What happens if hydraulic oil is overfilled?

The overfilling of a reservoir can cause foaming, aeration, and a rise in internal pressure because the fluid is not able to expand or degas. The air trapped in the reservoir compresses under load, resulting in an unstable or spongy cylinder as well as overheating, seal leaks at the reservoir's breather and shaft seals, and in extreme instances, the discharge of oil through the cap on the breather or dipstick. If left uncorrected, it can cause cavitation in the pump and reduce the life of the pump by the oxidation process.

It's a mistake that appears harmless. A little more oil isn't going to hurt, is it? In actuality, the hydraulic reservoirs are built with a precise headspace to serve an reason, and crossing the line of fill creates an array of issues which can be more destructive than running out of fuel. Knowing what happens inside the system will help you understand the reason "topping off just a bit more" is among the most common and preventable reasons for premature failure of hydraulics.

Why does reservoir headspace matter?

A hydraulic reservoir isn't simply a tank; it's an integral component of a system that is that is designed to perform three functions to store fluid, permit the air that's entrapped to rise from the reservoir, and allow the fluid space to expand when it warms up when it's in use.

The importance of air separation

If oil returns back to tank it's typically filled with microscopic air bubbles that have been that are triggered by motion of the cylinder rod as well as valve switching, or suction of the pump. The reservoir must have an open volume over the fluid line, so the bubbles may reach the surface and disappear before being returned to the pump's intake. The overfilling process eliminates that separation zone, which forces the oil to flow straight to flow back.

Allowance for thermal expansion

Hydraulic fluid expands when it gets warmer from operating temperature to ambient and on many systems, this could mean a increase of 40-60 degrees Fahrenheit (22-33degC) in a full time. Reservoirs are constructed with this expansion built-in. A tank that is overfilled has no place for the expanding volume to move, so pressure builds inside and pushes fluid out of the weakest part which is usually breathing cap the filler neck, or shaft seal.

What happens in the system that is overfilled?

Aeration and foaming

In the absence of a sufficient settling area, air bubbles stay suspended within the oil instead of bursting out. The aerated fluid then gets into the pump's intake, because air is compressible unlike oil, and the pump will end up creating gas pockets within the liquid. This causes spongy and irregular actuator movement the cylinders stall, jump or react in a different way to inputs, even if the valves and pump work properly.

Cavitation of the pump and erosion

A euphoric fluid that enters the pump's intake causes an event that is closely linked to cavitation. When air bubbles trapped in the pump's inlet collapsing violently under the pressure of high pressure within the pump's chamber, they produce localized shockwaves and heat that corrode and wear away the metallic surfaces on the pump's internal componentsvanes, gears or pistons, depending on the type of pump. The erosion process is gradual and irreparable. A pump that is running for a long time with excess aeration can experience wear and tear that would be equivalent to a longer life span in normal conditions.

Overheating

Air is not a good conductor of heat when compared to oil. When air entrained displaces some of the that is actually performing heating, the system's total capacity to dissipate heat decreases. Together with the internal churning and the compression of air pockets that are trapped Overfilled systems typically perform more than they should, and this in turn increases the oxidation process and also causes additive breakdown within the fluid.

Leakage from outside through sealings or breathers

Since the fluid expands as it absorbs heat and lacks headspace to absorb the growth, the pressure in the tank increases. Most reservoirs release this pressure via a breather cap that is rated to a certain pressure differential. Overfilling causes pressure to rise above the limit, forcing oil to escape through the breather cap gasket, filler cap, and dipstick tube. For pumps where the shaft seal is beneath the fluid line an extended overpressure may cause oil to flow past the seal lip of the shaft which can cause obvious leaks in the housing of the pump.

False readings of fluid levels and delayed detection

A system that's overfilled may conceal the fact it's losing oil elsewherean external leak that is slow could go unnoticed for a longer period of time since the reservoir was initially over the threshold. This can delay the identification of the root cause until the damage has already begun to show.

What can you tell if a system is overfilled?

• Foamy or milky-oil appearance when seen through a view glass, particularly shortly after the start of the program.
• Seepage of oil around the cap on the filler or breather in particular after the machine is operating and the fluid has heated up.
• Erratic or jerky cylinder extension/retraction, especially at startup before the system reaches operating temperature.
• Unusual pump sounds that sound like an unsettling whining sound that is consistent with cavitation.
• Fluid level that is clearly over what is indicated by the "full" or "max" mark on the dipstick or sight glass in the event of cold temperatures.

Correction of a reservoir that is overfilled

1. Shut down your system's temperature to drop down to an ambient temperature, so that the fluid volume is reflective of its actual state of rest rather than its expanded thermal state.
2. Make sure the fluid level is in line with the cold-fill specifications of the manufacturer and not just a generic "full" line, since the correct levels of fill differ based on the design of the reservoir as well as the configuration of the cylinder.
3. Remove excess fluid slowly via the drain valve in the tank until the level is within the mark specified, then capturing the oil that has been drained for appropriate disposal or reuse in the event that it's not out of spec.
4. Examine the cap of the breather and shaft seals for indications of oil residue or stress which could indicate that the overfill has had already caused leakage of a minor degree.
5. Perform an unintentional test and recheck the level when the fluid is at a normal operating temperatures, and confirm that there is still enough headspace the full temperature of thermal expansion.

Filling the right level the first time

The majority of reservoir sight glasses indicate the proper cold-fill level using an arrow or a rangeand filling it to the mark when the system is cold and cylinders pulled back (or extended, based on the intended design of the system) is the preferred benchmark. Since retracted or extended cylinder positions alter the amount of fluid that is moved from or into the reservoir, be sure to verify the documentation of your equipment for the correct conditions prior to filling up. If you are unsure, filling up until the midpoint of the sight glass using the cylinders in their standard "at rest" position is an acceptable option; however, the specifications of the manufacturer should always prevail.

Do you think that overfilling the hydraulic oil of your pump could harm your pump?

Yes. The overflowing reservoirs encourage aeration and the aerated oil that enters the pump's inlet can cause cavitation-like erosion of the internal components of the pump that cause wear and shorter lifespan of the pump.

What is too excessive when it comes to hydraulic oil levels?

Any level of fluid above the manufacturer's mark "full" or "max" line on the dipstick or sight glass and checked using the system at room temperature and a dipstick cold, is considered to be filled. A slight excess decreases the reservoir's capacity for air separation as well as capacity for thermal expansion.

Do hydraulic oils that are overfilled let out on their own?

Often, yes. As the oil heats and expands, but there is no headspace to support it, the pressure inside increases until it squirts the oil through the breather cap filler neck gasket as well as shaft seals.

Is it okay to take a small amount of oil out in the event that I've filled up the reservoir too much?

Yes, it is true that draining excess fluid via the drain valve of the tank until the level is at the cold-fill mark that is specified is the best solution. Always allow the system to cool down first so that the reading accurately reflects the liquid level at rest.

Does overfilling impact the life of the hydraulic fluid, and not only on the equipment?

Yes. The heat and aeration caused by a system that is overfilled increase the rate of oxidation and decrease the amount of additives within the fluid, reducing its service lifespan even if there is no hardware damage.