How Often Should You Check or Recharge a Hydraulic Accumulator?

The majority of gas-charged hydraulic accumulators must be inspected for pressure before charging at least every three to six months, under normal operating conditions. This should be followed by an inspection of the entire system at least once per year. Safety-critical or high-cycle systems (mobile presses, equipment, and injection molding equipment) generally require periodic checks as do stationary systems that are not heavy-duty. may go as long as six to twelve months without checking. The loss of nitrogen that exceeds 10 percent from initial charge pressure requires immediate recharge, as accumulators that are not charged have less stored energy, are cycled faster than they were designed to, and are more susceptible to damage to the diaphragm or bladder.

The importance of pre-charge pressure on accumulators?

The accumulator is able to store the energy it needs by compressing nitrogen inside diaphragms, bladders, or pistons and then releasing hydrocarbon fluid located on the other side. The gas pre-charge tension—usually set to around 90 percent of the system's minimal working pressure—decides the accumulator's ability to be able to absorb and how much it can release as well as the way it behaves under fluctuating load.

If pre-charge drops, it can cause a number of problems. The volume of fluid that can be used by the accumulator decreases, meaning it will not buffer high pressures or sustain the flow of pump dead-time according to how it was designed. The diaphragm or bladder moves further each time by ramming into the poppet valve or shell more vigorously, which increases the wear and fatigue of elastomers. In accumulators made of pistons, too much travel could result in the piston being forced to top out several times, causing damage to seals and bore surfaces.

It's not a sign of a unit that's defective in the first place—some permeation of elastomer bladders can be normal and will happen in time. The aim of a check-and-recharge plan isn't to spot an issue, but to ensure that the accumulator is functioning within its intended pressure band prior to the performance or life of the component being impaired.

Typical nitrogen loss rates

Gas loss rates differ based on the type of accumulator used and the material of the bladder:

• Bladder-type accumulators usually lose about 1-2% of pre-charge pressure each month in normal conditions. This translates to approximately 10-15% per year.
• Diaphragm-type batteries generally retain pre-charge better because of the smaller surface area. Typically, they lose less than 10% per year.
• Piston-type accumulators depend on piston seals, not permeation through elastomer. They can store a charge for longer periods, up to a year, in the event that seals are maintained.

The figures are based on correctly identified nitrogen (not compressed air, which permeabilizes faster and increases the risk of oxidation) A gauge-and-valve assembly in good working order and operating temperatures within the manufacturer's range. The high temperatures of the ambient or fluid increase gas permeation and reduce the time between inspections.

Check intervals recommended by the application

High-cycle industrial equipment (Presses, injection molding, test rigs)

Review the pre-charge every month or in line with current preventive maintenance cycles. These systems run thousands of times each shift; therefore, even the smallest pre-charge drop can add up quickly into shorter bladder life and irregular cycles.

The mobile and off-highway machinery (Excavators, cranes and agricultural machinery)

Make sure to check each one to three months, particularly prior to the time of season when equipment has been in a stale position for a while. Temperature fluctuations between operation and storage cause pressure readings to fluctuate, and heavy routines put greater mechanical strain on poppet and bladder components.

Industrial stations (Standard hydraulic machines, power units)

Make sure to check it each 3 to six months in the course of your routine preventative maintenance. Connect this to your existing filters or oil sampling intervals to make it an integral part of the maintenance check rather than an individual task that is simple to forget.

Emergency backup and safety accumulators (Brake systems and emergency steering systems, shut-down circuits)

Make sure to check every month at a minimum, with certain regulatory or OEM requirements requiring regular confirmation. These accumulators could be in full charge for lengthy periods of time without cycling, which could disguise a gradual leak until the time that stored energy is required, making frequent verification not a requirement, but rather an option.

Pulsation-dampening or low-duty applications

Examine each six to twelve months to ensure that the system is operating at a low pressure, a high cycle rate, or a stable temperature. Even in this case, an annual check is best treated as a floor and not as a goal to extend further.

