Information Notice No. 92-64: Nozzle Ring Settings on Low Pressure Water-Relief Valves
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
August 28, 1992
NRC INFORMATION NOTICE 92-64: NOZZLE RING SETTINGS ON LOW PRESSURE
All holders of operating licenses or construction permits for nuclear power
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to problems that could result from the inadequate
control of maintenance of low pressure water-relief valves in operating
nuclear power plants. It is expected that recipients will review the informa-
tion for applicability to their facilities and consider actions, as appropri-
ate, 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 August 8, 1991, at the Shearon Harris Nuclear Power Plant, an operator
started a second component cooling water (CCW) pump to support diagnosing a
problem with a flow indicator. The operator immediately noted a decreasing
level in the CCW surge tank and began manually filling the system. Additional
operators were dispatched to the area, and they reported that various CCW
relief valves were lifting. Even though the control board operator initiated
actions to reduce the system pressure from 140 psig to 120 psig (the reseat
pressure for these valves was 135 psig in the vendor test reports), the
leakage continued until the second CCW pump was secured and the system
pressure dropped below 105 psig. These valves were manufactured by the Crosby
Valve and Gage Company.
In its review, the licensee found that the nozzle rings on various CCW valves
had incorrect settings which resulted in a reseat pressure lower than the
system design specified. The licensee reviewed the maintenance history and
found that these valves had been disassembled and rebuilt before the initial
startup in 1986. During these activities, the nozzle rings had been
improperly set. Procedural guidance on the setting of the nozzle rings was
lacking and the manufacturer's design data and technical manual were difficult
August 28, 1992
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Subsequent to the event, the licensee identified 34 low pressure relief valves
on various safety related systems that required verification of the nozzle
ring settings. These valves were chosen because they had previously been
subjected to maintenance which may have disturbed the nozzle ring settings.
The 16 valves checked as of May 1992 all had incorrect settings varying from a
few notches to over 170 notches (method used to set nozzle rings). The
licensee intends to check the remaining valves during forthcoming plant/system
On July 23, 1991, at the Trojan Nuclear Plant, a CCW relief valve lifted
during a system pressure surge and did not reseat until it was manually
isolated. The licensee investigated the incident and found the nozzle ring
settings in the relief valves in both CCW trains substantially different from
the required setting. While conducting preliminary tests, the licensee found
the reseat pressure to be from about 60 to 80 psig, while the expected value
was 135 psig. This event was cited by NRC as part of a severity level III
violation of 10 CFR Part 50, Appendix B, Criterion XVI.
The licensee found that the incorrect ring settings were the result of lack of
procedural controls to set and maintain the nozzle ring settings on certain
plant relief valves. In 1989, the licensee had established controls to return
the nozzle rings to their "as-found" position after maintenance was performed.
However, this was not sufficient to assure that ring settings were in
accordance with vendor test data. Subsequent to this event, the licensee
checked the nozzle ring settings on 54 low pressure relief valves in various
safety related systems that previously had maintenance. Thirty valves had
incorrect settings; twelve of these settings were significantly outside the
In December 1989, during operations to start up Unit 1 at the Braidwood
Station, a suction relief valve in one of the RHR trains suddenly opened as
the operators were drawing a bubble in the pressurizer. In the first
18 minutes after the valve opened, the pressure in the primary system dropped
to about 270 psig, which was well below the expected reseat pressure for the
suction relief valve. Approximately 67,000 gallons of reactor coolant system
water were released to the radwaste holdup tanks before the operators isolated
the RHR train having the open relief valve about 2 hours later.
About 18 months before the event, the licensee had performed maintenance on
the affected RHR relief valve prior to its being installed in the plant. At
that time, the licensee discovered four RHR suction relief valves at the site
that had nozzle ring settings considerably different from those specified in
the vendors's valve test package. A temporary set of instructions was
developed to return the ring settings to the vendor's specified value using a
different reference point and notch count instead of the "as found" setting
and reference point as directed in the existing maintenance procedure. Both
sets of instructions were contained in the same work package used to perform .
August 28, 1992
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the corrective maintenance for the valve that subsequently stayed open for an
extended time. The mechanic apparently confused the two sets of instructions,
adjusting the nozzle ring about 220 notches from its proper position.
In 1986, one of the RHR suction relief valves opened following a pressure
spike to 380 psig and failed to reseat until the pressure decreased to
260 psig (expected reseat pressure was 342 psig) about 10 minutes later when
the affected RHR train was isolated. Upon disassembling the valve, the
licensee noted that the nozzle ring was unmovable. The nozzle ring was jammed
in a locked position about 225 notches from its proper position.
