Hot Air & Slippery Stuff -104

Accumulators – General

Gas charged accumulators are a common feature in 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 as pre-charge pressure must be maintained for proper functioning and optimum service life. And periodic inspection, testing and certification rules 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 it’s path. The only real difference between pumping up and 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 the rupture of the blader thats dangerous, it’s the potential rupture of the shell of the accumulator that is 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 applications is typically 90% of the 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.

That 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 and Pressure Vessel code VIII, division 1.

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

So if you own 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 situation.

by Brendan Casey: