Lockout, tagout, and control hazardous energy on the shop floor
OSHA instructs maintenance personnel to lockout, tagout, and control hazardous energy. Some don’t know how to take this step, which is different for every machine. Getty Images
Merely shutting off a machine’s power isn’t enough, says OSHA
Among people who use industrial equipment of any sort, lockout/tagout (LOTO) is nothing new. Nobody would dare perform any sort of routine maintenance or attempt a repair on a machine or system unless it were disconnected from its power source. This is just common sense and a requirement stipulated by the Occupational Safety and Health Administration (OSHA).
Before undertaking a maintenance task or a repair, disconnecting the machine from its power source—usually by switching off a circuit breaker—and locking the door of the circuit breaker panel is straightforward. Adding a tag that identifies the maintenance technician by name is likewise a simple matter.
If the power source can’t be locked, using only a tag is the alternative. In either case, with or without a lock, the tag indicates that maintenance is under way and the equipment is not to be energized.
However, this isn’t the end of LOTO. The overall goal is more than simply disconnecting power. The goal is to deplete or release all hazardous energy—in OSHA parlance, to control hazardous energy.
A common saw illustrates two temporary hazards. After a saw is shut off, the blade continues to run for several seconds, stopping only when the momentum stored in the motor is depleted. The blade stays hot for several minutes until the heat dissipates.
Just as the saw stores mechanical and thermal energy, the works that run industrial machines—electric, hydraulic, and pneumatic—usually can store energy for long periods of time. Depending on the sealing capability of a hydraulic or pneumatic system, or the capacitance of an electrical circuit, energy can be stored for surprisingly long periods of time.
Stored Hazardous Energy
Industrial machines of all sorts work with vast amounts of energy. A typical steel, AISI 1010, resists bending forces up to 45,000 PSI, so machines such as press brakes, stamping presses, punching machines, and tube and pipe benders have to deliver force by the ton. If the electrical circuit that feeds the hydraulic pump system is shut off and disconnected, the hydraulic portion of that system might still be capable of delivering 45,000 PSI. On a machine that uses dies or blades, that’s more than enough to crush or sever a limb.
A bucket truck that is shut down with the bucket in the air is just as dangerous as a bucket truck that isn’t shut down. Open the wrong valve and gravity takes over. Likewise a pneumatic system can retain substantial energy when shut off. A modest-sized tube bender can draw as much as 150 amps of current. As little as 0.040 amp can stop a heart.
Releasing or depleting the energy safely is the crucial step after shutting down the power and LOTO. Releasing or depleting hazardous energy safely is a matter of understanding system principles and the specifics of the machine that needs maintenance or repair.
Hydraulic systems come in two varieties: open loop and closed loop. In industrial settings, common pump types are gear, vane, and piston. The cylinder that runs the tool can be single-acting or double-acting. A hydraulic system can have any of three valve types—directional control, flow control, and pressure control—and each of these types comes in several varieties. This is a lot to be aware of, so a thorough understanding of every component variety is necessary to eliminate the risks associated with the energy.
“A hydraulic actuator might be driven by an all-ports-blocked valve,” said Jay Robinson, owner and president of RbSA Industrial. “A solenoid opens the valve and, while the system is running, hydraulic fluid flows at high pressure to the device and at low pressure to the tank,” he said. “If the system develops 2,000 PSI and the power is shut off, the solenoid goes to the center position and blocks all the ports. Oil can’t move and the machine is stopped, but the system can have up to 1,000 PSI on each side of the valve.”
In some cases, a technician who attempts to carry out routine maintenance or perform a repair is at immediate risk.
“Some companies have very generic written procedures,” Robinson said. “Many of them state that the technician should disconnect power, lock it out, tag it out, and then hit the START button to activate the machine.” In this state, the machine might not do anything—it doesn’t load a workpiece, bend, cut, form, discharge a workpiece, or anything else—because it can’t. The hydraulic valves are run by solenoids, and solenoids need electricity. Pressing the START button or using the control panel to activate any aspect of the hydraulic system does nothing to actuate unpowered solenoids.
Second, if the technician understands that he needs to relieve hydraulic pressure by operating a valve manually, he might relieve the pressure on one side of the system and think that he has released all of the energy. In reality, the other part of the system can still be charged with up to 1,000 PSI. If that pressure is on the tool side of the system, the technician is in for a big surprise and possibly an injury if he proceeds with a repair activity.
“If that pressure gets released suddenly, the tool will actuate,” Robinson said.
Hydraulic fluid doesn’t compress much—only about 0.5% per 1,000 PSI—but in this situation, that’s irrelevant.
“If the technician releases that energy on the actuator side, the system is probably going to move the tool over its full stroke,” Robinson said. “Depending on the system, that stroke could be 1/16 in. or it could be 16 ft.”
A machine that lifts a load overhead can equally dangerous.
“A hydraulic system is a force multiplier, so a system that develops 1,000 PSI can lift a load that is much heavier, say 3,000 pounds,” Robinson said. In this case, the hazard isn’t an accidental actuation. The risk follows releasing the pressure and unexpectedly lowering the load. It might sound like common sense to find a way to lower the load before working on the system, but OSHA fatality records indicate that common sense doesn’t always prevail in these situations. In OSHA incident 1428747.015, “an employee was replacing a leaking hydraulic hose on a … skid steer and disconnected the hydraulic line and the pressure was released. The boom arms dropped rapidly and struck the employee, crushing his head, torso, and arms. The employee was killed.”
In addition to reservoirs, pumps, valves, and actuators, some hydraulic tools have an accumulator. As the name implies, it accumulates hydraulic fluid. Its job is to regulate the system’s pressure or volume.
