Hydac SK Series Piston Accumulators

​Piston Accumulators are a cost effective option for numerous functions involving energy storage, and sometimes shock absorption in a hydraulic or fluid system. They are well suited for applications needing:
High Pressure Ratios
Large Volumes of Oil
High Fluid flow rates
Volumes monitoring by way of piston position sensor or switch systems
​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 ftsec continuous
Improved accumulator efficiency
High life expectancy, low maintance requirements
Construction
HYDAC piston accumulators consist of:
A cylinder with a finely finished internal surface
An end cap on the gas side and fluid side, sealed with 0-rings
A lightweight metal piston
A variety of sealing systems are available depending on the application
Sealing Systems
Precise information about the proposed operating conditions is required in order to select the most appropriate sealing system. Important criteria for this selection are:
Number of actuations or cycles
Piston speed
Temperature fluctuation
Operating fluid
Cleaniness of fluid
Maintenance requirements
Seal Materials
The following sealing elastomers are available, depending on the operating conditions:
NBR (acrylic nitrile butadiene rubber)
FPM (fluoro-elastomer)
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 approx. 100°C
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)
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 84, 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 ftsec with short bursts, up to 15 ftsec (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 measurements 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 int he pressure differential between gas and oil side:
Graph 1: Δp max. ~ 125 psi
Graph 2: Δp 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.