Why Filter?
What End Up in a Filter?
How Wear Attacks
Stopping Comtaminants
Filter Families
How Do You Tell a Filter from an Orange?
Filter Sizing Considerations
Filtration: How Much?
Filter Circuits
Conclusion
Industrial filtration touches the air we breathe and the water we drink. HVAC systems filter office and plant air. Municipalities filter tap water to remove bacteria and particulate matter.
Nearly all the products we consume have passed through an industrial filter. The gasoline our cars use is filtered. Chemicals and other raw materials are refined with filters. These raw materials make nearly everything in your home. The antibiotic you take to cure an infection has to pass through a filter during its production. Computer microchips could not even be produced without the water and air made pure with industrial filters.
So, why would you filter? The most common reasons:
- Protect equipment.
- Refine a product
- Reduce maintenance or extend the life of a product
- Reduce costs
- Protect patients
- Protect the public
- Increase product shelf life
- Safety
- Improve performance
Contaminants differ by industry. If you are filtering beer, you may want to remove diatomaceous earth fines, insoluble haze and yeast. If you are filtering lube oil, you may want to remove ferrous metals and other hard contaminants. What you want to filter out depends on your industry and fluid or gas.
The sources of contamination include:
- Dust and other airborne contaminants
- Bacteria
- Seals
- Hoses
- Pumps
- Valves
- Bearings
- Reservoirs
- Fluids
- Hydraulic motors
- Weld slag
- Component disassembly/assembly
- Casting Sand
- Poor Maintenance practices-shop rags
All of these are sources of contamination. Pumps, hoses and other system components have built-in contamination, and small and large particles inside these components end up in your fluid. As all of these components wear, they introduce more contamination to your system.
The environment around your system also adds to the contaminant load. Some plants generate dust and other contaminants. Your system ingests these contaminants through seals and breathers. And when you perform maintenance it is easy to introduce large amounts of contamination during the repair process.
Not all processes are concerned with wear-generated particles. These particles are usually a concern in closed-loop industrial applications such as lube oil. There are different types of wear: abrasive wear, adhesive wear, fatigue wear and corrosive wear.
Studies indicate that over 80 percent of system failures are caused by contaminants. Contamination levels can be reduced and managed by proper filtration. This can help you avoid system failures and costly downtime.
Abrasive Wear-When a contaminant is forced between two surfaces, it generates new contaminants. This happens when the particle is nearly the same size as the dynamic clearance of its path. Abrasive wear is the primary source of contamination in hydraulic oil streams.
Adhesive Wear- Metal-on-metal contact can cause the surfaces to cold-weld together; when this weld is broken contaminants are released. This type of wear occurs with bushings and journals. The adhesion is caused by high heat. The wear changes the metal's surface, and the new, rougher surface is more prone to adhesive wear.
Fatigue Wear-Over time, bearings and other equipment surfaces become stressed. Microscopic cracks will form, and eventually the load will cause particles to be generated.
Corrosive Wear-Water changes the chemical make-up of hydraulic oil. The result is metal fatigue as exposed system parts begin to corrode. Water increases the oxidation rate of hydraulic oil, causing it to become acidic. Acidic oil corrodes metal.
How does a filter stop a contaminant? There are three main ways a filter stops a contaminant:
- Inertial impaction
- Diffusional interception
- Direct interception
Inertial impaction-Particles traveling in a fluid have a mass and velocity. The fluid will follow the path of least resistance. Some particles will impact onto the filter medium and be caught due to their inertia driving them into the filter. This is not a primary mechanism for particle retention. It is more common in gas streams than fluid streams.
Diffusional interception-Extremely small particles will bounce around randomly in both fluid and gas streams as they strike liquid or gas molecules. This motion is called Brownian Motion and is more pronounced in gases. The random path of the particle increases the chance it will strike the filter and be captured.
Direct Interception-If the particle is larger than the pore of the filter, it is retained. For example, the screen on your screen door is mesh. It allows air to pass but keeps insects and anything larger than the mesh out. Filters work in the same way; however, the flow path is not necessarily straight. The pores can be infinitely smaller, and there can be layer after layer of media for the liquid/gas to pass through. Direct interception is the most common form of retention in both gas and liquid service. Most filters maximize their direct interception with torturous flow paths, which increase the filters retention capability.
There are hundreds of different types of filters. All of these filters fit into one of five filter families. These are commonly accepted industry classifications.
- Surface
- Depth
- Hybrids
- Crossflow
- Coalescers
Surface Filters-These filters work like screens. There is one layer of media. Particulate is captured on it. Surface filters generally have a low surface area and therefore a shorter life than other filter types. Most porous stainless steel filters are surface filters.
Pleated Surface Filters-The same flat media that surface filters are made from is pleated around a filter core. A wrap material or filter cage keeps the filter pleated. Nylon, ptfe, pvdf, and other surface-type membranes are commonly pleated.
Depth Filters-a depth filter is thick and captures particles throughout its media; it consists of either a fixed or random pore structure. Generally, depth filters have higher surface area and longer life than surface filters. Depth filters are constructed from different membrane types that offer a range of chemical compatibilities and pore sizes. Generally, these materials are melt-blown onto a filter core.
