United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 85-100: Rosemount Differential Pressure Transmitter Zero Point Shift

                                                       SSINS No.:  6835 
                                                          IN 85-100 

                                UNITED STATES
                           WASHINGTON, D.C. 20555

                              December 31, 1985

Information Notice No. 85-100:  ROSEMOUNT DIFFERENTIAL PRESSURE 
                                   TRANSMITTER ZERO POINT SHIFT 


All nuclear power reactor facilities holding an operating license (OL) or a 
construction permit (CP). 


This information notice is provided to alert licensees of a potentially 
significant safety problem involving a loss of high pressure injection (HPI)
flow indication because of a shift in the zero point of a Rosemount (Model 
1153, Series B) alphaline differential pressure (flow) transmitter from the 
depressurized calibration condition to the pressurized operating condition. 

It is suggested that recipients review this information for applicability to
their facilities and consider actions, if appropriate, to preclude similar 
problems occurring at their facilities. However, suggestions contained in 
this information notice do not constitute NRC requirements; therefore, no 
specific action or written response is required. 

Description of Circumstances: 

The Rancho Seco Nuclear Generating Station experienced an unplanned reactor 
trip on October 2, 1985. Following the reactor trip, a rapid cooldown of the
primary system occurred. The rapid cooldown caused a decrease in pressurizer
level. As prescribed by plant operating procedures for pressurizer level 
decreasing below 100 inches, HPI was manually initiated through the "A" HPI 
valve and reactor coolant system (RCS) Cold leg loop "A" nozzle. Although 
the above actions increased flow to the RCS, the pressurizer level continued 
to decrease. The operator opened the remaining three loop HPI valves, 
allowing HPI flow through all four paths to the RCS. At this point, an 
erroneous zero flow reading was observed on the "A" HPI flow indicator. This 
loss of loop "A" HPI flow indication was unexpected by the operators. To 
further augment the HPI supply, an additional HPI pump was started and the 
loop "A" HPI indicated flow increased to about 80 gpm. 

The licensee's investigation identified the root cause, which was a shift in
the zero point of the flow transmitter. This shift occurred as the device 
was calibrated with the system depressurized and then brought to system 
operating pressure. In the case of the "A" transmitter, this shift was found
to result in zero flow indication at 2500 psig with as much as 80 gpm (per 
loop) of 


                                                         IN 85-100 
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                                                         Page 2 of 3 

actual flow. The zero shift in the "B" and "D" transmitters were found to 
result in indicated flows slightly above actual at 2500 psig under very low 
flow Conditions. These differential pressure transmitters (Rosemount Model 
1153) have a characteristic which affects the zero setting as the process 
pressure changes. According to the Rosemount literature, this zero shift 
causes a constant differential pressure offset over the transmitter range. 
This zero shift can be in either direction and can be as much as 0.66 
percent of full transmitter range for each 1000 psi of static pressure 
change. Static pressure changes caused by valve throttling are dependent 
upon the relative position of the flow transmitter and the throttle valve in 
the HPI line. Although the amount of shift differs for each transmitter, the 
error is repeatable (within the limits of the repeatability specification). 
Since the differential pressure across the flow detector (Venturi) decreases
exponentially at lower flow (proportional to the square of the HPI flow) and
the zero shift is constant, the net effect of the above is to generate a 
large error at low HPI flows under normal reactor operating pressure 

In a deficiency report in accordance with 10,CFR Part 21, the licensee 
reported that the Rosemount instruction manual, number 4302 revision B, of 
July 1982 does not address zero shift due to process pressure changes. How-
ever the Rosemount performance specification report does address this mech-
anism for zero shift. The licensee also reported that these differential 
pressure transmitters also were used in at least seven other applications in
the plant for either level or flow indication. The licensee has taken 
several actions to minimize any adverse effects of this shift in the zero 
point of the transmitters. These are: 

     o    Operators have been informed of the characteristic of the flow 

     o    Each of the HPI flow transmitters has been calibrated such that, 
          with the worst case zero shift, the actual flow will be greater 
          than the indicated flow. 

     o    The "A" and "C" HPI flow transmitters had the largest zero shift 
          and these transmitters are scheduled to be replaced. 

     o    All procedures and pertinent documents were reviewed to ensure the
          resultant flow indication accuracy at low HPI flows is adequate. 

The licensee is still evaluating HPI flow indication to determine if addi-
tional actions are practical to compensate for this zero shift. In addition,
the zero shift characteristics of other Rosemount differential pressure 
transmitters will be determined. Corrective actions, when necessary, will be
similar to those actions taken for the HPI flow transmitters. 


                                                         IN 85-100 
                                                         December 31, 1985 
                                                         Page 3 of 3 

No specific action or written response is required by this information 
notice. If you have any questions about this matter, please contact the 
Regional Administrator of the appropriate regional office or this office. 

                                   Edward L. Jordan, Director 
                                   Division of Emergency Preparedness 
                                     and Engineering Response 
                                   Office of Inspection and Enforcement 

Technical Contact:  Henry Bailey, IE 
                    (301) 492-9006 

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