“An accumulator consists of two main components: a bladder inside a tank,” Robinson said. “The bladder is filled with nitrogen. During normal operations, hydraulic fluid enters and exits the tank as the system pressure increases and decreases.” Whether fluid is entering or exiting the tank, or if no transfer is taking place, is a matter of the pressure difference between the system and the bladder.
“The two types are shock accumulators and volume accumulators,” said Jack Weeks, founder of Fluid Power Learning. “A shock accumulator absorbs pressure spikes, whereas a volume accumulator prevents the system’s pressure from dropping when a sudden demand exceeds the pump’s capacity.”
To work on such a system without sustaining an injury, the maintenance technician has to be aware that the system has an accumulator and how to release its pressure.
In the case of a shock accumulator, a maintenance technician has to be especially wary. Because the bladder is charged to a pressure that is greater than the system pressure, a valve failure means that it could add pressure to the system. Also, they usually aren’t equipped with dump valves.
“There is no good solution to this problem because 99% of the systems don’t provide a way to verify a blocked valve,” Weeks said. However, a proactive maintenance program can provide a preventive measure. “You can add an aftermarket valve to bleed off some fluid anywhere pressure can develop,” he said.
A maintenance technician who notices that an accumulator bladder is low might be tempted to add air, but this is forbidden. The problem is that these bladders are outfitted with Schrader valves, which are the same valves as those used on car tires.
“The accumulator usually has a decal warning not to add air, but after a few years of operation, the decal is usually long gone,” Weeks said.
Another concern is the use of a counterbalance valve, Weeks said. On most valves, clockwise rotation increases pressure; on a counterbalance valve, it’s the opposite.
Finally, mobile equipment requires extra vigilance. Because of space limitations and obstructions, the designers have to be creative in how they lay out the systems and where they locate the components. Some of the components are likely to be hidden from view and hard to access, making routine maintenance and repairs more challenging than for stationary equipment.
A pneumatic system has nearly all of the potential hazards of a hydraulic system. A key difference is that a hydraulic system that springs a leak can create a jet of fluid with enough pressure per square inch to penetrate clothing and skin. In an industrial setting, “clothing” includes the sole of a work boot. Hydraulic fluid penetration injuries require medical attention and often hospitalization.
A pneumatic system is essentially as dangerous. Many people think, “Well, it’s only air” and don’t handle it carefully.
“People hear the pneumatic system’s pump running, but they don’t think about all the energy that the pump puts into the system,” Weeks said. “All that energy has to go somewhere, and fluid power systems are force multipliers. At 50 PSI, a cylinder with 10 sq. in. of surface area develops enough force to move a 500-lb. load.” Workers have been known to use such systems to blow debris off of their clothing.
“In many companies, that’s grounds for immediate termination,” Weeks said. The jet of air exiting a pneumatic system can peel off skin and other tissues right down to the bone, he said.
“If a pneumatic system is leaking, whether it’s at a coupling or through a pinhole in the hose, usually nobody notices,” he said. “Machinery is loud, workers have hearing protection, and nobody hears the leak.” Simply picking up a hose is risky. Leather gloves are necessary for handing pneumatic hoses, whether the system is running or not.
Another problem is that, because air is highly compressible, a pneumatic system that has been shut off can have enough energy stored up to run for quite some time, actuating a tool repeatedly, if a valve is opened on a charged system.
Volts, Amps, and Watts
Although the flow of current—the motion of electrons as they move through conductors—seems to be a world apart from physics, it’s not. Newton’s first law of motion applies: “An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction, unless acted upon by an unbalanced force.”
To this first point, every electrical circuit, no matter how simple, resists the flow of current. Resistance works against the flow of current, so when a circuit is off (at rest), resistance keeps the electrical circuit in a state of rest. When a circuit is switched on, current doesn’t flow through the circuit instantaneously; it takes at least a brief moment for the voltage to overcome the resistance and for current to flow.
By the same token, every circuit has some measure of capacitance, similar to the momentum of an object in motion. Shutting off a switch doesn’t bring the current to an immediate halt; the current stays in motion, at least briefly.
Some electrical circuits use capacitors to store power; this function is similar to that of a hydraulic accumulator. Depending on the rating of the capacitor, it can store electrical energy—hazardous amounts of electrical energy—for extended periods of time. A 20-minute discharge time isn’t out of the question for the electrical circuits used in industrial machinery, and some might require much more time than that.
For a tube and pipe bender, Robinson estimates at a 15-minute duration is probably enough time for the energy stored in the system to dissipate. This is to be followed by a simple check with a voltmeter.
“Attaching a voltmeter does two things,” Robinson said. “First, it lets the technician know if the system still has any power left. Second, it creates a discharge path. The current flows through the meter from one part of the circuit to the other, depleting any energy still stored in it.”
Under the best scenario, the technician is thoroughly trained, long on experience, and has access to all of the machine’s documentation. He has a lock, a tag, and a thorough understanding of the task at hand. Ideally he works with a safety spotter, providing an extra set of eyes watching for hazards and a way to summon medical assistance if something should still go wrong.
A worst-case scenario is a technician light on training and short on experience, working for an outside maintenance firm and therefore not familiar with the specific equipment, called in on a weekend or a night shift when the offices are locked and the equipment manuals are inaccessible. This is a perfect storm situation, and one that every company that has industrial equipment should go to great lengths to prevent.
Companies that develop, produce, and market safety equipment usually have a deep reservoir of industry-specific safety expertise, so a safety audit from an equipment vendor can help to make the workplace safer for routine maintenance tasks and repairs.