Accumulator As Hydraulic Shock Absorber

 

Accumulator

    The accumulator is an energy storage device: a device that accepts energy, stores energy, and releases energy as needed. Some accumulators accept energy at a low rate (low power) over a long time interval and deliver the energy at a high rate (high power) over a short time interval.

    Some accumulators accept energy at a high rate over a short time interval and deliver the energy at a low rate over a longer time interval. Some accumulators typically accept and release energy at comparable rates. Various devices can store thermal energy, mechanical energy, and electrical energy. Energy is usually accepted and delivered in the same form. Some devices store a different form of energy than what they receive and deliver performing energy conversion on the way in and on the way out.

Basic principles

    You have an accumulator with a hard shell. Normally carbon steel – very similar to the bucket that I showed you before, and you have an elastomeric diaphragm. This elastomeric diaphragm will make a barrier to a pre-charged nitrogen section. You can compare the pre-charged nitrogen with the air that you have in your balloon.

    The port is connected to the hydraulic system. To the hydraulic system, we will apply pressure in this portion and will be translated on the same action that you have with this Force. So basically, when you have the hydraulic system, you increase the pressure in the nitrogen area.

Basic Functions Of An Accumulator

1.      Starting with the pulsation dampening. On the hydraulic system, you have pulsation. This pulsation is coming basically from the hydraulic pumps. So the accumulator will make a dampening on this pulsation and will stabilize your system. You’ll reduce the noise, you’ll reduce the vibration of the system and you’ll have the system working this move.

2.      Also, the accumulator can keep constant pressure in your system. If you have a leakage, for example, you’re going to lose the pressure of the system. The accumulator will stabilize the pressure and you keep the pressure at a certain level until you can stop your system for maintenance.

3.      Another function of the accumulator is really to be an emergency source of power in your system. For example, when you have your system being applied on hydraulic brakes and you need a sudden release of pressure in your system, the accumulator will help you release this pressure whenever it is needed.

 

Hydraulic accumulator

A hydraulic accumulator is a pressure storage reservoir in which an incompressible hydraulic fluid is held under pressure that is applied by an external source of mechanical energy. The external source can be an engine, a spring, a raised weight, or a compressed gas.

An accumulator enables a hydraulic system to cope with extremes of demand using a less powerful pump, to respond more quickly to a temporary demand, and to smooth out pulsations. It is a type of energy storage device.

What Happens On The Hydraulic System?

We have here a schematic of a hydraulic system.

·        Hydraulic cylinder

·        Valve pumps

·        A tank

And we have added an accumulator in the system.

When the hydraulic system has no pressure, you have the pre-charge of the nitrogen using the whole cavity of the accumulator.

For example:

You have a shovel on your tractor and the shovel hits a stone. You have a huge force being applied here that will increase the pressure in the whole system. This pressurized oil will move to the accumulator and will increase the nitrogen pressure. So, this nitrogen inside the accumulator will work as a cushion. You have dampened the system using an accumulator.

Types of hydraulic accumulator

1. BLADDER ACCUMULATOR

The Bladder is the bread-and-butter. You can use bladder accumulators everywhere. Most of the hydraulic systems use bladder accumulators.

·        You have a bladder bag.

·        You have the pre-charge of nitrogen.

·        Connected to the hydraulic system.

Those accumulators are used in pulsation dampening where you have high frequency, especially in a small amplitude. A lot of applications, right? But this type of accumulator has a restriction. The bladder has a vulcanized seam, and this is the weak point of the bladder system. If you have high frequency and high cycle demand, you can have a rupture in this seam. This is the restriction of this type of accumulator.

2. DIAPHRAGM ACCUMULATOR

Then we can go to the diaphragm type accumulator.

·        Basically, the same where you have a carbon steel shell but instead of a bladder, you have a diaphragm.

·        Also, the pre-charge of nitrogen and this portion is connected to the system.

Very similar applications as the bladder type accumulator. However, the diaphragm accumulator has an advantage.

Since you don’t have a seam in the diaphragm, you don’t have the restrictions that you have with the bladder type accumulator.

So applications pretty much the same, but this one is really a reliable accumulator, especially when you have high cycle demands. Applications with 1 million, 2 million, 3 million cycle demands – this is where to use a diaphragm accumulator.

3. PISTON ACCUMULATOR

• Instead of a bladder or a diaphragm, we are using an aluminum piston to make a barrier with the nitrogen.
• You keep the nitrogen pre-charged.
• The system is connected to your hydraulic system.

But really you don’t have limits for this type of accumulator.

Since you machine the accumulator, you can make it in any size. You can make accumulators with a quarter gallon. You can make accumulators with 300 gallons. You can make accumulators going to 40,000 PSI.

Custom ports, custom design, and materials so the piston accumulator is really for limited applications where you can make custom design accumulators.

Accumulator as a hydraulic shock absorber

One of the important applications of accumulator is the elimination of hydraulic shock. Hydraulic shock is caused by the sudden stoppage or declaration of a hydraulic fluid flowing at relatively high velocity in a pipe line. By rapidly closing a valve creates a compression wave.This compression wave travels at the speed of sound upstream to the end of the pipe and back again to the closed valve, which causes an increase in pressure.The resulting rapid pressure pulsations or high pressure surges may cause damage to the hydraulic system components. If an accumulation is installed near the rapidly closing valve, thepressure pulsations or high pressure surges are suppressed.

    A hydraulic accumulator is pre-charged with dry nitrogen. Some type of separating device such as a piston, bladder or diaphragm is used to separate the nitrogen from the hydraulic oil inside the accumulator. A bladder (Figure 1) or diaphragm type is recommended to absorb shock. Both of these accumulators contain rubber elements that will be compressed when the hydraulic pressure rises above the dry nitrogen pre-charge. Depending on the system, the accumulator should be pre-charged to 100 psi below to 200 psi above the maximum operating pressure in the system. Accumulators that are used for shock can be small in size, usually one quart to one gallon.

    The accumulator should be installed as close as possible to where the shock spike is occurring. For example, if the pressure spike takes place when a cylinder fully extends, the accumulator should be installed near the port connected to the full piston side of the cylinder.

Accumulators are often used to absorb high flow surges in return lines. In this case, the pre-charge should be lower than the maximum pressure rating of any return filters or heat exchangers located downstream. Any time an accumulator is used in the pressure line, an automatic and/or manual dump valve should be installed to bleed the hydraulic pressure down to zero once the system is turned off.

The cylinder is added two ports by a double acting Hydraulic cylinder. As the piston of the hydraulic cylinder moves reciprocally due to excitations of oscillating road profiles, the fluid in the cylinder will be forced to flow into the fluid chamber of the connecting hoses in front of the motor. Driven by the pressurized flow, the hydraulic motor will produce rotary motion which will drive the DC generator to produce electricity. in order to realize the continue flow for driving the Hydraulic motor continuously, the flow direction is controlled by the four check valves and the pressure oscillation is adjusted by the hydraulic accumulator.