Reducing the dangers of stored energy

J. Eric Freimuth • Hydraulic Training Associates

Accumulators are devices that store energy in the form of fluid under pressure. Because of their ability to store excess energy and release it when needed, accumulators help improve hydraulic efficiency. Industrial hydraulic accumulators are typically classified as hydro-pneumatic. They apply a force to a liquid by using compressed gas and are most commonly available as bladder and piston type designs.

The hydraulic energy from the system exerts force against the piston or bladder, which isolates the gas from the fluid. The nitrogen is compressed as the piston or bladder travels from the force.

The compressed nitrogen then becomes stored potential energy. When the system pressure drops below the compressed nitrogen pressure, the nitrogen rapidly expands, forcing the hydraulic fluid out to the system. The velocity of this discharge is dependent on the pressure drop or differential across the piston or bladder.

Piston accumulators are available as gas-charged or spring-loaded designs. A hydro-pneumatic accumulator has a fluid compartment and a gas compartment, with a gas-type element, such as a bladder, separating the two. The bladder is charged through a gas valve at the top of the accumulator, while a poppet valve at the bottom prevents the bladder from extruding into the pressure line. The poppet valve is sized so that maximum volume metric flow cannot be exceeded.

To operate, the bladder is pre-charged with nitrogen to a pressure specified by the manufacturer according to the operating conditions. When the system pressure exceeds the gas pre-charged pressure, the poppet valve opens and hydraulic fluid enters the accumulator. The changing gas volume in the bladder determines the useable volume or useful fluid capacity.
Accumulators store energy that can be used during power failure or when additional energy is needed. In certain situations, additional flow may be needed. An accumulator can be used to supplement the flow rate of a pump.

Being able to identify an accumulator in a hydraulic system is the first step to safety. It’s important that accumulators are labelled properly to eliminate misidentification. The accumulator label should contain the following: The manufacturer’s name, part number, year of production, model and serial number, maximum allowable pressure in psi or bar, volume in gallons or liters, and pre-charge pressure.
Understanding everything that makes up an accumulator is critical, too. For example, two millwrights were doing maintenance service on a rock crusher. They mistakenly removed the fill valve (which resembles a bicycle stem) from the bladder accumulator, thinking it was the vent plug for a pressure filter. The valve hit the ceiling 60 ft above the crusher and was never seen again. The millwrights narrowly escaped injury. To

the inexperienced, the pressure filter might look like an accumulator.

Hydraulic systems are complex, so identifying stored energy locations in them can be difficult and dangerous. The energy of hydraulic fluid under pressure and the loads this energy supports are extremely hazardous.

The accidental release of this energy has been the cause of many injuries and death throughout all industries. It is important to have an understanding of fluids under pressure including an in-depth study of hydraulic accumulators, their design, application and dangers when working with them.

Accumulators are one of the most dangerous components in a hydraulic system. The physical exposure to the accidental release of this stored energy can be fatal. When discharging and charging an accumulator in a confined area, you should always monitor oxygen levels. Nitrogen does not contain oxygen; it has no taste or smell, and will displace breathing air. Always consult an expert when working with accumulators; the life you save may be your own.

Hydraulic Training Associates
htahydraulics.com

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Filed Under: Fluid Power World Magazine Articles
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