What an Accumulator Stores and Why It Survives a Lockout

Why this matters

A hydraulic accumulator holds usable energy with the machine switched off, the disconnect locked out and the pump not turning. Opening the electrical disconnect removes the source of new energy; it removes none of the energy already stored. That is why 29 CFR 1910.147 treats stored energy as a separate step rather than a consequence of isolation: at (d)(5)(i) the standard requires that after the energy isolating devices are locked out, all potentially hazardous stored or residual energy be relieved, disconnected, restrained or otherwise rendered safe. A machine with an accumulator is not de-energized because it is unplugged.

The number that makes this concrete is how far an actuator can still travel after everything is locked, and it is calculable in about four lines from three values you can read off the machine. On the example below it is thirteen inches of cylinder stroke at over twenty thousand pounds of force, with the power off. This card is that arithmetic and the health number behind it. The discharge procedure is its own document and is referenced at the end.

What is inside one

An accumulator is a pressure vessel divided into two volumes: a gas side charged with dry nitrogen only, and an oil side connected to the circuit. Compressing gas is what stores the energy; the oil is just the medium that carries it back out.

Never charge an accumulator with compressed air or oxygen. Air compressed rapidly against a film of hydraulic oil reaches ignition conditions, and an oxygen atmosphere in contact with hydrocarbon oil is worse. Nitrogen only, from a regulated cylinder with a charging assembly rated for the pressure, and the accumulator's own nameplate states the maximum.

Three constructions cover almost everything in the field:

• Bladder. A flexible bladder holds the gas inside a steel shell, with a poppet at the oil port that closes when the bladder fully expands. Fast responding, and the poppet is what protects the bladder from being extruded into the port. Nitrogen slowly permeates the bladder material, so precharge falls over time even with nothing wrong.
• Piston. A sliding piston with seals separates gas from oil. Handles larger volumes and higher compression ratios, tolerates being fully discharged, and can carry a position sensor. Seal friction means the gas and oil pressures differ slightly and the response is a little slower.
• Diaphragm. A small welded or threaded shell with an elastomer diaphragm. Small volumes, low cost, common for shock damping rather than energy storage.

What an accumulator is used for

Four jobs, and they call for different sizing:

• Supplementing pump flow for a short, fast movement the pump could not feed on its own, which lets a machine use a smaller pump.
• Holding pressure on a clamp or a brake while the pump unloads or shuts off, which removes exactly the standby heat the card on what a hydraulic system is trading away computes.
• Emergency function, deliberately keeping a brake or a stopping motion alive after a power loss. This one is the reason you cannot simply drain every accumulator you meet as a matter of policy.
• Absorbing shock or pulsation from a fast valve closure or a pump's own ripple.

Note the third against the second: on some machines the stored energy is a safety feature, and discharging it is exactly what you must not do until the machine is in a state where losing that function is acceptable.

Precharge is the health number

Precharge is the gas pressure with the oil side at zero. Everything the accumulator does follows from it, and it drifts down over a unit's life through permeation and valve seepage, so it is a maintenance reading, not a commissioning one.

The relationship worth carrying: for a slow, near-isothermal cycle, gas pressure times gas volume is constant, in absolute pressure, so the oil volume the accumulator holds at any system pressure is the shell volume minus the gas volume at that pressure.

Two consequences that point in opposite directions, and both matter:

• Below the minimum system pressure, usable volume rises in direct proportion to precharge. Lose 18 percent of precharge and you lose 18 percent of the oil the accumulator can deliver over the working band.
• Above the minimum system pressure, usable volume falls again, because the accumulator empties completely before the system reaches its minimum, and on a bladder unit the bladder then expands fully and strikes the poppet on every single cycle, which is the mechanism that cuts bladders.

Usable volume therefore peaks when precharge equals the minimum system pressure, and standard practice deliberately sets it somewhat below that, commonly in the region of 80 to 90 percent of minimum working pressure on a bladder unit, trading a little capacity for bladder life. The accumulator and machine builder documentation own the specified value; that band is the shape of the rule, not a substitute for their number.

Checking precharge: two corrections almost everyone skips

Correction one: the oil side must be at zero. With system pressure up, the bladder is compressed off the poppet and the gas pressure equals the oil pressure. A gauge on the gas valve then reads system pressure, and it will look like a perfect precharge on a completely flat accumulator. The reading is only a precharge reading after the oil side has been bled to zero and left open.

Correction two: temperature. The gas is at whatever temperature the machine is at, and at fixed volume, pressure tracks absolute temperature. A precharge read on a hot machine reads high against a specification written at a reference temperature. Use degrees Rankine, which is degrees F plus 460 for this purpose, and correct before you compare.

Worked example: a one gallon bladder unit on a clamp circuit

Shell volume 231 cubic inches, which is one gallon. System maximum 3,000 psig, system minimum 2,000 psig. Specified precharge 1,800 psig at a 70 F reference. The circuit feeds a 3 inch bore cylinder, 7.07 square inches of piston area.

