Information Notice No. 84-31: Increased Stroking Time of Bettis Actuators Because of Swollen Ethylene-propylene Rubber Seals and Seal Set
SSINS No.: 6835
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
April 18, 1984
Information Notice No. 84-31: INCREASED STROKING TIME OF BETTIS
ACTUATORS BECAUSE OF SWOLLEN ETHYLENE-
PROPYLENE RUBBER SEALS AND SEAL SET
All nuclear power facilities holding an operating license (OL) or
construction permit (CP).
This information notice is being provided as a notification of potentially
significant problems pertaining to actuators manufactured by the G. H.
Bettis Company. One problem involves the use of ethylene-propylene rubber
(EP) in contact with a lubricant, Mobil 28 grease, which deformed the seals.
The other problem is seal "set" when actuators are not exercised frequently.
The NRC staff expects recipients will review this notice for applicability
to their facilities and consider actions, if appropriate, to preclude
similar problems from occurring at their facilities. However, suggestions
contained in this information notice do not constitute NRC requirements and,
therefore, no specific action or written response is required.
Description of Circumstances:
The G. H. Bettis Company is a supplier of actuators used principally in
heating, ventilating and air-conditioning (HVAC) safety-related and
non-safety systems. The G. H. Bettis Company notified the NRC via a Part 21
report* that their NCB series, N52X, N72X, N73X series, and the NT310-SR4
and 5 and NT312- SR5 actuators had potential stroking times of greater than
the required 15 seconds because the EP elastomers in contact with the Mobil
28 grease lubricant were swollen. (The 15-second stroking time was used by
Bettis as typical of customer requirements.) Concern exists that not all the
licensees who are end users have been notified.
The problem as Bettis explains it is "... the actuator seals (ethylene-pro-
pylene) swell when in contact with the Mobil 28 grease currently used in the
manufacture of 'N' ,series actuators. Seal swell increases seal loading
causing greater time required to initialize motion. This problem is a
function of seal contact area as it relates to the force available from the
actuator piston or spring. As a result the larger the actuator the smaller
*G. H. Bettis, "10 CFR 21 Report No. CAR #0023 Advisory Notification,"
February 13, 1984, Waller, Texas.
April 18, 1984
Page 2 of 2
Bettis' original recommendation to resolve the swelling problem was to
replace the EP with Viton-A every 5 years or sooner. An architect-engineer
was concerned about the use of Viton in potentially high-radiation fields;
however, not all the seals were used in high-radiation fields or in
actuators for which the additional force needed to overcome the effects of
swelling would present a problem. Where it is necessary to replace swollen
seals, Bettis recommends replacing them with new EP seals and using
Dow-Corning Molykote 44 grease, a silicon based lubricant which Bettis
states has been shown to cause no seal degradation and provides at least
equal resistance to wear in metal to metal contact.
Concerning the use of rubber in its many forms and many applications in a
nuclear plant, it must be noted that the form of rubber used must be
designed for the service expected and the environment in which it must
function including selection of lubricants. This information notice deals
with EP rubber in Bettis actuators. It may also apply to other equipment in
similar service and similar environments; but it is not intended as a
universal statement on the use of EP and Viton and lubricants used with
The G. H. Bettis Company also identified another problem that could
adversely affect stroking time. Their report states that "...the magnitude
of stroking time degradation is related to the elapsed time between actuator
cycles. The longer the actuator remains stationary the more 'set' the seals
take. The 'set' characteristic causes the seal to form an intimate contact
with the sealing surfaces, further increasing the time required to
initialize stroke. Once the actuator begins to stroke, the seals begin to
recover their original shape, thus freeing the unit up. Stroking the
actuator three or more complete cycles using pressurized gas will cause the
seals to recover sufficiently to reduce stroking time to a minimum. No seal
degradation has been traced to periodic actuator stroking, quite the
opposite has been experienced. Frequent stroking tends to extend seal life
resulting in longer actuator cycle life." The manufacturer recommends that
"Units should be stroked or exercised at intervals of no more than 15 days."
No written response to this information notice is required. If there are any
questions regarding this matter, please contact the Regional Administrator
of the appropriate NRC Regional Office or this office.
Edward L. Jordan, Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contact: M. S. Wegner, IE
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