The elastomer compounds selected for use in the Hydraulic Shock and Sway Arrestors are chosen to perform in nuclear power plant environments. The environments include:
- high temperature
- radiation
- steam and humidity in accordance with specifications
In addition, the seal must retain the working fluid of the Arrestor to enable it to be operational at any time.
To date there are no known elastomers that can meet the forty (40) year design life criteria specified by most users. However, the objective is to provide and adequate seal material that will provide the longest possible life and mini- mal maintenance.
To appreciate the difficulty of seal selection, it is necessary to understand the conditions which deteriorate the seal to a point where it is no longer considered functional. Both radiation and high temperature are the most detrimental of environmental conditions. Both are energy sources which work on the elastomer’s molecular structure eventually causing chain scission and modification of the mechanical properties. Of the Ethylene-Propylene family, compound E740-75 exhibits the best radiation resistance. Ethylene- Propylene as a generic material has excellent high temp- erature resistance and is usually applied in applications between -65°F and +300°F for continuous service. These temperature limits are general and must be considered in conjunction with the media the elastomer must seal.
A methyl phenyl polysiloxane silicone fluid (1154) is utilized as the working fluid in HSSA units. This material has been tested in high temperature and radiation environ- ments in conjunction with various EP seal compounds and has been found most compatible with the following:
| E4248 A 75 | (E740-75) |
|---|---|
| E4207 A 90 | (E652-90) |
| E740-75 | |
| E93 B1 | (E740-75) |
These compounds are expressly used in static, dynamic, scraping and auxiliary sealing applications throughout the Hydraulic Arrestor. Compound E740-75 is generally requested in radiation environments but is in reality strictly an ‘O’ ring compound. Compound E4248 A 75 is the packing compound equivalent to E740-75. The manufacturing process involved in dynamic packing molding requires a modification of the ‘O’ ring compound to produce accept- able seals. E4248 A 75 does exhibit the same response to radiation, heat and fluid as it’s counterpart E740-75. In a similar manner, compound E4207 A 90 is equivalent to ‘O’ ring compound E652-90. However, this 90 durometer elas- tomer is not used in the fluid media of the Arrestor and is restricted to scraping or auxiliary sealing service. Compound E93 B1 is again equivalent to E740-75 and the seals molded from it are used in the quick fill (filler plug) fitting used on all HSSA unit pressurized reservoirs.
Another consideration is seal selection is durometer-Shore A hardness. The major portion of the life span of a snubber is spent in static service. For Hydraulic Snubbers this means low or no pressure to activate pressure responsive seals. Low durometers are selected to maintain an adequate pressure seal. In scraping or auxiliary sealing service, such as rod wipers and back up washers, high durometer materials are sought. These components are chosen to have a Shore A 90 durometer hardness, to safeguard against seal extru- sion and to wipe the piston rod clean of normal debris.
The useful shelf and service lives of the Ethylene-Propylene seal is a function of the storage and various environmental conditions. The average shelf life of the E-P seals furnished by Bergen-Paterson is expected to be between 5 and 10 years but are dependant upon the following:
- Ambient temperature not exceeding 120°F (49°C)
- Exclusion of air (oxygen)
- Exclusion of contamination
- Exclusion of light (particularly sunlight)
- Exclusion of ozone generating electrical devices
- Exclusion of radiation
Generally, polyethylene bags stored in larger cardboard containers or polyethylene lined craft paper bags insure optimal storage life.
The service life of the seal commences with installation and is again considered in the 5-10 year category but is depend- ant upon radiation and heat energy doses. The following figure illustrates radiation effects.
Compound E740-75 Radiation Test Data
| Physical Properties | Original | 107Rads | 108Rads |
|---|---|---|---|
| Hardness, Shore A; pts. (chg. pts.) | 70 | +3 | +9 |
| Tensil Strength, psi (chg. %) | 2080 | +3 | -18 |
| Elongation, % (chg. %) | 233 | -17 | -18 |
| Modulus @ 100%, psi (chg. %) | 554 | +46 | — |
| TR-10,°F | -59 | -60 | — |
| Compression Set, 25% Deflection 93 Days @RT % of Original Deflection |
6.7 | 28.6 | 90.5 |
All values are typical and should not be used for specification limits.
The radiation dosage limit for ‘O’ rings may be as high as 1 X 108 Rads or as low as 1 X 107 Rads for packings but 1 X 107 Rads is expected to be the maximum normal accept- able radiation dosage. In terms of years, at normal plant design radiation levels, the service life approaches 10 years. In addition, temperature effects also determine service life. Operating below temperature limits of +200°F should pro- vide optimum life whereas exceeding that limit impairs service life. The other factor to be considered in service life is mechanical wear. Some application may dictate replace- ment of dynamic packings at earlier scheduled intervals than others due to wear alone.
Ethylene – Propylene materials are generally compatible with the following material:
- Phosphate-ester base hydraulic fluids
- Steam (to 400°F)
- Water/hot water
- Silicone oils and greases
- Dilute acids
- Dilute alkalies
- Ketones
- Alcohols
These materials are not compatible with:
- Petroleum oils
- Di-ester base lubricants
When in contact with these materials the Ethylene-Prop ylene compounds are severely degraded and/or destroyed due to excess swell, loss of mechanical properties or dis entegration.
Whenever a solvent is used to clean components of the Hydraulic Arrestor all traces (wetting) of the material should be removed prior to installation of new seals to assure that no contamination exists. There are numerous solvents available in the market and specific information regarding compatability of these with the Ethylene-Propylene seals may be obtained from the Product Engineering Department.
To summarize, the environment of the seal dictates its total life. In some cases 20 years may elapse from the time the seal was originally manufactured until it is retired from service. The 5 10 year storage and service lives, respectively, are projections based upon past history of similar elastomer compounds and service history of the Ethylene-Propylene seals themselves. The seal performance witnessed to date indicates that these projections are acceptable.