United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 90-65: Recent Orifice Plate Problems

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

                               October 5, 1990


Information Notice No. 90-65:  RECENT ORIFICE PLATE PROBLEMS


Addressees:

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

Purpose:

This information notice is intended to alert addressees to recent problems 
that have been identified with orifice plates.  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 do not constitute NRC 
requirements; therefore, no specific action or written response is required.

Description of Circumstances:

Recent licensee and NRC inspections have identified two problems associated 
with orifice plates.  Namely, the installation of orifice plates in the 
reverse direction and the deformation of orifices in the direction of flow. 

With respect to the first of the two problems, on August 8, 1988, operators 
at the San Onofre Nuclear Generating Station, Unit 1, discovered an error in 
several daily calorimetric power calibrations.  Further investigation 
revealed that one of the three feedwater flow indicators was providing flow 
indication that was lower than the actual feedwater flow.  This error 
resulted in the indicated power on the nuclear instrumentation system being 
adjusted as much as four percent lower than the actual reactor power.   The 
licensee determined that the orifice plate for the deficient flow indicator 
was installed backwards, causing the indicated flow to be less than the 
actual flow.  Subsequent evaluation showed that all accident scenarios 
described in the Final Safety Analysis Report produced acceptable results 
despite the error. In 1985, San Onofre Unit 1 reported a similar event (LER 
50-206/85-014). 

During an NRC walkdown of the containment spray system at the Farley Nuclear 
Plant in April 1989, inspectors found that an orifice plate in a flow 
element had been installed backwards.  The flow element provided spray 
additive tank flow indication in the control room.  The licensee 
subsequently found four other orifice plates, used in the charging system, 
the auxiliary feedwater system, and the containment spray system, that were 
reversed.  Similarly, in July 1989 the Shearon Harris Nuclear Power Plant 
found a reversed orifice plate.  This orifice plate also was used to 
determine the flow rate for the containment spray system spray additive 
tank.


9009280166 
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Reversed orifice plates have also been found at the Salem Nuclear Generating 
Station, Brunswick Steam Electric Plant, Waterford Steam Electric Plant, 
Surry Power Station, and North Anna Power Station.  At the Salem plant, in 
May 1989, the flow metering orifices for the high-head cold leg safety 
injection line  were found installed backwards.  After the orifices were 
correctly installed, the indicated flow rate was 15 percent greater.  At 
Brunswick, in February 1989, the licensee discovered a flow restricting 
orifice installed backwards in the high-pressure core injection minimum flow 
line, and in two other systems.  Waterford experienced problems with low 
indicated flow in the recirculation line of a high pressure safety injection 
pump.  The problem was traced to a reversed flow orifice in May 1989.  At 
Surry in August 1989, an NRC inspector discovered a reversed flow orifice 
after the completion of a corrective action program aimed at flow orifices.  
Further walkdowns by the licensee identified two additional flow orifices 
installed backwards.  At North Anna in September 1989, a total of nine flow 
orifices were determined to be installed backwards.  Only orifices which are 
accessible during power operation were inspected.  The root cause of these 
events has generally been determined to be inadequate procedures for and 
inspection of installations.

In February 1989, a second problem with orifice plates was identified at 
Brunswick, i.e., several flow restricting orifices were deformed in the 
direction of flow.  The orifices were located in the residual heat removal 
system, the core spray injection and minimum flow line, and the 
high-pressure coolant injection system.  Each of the orifice plates was 
manufactured from A240-316 stainless steel and was 1/8-inch thick.  This 
thickness was based on standard orifice design that did not consider the 
thickness needed to prevent deformation caused by flow and the differential 
pressure across the orifice plate.  In one instance, the orifice bore had 
increased from 1.021 inches to 1.088 inches, and the plate had ballooned 
outward approximately 0.5 inch.  Subsequently, the licensee evaluated the 
design of the orifice plates using allowable material stresses, orifice 
plate geometric configurations, and system flow rates.  The results of this 
evaluation indicated that the applied stress exceeded several times the 
allowable material yield stress.  The licensee determined that the root 
cause of the deformation was an inadequate design thickness specification.

Discussion:

An orifice plate is commonly used as a primary flow element, and produces a 
differential pressure from which a flow rate can be determined.  Orifice 
plates have a handle on which pertinent data is permanently marked, such as 
orifice diameter, flange size, pressure rating, and, as appropriate, the 
word "Inlet."  The two most common types of orifice plates are squared edge 
and beveled edge.  On a beveled edge orifice plate that is properly 
installed, the word "Inlet" faces the inlet direction and the beveled edge 
faces the outlet direction.  A square edged orifice plate is not dependent 
upon orientation to perform its function.  However, installation procedures 
for both types of orifice plates should be consistent.  A beveled orifice 
plate that is installed backwards would provide a lower differential 
pressure across the flow element, resulting in a flow rate measurement that 
is lower than the actual flow rate.
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Orifice plates are also used as flow restrictors, to provide a specific 
hydraulic resistance in a piping system to limit flow rate.  A failure of 
the orifice plate could cause excessive recirculation or bypass flow, which 
may decrease the flow being delivered to the main flow path during an 
accident.  The failure of the orifice plate could also reduce the hydraulic 
resistance in the system, increasing the total pump flow rate and the 
probability of centrifugal pump runout.  Runout could damage the internal 
components of the pump, lead to inadequate net positive suction head, and 
overload the pump motor.  Increased flow rates in the orificed line could 
also lead to increased pipe vibration and erosion.  In addition, permanently 
deformed orifice plates could provide inaccurate flow measurements.

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 NRR project 
manager.




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

Technical Contacts:  Scott Sparks, Region II
                     (404) 331-4187

                     Andrew Kugler, NRR
                     (301) 492-0834


Attachment: List of Recently Issued NRC Information Notices
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