Choosing the Right Oil Filter Location

Wes Cash, Noria Corporation
Tags: oil filters

Great emphasis has been placed on contamination control within lubricating systems. By limiting the amount of contaminants entering the lubricant, you can effectively extend the life of the equipment and the oil. Reducing the ingression rate is one part of achieving maximum machinery reliability; the other part is removing contaminants as quickly as they are introduced.

Perhaps the most common method of removing contaminants is utilizing filtration. Filters come in a variety of shapes and sizes and can be installed in multiple locations. A filter is the best line of defense to remove contaminants once they have entered the machine. With filtration systems, there are many options that must be considered to make sure the system is as efficient as possible.

One of the first considerations is the filter material. The construction material can make a difference in the filter’s ability to do its job well. For instance, fiberglass filters have more pores and thus generally have a higher dirt-holding capacity. Fiberglass also holds up to acidic environments better than traditional cellulose-type media. One of the benefits of cellulose is that it can absorb some water from the oil being filtered.


Filter location options

For each filter media decision, there are also filter elements that must be considered as well. Two common types of filter elements are surface-type and depth-type elements. With surface-type elements, the majority of particles are trapped on the filter’s surface. These are typically constructed of a single layer of material that oil flows through and that traps particles as they pass by.

As their name suggests, depth-type media have more depth than surface-type elements. In depth filters, oil flows in a tortuous path throughout the media, and particles are trapped throughout the depth of the filter. Since these filters require the oil to remain in contact with the media for a longer period of time, they also tend to have a higher differential pressure or pressure drop across the elements. Care must be taken when using these types of filters in a supply-line application, as they can reduce oil flow to lubricated components downstream and lead to equipment failures.

Both media and filter materials can be designed to work in a variety of applications and are regularly used in tandem with each other in different locations throughout a system. Depending on the system design and overall machine criticality, several filtration options must be considered to achieve the system’s target ISO cleanliness codes. The filter’s physical location in the lube system will make a difference in some of the decisions such as media type and construction material.

With full-flow filters or when filters are installed in the supply line, they offer direct protection to components downstream of the filtration system. Since these filters are in the path of the oil before it lubricates any components, they must be monitored for any signs of filter plugging or the filter going into a bypassed state. The term “bypass” is a reference to an internal valve that opens when pressure becomes too great. If the filter’s bypass valve is opened, the oil flows around the filter and continues through the system unfiltered. When installing full-flow filters, ensure you have differential pressure gauges to monitor the health of the filters and to confirm oil is still flowing through the system.

Return-line filters are common in most hydraulic systems. They are installed in the line after all lubricated components and before the main reservoir. While these filters don’t offer direct component protection like supply-line filters, they are often oversized, which can lead to longer filter life. Typically, these filters are installed in tandem, so if one becomes plugged, the line can be switched over to allow the plugged filter to be changed without disrupting lubricant flow. Aside from being oversized, return-line filters usually have high beta ratios. The beta ratio refers to the filter’s capture efficiency. The higher the beta ratio, the more efficient the filter is in capturing particles at the rated micron size.

Benefits of Off-line Filtration

Off-line filters are a relatively modern alternative or addition to conventional full-flow filters. These filters sit off the main operating system as a side loop from the reservoir. A necessary supplemental component to the off-line filter is a pump and motor. Because it does not depend on the hydraulic system, it can run independently, even when the main system is off. The following are a few additional benefits of off-line filtration:

  • Constant flow optimizes dirt-holding capacity and capture efficiency for a given type of filter
  • Easy to service “on the run” (filter changes, repairs, etc.)
  • Heat exchanges can be built in the loop
  • Sample ports can be installed for sampling on the run
  • Lowest cost to remove a gram of dirt (expensive pressure-line and surge-resistant filters are required)
  • Can double for an oil transfer system for adding makeup oil

Kidney-loop systems offer the best option for reaching total system cleanliness targets. Since they don’t require the system to be working in order for the filters to be active, they provide a more economical method of removing dirt. With kidney-loop systems, it is also easy to add additional lubricant conditioning and monitoring accessories, such as heat exchangers, temperature gauges, online water meters, etc. These accessories can help ensure that the health and life of the oil are monitored and controlled as much as possible.

Filter carts and other portable filtration devices can be used as a kidney-loop filtration system on most machines. These allow you to decontaminate the in-service lubricant as well as clean the oil as it is transferred from the drum into the system. An added benefit of portable systems is that they can be moved from one system to the next. Care should be taken to make certain that they are dedicated to a single lubricant to avoid any potential problems with cross-compatibility between different lubricant types.

To maintain and improve system cleanliness, a balance must be established to remove particles as quickly as they are ingressed. For this to be achieved, the correct filtration system must be employed. In most cases, utilizing multiple filter locations offers the best way to hit the cleanliness targets that the machine requires to operate at optimum reliability. By reducing the number of particles in the oil, you can extend the life of the machine and make your facility more profitable.


About the Author

Wes Cash is the director of technical services for Noria Corporation. He serves as a senior technical consultant for Lubrication Program Development projects and as a senior instructor for Noria's Oil Analysis I and Machinery Lubrication I and II training courses. He holds a Machine L...

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