All accumulators operate on the principle of accumulated energy.
In years gone by this was achieved using a deadweight. However, spring-type accumulators or hydro-pneumatic type accumulators are still used in modern hydraulic applications.
Hydro-pneumatic accumulators, which use hydraulic fluid to compress nitrogen gas and hence the name hydro-pneumatic, are the predominant accumulator type.
Of the four principal hydro-pneumatic accumulator types – namely bladder, diaphragm, piston, and metal bellows – we’ll discuss the bladder-type accumulator. Nitrogen gas is used to fill the bladder to a specified pressure through a gas valve at the top of the accumulator. This is known as gas pre-charge pressure or P0 whereas the volume of gas within the accumulator is known as effective gas volume or V0.
In this configuration, the bladder holds closed the poppet on the fluid port assembly and there is no hydraulic fluid within the accumulator. Once the system pressure increases above the gas pre-charge pressure, the poppet valve opens, the hydraulic fluid enters the accumulator, and the bladder is compressed.
Hydraulic fluid continues to compress the bladder if system hydraulic pressure increases. P1 is the minimum system operating pressure and V1 is the corresponding nitrogen volume at that pressure.
It is important to note that hydraulic system pressure and nitrogen gas pressure are always in equilibrium.
As this system pressure increases, the bladder and nitrogen gas continue to compress, which results in more hydraulic fluid being present in the accumulator.
At this point, the accumulator is storing hydraulic fluid at the maximum system operating pressure (P2). If the hydraulic pressure in the system drops, the bladder expands, forcing hydraulic flow from the accumulator back into the system.
Importance of accumulator pre-charge pressure
Hydro-pneumatic accumulators use the principle of potential energy in the form of compressing and expanding nitrogen gas to allow hydraulic fluid to be stored or expended in various applications.
The nitrogen gas that fills the accumulator before being connected to the hydraulic machine or equipment is set to a specified pressure. This is called gas pre-charge pressure.
Why is it important to have the correct nitrogen gas pre-charge pressure in an accumulator?
The simple answer is to ensure the accumulator operates as intended and to prevent premature failure.
But there's a little bit more to it than that! Here we discuss what can go wrong with the bladder-type accumulator. And the same principles apply to other hydro-pneumatic accumulators.
In order to understand how incorrect pre-charge pressure can cause premature accumulator failure, the minimum and maximum system operating pressures must be considered.
Typical system pressure increases and decreases with actuation. If the accumulator pre-charge pressure is too close to the minimum permitted operating pressure (P1), the bladder will regularly strike the poppet on the fluid port, which causes premature failure of either the poppet valve or the bladder. If the system pressure is too high in comparison to the pre-charged pressure, the bladder will be compressed, deformed, and folded beyond its operational limits, which may result in premature failure.
For this reason, the maximum pressure (P2) is determined in relation to the pre-charge pressure and is not necessarily the maximum design pressure of the accumulator. It's therefore critical that the accumulator has the correct pre-charge for the machine or application in order to avoid premature failure.
Calculating accumulator pre-charge pressure
In hydro-pneumatic accumulator applications, it’s vital that gas pre-charge pressure (P0) is calculated and set correctly.
However, we must start with the end state in mind in order to calculate what this pre-charge pressure should be. Here we demonstrate the calculations for a hydraulic energy storage application with a bladder type accumulator.
The equation P0 ≤ 0.9 x P1 tells us the pre-charge pressure should be 90 percent or less than the minimum system pressure (P1). The equation P2: P0 ≤ 4.1 tells us the maximum permitted pressure (P2) cannot be greater than four times the pre-charge pressure. This is also known as the pressure ratio.
Here is an example of determining the correct pre-charge pressure for the accumulator and the system. With a minimum system pressure of 50 bar, we can use the 90 percent rule to determine that our pre-charge pressure should be 45 bar or less. And with a maximum system pressure of 160 bar, we can determine that the pre-charge pressure should be 40 bar or more.
Therefore, a pre-charge pressure between 40 and 45 bar is acceptable.
Via the same method, we can determine the pre-charge pressure using the minimum system pressure of 40 bar and a maximum system pressure of 200 bar. We can see that when calculated the pre-charge pressure would need to be less than or equal to 36 bar but, at the same time, greater than or equal to 50 bar.
Clearly, this isn't possible. Therefore, in this scenario an adjustment to the minimum and maximum system pressures or a change in accumulator type may be required.
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