Part I – Oil Cleanliness: The Key To Equipment Reliability

EP Editorial Staff | September 1, 2007

In today’s highly competitive global economy, equipment reliability is more critical than ever. Fluid cleanliness is key to that reliability and, ultimately, uptime. As a result, companies that recognize the importance of fluid cleanliness are more capable of delivering productivity and profits than those that ignore this issue.

Over 70% of equipment failures can be attributed to contamination. The best course of action is to minimize the introduction of contaminants. It is estimated that the cost to remove contaminants is 5 to 10 times the cost to keep them out in the first place. Thus, any World-Class lubrication program begins with good storage and handling practices and the minimization of ingressed contaminants from the environment through effective seals, desiccant breathers and other practices.

ISO Cleanliness Code 
Before cleanliness standards can be established, one has to understand how cleanliness is measured. Most common methods today measure amount and size of particles with an optical particle counter. As fluids move past a laser light, particles in the path block the light and create a shadow that is measured by a photo sensor. The sensor, which has been calibrated with a test dust, reports the number of particles by size per ml.

The three major contaminants affecting equipment are particles, water and air. This series of articles will address the area of solid contaminants and focus on proper cleanliness levels required by equipment type and proper filtration practices to achieve these targets. This month, Part I discusses cleanliness codes and basic filtration principles. Subsequent installments will cover the setting of cleanliness targets and the best way to achieve these targets, as well as proper filtration techniques and their effect on equipment reliability. Most people equate cleanliness with hydraulic systems. While it is true that hydraulics require clean oil to be effective, many other applications also require clean oil, including gearboxes, turbines, paper machine oils, rolling element bearings, etc. This series will include case histories of different components in different industries.

Dark fluids and water contamination will not give good results with an optical particle counter. With these fluids, methods such as direct counting of particles on a patch through a microscope are used. Another method for counting particles in solutions and dark liquids is pore blockage, which equates particles and size by flow decay through a sensor screen of certain size pores (like 10 micron, for example). This technique will give different results than an optical particle counter, but it can be used on certain fluids as a good trending device. (Note: Cleanliness standards discussed in this article will focus on optical particle count numbers.)

0907_contaminant_concerns_img1Prior to 2000, optical particle counters were calibrated with AC Fine Test Dust (ACFTD). Although a new calibration technique with a Medium Test Dust (MTD) that was traceable by National Institute of Standards and Technology (NIST) was established and approved in December 1999, it gave a major difference in the calibration. There is a signifi- cant difference between the two calibrations in particle size distribution as measured by an electron microscope. For example, there were significantly more particles below 10 micron with the NIST calibration versus ACFTD. In order to keep the same ISO Cleanliness Table, measured particle sizes were adjusted to reflect this difference. Previously the size ranges reported by ACFTD were ≥ 2μm, ≥ 5μm and ≥ 15μm. The new method reports ≥ 4μm[c], ≥ 6μm[c] and ≥ 14μm[c]. The letter “c” after the code indicates that the calibration was based on the NIST method. Today, most oil analysis laboratories have converted to the NIST method and use the three number designations.

Equipment cleanliness standards are established by use of ISO 4406 illustrated in Table I.

The ISO cleanliness code is reflected as a three-number designation: = 4µm[c], = 6µm[c] and = 14µm[c]. Notice that for every increase of one ISO range number the number of particles doubles. This is very significant since very small increases in the ISO range particle number can result in very large increases in the actual number of particles. Remembering one range number such as 11 (which is 10 to 20 particles) allows you to construct a table by doubling the numbers for every increase in range number. The following example on how to convert particle sizes and amounts to the three-number cleanliness designation is shown in Table II. Assume the following particle sizes and amounts were measured with an optical counter.

Let’s look at an example of how much dirt can pass through a system. Consider a fluid being pumped at 65 gpm that has an ISO cleanliness code of 22/21/18 (which is typical of new unfiltered hydraulic oil). In one year, 8800 lbs of dirt would pass through this pumping system. How long do you think a pump would last in that environment? If the fluid is cleaned to a 16/14/11 (which is the typical fluid cleanliness required in a hydraulic system), only 9 lbs of dirt would pass through the pump in one year. A six ISO code change resulted in a 1000-fold increase in particulate contamination. From this example, we can clearly see that even small changes in the ISO cleanliness rating results in large change in particulate contaminants.


