OSHA instructs maintenance personnel to lock, tag, and control hazardous energy. Some people don’t know how to take this step, every machine is different. Getty Images
Among people who use any type of industrial equipment, lockout/tagout (LOTO) is nothing new. Unless the power is disconnected, no one dares to perform any form of routine maintenance or attempt to repair the machine or system. This is just a requirement of common sense and the Occupational Safety and Health Administration (OSHA).
Before performing maintenance tasks or repairs, it is simple to disconnect the machine from its power source-usually by turning off the circuit breaker-and lock the door of the circuit breaker panel. Adding a label that identifies maintenance technicians by name is also a simple matter.
If the power cannot be locked, only the label can be used. In either case, whether with or without a lock, the label indicates that maintenance is in progress and the device is not powered.
However, this is not the end of the lottery. The overall goal is not simply to disconnect the power source. The goal is to consume or release all hazardous energy-to use OSHA’s words, to control hazardous energy.
An ordinary saw illustrates two temporary dangers. After the saw is turned off, the saw blade will continue to run for a few seconds, and will only stop when the momentum stored in the motor is exhausted. The blade will remain hot for a few minutes until the heat dissipates.
Just like saws store mechanical and thermal energy, the work of running industrial machines (electric, hydraulic, and pneumatic) can usually store energy for a long time. Depending on the sealing ability of the hydraulic or pneumatic system, or the capacitance of the circuit, energy can be stored for an astonishing long time.
Various industrial machines need to consume a lot of energy. The typical steel AISI 1010 can withstand bending forces of up to 45,000 PSI, so machines such as press brakes, punches, punches, and pipe benders must transmit force in units of tons. If the circuit that powers the hydraulic pump system is closed and disconnected, the hydraulic part of the system may still be able to provide 45,000 PSI. On machines that use molds or blades, this is enough to crush or sever limbs.
A closed bucket truck with a bucket in the air is just as dangerous as an unclosed bucket truck. Open the wrong valve and gravity will take over. Similarly, the pneumatic system can retain a lot of energy when it is turned off. A medium-sized pipe bender can absorb up to 150 amperes of current. As low as 0.040 amps, the heart can stop beating.
Safely releasing or depleting energy is a key step after turning off the power and LOTO. The safe release or consumption of hazardous energy requires an understanding of the principles of the system and the details of the machine that needs to be maintained or repaired.
There are two types of hydraulic systems: open loop and closed loop. In an industrial environment, common pump types are gears, vanes, and pistons. The cylinder of the running tool can be single-acting or double-acting. Hydraulic systems can have any of three valve types-directional control, flow control, and pressure control-each of these types has multiple types. There are many things to pay attention to, so it is necessary to thoroughly understand each component type to eliminate energy-related risks.
Jay Robinson, owner and president of RbSA Industrial, said: “The hydraulic actuator may be driven by a full-port shut-off valve.” “The solenoid valve opens the valve. When the system is running, the hydraulic fluid flows to the equipment at high pressure and to the tank at low pressure,” he said. . “If the system produces 2,000 PSI and the power is turned off, the solenoid will go to the center position and block all ports. Oil can’t flow and the machine stops, but the system can have up to 1,000 PSI on each side of the valve.”
In some cases, technicians who try to perform routine maintenance or repairs are at direct risk.
“Some companies have very common written procedures,” Robinson said. “Many of them said that the technician should disconnect the power supply, lock it, mark it, and then press the START button to start the machine.” In this state, the machine may not do anything-it does not Loading the workpiece, bending, cutting, forming, unloading the workpiece or anything else-because it can’t. The hydraulic valve is driven by a solenoid valve, which requires electricity. Pressing the START button or using the control panel to activate any aspect of the hydraulic system will not activate the unpowered solenoid valve.
Second, if the technician understands that he needs to manually operate the valve to release the hydraulic pressure, he may release the pressure on one side of the system and think that he has released all the energy. In fact, other parts of the system can still withstand pressures up to 1,000 PSI. If this pressure appears on the tool end of the system, the technicians will be surprised if they continue to carry out maintenance activities and may even be injured.
Hydraulic oil does not compress too much—only about 0.5% per 1,000 PSI—but in this case, it doesn’t matter.
“If the technician releases energy on the actuator side, the system may move the tool throughout the stroke,” Robinson said. “Depending on the system, the stroke may be 1/16 inch or 16 feet.”
“The hydraulic system is a force multiplier, so a system that produces 1,000 PSI can lift heavier loads, such as 3,000 pounds,” Robinson said. In this case, the danger is not an accidental start. The risk is to release the pressure and accidentally lower the load. Finding a way to reduce the load before dealing with the system may sound common sense, but OSHA death records indicate that common sense does not always prevail in these situations. In OSHA Incident 142877.015, “An employee is replacing…slip the leaking hydraulic hose on the steering gear and disconnect the hydraulic line and release the pressure. The boom dropped quickly and hit the employee, crushing his Head, torso and arms. The employee was killed.”
In addition to oil tanks, pumps, valves and actuators, some hydraulic tools also have an accumulator. As the name suggests, it accumulates hydraulic oil. Its job is to adjust the pressure or volume of the system.
“The accumulator consists of two main components: the air bag inside the tank,” Robinson said. “The airbag is filled with nitrogen. During normal operation, hydraulic oil enters and exits the tank as the system pressure increases and decreases.” Whether fluid enters or leaves the tank, or whether it transfers, depends on the pressure difference between the system and the airbag .
