Accumulators are necessary for good closed loop control. In a closed loop system the purpose of the accumulator is to keep the system pressure as constant as possible by supplying oil when the pump can’t respond fast enough to current flow demands. In the field I see 3 main problems with how accumulators are misused.
- The accumulators are misplaced. Often the accumulator is placed near the pump. The accumulator should be placed neared the valves. This is where the energy must be stored. This also minimizes pressure loses between the accumulator and the valves.
- The accumulators are sized too small. Obviously the accumulator should hold enough oil so that the accumulator will not empty but the oil does not store energy, the pressurized nitrogen does and it is critical that the size of the nitrogen volume changes as little as possible so the pressure changes as little as possible.
- The accumulators are pre-charged at too low a pressure. When this happens there will be more oil in the accumulator than necessary but the gas volume will be too small so that changes in the gas volume cause big pressure drops.
I haven’t had to setup an accumulator in a little while for a hydraulic motion system and i cant seem to find my hydraulics systems fundamentals book that i received when i was on my last training course. Is there a general rule of thumb or a formula for correctly setting the pre-charge pressure in an Accumulator? I am working on a customers system right now which has a 1500 PSI operating pressure and the accumulator precharge is 2100 PSI…which seems way to high.
JSINGLE, you are right how can the pre-charge be higher than the system pressure? If the precharge is higher than the system pressure there will be NO OIL in the accumulator since the bladder would push it all out into the system. That isn’t good unless the accumulator is being used to protect against hydraulic shock and not as a source of oil for when the pumps can’t keep up.
I like to get the precharge at about 90% of the system pressure so the accumulator is mostly filled with nitrogen but still has enough oil so the accumulator never goes empty. The Design Guide we offer has a very good article on accumulators.
Here is the magazine article on the topic written by me.
hydraulicspneumatics.com/200/Tec … cumulators
For some quick accumulator calculations, Hydac has an online calculator here:
hydac.com/de-en/service/onli … light.html
In almost all motion control cases, you want to select adiabatic for the thermodynamic conditions. You also have to be careful to select the units you are using, it defaults to all metric.
There is a tab on the web page called Operating Manual that gives you a link to download some PDF instructions on the usage of the program.
It’s geared towards general applications and it wants to pick the pre-charge (P0) for you but you can over-ride it’s selection. The manual talks about doing that.
I know that for simple systems where we need to close cylinders on loss of power it has been spot on for calculating accumulator sizes.
Thank you for the information Gentlemen,
In this application the accumulators are only being used to compensate for the pump response time during the initial portion of the motion profile. The pumps are sized to provide more than enough flow for the application for the remainder of the motion. The 90% number rang a bell in my head, but i just wanted to be sure. What a great article Peter! I have passed it along to our hydraulic designer to review and calculate the required precharge pressure (although it sounds like we will end up in the 90% range).