The accumulator is in need of recharging earlier than planned

Between the scheduled check-ins, keep an eye out for these indicators that indicate the pre-charge may have fallen below safe levels:

• The frequency of pump cycles is increased—The pump is activated more frequently to keep pressure in the system because the accumulator absorbs lower energy in each cycle.
• The fluctuation in pressure, also known as "chattering" in circuits, the accumulator is intended to smooth, like the dampening of pulsation on piston pumps.
• A slower response when using accumulator-assisted functions such as emergency steering or backup braking, which takes longer to achieve full pressure.
• The sound of metallic echoes coming from piston-type machines, which could mean that the piston is settling due to a low gas cushion.
• A visible fluid is seen at the gas valve on bladders or diaphragms, which is a sign of a ruptured bladder, rather than just a gas loss, and is a sign that the system needs to be shut down immediately and inspected.

All of these signs warrant an out-of-cycle inspection regardless of the location where the unit is on the schedule of maintenance.

How can you test the correct pre-charge pressure?

1. Separate the accumulator from the system hydraulic pressure by shutting off the valve. Ensure that the hydraulic side has been de-pressurized prior to starting work with the valve for gas.
2. Connect a calibrated charging and gauge unit to the valve—not a tire gauge, which isn't equipped with the precision and accuracy required.
3. Note the pressure reading and then compare it to the pre-charge specifications for the nameplate, making adjustments for ambient temperature variations from the initial charge temperature when the difference is significant.
4. Recharge using dry nitrogen only in the event that your reading is below the tolerance that is acceptable (commonly 10% as per the specification for the nameplate; however, OEM documentation should prevail over general advice).
5. Re-check the reading following charging and allow the temperature to be stable before taking a final confirmation reading. This is because gas temperatures rise by a small amount during charging. This can display an unusually higher pressure when it is cooled.

Always adhere to the charging method of the manufacturer of the accumulator and use only equipment that has been rated for nitrogen, as oxygen and compressed air create an inaccurate reading and, for oxygen, can pose a major fire hazard when combined with a hydraulic fluid that is under pressure.

The check should be built into an ongoing maintenance program

The most reliable maintenance programs do not rely on memory. The recording of each accumulator's ID, nameplate's pre-charge specifications, dates, check date, pressure, and recharge amount will create a line of trend across time. A device that is consistently losing 2 percent per month is operating normally. One that drops by 15% in two monthly checks may have developed an issue with the bladder or valve seat issue that should be investigated before it becomes a problem in the service.

Combining the accumulator inspection with a maintenance routine such as the oil test or a change to the filter or a springtime startup inspection -- makes it much more likely to happen according to schedule, instead of being the one task to be put off when there is a short time.

How can I determine what pressure my accumulator must have?

Look for the nameplate on the accumulator shell. It usually indicates the pre-charge pressure. You can also review the hydraulic schematic as well as OEM documentation. Pre-charge pressures are typically set to around 90 percent of the system's maximum operating pressure.

Can I make use of compressed air instead of nitrogen to charge an accumulator?

No. Compressed air is a source of oxygen, which can cause a combustible risk when it comes in contact with hydraulic fluids when heated and pressure is applied, as well as permeates into bladder materials faster than nitrogen, resulting in greater loss of pressure.

Does it happen if we continue to operate with an unsatisfactory pre-charge?

The accumulation system will run more often and absorb less energy per stroke and expose the bladder and piston to greater mechanical stress, leading to wear and increasing the likelihood of bladder rupture and piston seal rupture.

Do accumulator sizes determine how often I have to examine it?

The size of the accumulator has less effect on bladder surface area in relation to the volume and operating temperature. Smaller accumulators with proportionally higher elastomer area for each unit volume of gas will lose pre-charge more quickly than larger models similar to the type.

Does it make sense for a brand-new accumulator to experience a loss of pre-charge within the initial couple of months?

Yes. New bladders usually exhibit a slightly greater initial permeation rate when the elastomer is absorbed before stabilizing to a typical loss of 1-2% per month found in bladders in the long run.