Pleated Depth Filter-A filter media can be pleated, increasing the surface area and increasing the amount of media a fluid has to pass through to get downstream. These types of filters blend the benefit of increased surface area and the thickness of a depth filter media.
Crossflow Filters-These filters are available in different formats and use high pump rates to create shear at the membrane surface. Particulate is washed from the surface and back into the filtration stream, which is re-circulated. This is called the retentate stream. Fluid bleeds through the membrane and continues downstream; this is often referred to as the permeate stream. In this type of filtering, either the retentate or the permeate may be the final product.
Coalescers-Coalescing separates water from oil or gas. A coalescer creates a separation by collecting small droplets in its pore structure and joining these droplets until the their weight pulls them away from the gas. The outside of the element sometimes is hydrophobic, helping the water droplets to run down the element and collect in the sump. The dry gas continues downstream.
We all know about comparing apples and oranges: They are two different things. Filters can also be difficult to compare for several reasons:
- Different filter manufacturers use different tests to rate their filters
- Different industries prefer different rating systems
- Filter ratings are different for liquid and gas service for the same filter
- Dirt capacity can be measured several ways and also depends on the dust used to run the test and the test's protocol
- Nominal versus absolute ratings
- And more
All filter rating systems start at the same place: the micrometer. The micrometer is commonly called micron (µm). A micron is 1/1000 of a millimeter or 0.00003937 inches. For reference, a human hair is approximately 70 µm. The human eye cannot see anything smaller than 40 µm. This is the unit by which filter pore size is measured. Filters are referred to by their pore size: 5 µm, 3 µm, 40 µm, etc.
Assigning a pore size or a rating to a filter can be confusing. Two common ways of making this assignment are called nominal and absolute. A third rating system is called beta ratio.
Nominal Rating- The National Fluid Power Authority defines it as "an arbitrary micron value assigned by the filter manufacturer, based upon removal of some percentage of all particles of a given size or larger. It is rarely well defined and not reproducible." The standard removal of a nominal rating filter is 98 percent of the micron size the filter is rated and larger. A 5 µm depth filter that is nominally rated removes 98 percent of particles 5 µm and larger. An issue with this system is each filter manufacturer uses a slightly different testing methodology and may use different test dusts. These differences affect the test results, making comparisons difficult. Additionally, nominal ratings for paper filters indicate 50 percent efficiency and not 98 percent.
Absolute Rating- The National Fluid Power Authority defines absolute rating as "the diameter of the largest hard spherical particle that will pass through a filter under specified test conditions." Dr. David Pall, founder of Pall Corporation, developed this test around 1955, and the SAE adopted it shortly after to rate rigid mesh filters.
This testing methodology does allow easier comparison of filters from different manufacturers; however, not all manufacturers perform absolute rating tests on their filters. In larger micron applications this is often the case.
Most high-end filtration applications use the absolute rating system. For example, the microelectronics, pharmaceutical and aerospace industries are familiar with absolute rated filters and absolute rating is the standard. Instead of hard spherical particles, bacteria are used during absolute rating tests for filters used in pharmaceutical and microelectronic plants. Some of these filters are rated at and below .02 µm.
Beta Ratio-This methodology was developed at Oklahoma State University. Beta ratio is used mainly with hydraulic and lube oil fluids. Beta rating is measured by conducting a multipass test. You derive a filter's beta ratio by measuring the number of contaminants of a certain micron size upstream of the filter and downstream of the filter. You divide the number of particles upstream by the number of particles downstream to determine the beta ratio. The beta ratio subscript is the micron size at which you are measuring. A B3 200 means the filter removes 99.5 percent of all particles 3 µm and larger.
Beta Rating Percent Removal
Bx75 - 98.7
Bx200 - 99.5
Bx300 - 99.7
Bx1000 - 99.9
To size a filter you have to consider many factors, including:
- Nature of fluid/gas
- Flow Rate
- Temperature
- Pressure Drop and Delta P
- Filter Surface Area
Nature of Fluid/Gas-Look at the chemical composition of the fluid or gas you're filtering. Filters are made from many different materials and are not chemically compatible with all fluids.
Flow Rate-Flow rate determines how quickly a fluid/gas travels through your system. It is usually measured in pounds per square inch (psi). Viscosity, the resistance to flow of a fluid, affects flow rate. In general, the higher the viscosity of a fluid, the more pumping it will require to reach a desired flow rate. The higher the viscosity of a fluid, the harder it is to push through a filter, and the greater the pressure drop.
Temperature-If a fluid or gas is extremely hot it can destroy the filter. Most filters have a maximum operating temperature. There are filters designed to operate at high temperatures. High temperature operation increases the rate of corrosion and can reduce the life of filters and other system components. When you choose a filter, make sure the filter media and seals can handle the maximum and minimum temperatures of your system.
Pressure Drop-The filter housing and filter itself create some resistance to the flow of the fluid/gas in your system. There is a pressure drop between the upstream and downstream sides of a filter: This is called Delta P (?P). When you size a new filter and housing, you want a Delta P that is as low as possible. This is because the life of your filter is related to its Delta P.
