One Thing You MUST Know About Hydraulic Accumulators

Bladder and diaphragm accumulatorGas-charged accumulators are a common feature of modern hydraulic systems. They carry out numerous functions, which include energy storage and reserve, leakage and thermal compensation, shock absorption and energy recovery. But while accumulators present a number of advantages in hydraulic system operation, and can provide many years of trouble-free service, they are a maintenance item.

For example, the correct gas pre-charge pressure must be maintained for proper functioning and optimum service life. And periodic inspection, testing and certification can be required by law, because hydraulic accumulators are pressure vessels.

To get a proper prospective on this issue, a hydraulic accumulator must be compared with a gas cylinder. If a high-pressure gas cylinder ruptures, the explosive expansion of the compressed gas takes out anything in its path. The only real difference between pumping up an accumulator with nitrogen, and any other type of gas cylinder, is in an accumulator the gas is contained within a bladder, within the shell.

Here’s the thing: it’s not rupture of the bladder that’s dangerous, it’s the potential rupture of the shell of the accumulator that’s extremely dangerous. The bladder simply separates the gas from the oil. It’s the accumulator shell which must resist the gas pre-charge pressure.

Consider for example a storage application of a 4-liter accumulator where the minimum required system pressure is 242 bar or 3500 psi. The gas pre-charge for an accumulator in a storage application is typically 90% of minimum required working pressure. In this case, this equates to 218 bar or 3158 psi. Note that this is a higher pressure than many gas cylinders contain.

But not all accumulators are equal of course, and so the potential danger depends on the volume of the accumulator and its gas pre-charge pressure. In other words, the greater the volume and pre-charge pressure of the accumulator, the more closely it resembles a high-pressure gas cylinder. Which means, although it doesn’t happen often, the shell failure of such an accumulator is potentially just as dangerous as a ruptured gas cylinder or steam boiler.

And this is why accumulators are manufactured, tested and certified according to statutory standards. In the United States for example, the relevant standard is the ASME Boiler & Pressure Vessel Code VIII, Division 1.

All pressure vessels manufactured to these and similar standards are considered to have a finite service life depending on the number of pressure cycles experienced during normal operation. The typical design life for a hydraulic accumulator is 12 years.

In many jurisdictions, periodic inspection and re-certification is required. This particularly applies to hydraulic accumulators which have relatively large volumes and operate at high working pressures. Inspection may be at required at pre-determined intervals, such as every 2, 5 or 10 years, or when a certain percentage of usable design life is deemed to have been reached.

Depending on the volume and pressure rating of the accumulator, re-certification may involve one or more of the following: visual inspection, ultrasonic thickness testing and/or hydrostatic pressure testing.

So if you own or are responsible for a hydraulic machine that has an accumulator, not being up to date with its maintenance and testing requirements can be a dangerous and costly mistake. And to discover six other costly mistakes you want to be sure to avoid with your hydraulic equipment, get “Six Costly Mistakes Most Hydraulics Users Make… And How You Can Avoid Them!” available for FREE download here.

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