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Week4/Lesson 1 – Accumulators
Fluid Power Engineering Week4/Lesson 1 – Accumulators
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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
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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
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What is an accumulator anyway?
Different size accumulators
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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
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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
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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
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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
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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
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…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
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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
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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
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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
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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
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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
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Accumulator symbols Weight-loaded Spring-loaded Gas-loaded
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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End of Week 4/Lesson 1
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