Is it safe to run a hydraulic system on low fluid temporarily?

The fact is that running the hydraulic system using low fluid levels is not secure, even if it is temporarily. Insufficient reservoir levels allow air to get into the pump's intake, leading to cavitation, overheating, and increased wear within minutes of operating. While a system isn't likely to fail immediately, every hour of operation that is lower than the minimum fluid level reduces the life of components and increases the likelihood of a sudden cylinder failure. If the fluid level is low, it is best to replenish the level prior to running the system instead of "run it a little longer."

Production pressure-stricken operators often are asked this question if an alarm that is low-level goes off mid-shift or if a view glass shows fluid close to the bottom of the line. The truth lies in the fact that hydraulics are less tolerant of low fluids than other fluids for machines, since the reservoir is more than just a place to keep oil in storage—the reservoir also cools and deaerates and degrades the presence of contaminants. Knowing what happens within the system when the fluid levels drop explains why "temporarily" is a riskier term than it seems.

What happens in the system when the system is running low on fluid?

A reservoir for hydraulics is designed to fulfill three functions simultaneously: it supplies fuel to the pump, lets the entrained air escape from the reservoir, and releases heat from the tank's walls. If the level of the reservoir falls below the minimum designed, the three functions are affected all at once.

Cavitation in the pump's intake

The suction line for the pump sits close to the bottom of the tank, and it is dependent on a minimum depth of fluid to remain submerged. When the level is near that suction line, it will begin drawing air in with oil. This causes cavitation, the formation and collapse of vapor bubbles within the pump. Cavitation results in a distinct sound that resembles marbles and physically damages the internals of the pump, pitting vanes, gears, or the piston's surface within the shortest time frame.

Aeration and foaming

Even before air gets to the suction line, the low level of fluid hinders the ability of the reservoir to allow air to escape from the returning oil. Return lines discharge turbulent, aerated fluids into the tank. The full reservoir allows the oil enough time and space to let air escape before it is redirected back to the pump. With less volume of fluid, air is less likely to separate from itself, which means it is recirculated throughout the system. The compressed oil can be compressed, which can result in delays or spongy response to actuators, as well as degrade the strength of the film that lubricates the oil.

The loss of heat dissipation

Reservoir volume is an important aspect of a system's heat balance. In a tank that is running low, there's less thermal mass and less area of the fluid that is exposed to cooling from the ambient, and thus the temperature of the oil rises quicker when it is under stress. Temperature increases accelerate the process of oxidation and break down the additive package and thin the oil below the intended viscosity level—thereby increasing the wear issues that are already caused by cavitation and aeration.

What is the reason "temporarily" doesn't reduce the risk as much?

It's tempting for one to believe that a quick run, for example, finishing the sequence, moving a heavy object away from the way, or waiting 20 minutes for the delivery of fluid is safe because the time of exposure is minimal. In reality, the damage mechanisms in this case don't decrease as time passes, like, for instance, the gradual wear of seals can.

The damage caused by cavitation is a consequence of the number of pump cycles that occur in air-conditioned conditions and not the number of hours that are in. A pump that is running at about a hundred RPM can record hundreds of thousands of cycles within one or two minutes of "temporary" operation. Pitting on piston shoes or vane tips may begin during that time, after which, once pitting has started, the process can increase in speed following that, as damaged surfaces can alter the film of oil further.

There's no method that can be trusted to ensure that the pump can "run gently" through a low-fluid situation. The reduction in flow or pressure does not eliminate air ingestion at the suction side, as that is governed by the level of fluid at the pump's inlet and not by the amount of force the pump is operating.

The risk in these situations is greater than the normal.

Certain operating conditions create a low-fluid runner to be more risky than a basic rule of thumb may suggest:

• A high ambient or already elevated oil temperature. Aeration and heat affect each other, and an oil run with low fluid levels during a hot shift or following prolonged heavy-duty cycling can be more dangerous than the same run in the cool, light-loaded system.
• Piston pumps, not vane or gear pumps. Piston pumps typically have more precise internal clearances and are more prone to damage from cavitation.
• Systems that do not have a low-level shutdown interlock. If the machine does not have an auto-cutoff feature and is dependent on the operator's observation of an indicator or sight glass or gauge, the system could be operating for a few minutes under the safe threshold before anyone reacts.
• Mobile equipment tilted upwards. The tilting can reveal the suction port, even if the level of fluid indicated appears good on a level surface and can result in the impression of a low-fluid state that the static gauge reading doesn't show.

What should you do instead of running out of fluid?

If a low-level warning is displayed, the most secure sequence is simple:

1. Stop the pump instead of ending the current cycle in the event that it's safe within the procedure.
2. Determine the reason for the low level—a real consumption gap due to the slow leak or a seal weep is not uncommon; however, an abrupt drop could indicate an issue with the hose, a fitting leak, or a blowout in the cylinder seal, which requires immediate attention, not just a quick top-off.
3. Finish with the appropriate fluid, using the appropriate viscosity grade and, most importantly, the brand or base oil type to prevent additive incompatibility.
4. Examine for discoloration or foaming in the reservoir prior to restarting the reservoir, as a low-level event can be a good signal to look at the condition of the fluid.
5. Normal operation can resume only after the level has returned to the specified range by the manufacturer on the dipstick or the sight glass.

A small amount of the appropriate fluid ready for this kind of situation rather than having to source it once the alarm goes off will eliminate the pressure of "just finish the job first."

The most important thing is the bottom line.

The operation of a hydraulic system with low fluid, even for a brief period, is exposing it to the risk of cavitation and the fluid to heat and aeration tension in a way that isn't scalable by reducing the run time. The most reliable and cost-effective solution to a low-level situation is to stop and top off the fluid, then take care of the root of the issue before proceeding—and not to consider the issue as something to be ignored until the present task is completed.

Can a pump be running low or dry before a problem develops?

The damage can start within seconds to minutes, as cavitation is linked to pumps, not to the amount of time that has passed. There's no standard safe duration, and the precise threshold is determined by the type of pump speed, type, and the level of fluid to be below that of the suction port.

Which are signs that indicate that equipment is short of fluid?

A common sign is an erupting sound or knocking emanating from the pump, delayed or spongy motion of the cylinder, rising temperatures of the oil, visible foaming in the reservoir's sight glass, and a decrease in system flow or pressure at the actuators.

Do low levels of fluid cause damage to other components besides pumps?

Yes. The overheated and emitted fluid increases wear on cylinders and valves and seals in the circuit because the weakening of the oil film and increased temperature affect every element that the fluid travels through, not only the pump.

Can I safely add a new type of hydraulic fluid during the event of an emergency?

It's generally accepted as a temporary emergency measure when the viscosity grade and base oil type are compatible, but mixing brands could impact the efficiency of additives in the long run. A complete fluid analysis or scheduled change in the fluid is worth planning for the following.

Can a low-level shutdown switch mean that it is safe to bypass fluid tests?

No. The shutdown interlock can be used as an insurance policy against the worst-case scenario, but not a replacement for regular check-ups on the level of fluid, as certain situations—such as tilting of equipment or an undiscovered leak—can result in a localized low-fluid leak at the pump's intake before a switch that regulates tank levels goes off.