1. Week4/Lesson 1 – Accumulators
Fluid Power Engineering
Week4/Lesson 1 – Accumulators

2. Accumulators In this lesson we shall Introduce hydraulic accumulators
Discuss an example application of a hydraulic accumulator
Examine the types of accumulators often employed in hydraulic circuits

3. What is an accumulator anyway?
It’s a little pressurized tank connected into the circuit
It is charged with fluid during the non-working part of the cycle
This fluid can then be combined with the pipe flow to increase the system’s total flow capacity during high-demand parts of cycle
In this way we can get away with a much smaller pump than would be needed to deliver max flow in real time
But this is only one reason to install an accumulator

4. What is an accumulator anyway?
Different size accumulators

5. Example: commercial airliner
A commercial airliner has a great need for hydraulic power…
…but only during short phases of flight
When the plane takes off…
The deployed flaps and wing slats need to be retracted
The landing gear needs to be retracted
Large deflections of control surfaces on the plane are needed due to the low speed and thus the need for large deflections

6. Example: commercial airliner
A commercial airliner has a great need for hydraulic power…
…but only during short phases of flight
Likewise, when the plane lands…
The flaps and wing slats need to be deployed
The landing gear needs to be deployed
Large deflections of control surfaces on the plane are needed due to the low speed and thus the need for large deflections
For stopping the plane need to apply brakes and also thrust reversers

7. Example: commercial airliner
A commercial airliner has a great need for hydraulic power…
…but only during short phases of flight
During cruise flight at speed, little demand for hydraulic power
Flaps/wind slats/landing gear stowed away
High speed means small deflections of control surfaces have a big effect
No braking

8. Example: commercial airliner
And remember, on an airliner…
…space and weight are critical…
…so it would really be nice to use a smaller pump than is needed for maximum demand
Enter the accumulator
When charged, supplies extra flow in times of need
And if pump is lost for one reason or another…
…stored pressure energy in accumulator is on hand

9. Three types of accumulators
There are three types of accumulators:
Weight loaded
Spring loaded
Gas loaded (gas spring)
2 and 3 both used spring energy to work
The compressed gas acts as a spring

10. …which compresses fluid here
Weight-loaded accumulator
Weights stacked here…
…push down on piston…
A weight-loaded accumulator maintains same pressure on fluid as fluid exhausts through the fluid port
…which compresses fluid here

11. Weight-loaded accumulator
Weight-loaded accumulator is oldest type of accumulator
But a weight-loaded accumulator is only good for fixed installations
Too big and heavy for mobile or aeronautical applications

12. Spring-loaded accumulator
Accumulator open at top to let air enter and exit through a filter
Spring in top
…pushes down on piston…
Disadvantage is that as spring expands, pressure drops in oil below
…which compresses fluid here

13. Gas-loaded piston accumulator
Can gang accumulators together for greater capacity
N2 charge put in here
Usually N2 because no moisture
Accumulator preloaded when empty, so that it exerts a pressure, even when most of the oil is gone
When system is in an inactive phase, pump pushes oil back into accumulator
Gas pushes down on piston…
Positive shut-off prevents N2 from entering oil system
…which compresses fluid here
Fluid enters and exits here

14. Gas-loaded bladder accumulator
N2 charge put in here
Usually N2 because no moisture
Gas pressure expands bladder…
…which compresses fluid here
Positive shut-off prevents bladder from squeezing into attached oil line
Fluid enters and exits here

15. Gas-loaded accumulators
There are even accumulators that have no separation between the gas and the oil
But these have the disadvantage of pushing the gas into the oil, i.e. causing gas entrainment
This makes the oil more compressible, which makes the response more spongy, oscillatory, and less precise

16. Accumulator symbols
Weight-loaded
Spring-loaded
Gas-loaded

17. Gas-loaded accumulators
There are even accumulators that have no separation between the gas and the oil
But these have the disadvantage of pushing the gas into the oil, i.e. causing gas entrainment
We’ll discuss this more when we investigate the properties of the oil
This makes the oil more compressible, which makes the response more spongy, oscillatory, and less precise

18. Accumulator as extra power source
This is a system in which the hydraulic pump supplies oil to a number of different actuators:
Also helpful is that if pump fails, still have some power to move motors a bit
Besides the extra power, the accumulators also help with reducing cross-talk, that is preventing demand by one motor from affecting the supply to other motors

19. Unloading circuit for pump
Auxiliary power for rapid retraction
Cylinder extends:
At extension, cylinder stays for some time for an operation to be performed
This allows accumulator to charge
Valve deactivates
Charged accumulator allows rapid retraction of cylinder
Unloading circuit for pump

