Oil Filter Carts: A Technical Overview

Jonathan Trout, Noria Corporation

Oil Filter Carts: A Technical Overview

Oil filter carts are a portable way to filter new and used oil, take oil samples, and transfer oil to and from storage containers and machines. Below, we discuss how to select, set up and use an oil filter cart.  


What Is an Oil Filter Cart?

Oil filter carts are a portable way to filter multiple types of new and used oil, take oil samples, and transfer oil to and from storage containers and machines. Oil filter carts are now considered a necessity when it comes to having an effective lubrication program thanks to their many benefits and uses. Filter carts can be customized to fit any plant or manufacturer's specifications and uses. Most filter carts include an oil sampling port with a bypass valve (discussed below) to let lubricants circulate or be transferred without being filtered. A reliable filter cart also includes a warning when the unit is saturated or needs to be serviced. This can hlep avoid filter failures, which can be a major problem if contaminants make their way past the filter and into critical machines.

Ways You Can Use a Filter Cart

  • Transferring lubricants to storage containers
  • Transferring filtered oil to a machine
  • Cleaning stored lubricants
  • Achieving new oil cleanliness standards
  • Reconditioning or decontaminating lubricants currently in use
  • Draining used oil from equipment
  • Providing contamination control functions like hose cleaning, power flushing, directional wand flushing and line flushing

Oil filter carts might be overlooked when it comes to new oil, but they offer many benefits when handling and storing it. Filtering new oil is always considered best practice, especially when the oil has been stored for over a year or has been stored outside.

Overall, filter carts are designed to be multifunctional, helping you easily transfer multiple types of oil and decontaminate new and used oil to meet viscosity and ISO cleanliness standards.


Selecting an Oil Filter Cart

The benefits of using an oil filter cart are numerous, but if you don't select the right one, you'll negate all of them and end up costing yourself money, time and manpower. It's important to consider a few things before you begin shopping for an oil filter cart.

  1. Target cleanliness: The most important part of any filter or filter unit is which contaminants are being filtered out. Know what your oil cleanliness targets are, which pieces of equipment you'll be using with the filter cart and to which types of contamination the equipment has historically been exposed. A good rule of thumb is for your finest filter to be around 3 microns for hydraulic fluid and around 6 microns for gear oils.

    Cleanliness codes Most modern oil filter carts use two filters, one for coarse particulate and one for fine particulate. This saves the lifespan of the fine particulate filter.

    ISO 4406 is the cleanliness code rating standard. This standard looks at particle count micron levels of those greater than 4, 6, 10, 14, 21, 38, 70 and 100 microns. ISO 4406:99 is the standard for fluid cleanliness. For this standard, a code number is assigned to three micron levels: greater than 4, 6 and 14 microns.

    While ISO codes show you whether the oil has reached target values, it doesn't do much in the way of showing trends. Therefore, ISO standards are great for setting key performance indicators (KPIs), but they should not be the primary measurement for evaluating used oil. All filter cart pumps have a minimum and maximum viscosity range, so be sure you specify the type of oil you'll be using with your filter cart. As you can see from the general target cleanliness level guidelines below, as viscosity increases, cleanliness levels decrease.

  2. Types of oil: The type of oil inside your machine plays a significant role in determining which oil filter cart you should buy. Oil filter cart pumps are designed for different viscosity levels. For example, if you try to pump viscous oil quickly, you'll likely encounter air bubbles forming inside the machine which damages the pump and oxidizes the oil. It's important to know the ISO VG (viscosity grade) oil rating for each type of oil you'll be filtering through a filter cart. Most plants use multiple oil filter carts for various applications due to the different types of oil they use.
  3. Flow rate: If you had to choose the most important aspect to consider before buying a filter cart, knowing the optimal flow rate would be it. It all comes back to money. More powerful oil filter carts cost more, so buying one with more power than you need means you're overspending. If your filter cart's flow rate is too low, you won't clean your oil enough in the allotted time, which could lead to machine damage.