Boyle's law is in absolute pressure, so every pressure below carries the atmospheric offset. At these pressures the 14.7 psi makes under one percent of difference, but it is the correct form and it is free.

What it holds when healthy.

• Precharge absolute: 1,800 plus 14.7 is 1,814.7 psia. Times 231 cubic inches is 419,200, the constant for this unit.
• Gas volume at 3,000 psig: 419,200 divided by 3,014.7 is 139.0 cubic inches. Oil in the shell: 231 minus 139.0 is 92.0 cubic inches.
• Gas volume at 2,000 psig: 419,200 divided by 2,014.7 is 208.1 cubic inches. Oil in the shell: 231 minus 208.1 is 22.9 cubic inches.
• Usable oil over the working band: 92.0 minus 22.9 is 69.1 cubic inches.

What it can still do with the machine locked out. The accumulator delivers oil until the bladder fully expands, which happens at precharge pressure, so the releasable volume is the 92.0 cubic inches it holds at 3,000 psig. That is an upper bound and it must be written as one: a slow bleed approaches it, while a fast dump cools the expanding gas, which lowers its volume at any given pressure and delivers less oil. So releasable volume is < 92.0 cubic inches, with no interval either side of it.

• Cylinder travel available: 92.0 divided by 7.07 square inches is 13.0 inches of stroke.
• Force behind it at the start: 3,000 psi times 7.07 square inches is 21,200 lbf.

Thirteen inches of travel at over ten tons, with the disconnect locked and tagged and nothing turning. That is the number to have in your head when someone says the machine is off.

Now the field reading. The gas valve is gauged with the oil side bled to zero and the bleed valve left open, on a machine that has been running, with the oil at 120 F. The gauge reads 1,620 psig.

• Correction one, already applied: this is a genuine precharge reading because the oil side is at zero. Had it been taken with system pressure up, the gauge would have read about 3,000 psig and told you nothing.
• Correction two, temperature: 1,620 plus 14.7 is 1,634.7 psia at 120 F. Corrected to the 70 F reference, that is 1,634.7 times (530 divided by 580), which is 1,493.6 psia, or 1,478.9 psig.
• Against the specified 1,800 psig, the unit is 321 psi low, which is 17.8 percent below specification.

What that costs, and the surprise in it. Because usable volume over the working band is proportional to precharge, it falls by the same 17.8 percent: 69.1 cubic inches becomes 56.8. Run the arithmetic straight through and it confirms itself, 1,493.6 divided by 1,814.7 is 0.823, and 56.8 divided by 69.1 is 0.823. The machine's fast stroke is now fed by four fifths of the oil it was designed for, which shows up as a slower or incomplete movement at the moment the accumulator is meant to help.

The surprise is on the hazard side. A low precharge leaves more room for oil at maximum pressure, so the shell now holds 231 minus (345,000 divided by 3,014.7), which is 116.6 cubic inches at 3,000 psig, against 92.0 when healthy. That is 16.5 inches of cylinder travel rather than 13.0. The unit's performance has fallen and its stored hazard has grown, and those two move in opposite directions on purpose, not by coincidence. Never read a weak accumulator as a safe one.

The failure mode. The version that hurts people is the technician who bleeds the visible pressure gauge on the manifold, sees zero, and starts work. The manifold gauge may be isolated from the accumulator by a check valve or a shuttle, in which case it reads zero while 92 cubic inches of oil sits behind it at 3,000 psi waiting for the first fitting anyone cracks. The version that costs money is the opposite: a machine that has slowed down over two years gets a new pump, a new valve and a new cylinder, and nobody gauges the accumulator, which has quietly permeated its way down to two thirds of its precharge.

What changes the reading

• A piston unit rather than a bladder unit. Seal friction means gas pressure and oil pressure differ by the amount it takes to move the piston, and a piston unit can be fully discharged without the poppet-strike damage mechanism, so the precharge target and the failure signature both change. The unit's own documentation governs.
• A fast cycle rather than a slow one. The isothermal arithmetic above is the slow case. A machine that discharges the accumulator in a fraction of a second is closer to adiabatic, and delivers measurably less oil than these figures predict. Size against the manufacturer's own curves for a fast duty; use the isothermal figure for the hazard, where it is the conservative direction.
• An accumulator that is part of a safety function. Where the stored energy runs an emergency brake or a controlled stop, discharging it is a hazard of its own, and the sequence has to be agreed against the machine builder's documentation before anything is bled.

References

• 29 CFR 1910.147, the control of hazardous energy, including the requirement at (d)(5)(i) that stored and residual energy be relieved, disconnected, restrained or otherwise rendered safe after the isolation devices are locked out
• Accumulator manufacturer documentation and nameplate for shell volume, maximum working pressure, specified precharge and its reference temperature, and the correct charging assembly
• Machine builder documentation for whether a given accumulator serves a safety function and what sequence its discharge requires
• See related: Discharging a Hydraulic Accumulator Before Any Work Begins; What a Hydraulic System Is Actually Trading Away; What Pressure and Flow Each Decide in a Hydraulic Circuit