20. Can adjust at what pressure valve switches
A modification
Could replace PRV with pilot-operated 2/2 valve
At pmax , pilot switches valve to bypass
At pmin , pilot valve switches valve
Can adjust at what pressure valve switches

21. Accumulator to maintain pressure
Or if you have a system that sits idle for long periods of time…
The accumulator maintains pressure when system is idle
At pmax , pilot valve switches valve
The switch turns the pump off when p reaches pmax
This is our normal unloading circuit
…and it turns the pump on when p drops below pmin

22. Accumulator as emergency power source
If the pump fails or electricity to pump drive fails and need to retract cylinder…
3/2 valve switches to A-T
Accumulator retracts cylinder
If power fails, solenoid will deactivate
Pump must work against this pressure when cylinder extends…
But cylinder extension is also charging accumulator

23. The pressure spike will be absorbed in the accumulator
Accumulator as pressure absorber
This system has an emergency shut-off valve
If the valve is shut suddenly, the pressure will rise due to the momentum of the fluid in the conduit
The pressure spike will be absorbed in the accumulator
As such, the accumulator acts like a capacitor in an electric circuit

24. How to size an accumulator?
We’ll use a piston accumulator as an example
Though a bladder accumulator works the same way N2 N2
Fluid
Fluid
State 1: Accumulator full of fluid
State 2: Accumulator empty of fluid

25. How to size an accumulator?
Use Boyle’s Law
𝑝 1 ∙ 𝑉 1 = 𝑝 2 ∙ 𝑉 or 𝑝 1 𝑝 2 = 𝑉 2 𝑉 1
Here we’re assuming DT is small (slow, isothermal discharge)
When the accumulator empties, so that 𝑉 2 𝑉 1 >0 …
… 𝑝 1 𝑝 2 >0 , the pressure goes down
Example: 𝑉 2 =2∙ 𝑉 1 ⇒ 𝑉 2 𝑉 1 =2= 𝑝 1 𝑝 2
So 𝑝 2 = 1 2 ∙ 𝑝 1 ⇒ Doubling the volume halves the pressure

26. Sizing accumulator
We want to use the accumulator below for extra flow to extend or retract the cylinder
At ends of stroke, pump charges accumulator QA
When valve actuates, flow comes out of accumulator
Qpmp
Qacc
QA = Qacc + Qpmp

27. Sizing accumulator 𝑄 𝐴 = 𝑄 𝑎𝑐𝑐 + 𝑄 𝑝𝑚𝑝 =𝑣∙ 𝐴 𝑐𝑦𝑙
If we integrate this over time to extend cylinder
𝑉 𝑐𝑦𝑙 = 𝑉 𝑎𝑐𝑐 + 𝑉 𝑝𝑚𝑝 =𝑆∙ 𝐴 𝑐𝑦𝑙
This tells us how much flow must come out of accumulator and pump during the extension

28. Sizing accumulator
But there is another problem to consider. As the N2 in the accumulator expands, its pressure drops.
We want the pressure in the accumulator to remain above the pressure in the cylinder at the end of its stroke
Otherwise, at the end of the piston stroke, the pump will be feeding both the cylinder and charging the accumulator
This will cause the cylinder extension to slow down
The empty accumulator is charged to a certain pressure with N2
This pressure must be greater than the pressure of the cylinder at the end of its stroke

29. Sizing accumulator
There will be a pressure loss in the conduit between the accumulator and the cylinder
So place the accumulator near the cylinder to lower this loss
Also, size the pipe between the accumulator and cylinder to accommodate the extra flow from the accumulator
We’ll see later how to calculate pressure drops in pipes/tubes/hoses

30. The load force has to be known
Sizing accumulator
At end of piston stroke still need pacc > pA
𝑝 𝐴 = 𝐹 𝐿 𝐴 𝑐𝑦𝑙
The load force has to be known
𝑝 𝑎𝑐𝑐 = 𝑝 𝐴 +∆ 𝑝 𝑎𝑐𝑐−𝑐𝑦𝑙
This gives us the pre-load needed on accumulator to prevent inflow into accumulator at end of piston stroke
This shows the reasoning needed to size an accumulator
You need to look ahead and consider all states of the process
You need to calculate preload on accumulator

31. Outside learning
To better understand this subject matter, view the following videos
Don’t forget to turn the closed-captioning on to be able to understand better the details of the lectures
Watch:
Accumulators
Mechanical hydraulic basics course – accumulators
Piston accumulators

32. End of Week 4/Lesson 1