Hydraulic accumulators are pressure vessels that perform many tasks within a hydraulic system, and the safety dangers associated with such components are often overlooked. In contrast, personal protective equipment, chain guards, safety harnesses, and lockout-tagout procedures are typically covered in depth.
These vessels are responsible for maintaining pressure, storing and recapturing energy, and reducing pressure peaks while also dampening shock, vibration, and pulsations. As they are tasked with completing many procedures, there are many safety dangers associated with them. With this in mind, this blog will provide a brief overview of the safety dangers of hydraulic accumulators as well as the best practices and safety solutions for handling them.
An accumulator is a storage device in a hydraulic circuit. The two most common are bladder and piston types. A bladder hydraulic accumulator consists of a rubber balloon that separates the hydraulic oil from the dry nitrogen. Dry nitrogen is used to fill the bladder to a pre-charge level that is typically ½ to ? the maximum pressure in the system.
When the pump is turned on, the nitrogen is compressed to meet the maximum pressure in the system. The pump compensator spring has a setting that determines the maximum pressure when a pressure-compensating pump is utilized. The relief-valve setting controls the pressure in a fixed-displacement pump circuit.
In a piston type accumulator, the piston separates nitrogen from the hydraulic oil. When the oil is delivered into the accumulator, the piston rises until it reaches maximum pressure. In that instance, the hydraulic and nitrogen pressures will be equal. When the pump is turned off, the pressurized fluid in the accumulator should be released back to the tank. This can be achieved with the use of an automatic or manual dump valve.
If the fluid is not dispersed back to the tank through the dump valve, the accumulator will remain pressurized. As a result, the accumulator becomes a one-shot hydraulic pump. If the valve were to move slightly in any direction, the pressurized fluid would be directed to operate the cylinder or hydraulic motor. This will cause the load to move, which can be dangerous or even deadly to maintenance workers.
Additionally, dump valves can be operated hydraulically or electrically. In an electrical dump valve, when there is no electrical power provided to the solenoid, the valve spring moves the spool to an open position. The lack of power allows any pressurized oil in the accumulator to make its way back to the tank. The solenoid on the valve is typically wired into the electric motor starter. From the moment the motor is started, voltage is applied to the valve solenoid, enabling the valve to close. Like other components within these devices, the dump valve may fail. Some of these failures include not opening which can cause a loss of speed, or the inability to close which will maintain the accumulator in a pressurized condition. With manual dump valves, operators should be aware that these components must be opened to release the pressurized fluid in the accumulator back to the tank.
With regard to the kind of air that accumulators are charged with, dry nitrogen should always be used due to the fact that it does not react with other chemicals. As such, there is typically a safety label on the accumulator indicating that only nitrogen should be used for pre-charging. If an accumulator is filled with compressed air, the result would be an explosion and/or fire.
Proper mounting and removal is also detrimental to an accumulator’s optimal functionality. An accumulator should be correctly clamped to a mounting fixture. When assembling the accumulator after replacing the bladder, the retainer ring must be fitted around the circumference of the poppet valve, and both components should be placed into the accumulator shell. If the accumulator is not properly clamped, the retainer ring may not completely fasten, causing the poppet valve to detach from the accumulator. In the case that this happens, the accumulator shell may shoot up. With regard to the removal or storage of the accumulator, the nitrogen should be released and a protective cap placed over the valve.
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