Let’s see how hydraulic accumulator can help in hydraulic system
Let us consider an example, a hydraulic press is tasked with compressing a work piece. It needs a lot of force (know why) to do that, but only with a low speed. It means, we need high pressure with low flow rate pump. The recirculation after pressing means piston for retraction does not need much force, but it needs to go quickly, which means low pressure and high flow rate pump. To full fill both condition we need to adjust pump setting which is pretty expensive.
But, hydraulic accumulator gives the solution. We can use a pump having high pressure and low flow rate with addition of accumulator to the delivery of pump.
As shown in the figure 4 accumulator is installed between pump and direction control valve. The pump’s flow rate is enough for pressing at low speed. The high pressure during pressing compresses the gas in the hydraulic accumulator and it charges itself with liquid. As soon as retraction of piston starts due to less load restriction pressure will drop in circuit.Therefore, The gas in the hydraulic accumulator expands, pushes out the liquid and increases the flow rate. The press can therefore return faster than with just the pump’s flow rate.
Types of pre-charged gas accumulator.
- Bladder type
- Diaphragm type
- Piston type
The use of these various accumulator depends on pressure and volume required in system. Bladder and diaphragm type accumulator are use for moderate pressure and less volume (0.5 to 500 litres). But its response time is quick. On the other hand piston type accumulator is using for high pressure and large volume (more than 500 litres). But it has low response time because of piston large mass. Lastly, pre-charged gas accumulator should mount in specified position as per design for better result.
2. Spring loaded hydraulic accumulator working principle
In the spring loaded hydraulic accumulator, there is a spring along with container & movable piston. Spring loaded accumulator can mount in any position. However, the spring force is not easy to adjust. These springs create the required pressure on the hydraulic piston to pump out fluid. The amount of pumping depends upon the compression rate of the spring. The main advantage of this type of hydraulic accumulators is that they are comparatively small in size, have a low weight and are easy to assemble in the hydraulic system and at the same time cost effective. They can be used as mobile hydraulic accumulators due to their lightweight. Because of limitations on spring sizes, the use of this accumulator is impractical where large volume of fluid is required.
3. Weight loaded hydraulic accumulator working principle
The weight loaded type was the first used but is much larger and heavier for its capacity than modern piston and bladder types. On the other hand it is the only type of accumulator where the pressure is constant, whether the chamber is full or nearly empty (you can know here why). It is heavy & bulky, making the use limited. For example, it is used in heavy presses where constant pressure is required.
Example: Accumulator use for damping in automobile.
For damping of undesirable pressure spikes and fluctuation we can use accumulator. For example, hydro-pneumatic shock absorber in vehicles as shown in figure 7. It basically consist of a hydraulic cylinder and an gas pre charged accumulator. If bumps in the road push the wheel upwards, the pressure in the hydraulic fluid increases abruptly. It is pushed into the hydraulic accumulator and compresses the gas present there. As a result the gas acts like a spring. The shock’s pressure and motion are absorbed and only transmitted to the vehicle in dampened form.
As the wheel’s pressure on the hydraulic fluid attenuates after the shock, the gas in the hydraulic accumulator expands again due to the attenuated back pressure and pushes the liquid back into the cylinder.
The above example is also applicable in damping in hydraulic system. When hydraulic direction control valve changes position rapidly then accumulator comes into play and absorb the sudden pressure fluctuation.
Maintenance of Accumulator
Hydraulic pre charged does not require much maintenance but gas pressure is to be regularly check.
For example. In a piston type accumulator if the charging pressure is too high, the hydraulic fluid will be completely pushed out of the accumulator at minimum operating pressure. The piston strikes the hydraulic connection and can be damaged.
The opposite happens when there is too little charging pressure. The piston is pushed completely upward against the gas valve at maximum operating pressure, and can also be damaged.
Similarly, in bladder and diaphragm type the separating membrane can be damaged due to incorrect charging pressure.
On the other hand, due to incorrect gas charged the usable volume of hydraulic fluid needed for the system changed. This results alter behaviour of the system and can damage other hydraulic components.
Procedure of gas charging
- Firstly, the hydraulic system must not be pressurised
- The charging equipment should directly connected to the accumulator gas valve.
- There must an integrated gauge in the charging kit which will the show prevailing gas pressure of the accumulator.
- To charge, connect a commercially available gas cylinder to the equipment using a flexible hose.
- There should be two control valve for controlling amount of gas in accumulator. One will control gas cylinder and one will control accumulator gas.
- After connecting charging kit, first open check nut of accumulator. Then open cylinder control valve little and open the accumulator gas control valve. After that check for leakage and then close completely.
- If there is no leakage the then completely open the cylinder gas control valve. And by accumulator gas control valve charge the accumulator with required pressure. Then tight the check nut. Do not over tight and damage the check nut.
- Then First close the cylinder control valve and then remove the charging kit from accumulator gas valve.
Safety regarding accumulator
- Do not weld the accumulator. It may damage the accumulator and cause serious injury.
- Do not drill for fastening with bolts.
- Only secure the accumulator instructed by manual like secure with strap in instructed position.
- Before maintenance release the accumulator pressure securely. It may stored the pressure energy even in stop condition and cause serious injury.
Hope you enjoyed this article. See you in next article. Till then bye bye.