United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 84-31: Increased Stroking Time of Bettis Actuators Because of Swollen Ethylene-propylene Rubber Seals and Seal Set

                                                       SSINS No.:  6835 
                                                       IN 84-31 

                               UNITED STATES 
                       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 

Addressees: 

All nuclear power facilities holding an operating license (OL) or 
construction permit (CP). 

Purpose: 

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 
the effect." 

*G. H. Bettis, "10 CFR 21 Report No. CAR #0023 Advisory Notification," 
February 13, 1984, Waller, Texas. 


8403140286 
.

                                                            IN 84-31 
                                                            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 
these materials. 

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 
                    (301) 492-4511 

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