Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
SSINS No.: 6835
IN 85-100
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
December 31, 1985
Information Notice No. 85-100: ROSEMOUNT DIFFERENTIAL PRESSURE
TRANSMITTER ZERO POINT SHIFT
Addressees:
All nuclear power reactor facilities holding an operating license (OL) or a
construction permit (CP).
Purpose:
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
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IN 85-100
December 31, 1985
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
conditions.
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
indication.
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.
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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
Attachment: List of Recently Issued IE Information Notices
