United States Nuclear Regulatory Commission - Protecting People and the Environment

IE Circular 76-05 - Hydraulic Shock and Sway Suppressors - Maintenance of Bleed and Lock-up Velocities on ITT Grinnell's Model Nos. - Fig. 200 and Fig. 201, Catalog PH-74-R

IE Circular 76-05                               October 8, 1976



Recent information has become available related to improper lock-up and
bleed  rates of certain ITT Grinnell shock and sway suppressors
(snubbers). The  control block or valve block on the snubbers involved is
one which contains the mechanisms for a "dual orifice type" snubber valve
arrangement. These control blocks or valve blocks can be identified by
four locking nuts on the surface of the block. These nuts lock the stem
of the valve control into  position. Rotation of the stem modifies the
orifices which control the lock-up and bleed rates. The units in question,
at this time, are those snubbers which have control block or valve block
serial numbers from B-0001 through B-2000. These snubbers include 1 1/2
inch diameter (3 kip capacity) through 6 inch diameter (72 kip capacity)

ITT Grinnell first identified this problem in connection with their own
testing program during the last two weeks of August 1975. The origin of
the problem was traced by Grinnell to November of 1974 when a design
change (known as Revision B) was initiated on Model Nos. - Figure 200 and
Figure 201, Catalog PH-74-R snubbers, along with the addition of a
Grinnell design specification for lock-up and bleed velocities. These new
rates were to be 8 in/min. plus or minus 2 in/min. for lock-up and 4
in/min. plus or minus 1 in/min. for bleed. At that time production testing
of completed snubbers  included within the above series of serial numbers
was completed on the basis of "go" or "no-go" testing and did not
specifically determine the lock-up and bleed rates for each snubber.


Grinnell has determined that all snubbers involved should lock under
seismic events as designed, but perhaps at a lower velocity. Additionally,
the bleed rate has been determined to exist at a value as low as 1/8
in/min. This  could result in increased piping stresses depending on the
specific design. With the uncertainty of specific lock-up and bleed rates,
there is the possibility of adverse effects on the piping systems or
components the snubbers were designed to protect. It has been stated,
however, that to the  best of Grinnell's knowledge, there were no lock-
up and bleed criteria defined by their customers for the plants identified
as having utilized snubbers from the B-0001 through B-2000 series.

Even though lock-up and bleed rates on snubbers with adjustable orifices
are now determined prior to shipment from the manufacturer, there is
always the possibility that subsequent removal or tightening of the
locking nuts on the control block or valve block will modify the orifice
setting, thus affecting lock-up and bleed rates. At least two models of
Grinnell's snubber control valves use some type of seal on the locking
washers and in some cases these may have been replaced in the field to
change the seal material or correct small leaks. In these cases, if proper
procedures were not followed to maintain the stem portion in its original
position, the orifice setting could change. The tightening of a lock nut
itself could result in a change to the orifice. ITT Grinnell makes the
following statement in the printed material  furnished with each snubber
assembly: "Adjustment of snubber valve requires  equipment capable of


The recommended corrective action outlined by ITT Grinnell is to replace
the entire valve assembly (2 required per snubber) known as the "barrel"
in the case of the lock-up control, and the pins or stems (2 required per
snubber) used to control the bleed rate. ITT Grinnell has prepared
documents which prescribe the replacement procedures. The replacement
parts are precalibrated.  Therefore, field calibration of the entire
snubber unit will not be necessary  if the replacement parts are installed
in accordance with the prescribed procedures. The tolerances on the
precalibrated pins or stems associated with the bleed mechanism will
exceed the original value of 4 inches per minute plus or minus 1 inch/min.
The new bleed rate will be 4 inches/minute plus or minus 2 inches/minute.

Corrective action could also consist of recalibration of the existing
snubbers  with adjustments made to bring the operating characteristics of
the units back  within the tolerances specified by ITT Grinnell.
Monitoring of relative  rotation of the lock washer and stem could in the
future be carried out if  position marks were provided on the parts after

ITT Grinnell has identified the following facilities which have licenses
under  10 CFR 50 which received snubbers within the group defined by the
bounding  serial numbers.

                              Number of Grinnell Snubbers
     Plant                   Model Nos.-Fig. 200 & Fig. 201

Farley Nuclear Plant                      239
Millstone Point Unit #2                   185
North Anna                                155
Davis Besse Unit #l                       111
D. C. Cook                                 78
Calvert Cliffs                             66
Beaver Valley                              53
Diablo Canyon                              41
Ginna                                      8
Kewaunee                                   2
Peach Bottom                               2
Browns Ferry                               1


ITT Grinnell has also informed the utilities who own these facilities 
regarding this matter; however, some of the series may be in use or
intended  for use at other reactor facilities as a result of utilities
loaning or  selling units to other utilities or by the fact that
subcontractors or other  suppliers might be involved.


For all power reactor facilities with an operating license or a
construction  permit using or intending to use the above described
snubbers on safety  related systems or components:

1.  Determine if any of the subject snubbers are installed or scheduled
    to be  installed in safety-related systems at your facility.

a.  Provide a list containing each snubber identified and indicate the
    piping  system or component with which it is associated.

b.  Indicate for each snubber its history of testing and test results as
    well  as its maintenance history.                     

c.  For each snubber list the current required lock-up and bleed rates and
    what  rates the snubbers were specified to meet by the original
    construction/purchase specification. Indicate the organization, by
    name, that  was responsible for the preparation of the
    construction/purchase  specification.

d.  If replacement parts are utilized instead of recalibration of the
    snubbers,  provide an assessment of the effect of a bleed rate of 2
    inches/minute which  represents the new lower bound.

2.  For snubbers within the above described series that do not have 
    documentation available defining the lock-up and bleed velocities,
    provide a  description of the actions to be taken to substantiate
    these characteristics.  Also, provide the schedule to complete these
    verifications and modifications,  if needed.  


3.  Review maintenance and modification procedures for the above described 
    series of snubbers where such actions could affect the performance 
    characteristics of the snubbers. Such actions include but are not
    limited to  the disassembly of snubbers, replacement of seals,
    changing hydraulic fluid  and the changing of any valve springs.
    Modify the procedures, as appropriate,  to assure that any future
    maintenance or modifications will not change the  performance

4.  Provide a report which contains the responses to items 1, 2 and 3
    above  within 90 days of the date of this Circular. Reports should be
    submitted to  the Director of the NRC Regional Office and a copy
    should be forwarded to the  NRC Office of Inspection and Enforcement,
    Division of Reactor Inspection  Programs, Washington, D. C. 20555.

Approval of NRC requirements for reports concerning possible generic
problems  has been obtained under 44 U.S.C. 3152 from the U.S. General
Accounting  Office. (GAO Approval B-1802S5 (R0062), expires 7/31/77).

Page Last Reviewed/Updated Wednesday, July 08, 2020