While most of us are aware that contamination in hydraulic oil is a major cause of failure in hydraulic systems that should be addressed, it is common for maintenance and reliability professionals to underestimate just how clean hydraulic fluid should be in order to achieve reductions in downtime and increases in asset life.
However, if you have set a good target, taking action to achieve it can be a learning process as well. Achieving your cleanliness targets requires controlling two main variables: how much contamination is getting into the hydraulic fluid over time and how much contamination is being removed over time.
Often, when we consider how to achieve and maintain a desired cleanliness level, we think first about oil filters. While filters certainly play a role in controlling contamination, it is more cost-efficient to exclude contaminants—that is, keep them from entering your hydraulic oil in the first place—than it is to remove those same contaminants through filtration once they have entered the oil. The cost of replacement filters, accelerated oil degradation, damage to machine surfaces and components or even unexpected valve failure can occur when we allow contaminants into our fluids, even if we think we are “filtering them out” relatively quickly.
Part of the reason that filtration does not solve the problem of contamination is the size of contaminants and how that impacts hydraulic system performance and asset life.
Small yet destructive particles
Contaminant particles in hydraulic systems are measured in microns. One micron is one-millionth of a meter. The clearances inside most pumps and valves are approximately 0.0004 inches.
How small is this? Consider that a grain of salt is 0.0039 inches or 100 microns. The lower visibility of the human eye is 40 microns (0.00158 inches).
A red blood cell is 0.0003 inches or 8 microns. A particle much smaller than what the human eye can see is capable of causing a hydraulic system failure.
Hydraulic filters should be selected to protect the most critical components in the hydraulic system. Today’s filters are assigned a beta rating to determine their efficiency.
The beta rating represents the number of particles that enter the filter relative to the number of particles that are flowing out. Even the best filters cannot capture everything. That is why proper breather selection is such an important first step.
Breathers for reservoirs
Whenever the oil level drops in a reservoir, atmospheric air will flow through the breather. Many hydraulic units contain an inexpensive breather that doubles as a fill cap, but oil should never be added to the system without being filtered.
Not only is it important to remove solid contaminants from the air but also to keep moisture out of the tank. Air contains water vapor, which can turn into liquid moisture once it cools down inside the tank.
A desiccant breather can be used to remove the moisture from the air before it enters the tank. Through the use of an adapter, this type of breather can be mounted on the same base as the existing, old-style breather.
The desiccant crystals will change color as the moisture is absorbed. Most desiccant breathers contain a 3-micron internal filter for removing solid particles from the air. Your selected breather should also have a visual dirt alarm to indicate the condition of the particulate filter.
Suction filters protect pumps
The purpose of a suction filter is to prevent large particles from entering the pump. This filter may be in the form of a strainer located underneath the fluid level. These strainers usually have a 74- or 149-micron rating.
Suction strainers should be removed from the reservoir at least twice per year and cleaned or changed. Often there will be a suction filter access flange located where the pump suction line enters the reservoir. This permits the removal of the strainer without draining all the oil from the tank.
Pressure line filters
As pumps operate, metal breakdown occurs. When the pressure exceeds 2,200 psi in a system with a fixed displacement pump, a pressure filter should be mounted in the pump outlet line.
This will filter the metal particles prior to being directed to the system. When a variable displacement pump is used at pressures higher than 1,500 psi, a filter should usually be installed in the pressure line.
Pressure filters are also commonly used immediately upstream of proportional valves and servo valves. This is due to the extremely tight clearances inside the valves. The majority of these filters are of the non-bypassing type.
The filter should be mounted as close as possible to the valve. It is imperative to change these filters on a regular basis to prevent collapsing of the element, resulting in a catastrophic failure of the valve.
Typical installation locations for common oil filter types.
Return line filters
Filters are often connected in the return lines of a system’s directional valves. This allows the oil that exhausts out of the cylinders and motors to be filtered before returning to the reservoir.
However, this type of filtration is only effective if at least 20 percent of the system volume is ported through the element in one minute. For example, with a pump volume of 100 gallons per minute (GPM), a minimum of 20 GPM should flow through the return filter.
A filter maintenance schedule can be established by initially sampling the oil for several weeks or months.
Case drain filters
Any oil that bypasses a variable displacement pump or an externally drained hydraulic motor will flow through the case drain line and into the tank.
The oil that bypasses will contain contaminants generated by the metal breakdown in the pump and motor. A small filter can be installed in the case drain line to remove the contaminants. Prior to installing these filters, the rating of the pump or motor shaft seal should be checked.
For most variable displacement pumps, the shaft seal rating is 10-15 psi. Hydraulic motor shaft seals usually have a higher rating (approximately 50 psi). An internal or external check valve should also be used to allow the oil to bypass when the element becomes contaminated.
Kidney-loop systems
A kidney-loop system consists of a separate pump and filter. Frequently, a heat exchanger is also connected in the loop. The pump constantly recirculates the oil in the reservoir through the filter and cooler.
These filters can be quite large. The reservoir volume should be turned over five to seven times in one minute by the recirculating pump. For example, if the reservoir holds 3,000 gallons, to filter the oil five times (15,000 gallons) in one minute, a 250 GPM pump will be required.
Separation systems
While kidney-loop systems can be larger than a conventional filter, there are other system types, such as oil reclamation or regeneration systems that can bring additional benefits and efficiencies. As more organizations focus on bringing circularity to their operations, the circular use of oil has become a larger topic of discussion and interest.
Oil regeneration systems like the RecondOil Box use depth filtration combined with a separation technology, allowing users to remove even the smallest nanoparticles that conventional filtration systems cannot capture. This returns hydraulic fluid to a “like new” state of cleanliness that allows it to be used again without negatively impacting performance. In fact, keeping in-service hydraulic fluid highly clean down to the nanoparticle level has shown benefits for seal life, asset life, and other associated efficiencies.
Conclusion
There is no “correct” number and type of filters that should be used in a given hydraulic system. Your resources, goals, and needs should all be considering in making the right choice, but by taking a thoughtful approach to both excluding and removing contaminants, it is possible to mitigate common causes of hydraulic system failure and reduce the total cost of ownership for critical hydraulic assets.