with nitrogen gas filling
Hydraulic accumulators are used in hydraulic power workholding applications as energy storage for compensation of internal leakages or to compensate the volume in the case of temperature changes.
With intermittent cycles the rating of the pump and thereby energy can be saved. During the breaks the pump refills the hydraulic accumulator. If required, a higher flow rate is available for a short time.
Compensation for internal leakage
In power workholding most of the pressure generators work in a cycling mode controlled by a pressure switch. If hydraulic devices with internal leakages are connected, e.g. spool valves or rotary valve couplings, this leads to frequent switching on and off of the electrical drive motor. The use of a small hydraulic accumulator considerably reduces the number of switching cycles and saves the material as well as energy.
Volume compensation in case of temperature changes
If hydraulic clamping systems will be separated from the pressure generator, there are considerable changes of the clamping pressure in the case of temperature variations (reference value ±10 bar at ±1°C). A small hydraulic accumulator mounted at a protected place on the fixture, causes a volume compensation and reduces pressure variations. In addition a small leakage will not immediately result in a pressure drop. A pressure gauge for pressure control should be installed in any case.
Liquids are more or less incompressible and therefore are not in the position to store pressure energy. For hydraulic accumulators the compressibility of nitrogen is used to store liquids. A gas-tight diaphragm separates the liquid zone from the gas zone.In the bottom of the diaphragm there is a valve disk that avoids a damage of the diaphragm if the hydraulic accumulator will be completely discharged.
Nitrogen is filled in at the screw plug and provided with the required preload. For this purpose an appropriate filling and testing fixture is required. The offered hydraulic accumulators correspond to the regulations of article 3 paragraph 3 of the directives for pressure devices 97/23/EG and are not allowed to bear the CE sign.
For hydraulic accumulators the applicable regulations at the place of installation have to be considered before start up and during operation. The operator is exclusively responsible for the intended use and compliance of these regulations. In Germany the Classification as per German Health and Safety at Work Regulations (BetrSichV) is valid as legal basis. For the offered accumulator size the following is valid:
All works at the hydraulic or pneumatic ports of the hydraulic accumulator must only be effected by trained experts.
An expert is not required for the first acceptance.
1. Nominal volume
The nominal volume is the effective gas volume of the hydraulic accumulator. The maximum storable oil volume is approx. 10 % smaller.
2. Maximum operating pressure
The maximum operating pressure must not be exceeded in any operating mode. For this purpose a suitable safety valve is required for pressure limitation (see safety devices).
3. Gas preload
The gas preload is the nitrogen pressure at a room temperature of 22 °C, without oil filling. The hydraulic accumulator can only take hydraulic oil after exceeding this pressure.
4. Recommended range of operating pressure
In this range the hydraulic accumulator works with the best degree of efficiency with optimum life of the diaphragm.
V0 = nominal volume = max. gas volume
p0 = gas preload
V1 = gas volume at p1
p1 = min. operating pressure ≥ 1.1 x p0
V2 = gas volume at p2
p2 = max. operating pressure ≤ 8 x p0 for 9606-10X ≤ 3…4 x p0
6. Stored oil volume
Starting from the maximum operating pressure until the complete discharge of the hydraulic accumulator the stored oil volume is ∆VOil = V0 – V2
7. Pressure-volume diagram
The compression and expansion processes in hydraulic accumulators are subject to the laws of polytropic state change of the gas. The temperature and the time-dependent course are of decisive importance.
Charge and discharge are effected very slowly, so that there is sufficient time for a complete temperature compensation.
Application : compensation of leakage oil or volume in the case of temperature changes (see use)
Charge or discharge is effected very quickly. The nitrogen is considerably heated or cooled.A quick temperature compensation with the environment is not possible.
Application : Energy storage (see use)
8. Hydraulic accumulators in power workholding
In hydraulic power workholding hydraulic accumulators are mostly used for the compensation of leakages or of the volume in the case of temperature variations. Charge of the accumulator is made very quickly, that means adiabatically, but the discharge is relatively slow, that means isothermically. If the power unit works in a cycling mode, after the clamping process there will be several reswitchings until the pressure is constant.
Reason: The quick adiabatic pressure build-up heats the nitrogen.
If it cools again through the accumulator body, the pressure in the system drops and oil has to be resupplied one or two times. At the end as much oil is supplied into the hydraulic accumulator as in case of a mere isothermic compression.
Do not uncouple clamping systems with coupling unit immediately after clamping, but wait approx. 15 seconds until the clamping pressure is constant.
Diaphragm accumulators are in fact maintenance free. However to allow trouble-free function and a long life the following checks have to be realised:
The hydraulic accumulators are delivered with the desired gas preload and are marked correspondingly. On request also other preload pressures are available.
Check preload pressure
The preload pressure has to be checked
If no filling loss is observed, yearly checks are sufficient. If no test and filling device is available, the filling pressure can also be checked at the hydraulic side:
Change preload pressure
This is only possible with an appropriate test and filling device. Please contact us.
The service life of diaphragm accumulators depends on the width of pressure variations and the number of load changes. Similar to high-pressure hoses, you can assume a life of 6 years with conventional use.
A hydraulic accumulator is in the position to supply a high flow rate within very short time. Since this is not required or desired for the most applications, the flow rate should be throttled to save the accumulator diaphragm. A flow control valve has to be installed so that the outflowing hydraulic oil will be throttled, but a quick charge of the accumulator will be guaranteed in opposite direction (see hydraulic circuit diagram).
Hydraulic accumulators are subject to the valid national regulations and decrees at the installation location. In Germany, the "Technical Regulations for Pressure Reservoirs" (TRB) apply. These regulations demand the following equipment:
1. Pressure relief valve
The pressure relief valve shall protect the hydraulic accumulator against a pressure increase by more than 10 % of the maximum operating pressure.Adjustment has to be effected with the maximum flow rate of the power unit. The reaction pressure of the pressure relief valve should be a little bit higher than the nominal pressure of the hydraulic accumulator.
The valve spindle of the pressure relief valve has to be secured against adjustment in the direction of higher pressure by means of distance plates and/or lead-sealing.
The pressure relief valve of the power unit must not be adjusted above the maximum operating pressure of the hydraulic accumulator. In the case of "small accumulators" with a nominal volume below 100 cm³ pressure safeguard can be realised by the pressure relief valve at the power unit, when the adjusting spindle is secured against exceeding the maximum operating pressure. If "small accumulators" are located on workpiece pallets, that will be uncoupled from the power unit, a pressure relief valve has to be provided on each pallet.
2. Relief device
Before realising maintenance works at the hydraulic system or the fixture the hydraulic accumulator should be completely discharged. There are two possibilities:
To screw completely out the valve spindle of the pressure relief valve in the direction of low pressure or to open an installed shut-off valve (see example).
3. Pressure gauge
The pressure gauge shall indicate the actual pressure in the hydraulic accumulator. For this purpose a direct supply line has to be mounted. The pressure gauge at the power unit is not suitable for that purpose. The maximum operating pressure of the hydraulic accumulator shall be indicated by a marking on the pressure gauge scale. Alternatively also a labelled plate or tag can be fixed.
4. Connection of a test pressure gauge
For regular pressure tests a test pressure gauge can be connected.
5. Shut-off valve
By means of the shut-off valve the hydraulic accumulator can be separated from the power unit and the fixture in order to realise adjusting and maintenance works without danger.