Seal Materials
The following sealing elastomers are available, depending on the operating conditions:
- NBR (acrylic nitrile butadiene rubber)
- FPM (fluoroelastomer)
- PUR (polyurethane)
Suitable materials are also available for low temperature applications.
Fluids
The following sealing materials are suitable for the fluids listed below:
NBR, resistant to:
- Mineral Oils (HL and HLP)
- Non-flammable fluids from groups HFA, HFB, and HFC
- Water and seawater up to approximately 100ºC
NBR, not resistant to
- Aromatic hydrocarbons
- Chlorinated hydrocarbons
- Amines and ketones
- Hydraulic fluids from the HFD Groups
FPM, resistant to:
- Mineral Oils (HL and HLP)
- Hydraulic fluids from the HFD Groups
- Fuels as well as aromatic and chlorinated hydrocarbons
- Inorganic acids (but not all, please contact HYDAC)
FPM not resistant to:
- Ketones and amines
- (Anhydrous) ammonia
- Organic acids such as formic acid and acetic acid
PUR resistant to:
- Mineral Oils (HL and HLP)
- Non-flammable fluids from the HFA group
PUR not resistant to:
- Water and water-gylcol mixtures
- Alkalis
- Acids
Corrosion Protection
For use with certain aggressive or corrosive fluids, or in a corrosive environment, HYDAC offers protective coatings and corrosive resistant materials (i.e. stainless steel) for the accumulator parts that come in contact with the fluid, or are exposed to the hostile environment.
System Mounting
HYDAC piston accumulators may operate in any position. Vertical installation is preferable with the gas side up. We recommend the use of our mounting components, which are detailed on page H21, to minimize risk of failure due to system vibrations.
Effects of Seal Friction
The permissible piston velocity depends on the sealing friction. Higher piston velocities are possible where there is less sealing friction.
HYDAC piston accumulators with low friction piston seals allow continuous operating velocities of up to 12 ft/sec with short bursts, up to 15 ft/sec (see type 2 piston).
Small pressure differentials between gas and oil side improve the effectiveness of HYDAC piston accumulators. To emphasize the friction effect on the pressure curve during an accumulation cycle, measurements with various sealing systems are illustrated.
The measurement graphs below are a true representation of the gas and oil pressure of piston accumulators with different sealing systems. The comparison of these two measurements clearly shows the difference in the pressure differential between gas and oil side:
Graph 1: ∆p max. ≈ 125 psi
Graph 2: ∆p max. ≈ 14.5 psi
The effect of the sealing friction on the working pressure is particularly striking in traditional piston designs. Abrupt piston movements (the stick-slip effect) are caused by the seal friction as shown in Graph 1. The low sealing friction of HYDAC type 2 pistons drastically reduces the stick-slip effect therefore maximizing piston responsiveness.
Advantages of Using the Low-friction Sealing System (type 2):
- Minimum friction.
- Suitable for low pressure differentials.
- No start-up friction, no stick-slip.
- Low noise, no vibration.
- High piston speeds up to 12 ft/sec continuous.
- Improved accumulator efficiency.
- High life expectancy, low maintenance requirements.