    You should be able to pump up to 10 percent of your machine's reservoir volume per minute. You can also calculate flow rate by multiplying the oil volume by the number of times you want it to pass through the filter and then dividing that number by the number of available minutes or seconds. Most suppliers have standard filter carts with a flow rate of 5, 10 and 15 gallons per minute (GPM). Filter carts for hydraulic fluids with an ISO VG of less than 68 usually have a flow rate of 2, 5, 10 or 20 GPM.

  4. Intended use: As mentioned earlier, routine filtration isn't the only application for modern oil filter carts. You can employ them for oil sampling, transferring oils and more. Many of these alternate uses might require a bit of customization to the filter cart. Oil sampling from a filter cart, for example, requires installation of a bypass valve so you can bypass the filters and get a true representation of the oil inside the machine.
  5. Place of use: While oil filter carts are portable for most applications, you might need to use them on an elevated machine or one located on extremely rough terrain. In these cases, you should consider the weight of the filter cart or choose one that can be carried. Consider the power source as well. Most oil filter carts are electric, but some use an air-driven motor, which is good for areas with high levels of dust.

How to Properly Set up an Oil Filter Cart

As we touched on earlier, oil filter carts are used for multiple functions and are customizable to meet these various applications. When setting up your filter cart for use, consider these important factors:

  • Filter selection and run time: If you've ever changed the air filter in the return duct of your home, you know there are numerous air filter types and sizes, each with a different filter performance rating (FPR) and minimum efficiency reporting value (MERV). These ratings are intended to show the size of particles the filter will filter out. What many people don't realize is that if you have an older heating, ventilation and air conditioning (HVAC) unit, your system might struggle to pass air through a fine particulate filter. Similarly, filters for oil filter carts come in multiple sizes, and even those with the highest efficiency rating may be too fine to decontaminate your lubricant properly.

    Filters are made so the fluid being passed through them is stripped of particles greater than the pore size of the filter material. During this process, however, a pressure differential is created due to the filter material's pore size and surface area, the fluid's viscosity and the number of contaminants already captured in the filter.

    filter cart

    Oil filter carts generally have two types of filters – one is a coarser filter and the other is finer. To select the proper filter for your oil filter cart, you need to choose one based on the viscosity and temperature rating of the oil you'll be filtering. Even small temperature fluctuations can have a big impact on viscosity. For example, if a gear oil with a viscosity of 400 centistokes (cSt) at 100 degrees F is filtered at 68 degrees F, the viscosity nearly doubles.

  • Manual filter bypass: A bypass valve installed on your oil filter cart has multiple uses. The first use directly relates to the pressure differential caused by the oil passing through the filter. If the pressure is too high, the bypass valve will open, allowing the oil to bypass the filter. This is why selecting the right filter is important. Additionally, installing a bypass filter on your oil filter cart enables you to take oil samples from your machinery for testing. Manually turning on the bypass valve lets your oil sample bypass the filters, giving you a true representation of the oil in your system.
  • Quick connects: Ensuring your oil filter cart is set up with quick connects instead of drum wands greatly helps with controlling contamination. Drum wands are difficult to keep clean and require opening the machine to use, allowing airborne contaminants to enter the machine. Using quick connects lets you keep the machine closed during filtration. You can install different types of quick connects on multiple filter carts, making sure each filter cart is being used for the specific type of oil for which it was intended. Using quick connects of different types, sizes or colors makes it easy to keep cross-contamination at bay.
  • Oil sampling valve: One or more oil sampling valves should be installed for convenient oil testing.
  • Identifying and warning indicator: Visual indicators like differential pressure gauges tell you when to change the filters. Additionally, it's a good idea to get a filter cart with a digital, light-emitting diode (LED) warning indicator for low, medium and high contamination levels. Many digital early warning indicators can sync up to self-diagnostic software to display any indication of particle detection. This software is typically an optional feature.

