/CR 5851, "Long Term 
Performance and Aging Characteristics of Nuclear Plant Pressure 
Transmitters."  The staff has completed a part of this effort, and the 
results of tests conducted on pressure sensor response time testing were 
published in an ISA (Instrument Society of America) transaction 91-720, 
"Response Time Testing of Pressure Transmitters in Nuclear Power Plants."  
This publication addresses various causes for delays in sensor response and 
documents that significant time delays can occur.

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 (NRR) project manager.




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

Technical contacts:  Thomas Koshy, NRR
                     (301) 504-1176

                     Iqbal Ahmed, NRR
                     (301) 504-3252

Attachments:  
1.  Figure 1.  Test Configuration that Excludes the
      Effect of Snubbers  
2.  List of Recently Issued NRC Information Notices 


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