New Gear Harvesting New Gear
Accumulated Risk

Pre-Charging

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 types 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 piston in a piston type accumulator (Figure 2) separates the nitrogen from the hydraulic oil. The nitrogen pre-charge is usually 1/2 to 2/3rds of 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 of the pump compensator spring determines the maximum pressure when a pressure compensating pump is used.  The relief valve setting determines the maximum pressure in a fixed displacement pump circuit. In Figure 3, the bladder accumulator has been pressurized to 2,000 psi.

When the pump is turned off, the pressurized fluid in the accumulator must be released back to the tank.  This is done either by an automatic or manual dump valve.  If this pressurized fluid is not bled back to the tank through the dump valve, then the accumulator remains 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, of course, results in the load moving, which can be hazardous or deadly to maintenance or operating personnel.  

Auto Dump Valves

Many systems use automatic dump valves. These are operated either hydraulically or electrically.  A common type 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 now 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 and /or pressure.  However, if the valve fails closed, then the accumulator remains pressurized.

Consider what happened at a plant a few years ago.  This system had an electrically controlled dump valve, which opened once the pump was turned off.  Everyday at 3:30, the plant was shut down for 30 minutes for changing shifts.  During this time, the operator would frequently change the knives on the chipping heads.  The operator followed all the mill 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 tank through the dump valve. If the operator had looked at the gauge in this one particular instance, he would have seen that there was 1,500 psi still locked in the hydraulic lines.  What he did not know was that the accumulator dump valve had failed closed.  While knives were being changed, a co-worker crawled over the infeed conveyor, which enacted a photo eye.  An electrical signal was then sent to 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 0 psi when the machine was turned off, he might be alive today.

Manual Dump Valves

Other systems have only a manual type valve, which 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 one particular machine.  One week, the Class I mechanic was on vacation, and the trainee had to change a cylinder on the machine.  When 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.  Today he wears a special lens due to the 40% vision loss as a result of the accident.  If a written procedure to achieve a zero energy state had been in existence, then this accident would not have happened.

Dry Nitrogen or Air?

Accumulators should always be charged with dry nitrogen, never oxygen or compressed air.  Dry nitrogen, while technically not an “inert” gas, does not react readily with other chemicals.  Oxygen and compressed air, as we all know, aid combustion.  Most accumulators have a safety sticker on the accumulator shell warning that only nitrogen should be used to pre-charge the accumulators. One of our consultants was working with a plant a few years ago, and found an accumulator in the mill labeled “Danger – Compressed Air” as shown in Figure 5.  This was found only two days after an OSHA inspection of the plant!  

Why would anyone put this sign on an accumulator?  Could it be because many people have a well at their homes, which has an accumulator that is pre-charged with air? The person that put this sticker on the accumulator most likely recognized that the Schrader valve used to refill with nitrogen looks 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.  But 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.

Mounting and Removal

The accumulator should be properly clamped to the mounting fixture.  In Figure 6, a breakdown of the accumulator, minus the bladder, is shown.  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.  The nut then tightens the poppet valve and retaining ring to the 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 cause the shell to take 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 should be installed over the Schrader valve. One plant only had a single accumulator, and every time the accumulator needed pre-charging, they removed it, laid it in a pick-up truck, and drove it to the nearest shop.  If the Schrader valve would have broken off or the retaining ring failed during this transport, the accumulator would have once again acted as an unguided missile.  

One of the most common remarks made by students after they attend our training workshops is “I had no idea about the dangers of accumulators”.  Don’t wait until some one is injured or killed to educate your mill personnel on accumulators.  If you do not have anyone qualified to speak on this subject, please contact our company for a safety presen-tation at your plant.

Al Smiley Jr. is a hydraulic systems specialist working with the forest products sector across North America to maximize mill efficiency and minimize downtime and operating costs. He wrote this article specifically for Canadian Wood Products Magazine, and can be reached at GPM Hydraulic Consulting, Inc., P. O. Box 376, 797 Ridge Rd., Monroe, GA 30655, 770-267-3787, Fax: 770-267-3786,gpm@gpmhydraulic.com, www.gpmhydraulic.com