How to Properly Use an Oil Filter Cart

Filtering or transferring oil using an oil filter cart allows you to perform the process while the machine or equipment is still running. One of the main purposes of utilizing an oil filter cart is to control contamination. When you use a filter cart, oil cleanliness is diluted by the dirty oil in the sump, since you're taking oil from the dirty sump, filtering it and returning it to the dirty sump. To combat this dilution, the amount of oil in the tank must pass through the filter approximately seven times to get the same cleanliness you would get from a single-pass filtration system. The filtration time calculation looks like this:

Filtration time = (Oil Volume / Pump Capacity) x 7

cleanliness levels

For example, if you have a 40-gallon tank and a filter cart that pumps 5 gallons per minute, you need to run the filter cart for 56 minutes to equal a single-pass filtration (40 gallons / 5 GPM) x 7. Below are common time requirements to achieve certain cleanliness levels (pre-calculated).

Oil filter carts use what's called offline filtration, so machinery can remain in operation while you filter the oil or take a sample. Offline filtration is a more modern alternative to full-flow filtration. In addition to being able to filter and sample oil while the machine is still running, you can use kidney-loop filtration if the machine is down (hydraulic systems). Below are common ways to perform offline filtration.

  • Kidney-loop filtration: Kidney-loop filtration is the best method of offline filtering and can be done using a low- or high-pressure filter cart system. Kidney-loop filtration for low-pressure systems (15-150 pounds per square inch) and high-pressure systems (50-3,000 pounds per square inch) works similarly in that a motor, pump and filters form an independent unit that can run intermittently or continuously. You'll want a kidney-loop system with two filters to be effective (one for large particulate and one for fine particulate).

    As previously mentioned, flow rates vary widely, and your machine's flow rate should be determined to select the right filter. For high-pressure systems, it's a good idea to install a flow-control valve (0.5 to 2 GPM) to reduce the pressure before the oil flows into the filter. Offline kidney-loop filters offer a lot of advantages such as:

    • Continuous flow through the filter
    • Low pressure lets you use finer filters at affordable costs
    • Machinery doesn't need to be shut down to change the filter
    • Flexibility
    • Can adapt to new oil and oil entering or leaving the reservoir
    • Lengthens pressure- and return-line filters
  • Offline filtration of high- and low-viscosity lubricants: When filtering high- and low-viscosity oils through a filter cart, make sure you know the target cleanliness level for the specific type of equipment on which you're working. As a guide, a 10-micron filter is capable of producing an ISO 17/15/12 oil cleanliness level. If your optimum cleanliness level is less than this, you can use a lower micron filter. Once you know your target level:
    • Make sure you're using a dedicated oil filter cart for the type of lubricant you're filtering. This prevents cross-contamination.
    • Ensure you have pressure-venting valves installed for high-pressure systems.
    • Confirm that the filter cart has bypass loops installed.
    • Install different types of quick connects on all filter carts to ensure each filter cart is only being used with the oils for which they are specified.
    • Put the matching ends of all quick connects on the appropriate equipment where they will be used. Using quick connects of different types and sizes makes it impossible to connect the filter cart to the wrong equipment.
    • Ensure you're using the right filter based on the viscosity and temperature. A high-viscosity oil like an ISO VG 220 or greater requires a low flow rate to avoid high differential pressures. It's also important to know how the operating and ambient temperature can affect viscosity. If the machine is located outdoors, assume higher viscosity in cold winter months.
    • It's always good to keep in mind that differential pressure can be halved by doubling the length of the filter element or putting two elements next to each other. You can use a 3-micron filter with high-viscosity oils if the temperature allows and your target cleanliness level requires it.

    Most oil filter cart manufacturers offer filter carts specific to high- and low-viscosity oils. This makes it easy to keep your oil filter cart setup straight. For example, a low-viscosity oil filter cart will already have the correct filter, valves and pressure settings installed for use on machines with low-viscosity oil.

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