I’m going to attempt the impossible: I am going to explain the fundamentals of hydro-pneumatic accumulators without using any mathematics. I will use some numbers where needed, but the unfortunate reality is that the proper application of accumulators does require the manipulation of equations. Accumulators are a versatile and valuable tool, but because of the lack of understanding around their use—and the fact that few people are proficient at applying them properly—they’re being underused. By the end of this article, I hope to have instilled a solid foundation on the theory of operation behind accumulators.
Hydraulic accumulators are able to provide a handful of functions: Energy storage, leakage compensation, and vibration and shock reduction. These functions can be used for various applications and purposes, although energy storage is by far the most common. There are few hydraulic systems so perfect that an accumulator would not improve it, with perhaps the exception of extremes in high-demand, cost or lightness.
Hydraulic fluid, whether it be oil, water or synthetic composition, is not very compressible. We are taught that it’s not compressible, but everything is, even diamond and tungsten. It’s just that some matter is more compressible than others, and in fact, hydraulic oil will compress less than 0.5% per 1,000 psi. So at an astounding pressure of 10,000 psi, oil will be compressed by a measly 4%. In actual hydraulic systems, the compression can actually be higher due to entrained air within the oil.
As you can see, any attempt to store energy by compressing oil is fruitless. Although decompression of a high volume of high pressure fluid is a definite concern, as a lot of energy can be released, that energy release typically happens in fractions of a second. Large, high pressure systems, such as on press brakes or massive shears, require subcircuits to control this decompression. Even when decompression is allowed to occur slowly, it is never long enough to make useful work of the energy being released.
Gases are highly compressible, however, and when a gas is compressed into a confined spaced where pressure outside its container is lower, the gas will do everything in its power to expand to equalize with ambient pressure. The pressure energy stored in a compressed gas is inversely proportional to the size of the new space the gas occupies. For example, taking ten cubic feet of ambient air and putting it into a one cubic foot box will increase pressure ten fold (always remember that ambient, absolute pressure must be used in that calculation).
Pneumatic systems take advantage of the pressure differential between compressed air and the atmosphere. Air compressors “suck” in ambient air, and then compress it to 1/7th to 1/11th of its original volume to achieve between 90 and 150 psi. This compressed air is stored and/or distributed, where it takes advantage of pressure differential to create mechanical force in pneumatic cylinders and motors. The higher the compression ratio, the more potential it has to do work, although there comes a point in pneumatic systems where compressing to higher than 150 psi starts to create more heat than anything. Remember when you squish down a volume of air, you’re basically taking all the air molecules and heat energy, and condensing it. Compressing air to a tenth of its original volume also increases temperature tenfold (Charles’ Law).
Typical pneumatic system pressure does little to provide motivation in hydraulic systems, however. Even at 150 psi, which is high for a pneumatic system, you couldn’t even turn a large displacement orbital motor with no load on it. So if pneumatic systems can’t efficiently reach 200 psi, how can we use gases to store energy in systems with 3,000 psi or more?
Hydro-pneumatic accumulators use compressed nitrogen gas, both because it is relatively inert and is the most abundant gas in our atmosphere. Nitrogen has no magical properties allowing it to be compressed without heat, but nitrogen compression systems are typically large, efficient and expensive. They tend to work slowly, in multiple stages. This allows the compression ratio of each stage to be moderate, and allows for cooling between the stages. Once compressed, the nitrogen can be stored in large holding tanks or right into the nitrogen cylinders for distribution to end-users. Typically the tanks are charged to 5,000 psi, which is plenty to fill most accumulators.
Once an accumulator is installed, it is ready to be charged. A special hose and charge head, which both typically come in a kit, are used to connect the nitrogen cylinder to the accumulator’s gas fitting. The charge head will have a pressure gauge on it to read the pressure inside the accumulator (there is usually a gauge on the cylinder, too). As the valve is opened to allow nitrogen to enter the accumulator, the rush of gas can be heard as it fills quickly at first. The pressure differential lessens as it becomes filled, and the valve is closed when preset pressure is achieved.
