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
- Volume monitoring by way of piston position sensor or switch systems
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 o-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
- Cleanliness of fluid
- Maintenance requirements
Piston Types
TYPE 2 Without Check Valves
Application (without Check Valve)
Low-friction design for higher piston speeds, slow movements without stick-slip effect and high number of actuations (millions). Actual cycles achieved will vary with operating parameters. Notes: Filtration ≤ 10 µm absolute. (ISO 17/15/12) Max. continuous velocity = 12 ft/sec
TYPE 2 With Check Valves
Application (with Check Valve)
The addition of a check valve drastically reduces the oil pumping to the gas side of the piston.
TYPE 3
Application
Actual cycles achieved will vary with operating parameters.
Notes: Filtration ≤ 10 µm absolute. (ISO 17/15/12)
Max. continuous velocity = 3 ft/sec
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.
Advantages of HYDAC Piston Accumulators
- Complete size range from 1 qt. to 100 gallons nominal volume.
- High ratios possible between precharge pressure and maximum working pressure.
- High flow rates - up to 4700 gpm from one accumulator.
- Power savings.
- Gas-proof and leak-free.
- No sudden discharge of gas when seal is worn.
- Space efficient.
- Piston location monitoring available.
SK 210 Series
(Non-ASME) 3000 psi
Advantages
The piston accumulator series SK210 is an intermediate bore diameter with repairable design. They are HYDAC certified, designed in accordance with ASME pressure code. Features of this series are:
- Bore Diameter up to 6” ID
- Sizes from 1 quart to 15 gallons
- Largest range of standard models for quick delivery times
- Military Style Gas Valve, repairable
Application
- Mobile Hydraulic
- Industrial Hydraulic
SK 280 Series
(Non-ASME) 4000 psi
Advantages
The piston accumulator series SK280 is a weight optimized, nonrepairable design. The non-repairable design and special production process of these HYDAC accumulators save cost, making this series an economic option.
- Cost-effective – due to the non-repairable design and an optimized production process
- Weight reduced series
- Reduced installation space
- Standard-gas valve (HYDAC Version 1) with integrated M28x1.5 male thread
- Quick delivery for models with standard connection
- SAE fluid ports are available
- PED/CE pressure code certification
Application
- Mobile Hydraulic
- Weight Sensitive Industrial Hydraulic
SK 350 Series
(Non-ASME) 5000 psi
Advantages
The piston accumulator series SK350 are an intermediate bore diameter with repairable design. They are HYDAC certified, designed in accordance with ASME pressure code. Features of this series are:
- Bore Diameter up to 6” ID
- Sizes from 1 quart to 15 gallons
- Largest range of standard models for quick delivery times
- Military Style Gas Valve, repairable
Application
- Mobile Hydraulic
- Industrial Hydraulic
SK 350 & 600 Series
(ASME) 3000 psi & 5000 psi
Advantages
The piston accumulator series SK350 & 600 are HYDAC’s most versatile series with a repairable design and large selection of options. The largest range of possible sizes, material construction, and other options are offered. Standard and Low Friction piston designs are available for superior performance and flow rates. Features of this series are:
- Bore Diameters from 2.4” ID to 19.3” ID
- Sizes from 1 quart to 200 gallons
- Largest range of possible sizes and material options
- Standard and Low Friction piston designs available
- Largest variety of gas and fluid port options
- A variety of piston position sensor monitoring systems are available
- ASME, CRN, PED/CE and other pressure code certifications are available
Application
- Heavy Mobile Hydraulic
- Industrial Hydraulic
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