Hydraulics question - accumilators

[Flamin_Squirrel]

Hi chaps, just trying to understand the intricacies of hydraulic systems, specifically how accumilators provide pressure during high demand.

Generally hydraulic pressure is 3000psi. If there is high demand for which the pumps can't keep up with the accumilator will provide it. To do this however, I imagine the accumilator pressure must be higher than 3000psi, or the supply pressure will drop?

Do the pumps/accumilator run at higher pressures, with pressure reducing valves to maintain 3000psi? If so, what pressure do they run at?

Many thanks.

[jtr]

Hi there. Not an eng, but am a driver. From memory the Airbus accumulators are pre charged to 1450psi, which I assume will bump up to 3000psi when the system is operating.

[Truck2005]

Accumulators themselves do not supply the hyd pressure, they supplement it. They supply a constant back pressure, usually around 1600psi, which combines with the system pump pressure when selected. You could look at this as a stored potential energy which is always there in the system. If you can imagine a pump at one end of an aircraft and a service at the other end, if you only had a pump it would have to be a large one to enable a reliable pressure head at that service. By having acc pressure this guarantees a min pressure at all services at any time, (although I would be very worried if my guage was reading acc pressure only during flight)! Acc pressure also protects against any 'pulsing' that pumps may induce when on load.

[SNS3Guppy]

Think of an accumulator as a balloon, the kind you blow up at a children's party. Instead of air, it's filled with hydraulic fluid. Release the neck of the balloon, the air wants to escape under pressure; the pressure is created by the tension of the rubber in the balloon.

In an aircraft, an accumulator isn't made of rubber, but metal, and is either a sphere or a cylinder with a piston inside. The cylinder is pressurized with nitrogen, to a specific value (usually about a third or half of the working hydraulic system pressure). This pressure is called the precharge. Without any fluid in the cylinder, the piston is pushed all the way to the end at which the hydraulic fluid enters, and the nitrogen pressure is lowest. The nitrogen pressure, or precharge, does the same thing that the balloon rubber tension does in the children's party balloon. When hydraulic fluid is forced into the opposite end of the cylinder, pushing back the piston against nitrogen pressure, the nitrogen is compressed, increasing it's pressure, which in turn pushes back against the hydraulic fluid.

Cap off the hydraulic fluid,and the hydraulic system now holds the same pressure value as the nitrogen on the other side of the piston.

The accumulator is used to absorb surges in the hydraulic system. Rapid closing and opening of valves can cause "hammer" or slamming of the hydraulic system and components. The accumulator serves as a shock absorber, giving some elasticity to surges or sudden stoppages in fluid flow.

The accumulator is also used to supply pressure in the system for certain uses such as supporting a parking brake, or certain emergency functions that require a small pressurized source of fluid. The accumulator isn't a large supply of fluid, and thus can't flow a lot of fluid, but can hold small amounts of fluid under pressure.

A hydraulic accumulator can't operate at a higher pressure than the system pressure, because it's fed and supplied by the hydraulic system. Typical hydraulic system pressures are 1,500, 1,800, 2,000, and 3,000 psi systems. typical accumulator precharges are 750, 1,200 and 1,500 PSI.

Accumulators are supplied by system pressure and organized with check valves to allow fluid in from one direction, and out in another. This arrangement means that while hydraulic system pressure might be depleted due to fluid loss, for example, a minimum amount of pressurized fluid might be remaining to allow for one brake application during a landing with a fluid loss.

Accumulators are sometimes used in propeller driven aircraft to help unfeather the propeller in flight. These store oil or hydraulic fluid under pressure for use in driving the propeller mechanism out of the feathered position, to allow for in-flight engine starts.

Some landing gear systems use accumulator pressure to help hold landing gear up, or in some cases to provide for assistance in gear extension.

Hydraulic accumulators perform a similar duty for the hydraulic system that a battery does for an electrical system. The battery absorbs transient electrical surges, stores electrical energy, and provides a small, limited amount of electricity for certain purposes. Likewise,the accumulator absorbs hydraulic surges, stores hydraulic pressure, and provides it in small, limited quantities for certain specific purposes.

[Flamin_Squirrel]

Thanks chaps.

I knew accumilators wern't charged to a lower pressure rather than 3000psi, because if they were the piston/bladder wouldn't move to allow fluid to enter when the pumps came on line.

But this was what I was trying to wrap my head around - for the accumilator to provide the suplimental/potential energy, the pressure in the accumilator has to drop. So, does this mean the pressure drops below 3000 when suplimental pressure is needed?

[RetiredSHRigger]

F S

Read Guppys excellent post carefully :ok:and all becomes clear. Accumulator Pressure can only be checked with hyd pressure exhausted, and charged with Nitrogen typically to 1450 psi. When pumps are running indicated pressure will typically read 3300 psi, but pressure will fluctuate under load conditions with damping from the acc.

[Midland 331]

For what it's worth, the hydropneumatic suspension system used by Citroen on certain of their older cars uses "spheres" to provide suspension softness, and aircraft hydraulic accumulators sound not unlike them in that they are both pressure vessels in which fluid is kept under pressure by gas, the two separated by a "rubber" diaphragm.

Additionally, in the event of pump failure on a Citroen, a separate accumulator sphere provides braking power (for a while....).

There are plenty of diagrams of how their system works on the internet, and they may help to give background on hydraulic systems.

I speak from the experience of having run and maintained an "old school" Citroen XM since 2002.

r

[Dave Clarke Fife]

When are you going to write a book on everything you know SNS3Guppy? I find you make what can be hard to understand concepts very easy to understand. 10 out of 10 for your explanation.

Respect!!

(p.s maybe you can re-write Ace the Technical Pilot)

[dixi188]

As to the original question about high demand.

A typical aircraft hydraulic pump is of the constant pressure, variable displacement type.

This means that when there is little demand on the system the pump is effectively idling. When a high demand is required as in gear retraction or large control deflection, it takes time for the pump to react to this, so the accumulator provides the initial fluid pressure until the pump responds.

If you watch the system pressure indicator you will see a momentary dip in the pressure as the demand is made.

Hope this helps.

[Flamin_Squirrel]

Ahha, so the pressure drops below the 3000psi. Thanks dixi :cool: