Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
August 11, 1994
NRC INFORMATION NOTICE 94-56:INACCURACY OF SAFETY VALVE SET PRESSURE
DETERMINATIONS USING ASSIST DEVICES
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 a possible source of inaccuracy when adjusting
the set pressure of various safety valves using assist devices. 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
In August 1993, while testing the set pressure of main steam safety valves
(MSSVs) at the Palo Verde Nuclear Generating Station, the licensee noted that
the set pressure test results for eight of the first nine valves tested were
lower than expected. The set pressures ranged between 1.8 and 5.4 percent
below the nominal values, whereas the range allowed by the plant Technical
Specifications is +1 percent. The licensee considered these results to be
unusual because, historically, MSSV test results at Palo Verde tended to be
higher than the nominal setpoint. The MSSVs were tested in place on the main
steam header using operating steam pressure and a Trevitest assist device
which is manufactured by Furmanite America, Inc (Furmanite). The licensee
postulated that, since the set pressure values were all lower than expected,
and since the valves had been previously set at the Westinghouse Western
Service Center laboratory with full-pressure steam, the set pressures measured
using the Trevitest device could have been wrong.
Following the Trevitest testing of the MSSVs, the licensee arranged for
Furmanite to perform comparative testing to determine the difference between
the assist device testing and full-pressure testing. A total of 37 valves
were tested, and Furmanite found that in order to account for the offset
experienced between the two test methods, the equation used to determine the
Trevitest set pressure required modification. Specifically, the disk area
over which steam pressure acts was adjusted for the Palo Verde valves (Dresser
model 3707R).
9408050072. IN 94-56
August 11, 1994
Page 2 of 3
Discussion
The Trevitest device is one device which is used for measuring set pressures
of safety valves installed in place at system fluid operating pressure
conditions. The device is installed on the safety valve stem where the device
can provide the additional force necessary to balance the pressure and spring
forces on the valve disk. Thus, by measuring the system pressure and the
additional stem force provided by the assist device, the valve set pressure
may be computed, converting the assisting force into equivalent pressure by
dividing the assisting force by the disk area over which system pressure acts.
Therefore, the equation for determining the set pressure when using an assist
device can be written as:
This equation can include other factors to account for the weight of the
device or calibration factors to compute the stem assisting force. Furmanite
has stated that the only parameter in this basic equation that is not directly
measured is the disk area. This parameter should be the area of the disk
surface which steam pressure acts against. For the purposes of this equation,
Furmanite computed and used the mean seat diameter.
On the basis of the results of the comparative testing performed at the
request of the Palo Verde licensee, Furmanite adjusted the seat area to be
used for computing the set pressure. The adjusted disk area, based on the
comparative testing, was less than the area previously determined. The
specific amount of the resulting error in the set pressure measurement is
dependent on the relative magnitudes of the operating pressure when conducting
the testing and on the assisting force. In the comparative testing for the
Palo Verde MSSVs, the offset error was found to be between one and two percent
of the nominal set pressure. The magnitude of the error will vary for
different valve models.
Furmanite has recently sent this information to the eight other licensees that
have used the Trevitest device to test this specific MSSV model, as indicated
in its letter to the NRC, dated March 16, 1994. There are other Dresser and
Crosby MSSV and pressurizer safety valve (PSV) models that have been tested
using the Trevitest device at nuclear power facilities. Since the magnitude
of the adjustment described in this information notice applies only to the
Dresser 3707R model MSSV, the possible offset which could be experienced by
other Dresser or Crosby MSSV or PSV models is unknown at this time. An offset
error which is not properly accounted for can result in premature safety valve
actuation or result in subjecting the system to higher pressures than
calculated under transient conditions.
. IN 94-56
August 11, 1994
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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.
/s/'d by BKGrimes
Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation
Technical contacts: Charles G. Hammer, NRR
(301) 504-2791
Howard J. Wong, RIV
(510) 975-0296
Robert A. Benedict, NRR
(301) 504-1157
Attachment:
List of Recently Issued NRC Information Notices