Accumulator preset pressure is typically set at 90% of minimum working pressure. This is to allow maximum compression of the gas to store energy. If preset pressure is too low, the effect of the accumulator will be lazy, and the gas will easily compress and store little energy. If the preset pressure is too high, the gas won’t even start to store energy until system pressure is higher than preset pressure.
The accumulator stores energy any time system pressure is higher than precharge pressure. Although this can happen during a working cycle on the machine, circuitry is designed to fill the accumulator during off-demand, when pump flow is not allocated to the actuators. Let’s take a machine example and say the main relief valve is set at 3,000 psi, the machine work function requires 2,000 psi, and the accumulator is set at 1,800 psi.
When the system is turned on, while all control valves are closed, the pump (which is capable of 3,000 psi), will start flowing, and with 1,800 psi at the accumulator, it is the current path of least resistance. The accumulator will take full pump flow until pressure reaches 3,000 psi, where it will bypass over the relief valve. There is typically a check valve between the pump and accumulator, to ensure energy stays in the accumulator and does not try to push back through the pump or through the relief valve.
Often times, the relief valve is equipped with an unloading function that reads pressure on the accumulator side of the check valve, which will pilot the relief valve fully open to dump pump flow back to tank at low pressure. The unloading function can also be electric, where a pressure switch opens an unloading solenoid valve, or the pressure switch can be programmed to turn the pump motor off altogether.
At this point, the accumulator is ready to add its stored energy into the system, which is often combined with pump flow to increase peak output while the pump size remains smaller. With the pump running, and the directional valve open, the flow from the accumulator joins that of the pump to provide high flow to the actuator(s), but only until the 3000 psi accumulator pressure reaches system pressure, at which point it is nearly depleted and no longer supplementing flow, in this case, 2000 psi. The accumulator will supplement flow as quickly as it can based on pressure drop and flow calculations; accumulators are sometimes metered to prevent excessive flow from entering the system too quickly.
The short explanation of accumulator operation is this: Air bag is filled with gas, hydraulic fluid is squeezed into the space taken up by the gas, gas tries to push out the hydraulic fluid, and opening a downstream valve allows the gas to push out the hydraulic fluid. As I mentioned earlier, this is done to store energy, to compensate for leakage or to reduce shock or vibration.
Energy is the name of the game, and these days, anything to be done to save it is considered paramount. For decades, hydraulic systems have been using accumulators to store energy, although initially it was to “gain more from less.” Because a small pump could be used with an accumulator to provide high flow in lower duty cycle systems, size and cost are saved on the pump and prime mover. With high energy costs, this method of storing energy is economical and efficient, especially in systems that turn off the pump completely when demand is low.
Energy storage doesn’t have to be for used in continuous cycling, and sometimes accumulators are used for emergency energy during pump failure or loss of electrical power. The pressurized fluid in the accumulator can be used to open a mold or move the machine to a safe position where it can stay until power is restored or the malfunction corrected.
For use as leakage compensation, an accumulator can last for extended periods of time. A machine clamping function, for example, need not require the hydraulic system running and wasting energy while it pushes the clamp closed. An accumulator can provide constant clamping pressure, even while flow is slowly lost to leakage through piston seals or control valve clearances. When accumulator pressure drops to a critical point, a pressure switch will tell the pump to come on for only as long as it takes to refill the accumulator.
Because of the physical properties of hydraulic fluid, it is easy to transmit shock and vibration through the pipes, tubes and hoses of the system. Some pumps, for example, create pulses of pressure when the pistons or gears reach their outlet port. By adding a small accumulator at the outlet of the pump, the compressed gas can absorb these pulses like the struts on your cars suspension can absorb bumps in the road, providing smoother operation.
Sometimes pressure spikes are quite large, such as in the decompression of a large cylinder under high pressure, as discussed earlier. By adding an accumulator into the return line of these machines, decompression shock can be absorbed and prevent damage to the downstream components, which in a return line, are often not rated for high pressure.
Although each example of accumulator usage requires its own, unique equation to solve for critical parameters such as accumulator volume and precharge pressure, you don’t require these formulas to understand how and where to use an accumulator. But if you don’t understand the math involved, you will need to employ the services of someone who does. Accumulators are straight-forward in their application, but as they say, the devil is in the details.