Foreign Pressurized Water Reactor
In May 1985, about 25,000 gallons of reactor coolant were released to the
containment sump through an RHR suction relief valve. The primary system
pressure stopped decreasing about 30 minutes into the event when the RHR
system was isolated. The event was attributed to an improperly set nozzle
ring. The utility also found a broken disc insert pin when the valve was
All of the valves involved in the referenced events were Crosby relief valves
which are used in a number of nuclear plants in the U.S. The expected reseat
pressure for these particular valves was about 90% of the valve lift set
pressure. Other style relief valves, whether Crosby or another manufacturer,
may have different reseat pressures relative to the lift set pressure.
Those relief valves that do have nozzle rings (or guide rings as the case may
be) generally are set by counting notches on the ring from some specified
reference point as it is rotated about the nozzle. The nozzle ring is
initially set by the manufacturer during full-flow tests and locked into
position by a set screw that engages a notch in the ring.
An incorrect nozzle ring setting on a relief valve adversely affects the
valve's characteristics and can result in uncontrolled leakage or discharge
from the valve. A low setting (high reseat pressure) may cause the valve to
open and close intermittently, leading to valve failure and uncontrolled
leakage. A very high nozzle ring setting (low reseat pressure) may delay the
valve reseating until lower than expected pressures are reached as occurred in
the events noted above.
Some of the potential problems observed in setting the nozzle rings are:
(1) If the relief valves were worked on during plant construction or other
earlier operating period without adequate control of nozzle ring
settings, there is high likelihood that they are misadjusted. Shearon
Harris, Trojan, and Braidwood all found multiple, low pressure, relief
valves with incorrect ring settings on various safety related systems..
August 28, 1992
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(2) The misadjusted nozzle rings are not self-revealing until the valves are
challenged by a plant transient as illustrated in all the events cited
(3) Procedures which require that ring settings be returned to their "as
-found" positions may be inadequate. At Shearon Harris, Trojan, and
Braidwood the "as-found" ring settings were incorrect as a result of some
previous incorrect maintenance or test activity. The licensees found it
necessary to go back to the manufacturer's specified nozzle setting after
maintenance or testing is performed. 10 CFR Part 50 Appendix B,
Section III, addresses configuration control which includes the nozzle
ring setting on these valves. The ASME test code (PTC 25.3) for relief
valves directs that the rings be set to their original test data. Trojan
specifically flagged this problem.
(4) There may be confusion introduced by the use of two different reference
points (location from which to start notch count) in the vendor's
documentation for setting the nozzle rings. The reference points in some
cases may also be difficult to establish in the field.
(5) Specific directions for adjusting nozzle ring settings may cause
confusion. Including two different instructions for setting the nozzle
ring in the same work package may contribute to the confusion. Terms
such as counter-clockwise and up and down may not be adequate directions
when looking edgewise at a slowly moving ring. These problems are
discussed in the NRC staff's report AEOD/E90-02, "Crosby Low Pressure
Relief Valve Ring Problems," February 1990.
The licensees, noted above, have generally modified/improved their procedures
and instituted personnel training to address nozzle ring setting. To
eliminate errors, the valve manufacturer suggested training maintenance
personnel on the specific valve before they attempt to adjust it.
The Trojan plant found that procedures, even with the manufacturer's nozzle
ring setting stamped on the valve body, were not sufficient to preclude errors
in maintaining their steam safety valves. A mechanical location indicator or
similar device might preclude the gross errors observed in the ring settings
and also provide a means of independently verifying the ring position without
At Braidwood, a picture, taken through the valve discharge port while it was
on the bench, was used in conjunction with the valve drawing to determine that
the nozzle ring was set too high. However, Trojan and Shearon Harris checked
most of their ring settings with the relief valves in place so only the small
ring adjustment hole was available for observing the ring position.
Fiberoptic technology could be useful where the only opening to observe the
nozzle ring is small. .
August 28, 1992
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Related Generic Communications
Information Notice 90-18, "Potential Problem with Crosby Safety Relief Valves
Used on Diesel Generator Air Start Receiver Tanks"
Information Notice 90-05, "Inter-System Discharge of Reactor Coolant"
A more detailed account of problems with nozzle ring settings is contained in
the NRC staff's report AEOD/E90-02, "Crosby Low Pressure Relief Valve Ring
Problems," February 1990.
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.
ORIGINAL SIGNED BY
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical contacts: Sanford Israel, AEOD Francis Jape, RII
(301) 492-4437 (404) 331-4182
Mary S. Wegner, AEOD Melvin C. Shannon, RII
(301) 492-7818 (919) 362-0601
Attachment: List of Recently Issued NRC Information Notices
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