- Buyer's Guide
What do you normally cover in your plant’s safety meetings? Personal protective equipment, chain guards, safety harnesses and lockout-tagout procedures are common topics. When was the last time hydraulic accumulators were discussed? If your plant is like most, the answer is never. Why? In most plants, maintenance, production and safety managers are not aware of the dangers. Nevertheless, accumulators can present a safety hazard if the potential risks are not understood.
An accumulator is a storage device in a hydraulic circuit. It is the hydraulic equivalent of a capacitor in an electrical circuit. The two most common kinds of accumulators are the bladder and piston types. The bladder (Figure 1) is nothing more than a rubber balloon that separates the hydraulic oil from the dry nitrogen. Dry nitrogen is used to fill the inside of the bladder to a pre-charge level. The nitrogen pre-charge is usually half to two-thirds the maximum pressure in the system.
When the pump is turned on, the nitrogen is compressed to the maximum pressure in the system. The setting on the pump compensator spring will determine the maximum pressure when a pressure-compensating pump is used. The relief-valve setting controls the pressure in a fixed-displacement pump circuit.
In Figure 2, the bladder accumulator has been pressurized to 2,000 pounds per square inch (psi).
The piston in a piston-type accumulator (Figure 3) separates the nitrogen from the hydraulic oil. When oil is ported into the accumulator, the piston will rise until the maximum pressure is reached. The hydraulic and nitrogen pressures will be equal at that time.
When the pump is turned off, the pressurized fluid in the accumulator must be released back to the tank. This is accomplished by an automatic or manual dump valve. If this pressurized fluid is not bled back to the tank through the dump valve, the accumulator will remain pressurized. The accumulator then becomes a one-shot hydraulic pump. If a valve were to inadvertently shift, then the pressurized fluid would be directed to operate the cylinder or hydraulic motor. This results in the load moving, which can be hazardous or deadly to maintenance or operations personnel.
Figure 4. An electrical dump valve
Many systems incorporate automatic dump valves. These valves are operated either hydraulically or electrically. A common type of electrical dump valve is illustrated in Figure 4. When there is no electrical power to the solenoid, the valve spring shifts the spool to the open position as shown. This allows any pressurized oil in the accumulator to return to the tank. The solenoid on the valve is usually wired into the electric motor starter. When the motor is started, voltage is applied to the valve solenoid, causing the valve spool to shift closed. Flow from the pump and accumulator is then blocked back to the tank.
So if your systems incorporate automatic dump valves, why should you be concerned? Because like any other hydraulic component, these valves can fail. The valve may fail open, causing a loss of speed, or fail closed, maintaining the accumulator in a pressurized condition.
Consider what happened at one plant several years ago. This system had an electrically controlled dump valve that opened once the pump was turned off. Two cylinders were mounted on the machine to extend and retract two large chipping heads. When the machine was shut down, the operator would frequently change the knives on the chipping heads. The operator followed all the mill’s safety procedures for locking and tagging out the machine. There was no written procedure for checking the gauge at the accumulator to verify that the pressure inside the accumulator had been released back to the tank through the dump valve. If the operator would have looked at the gauge in this particular instance, he would have seen that 1,500 psi were still locked in the hydraulic lines. What he did not know was that the accumulator dump valve had failed closed.
While the knives were being changed, a co-worker crawled over the in-feed conveyor, making a photo eye. An electrical signal was then sent to the programmable logic controller (PLC), indicating that a log was on the conveyor. The PLC then sent a current signal out for the chipping head valves to shift. The accumulator discharged oil to the cylinders, which extended the chipping heads, crushing the operator. Had the operator been instructed to verify that the hydraulic pressure had bled down to zero psi when the machine was turned off, he might be alive today.
Figure 5. A mislabeled accumulator
Other systems have only a manual valve that must be opened to bleed the pressurized fluid in the accumulator back to the tank. In this case, all individuals working on or around the machine must know where the valve is located and that it should be opened. How much training does a new millwright or electrician get in your plant regarding hydraulic safety? Normally, little or nothing is said about releasing pressure in hydraulic accumulators.
One company hired a new mechanic who was being trained on the job by a Class I mechanic. The Class I mechanic failed to tell the trainee about opening a manual dump valve prior to working on a particular machine. When the Class I mechanic was on vacation, the trainee had to change a cylinder on the machine. After he loosened the fitting on the hose at the cylinder, 2,800 psi discharged from the line. His safety glasses were knocked off, and oil was injected into his eye. As a result of the accident, he now wears a special lens due to the 40-percent vision loss. If a written procedure to achieve a zero-energy state had been in existence, this accident would not have occurred.
Figure 6. A breakdown of the accumulator minus the bladder
Accumulators should always be charged with dry nitrogen, never oxygen or compressed air. While technically not an “inert” gas, dry nitrogen does not react readily with other chemicals. Oxygen and compressed air aid combustion. Most accumulators have a safety sticker on the shell warning that only nitrogen should be used for pre-charging.
A few years ago, one of our consultants was working with a plant and located an accumulator labeled “Danger: Compressed Air,” as shown in Figure 5. This was discovered only two days after an inspection of the plant by the Occupational Safety and Health Administration (OSHA). Why would anyone put this sign on an accumulator? Could it be because many people have a well at their homes with an accumulator that is pre-charged with air? The person who placed this sticker on the accumulator most likely thought that the Schrader valve used to refill with nitrogen looked very much like the accumulator in his well system, bicycle or car tire.
Also, notice that the actual warning sticker applied by the accumulator manufacturer is covered up by the piece of wood underneath the chain clamp. Fortunately, compressed air had never actually been used in the accumulator. If someone had ever filled it with compressed air, as the sticker suggests, the bladder could have ruptured, and the result would have been an explosion or possibly a fire at this plant. Needless to say, our consultant had this sticker removed immediately.
Figure 7. A properly clamped accumulator
An accumulator should be properly clamped to the mounting fixture. Figure 6 shows a breakdown of the accumulator minus the bladder. When assembling the accumulator after bladder replacement, the retainer ring is fitted around the outside of the poppet valve, and both are inserted into the accumulator shell. If the accumulator shell is not properly clamped, then failure of the retainer ring can cause the poppet valve to disconnect from the accumulator. This can result in the shell taking off like a rocket. Figure 7 shows a properly clamped accumulator.
Prior to removing and storing an accumulator, the nitrogen pressure should be released and the protective cap installed over the Schrader valve. One plant only had a single accumulator. Every time the accumulator needed pre-charging, it was removed, placed in a pickup truck and driven to the nearest shop. If the Schrader valve had broken off or the retaining ring failed during transport, the accumulator could have acted as an unguided missile.
Most workers are not aware of the dangers of accumulators. Don’t wait until someone is injured or killed in your plant to educate your personnel.