Finding and Releasing Stored Energy in a Lockout

Lockout stops new energy from reaching a machine. It does nothing about the energy already inside it. A cylinder can hold 800 psi with the pump off. A press ram can hang in the air with the disconnect locked. A motor drive can hold a deadly charge after the power is gone. OSHA's lockout standard, 29 CFR 1910.147(d)(5), requires you to relieve, disconnect, restrain or otherwise make that energy safe before work starts, and to keep checking if it could build back up. This is for authorized employees who can already run a basic lockout and now work on machines where the hazard is hard to see. Your machine's written procedure and the maker's manual always come first.

What You Will Be Able to Do

• Find energy that a dead control panel or a single gauge does not show.
• Choose a safe order for blocking, bleeding and discharging.
• Work out how much force a cylinder can still produce.
• Recognize energy that is building back up during a job.
• Diagnose a failed verification instead of working around it.

Where Stored Energy Hides

Get to the Lowest Energy State First

The easiest energy to release is the energy you never leave in the machine. During shutdown, before you isolate, use the machine's own controls to put it in its safest position. Lower the ram onto its blocks. Retract the cylinders. Run the hopper empty. Let the flywheel stop. Your procedure should spell out this position.

The hard case is a machine that stopped where it should not have, such as a jammed press with the ram up or a lift table stuck halfway. You cannot lower it the normal way. Treat that as a new job, not a routine lockout. Stop and get your lead, because the method needs to be planned.

Block Before You Bleed

Bleeding pressure is not always the safe direction. Oil pressure is often what holds a load up. Air is often what holds a clamp shut or an air-applied brake on. Vacuum may be what holds a sheet of glass on a lifter. Release that energy first, and the machine moves.

Work in this order:

1. Ask what each energy source is doing right now. Is it pushing, holding, or doing nothing?
2. Support or restrain anything that would move when that energy goes away. Use the pin, prop or safety block your procedure names.
3. Release the energy at the bleed or discharge point your procedure names.
4. Confirm it is gone with a gauge, a meter or a look at the part, from a safe spot.

Warning: Stay out from under a load while you bleed. If a block is not bearing the way you expected, you want to find that out from outside the danger zone.

How Much Force Is Still There

You can work out the force behind a cylinder from two numbers. It is worth doing, because the answer changes how people treat the machine.

Force = pressure x piston area, and piston area = 0.785 x bore x bore.

Worked example: A press has a 4-inch cylinder that runs at 2,000 psi.

1. Area = 0.785 x 4 x 4 = 12.56 square inches.
2. Force = 12.56 x 2,000 = 25,120 pounds.
3. That is more than 12 tons, from a cylinder only 4 inches across.

An air cylinder of the same size at 90 psi gives 12.56 x 90 = about 1,130 pounds. That is less, but it is plenty to crush a hand.

You can also run the math backward to see what a stopped machine is still holding.

Worked example: A lift table weighs 6,000 pounds with its load, and it sits on a cylinder with a 3-inch bore.

1. Area = 0.785 x 3 x 3 = 7.07 square inches.
2. Pressure = 6,000 / 7.07 = about 850 psi.
3. The gauge at the pump reads zero, because a check valve keeps the oil in the cylinder. Those 850 psi are real, and they are trapped between the valve and the cylinder.

Hydraulic Systems

Trapped oil. Check valves, counterbalance valves and closed-center control valves all hold oil in a cylinder after the pump stops. A gauge only tells you about the part of the circuit it is connected to. Know where your gauge sits on the hydraulic schematic, and which sections it cannot see.

Accumulators. An accumulator is a tank that stores oil under pressure so that the system has reserve power. Many have an automatic dump valve that drains them at shutdown. Others use a manual valve. Dump valves can fail, so confirm the drop on a gauge that reads the accumulator itself.

Gas-charged accumulators hold a second energy source. A nitrogen charge sits behind the oil, on the other side of a bladder or piston. It stays there after the oil gauge reads zero. Draining the oil makes the system safe to work on. It does not make the accumulator safe to open. Removing or opening one is a separate job that follows the maker's method.

Warning: A fine jet of hydraulic oil can pass through skin. It may look like a small puncture, but it is a surgical emergency, so get to a hospital and tell them what happened. Never crack a fitting to bleed a line, and never feel for a leak with your hand. Use the bleed point in the procedure.

Compressed Air

Closing a valve stops new air. The line past the valve stays full. Use a lockable valve that vents the downstream side, then watch the gauge fall to zero.

Look for air that the vent cannot reach:

• Closed-center valves block both cylinder ports, so air stays locked in the cylinder.
• Receivers and surge tanks past the isolation point hold a large reserve.
• Pilot-operated check valves hold a cylinder in place on purpose.

Air is springy because it compresses. Trapped air can drive a cylinder hard the moment something shifts, so never assume a part will move slowly.

Gravity and Springs

Gravity cannot be shut off or locked. Anything that can fall, tip, roll or swing has to be lowered or physically held. That covers more than presses:

• Dump truck and trailer beds.
• Loader arms and raised forks.
• Scissor lifts and lift tables.
• Counterweights on doors and machine guards.
• Inclined conveyors with product on them, which can roll backward with the drive off.

Use the safety block, prop or pin the maker provides, or one the procedure specifies for that load. A block has to be strong enough for the load and sit where it cannot kick out. A jack is for lifting, not for holding a load while someone is under it.

