Machinery Lubrication magazine published by Noria Corporation
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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 pres- surized fluid in the accumulator must be released back to the tank. This is accom- plished 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 inadver- tently 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. Automatic Accumulator Dump Valves 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 accumu- lator is then blocked back to the tank. So if your systems incorporate auto- matic 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 elec- trically controlled dump valve that opened once the pump was turned off. Two cylin- ders 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. Manual Dump Valves 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 electri - cian 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 acci- dent, he now wears a special lens due to the 40-percent vision loss. If a written proce- dure to achieve a zero-energy state had been in existence, this accident would not have occurred. Dry Nitrogen or Air? 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. HYDRAULICS Figure 4. An electrical dump valve Figure 5. A mislabeled accumulator 28 | November - December 2015 | www.machinerylubrication.com