Safety at what cost?
Hydraulic systems carry their own dangers, but few in mill management or on safety committees know what they are.
by Al Smiley Jr.
Q. What safety procedures should my maintenance department be following when working on or around hydraulics?
Most production and maintenance personnel are not aware of the dangers surrounding hydraulic systems, even with the system turned off. The problem is that mill management and plant “safety” personnel often don’t know either! The thinking is that if the system is turned off and the electric motor is locked and tagged out, then the machine is safe. Yet this is one time that going with the flow can hurt you, making it imperative that all personnel be made aware of the dangerous conditions that exist when working on or around hydraulics.
The hydraulic accumulator is the most dangerous hydraulic component in the system. Students who attend our training workshops are amazed to find that they have been working around hydraulics for years unaware of this hazard.
The accumulator stores hydraulic oil and is primarily used to supply an additional volume of oil when needed to move rams and cylinders at high speed. For example, a 5,000-gallon accumulator supplies oil to close an oriented strand board (OSB) press in eight seconds. The accumulator delivers approximately 400 gallons of oil per second each time the press is closed. Although most systems do not use an accumulator of this size, the function is the same for bladder and piston types when used to provide volume.
The problem comes when the hydraulic system is turned off. Unless the pressurized fluid in the accumulator is manually or automatically released back to tank, the accumulator will operate as a “one shot” hydraulic pump. This means that if a valve were to inadvertently shift, then the accumulator would supply oil to the cylinder or ram, causing it to move.
Most systems have a manual valve or some type of automatic dump valve to direct the oil in the accumulator back to tank when the system is turned off. The OSB press previously mentioned uses a large motorized valve to block flow to the rams during maintenance.
To verify that the pressure has bled down to zero once the system is turned off, the gauge located nearest the accumulator should be observed. A “test and try” procedure should also be done to make sure that the cylinders or rams will not move with the pump off. I know of one fatality that occurred because an operator did not check the pressure gauge before working on the machine. The automatic dump valve failed to open and release the pressure in the accumulator back to the tank when the machine was turned off. A co-worker crawled near a photo eye, causing the directional valve to shift and allowing the accumulator volume to extend the cylinders and kill the operator.
The second risk around undrained accumulators is exposure to pressurized fluid when removing a line or other system component. At a particleboard plant a millwright-intraining cracked the fitting on a hose that contained 2,800 psi. This pressure remained because he was not properly trained in safety procedures on the machine. The pressure fluid came out of the fitting, knocked his safety glasses off and shot into his eye. The result was a 40% loss of vision in the eye, because he was not properly trained in manually bleeding the pressure out of the accumulator.
Other hydraulic devices may cause oil to remain locked in a cylinder of a hydraulic motor. The two most common are the counterbalance and pilotoperated check valves. These two valves are commonly used to lock oil in a cylinder to prevent it from drifting down when the system is turned off. Prior to working under or on the machine, the load should be fully lowered, porting the pressurized fluid back to the tank.
Overall, specific procedures should be written about bleeding the pressure down in each individual system. Gauges need to be located in the proper places for checking the pressure.
Getting hosed?
Another common safety hazard in many plants involves hydraulic hoses. Each time a hose is replaced,
it gets a little longer because the mechanic doesn’t want to make it too short. The hose will eventually end up rubbing on another hose, catwalk, beam, etc. (Figure 1). This will obviously cause premature hose failure, endangering anyone in the area. Several manufacturers make hose sleeves that can be used if
hose rubbing cannot be avoided.
I was recently consulting with a medium density fibreboard (MDF) plant on their press hydraulics and was standing near a two-inch hose that was five feet long. When the press closed, the hose jumped quickly, rising about a foot from its original position. This hose and others like it should be clamped to eliminate hazards to mill personnel.
In my 30 years in the industry I have never heard of a safety meeting held specifically to discuss the hydraulic dangers in a mill. Mill managers and supervisors should implement hydraulic-specific training programs conducted by in-plant personnel and outside vendors to assure the safety of production and maintenance workers.
Hydraulics is a regular column written specifically for Canadian Wood Products. Al Smiley Jr. is the founder of GPM Hydraulic Consulting Inc., and has provided in-plant troubleshooting, consulting, and training services to wood products companies in the US and Canada since 1987. He is registered with the Fluid Power Society as a Fluid Power Specialist. Send questions to: Hydraulics, 90 Morgan Rd., Unit 14, Baie d’Urfé, QC, H9X 3A8 or by fax to (514) 457-2558. For information about GPM Hydraulic Consulting, call (770) 464-0777.