Springs are harder, because you cannot bleed one. Some are safe as they sit. A spring-set brake pushes the brake on, so losing air or power leaves it applied. Others are the hazard, such as the spring in a dock leveler or the torsion spring on an overhead door. If you need to take a spring apart or release it, use the maker's tool and method. Those parts can throw hardware with enough force to kill.

Rotating Parts

A machine can keep turning long after the power is off. Wait for all coasting parts to stop, and look to confirm it. Saw blades, grinding wheels and flywheels often have no sound or vibration you can trust.

Parts can also start turning again with no power at all. Wind or a building draft can spin a large fan. A loaded conveyor can back-drive through its gearbox. Your procedure may call for a pin, strap or brake to hold them.

Electrical Energy

Opening the disconnect is only the start. Check for these:

• Drives and power supplies. A variable frequency drive changes AC to DC and stores it in capacitors. On a 480-volt supply, the DC bus sits near 480 x 1.41 = about 680 volts. It can stay dangerous long after the disconnect opens. The label on the drive states the wait time.
• Second sources. Control power from another panel, a UPS, or a generator can keep part of a machine live.
• Batteries. These cannot be turned off. Forklifts, floor scrubbers and backup systems all carry them.
• Solar panels. They make voltage whenever light hits them.

Follow the maker's discharge procedure, wait the stated time, then confirm zero. Under 29 CFR 1910.333(b)(2)(iv), testing for voltage is done by a qualified person, using test equipment suited to the circuit. A qualified person is someone trained on that equipment and its hazards. If that is not you, watch it done before you go near the equipment.

Heat, Cold and Pipe Contents

Steam at only 15 psig is about 250°F. Hot oil can run much higher. Liquid nitrogen is about -320°F. Closing a valve does not cool the pipe or empty it. Lock the valves, vent and drain the line, and give it time. Then check the temperature before you open anything.

A single closed valve can leak. Where the contents are hazardous, your procedure may call for one of these:

• Double block and bleed: two closed valves, with a bleed valve open between them. If the first valve leaks, the flow goes out the bleed instead of on to you.
• Blank or blind: a solid plate bolted into the line. A bolted blank counts as a lockout device under the standard.

Watch for two traps:

• Liquid sealed between two closed valves. As it warms, it expands. It can burst the pipe unless a vent is open.
• Low spots. Draining and purging do not clear every section of pipe.

The second one has killed people. In a 2001 steel mill incident in Indiana, investigated by the U.S. Chemical Safety Board, a crew purged a gas line before removing a valve. A 300-foot stretch of the line sat 3 feet lower than the rest and still held reactive liquid. Steam heated it, the pipe ruptured, and the fire killed two people and injured four. Drains in that low section could have removed the liquid. Walk the line and read the drawing, rather than trusting that the purge reached everything.

Energy That Comes Back

Stored energy is not always gone for good. A valve can leak by. A line can refill from a tank. A hot vessel can raise pressure in a sealed section. OSHA requires you to keep verifying isolation until the work is done, or until the risk of that buildup is gone. The standard calls this reaccumulation.

If it could happen on your job, the procedure should say:

1. What to watch, such as a gauge, an open bleed or a test instrument.
2. How often to check, and who is responsible.
3. What reading means stop.

Worked example: A pipefitter is working on a pump in a hot water system. The line is double blocked, and the gauge reads 0 psi at the start. An hour later it reads 15 psi. Nobody touched a valve, so a valve is leaking by. The right move is to stop work, get clear, and tell the person running the lockout. Bleeding it down and carrying on would hide the fault. The next surge could come while the pump is open.

When Verification Fails

A failed check is information. Stop, get clear, and find out why. Do not work around it.

Each one means the written procedure may be wrong as well. Report it so that the next person does not meet the same surprise.

Common Mistakes

Check Your Understanding

1. A hydraulic lift gate holds a 3,500-pound load on a 2.5-inch cylinder. What pressure is trapped in the cylinder?

Area = 0.785 x 2.5 x 2.5 = about 4.9 square inches. Pressure = 3,500 / 4.9 = about 714 psi. The gate is holding that pressure whatever the pump gauge shows.

2. A clamp uses air to hold a heavy casting in a fixture. You are about to bleed the air. What comes first?

Support the casting, because the air is what holds it. Then bleed.

3. A drive's label says to wait 15 minutes after power is removed. A coworker says 5 minutes is plenty, since the fan has stopped. What do you do?

Wait the full time on the label. The fan tells you nothing about the capacitors. A qualified person then confirms zero.

4. You open a line under a double block and bleed, and a thin stream starts from the bleed valve an hour into the job. What does it tell you?

The upstream valve is leaking, and the bleed is doing its job. Get clear and report it, because the isolation may need another blank or a further valve closed.

5. A large exhaust fan is locked out, but the blades start turning slowly when an outside door opens. Is that a problem?

Yes. Air movement can drive the fan with no electrical power. It needs a mechanical hold, such as a pin or strap, before anyone reaches in.

Key Takeaways

• A lock stops new energy. You still have to deal with what is inside the machine.
• Lower the machine to its lowest energy state during shutdown.
• Find out what each energy source is holding, then block before you bleed.
• Force is pressure times area, and the numbers are bigger than most people expect.
• Testing for voltage belongs to a qualified person, after the stated wait time.
• If energy could come back, keep checking until the work is done, and treat a failed check as a reason to stop.