“The two types are impact accumulators and volume accumulators,” said Jack Weeks, founder of Fluid Power Learning. “The shock accumulator absorbs pressure peaks, while the volume accumulator prevents the system pressure from dropping when the sudden demand exceeds the pump capacity.”
In order to work on such a system without injury, the maintenance technician must know that the system has an accumulator and how to release its pressure.
For shock absorbers, maintenance technicians must be especially careful. Because the air bag is inflated at a pressure greater than the system pressure, a valve failure means that it may add pressure to the system. In addition, they are usually not equipped with a drain valve.
“There is no good solution to this problem, because 99% of systems do not provide a way to verify valve clogging,” Weeks said. However, proactive maintenance programs can provide preventive measures. “You can add an after-sale valve to discharge some fluid wherever pressure may be generated,” he said.
A service technician who notices low accumulator airbags may want to add air, but this is prohibited. The problem is that these airbags are equipped with American-style valves, which are the same as those used on car tires.
“The accumulator usually has a decal to warn against adding air, but after several years of operation, the decal usually disappears long ago,” Wicks said.
Another issue is the use of counterbalance valves, Weeks said. On most valves, clockwise rotation increases pressure; on balance valves, the situation is the opposite.
Finally, mobile devices need to be extra vigilant. Due to space constraints and obstacles, designers must be creative in how to arrange the system and where to place components. Some components may be hidden out of sight and inaccessible, which makes routine maintenance and repairs more challenging than fixed equipment.
Pneumatic systems have almost all potential hazards of hydraulic systems. A key difference is that a hydraulic system can produce a leak, producing a jet of fluid with enough pressure per square inch to penetrate clothing and skin. In an industrial environment, “clothing” includes the soles of work boots. Hydraulic oil penetrating injuries require medical care and usually require hospitalization.
Pneumatic systems are also inherently dangerous. Many people think, “Well, it’s just air” and deal with it carelessly.
“People hear the pumps of the pneumatic system running, but they don’t consider all the energy the pump enters the system,” Weeks said. “All energy must flow somewhere, and a fluid power system is a force multiplier. At 50 PSI, a cylinder with a surface area of 10 square inches can generate enough force to move 500 pounds. Load.” As we all know, workers use this This system blows off the debris from the clothes.
“In many companies, this is a reason for immediate termination,” Weeks said. He said that the jet of air expelled from the pneumatic system can peel skin and other tissues to the bones.
“If there is a leak in the pneumatic system, whether it is at the joint or through a pinhole in the hose, no one will usually notice,” he said. “The machine is very loud, the workers have hearing protection, and no one hears the leak.” Simply picking up the hose is risky. Regardless of whether the system is running or not, leather gloves are required to handle pneumatic hoses.
Another problem is that because air is highly compressible, if you open the valve on a live system, the closed pneumatic system can store enough energy to run for a long period of time and start the tool repeatedly.
Although electric current—the movement of electrons as they move in a conductor—seems to be a different world from physics, it is not. Newton’s first law of motion applies: “A stationary object remains stationary, and a moving object keeps moving at the same speed and in the same direction, unless it is subjected to an unbalanced force.”
For the first point, every circuit, no matter how simple, will resist the flow of current. Resistance hinders the flow of current, so when the circuit is closed (static), the resistance keeps the circuit in a static state. When the circuit is turned on, current does not flow through the circuit instantaneously; it takes at least a short time for the voltage to overcome the resistance and the current to flow.
For the same reason, every circuit has a certain capacitance measurement, similar to the momentum of a moving object. Closing the switch does not immediately stop the current; the current keeps moving, at least briefly.
Some circuits use capacitors to store electricity; this function is similar to that of a hydraulic accumulator. According to the rated value of the capacitor, it can store electrical energy for a long time-dangerous electrical energy. For circuits used in industrial machinery, a discharge time of 20 minutes is not impossible, and some may require more time.
For the pipe bender, Robinson estimates that a duration of 15 minutes may be sufficient for the energy stored in the system to dissipate. Then perform a simple check with a voltmeter.
“There are two things about connecting a voltmeter,” Robinson said. “First, it lets the technician know if the system has power remaining. Second, it creates a discharge path. Current flows from one part of the circuit through the meter to another, depleting any energy still stored in it.”
In the best case, technicians are fully trained, experienced, and have access to all documents of the machine. He has a lock, a tag, and a thorough understanding of the task at hand. Ideally, he works with safety observers to provide an additional set of eyes to observe hazards and provide medical assistance when problems still occur.
The worst-case scenario is that the technicians lack training and experience, work in an external maintenance company, are therefore unfamiliar with specific equipment, lock the office on weekends or night shifts, and the equipment manuals are no longer accessible. This is a perfect storm situation, and every company with industrial equipment should do everything possible to prevent it.
Companies that develop, produce, and sell safety equipment usually have deep industry-specific safety expertise, so safety audits of equipment suppliers can help make the workplace safer for routine maintenance tasks and repairs.
Eric Lundin joined the editorial department of The Tube & Pipe Journal in 2000 as an associate editor. His main responsibilities include editing technical articles on tube production and manufacturing, as well as writing case studies and company profiles. Promoted to editor in 2007.
Before joining the magazine, he served in the U.S. Air Force for 5 years (1985-1990), and worked for a pipe, pipe, and duct elbow manufacturer for 6 years, first as a customer service representative and later as a technical writer (1994 -2000).
He studied at Northern Illinois University in DeKalb, Illinois, and received a bachelor’s degree in economics in 1994.
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Post time: Aug-30-2021