Filtration basics
Once the ISO cleanliness number has been established for a particular equipment type, the fluid needs to be cleaned to achieve that target through filtration. As noted in the opening sidebar, subsequent articles in this series will discuss how to set the cleanliness targets and filtration systems to achieve these targets. In the remainder of this article, however, we will be introducing basic filtration principles.

Fig. 1 illustrates the two major filter categories—surface filters and depth filters.

Surface filters are not particularly effective in systems with low-solid and large contaminants. They are usually made of woven wire or pleated paper with a consistent pore size that provides the fluid with a straight path.

Depth filters make it more difficult for a particle to pass through, thus they provides better filtration than surface filters. Depth filters incorporate cellulose, metal or glass fibers that are stacked to provide media height. Both glass and metal can have a graded (tapered) density in pore size to provide greater and more effective filter utilization. The use of finer fibers has resulted in major advances in filtration technology. Each fiber type provides different performance characteristics.

Filters can be rated either “Nominal” or “Absolute.”

  • The Nominal rating is normally used with paper filters. It is an arbitrary rating assigned by the manufacturer as to the largest particle that will pass through the filter (for example, a 10-micron nominal filter). These filters typically will only remove 50% of the particles in their size range. Since this rating is not based on actual laboratory data, it is not very useful in establishing equipment cleanliness standards.
  • The Absolute rating of a filter means that laboratory data was provided in the filter rating through the ISO 16889 Multi-Pass Filter Test shown in Fig 2. This test is used in filter development to measure the performance properties of different filters under laboratory conditions. It is used to calculate the Beta Ratio (as illustrated by Fig. 3) as follows:

0907_contaminant_concerns_img3Assume we are evaluating the filter in Fig. 3 on its ability to remove particles >10 microns and 400 particles in this size range enter the filter and two particles >10 microns pass through it.

The ISO 16889 Multi-Pass Test is conducted as follows:

  • Optical particle counters are installed both upstream and downstream of the filter to measure the number of certain sized particles entering and passing through the test filter. Circle 72 or visit Fig. 2. The ISO 16889 Multi-Pass Filter Test (Source: HY-PRO Filtration)
  • NIST test dust is injected into a circulating fluid at an average rate of 3mg/l to 10mg/ml. Rates are varied by different filter manufacturers. A low-viscosity test fluid is circulated at 15-30 gpm.
  • All particles and their sizes are measured before and after the filter. Flow continues until the terminal pressure drop of the filter is reached, which varies by different filter manufacturers, and ranges from 60-100 psid. The terminal pressure drop is defined as when the OEM says this is the maximum drop across the filter before it is changed.
  • A Beta Ratio is calculated at every 10% of the terminal pressure drop and a weighted Beta Ratio is reported as the final result.
  • Dirt Holding Capacity, another important factor in filter performance, is calculated as the total amount of test dust the filter retained during the total run.
  • In order to better simulate actual field conditions, some filter manufacturers vary the flow rates during the test run.

Some filter manufacturers have varied the multi-pass test by varying the flow rate to more closely simulate actual hydraulic conditions. The efficiency of a filter is calculated as follows:

β – 1/β x 100

A filter with a Beta Ratio of 200 has an efficiency of 99.5%, while a filter with a Beta Ratio of 1000 has an efficiency of 99.9% This doesn’t sound like much of an increase for a five-fold increase in Beta Ratio, but with the large number of particles in most systems it can have a large effect on the ISO Cleanliness Rating.

The term today for the absolute rating of a filter refers to a Beta Ratio of at least 200 and filter manufacturers are moving to 1000. In the past, a filter was considered absolute if its Beta Ratio was 75. Today the most important factor in a filter’s performance is not its absolute rating, but how it performs in attaining a certain ISO Cleanliness Code.

Fluid cleanliness is vital in achieving equipment reliability and filtration is a key component in achieving cleanliness goals. Understanding basic filtration concepts is necessary in making decisions on how to achieve system cleanliness. The next article in this series will discuss setting and attaining cleanliness targets with filtration.

The author wishes to thank Mike Boyd of Fluid Solutions and Aaron Hoeg of HY-PRO Filtration for their assistance in the preparation of this article.

Contributing editor Ray Thibault is based in Cypress (Houston), TX. An STLECertified Lubrication Specialist and Oil Monitoring Analyst, he conducts extensive training in a number of industries. E-mail:; or telephone: (281) 257-1526. LMT




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