As the filter captures contaminants, the Delta P will rise. There is a point when the filter reaches what is called the "knee" of the curve (Flow versus ?P). At this point the filter's ?P begins to increase rapidly. This is when you should replace your filter.
Surface Area-In general, filters with greater surface area have longer life.
You can never make your filtration process too clean. However, you want to set targets that are realistic and meet or exceed industry practice.
A simple way to determine filtration is to filter just smaller than the contaminant you want to remove. If you need to remove particulate fines and more than 99.5 percent of them are 5 µm and larger, 5 µm filtration will work well.
A more complicated process scenario may require some experimentation and fluid analysis to determine the proper filtration level. Other processes like pharmaceutical demand ultra-fine filtration to remove or stop bacteria from contaminating the final pharmaceutical product. In these cases you know the size of the bacteria or organism that you want removed and filter below its size to ensure pure product.
Most lube and oil filter applications have cleanliness levels recommended by the International Standard Organization. The ISO protocol to determine an ISO code is ISO 4406. Manufacturers recommend cleanliness levels to protect their products and publish them in owner's manuals.
An ISO cleanliness level is a two-number rating such as 16/13. The first number is the cleanliness level at 5 µm and the second is 15 µm. A new code is under consideration by ISO. This new code will add a 2 µm cleanliness code number, making the code a three-number code such as 17/15/12. The lower the code numbers, the cleaner the target. A component that has a recommendation of 13/11 requires cleaner fluid than a component that needs 17/14. The chart below shows the correspondence between number of particles and the ISO rating number. The second chart shows general recommendations to protect standard fluid power components.
|
|
|
|
|
Servo Valve
|
<1000 psi (<69 bar) |
17/15/12 |
|
1000-3000 psi (69-207 bar) |
16/14/11 |
|
>3000 psi (>207 bar) |
15/13/10 |
|
Proportional Valve
|
<1000 psi (<69 bar) |
19/17/14 |
|
1000-3000 psi (69-207 bar) |
18/16/13 |
|
>3000 psi (>207 bar) |
17/15/12 |
|
Directional Control Valve |
<1000 psi (<69 bar) |
20/18/15 |
|
1000-3000 psi (69-207 bar) |
19/17/14 |
|
>3000 psi (>207 bar) |
18/16/13 |
|
Vane Pump |
<1000 psi (<69 bar) |
20/18/15 |
|
1000-3000 psi (69-207 bar) |
19/17/14 |
|
>3000 psi (>207 bar) |
18/16/13 |
|
Piston Pump |
<1000 psi (<69 bar) |
19/17/14 |
|
1000-3000 psi (69-207 bar) |
18/16/13 |
|
>3000 psi (>207 bar) |
17/15/12 |
|
Gear Pump |
<1000 psi (<69 bar) |
20/18/15 |
|
1000-3000 psi (69-207 bar) |
19/17/14 |
|
>3000 psi (>207 bar) |
18/16/13 |
|
New/Unused Fluid |
<1000 psi (<69 bar) |
20/18/15 |
|
1000-3000 psi (69-207 bar) |
20/18/15 |
|
>3000 psi (>207 bar) |
20/18/15 |
1
ISO Cleanliness Codes. The ISO cleanliness codes describe the number of particles of a certain size range per milliliter of fluid. The first number in the three number code describes the number of particles greater than 2 microns in size, the second number describes the number of particles greater than 5 microns in size, and the third number describes the nmber of particles greater than 15 microns in size. Note: the left number is always greater than the center number which is always greater than the right number. Refer to the
ISO Chart belowfor a description of the particle size ranges that correspond to the individual ISO 4406 Code Rating.
Pressure Filter-These filters are used to protect sensitive downstream components like servo valves. Pressure filters are installed downstream of the system pump before the component they protect.
Return Line Filters-These filters are usually just upstream of the reservoir. They remove contamination that has built up in the fluid before returning the fluid to the reservoir. It is important to size these filters to handle the largest surge flow rate that large rod cylinders and other components can create.
Kidney Loop Filtration-These filters are installed separately from the system. A dedicated pump constantly pulls fluid off the reservoir, runs the fluid through the filter and returns the fluid to the reservoir. This is an extra level of protection, and in some cases it is used as a polishing filter.
Stage Filtration-The fluid/gas is passed through several steps of filtration, from coarse to fine. This is also called a filter train. The simplest format of a staged system is a pre-filter upstream of a final filter. The pre-filter captures the larger particles, helping the finer, more expensive final filter last longer.
Vent Filtration-Tanks and reservoirs need to breath to relieve head pressure and stop the formation of a vacuum. Vent filters allow gas to escape while keeping out contaminants such as water and dirt.
Filters touch us every day by protecting and refining thousands of products. To properly select a filter, you first must have a basic understanding of filtration and an understanding of the type of filtration you require. This document is intended to give you an introduction to the basics of filtration. For more specific information or for assistance with a specific application, please e-mail us at info@efilter.com.
