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Capital Equipment Maintenance in Industrial Filtration

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Why Is Capital Equipment Maintenance Important?


Capital equipment is typically one of the most expensive assets for any business and as such, regular maintenance will extend the life of capital equipment, reduce capex, and mitigate the risk of unplanned downtime. In our article, why is filtration important, we discussed how 70% of component replacements or 'loss of usefulness' is due to surface degradation. In hydraulic and lubricating systems, 20% of these replacements result from corrosion with 50% resulting from mechanical wear. This underscores the vital importance of filtration and industrial lubrication.


Assets have value – if they are maintained well the value of capital equipment will not depreciate as quickly


Defer replacement costs – well maintained capital equipment will last longer, so you will not need to replace them as often


Minimize breakdowns – equipment that breaks can completely disrupt productivity and cost the business a lot of money. Maintaining the equipment will reduce the likelihood of any breakdowns


mechanical wear loss of usefulness



Sources of Contamination


Built in contaminants from components:


  • Assembly of system
  • Cylinders, fluids, hydraulic motors, hoses and pipes, pumps, reservoirs, valves, etc


Generated contaminants:


  • Assembly of system
  • Operation of system
  • Break-in of system
  • Fluid breakdown



External ingression:


  • Reservoir breathing
  • Cylinder rod seals
  • Bearing seals
  • Component seals


Contaminants introduced during maintenance:


  • Disassembly/assembly
  • Make-up oil



The Micrometre “µm”



‘Micron’ = micrometre = µm


The micrometre is the standard for measuring particulate contaminants in lubricating and fluid power systems.


1 micron = 0.001 mm (0.000039 inch)

10 micron = 0.01 mm (0.0004 inch)

Smallest dot you can see with the naked eye = 40 µm

Thickness of a human hair = 75 µm

micrometer diagram



Types of Contamination





Hard, translucent particles often associated with atmospheric

and environmental contamination, e.g., sand, dust.


Bright Metal


Shiny metallic particles, usually silver or gold in colour,

generated within the system. Generated contaminants are

products of wear and often cause additional component wear

and accelerated fluid breakdown.


Black Metal


Oxidized ferrous metal inherent in most hydraulic and

lubricating systems; built-in contaminant and genereated

within the system by wear.







Dull orange/brown particles often seen in oil from systems

where water may be present, e.g., oil storage tanks.




Contaminants most commonly generated from paper and

fabrics, e.g., shop rags.


Cake of Fines


Very large concentrations of ‘silt’-size particles coat the analysis

membrane and build-up into a cake. The cake obscures the

larger particles on the membrane making contamination

evaluation impossible.



Water Contamination in Oil



Water contamination in oil systems causes:


  • Oil breakdown, such as additive precipitation and oil oxidation
  • Reduced lubricating film thickness
  • Accelerated metal surface fatigue
  • Corrosion


Sources of water contamination:


  • Heat exchanger leaks
  • Seal leaks
  • Condensation of humid air
  • Inadequate reservoir covers
  • Temperature reduction causes dissolved water to turn into free water





water contamination in oil

To minimize the harmful effects of free water, water concentration in oil should be kept as far below the oil saturation point as possible.


10,000 PPM - 1%

1,000 PPM - 0.1%

100 PPM -  0.01%



Filter Types and Locations



Pressure Line


  • To stop pump wear debris from travelling through the system
  • To catch debris from a catastrophic pump failure and prevent secondary system damage
  • To act as a Last Chance Filter (LCF) and protect components directly downstream of it


Return Line


  • To capture debris from component wear or ingression travelling to the reservoir
  • To promote general system cleanliness


Kidney loop/off-line


  • To control system cleanliness when pressure line flow diminishes (i.e. compensating pumps)
  • For systems where pressure or return filtration is impractical
  • As a supplement to in-line filters to provide improved cleanliness control and filter service life in high dirt ingression systems 




Reservoir Air Breather


  • To prevent ingression of airborne particulate contamination
  • To extend system filter element service life
  • To maintain system cleanliness


Additional filters should be placed ahead of critical or sensitive components


  • To protect against catastrophic machine failure (often non-bypass filters are used)
  • To reduce wear
  • To stabilize valve operation (prevents stiction)


Flushing Filter


  • To remove particles that have been built-in to the system during assembly or maintenance before start-up
  • To remove large particles that will cause catastrophic failures
  • To extend ‘in-service’ filter element life  


lube and hydraulic filter locations



Flushing Recommendations


The aim of flushing is to remove contamination from the inside of pipes and components that are introduced during system assembly or maintenance. This is accomplished by passing clean fluid through the system, usually at a velocity higher than that during normal operation to pick up the particles from the surface and transport them to the flushing filter. Omission or curtailment of flushing will inevitably lead to rapid wear of components, malfunction and breakdown.


Reynolds Number (Re): A non-dimensional number that provides a qualification of the degree of turbulence within a pipe or hose.


laminar flow vs turbulent flow


Laminar Flow - Re < 2,000

Transitional Flow - Re 2,000 - 4,000

Turbulent Flow - Re > 4,000


For effective flushing procedures the Reynolds Number (Re) should be in excess of 4000. The flow condition in a pipe or hose can be assessed using Reynolds Number (Re) as follows:


reynolds number


Re = Reynolds Number

U = Mean flow velocity (m/s)

d = Pipe internal diameter (mm)

V = Kinematic viscosity of fluid in cSt (mm2 /s)

Q = Flow rate (L/min)



Cyclic Stabilization Test for Measuring Filter Performance




As opposed to ISO 16889 that only tests filters under steady state conditions, the Cyclic Stabilization test is used to evaluate hydraulic filter performance under typical stressful operating cyclic conditions such as:


  • Flow surges
  • Pressure peaks
  • Cold starts



cyclic stabilization test



Cyclic Stabilization Test (CST) measures a filter’s ability to clean up a contaminated system under cyclic flow (25 to 100% of rated flow) and contaminant loading conditions.


CST ISO 4406 Cleanliness Code ratings are based on the stabilized cleanliness achieved at 80% of the net terminal pressure drop, considered the worst operating condition.




Because of technological developments related to the products, systems, and/or services described herein, the data and procedures are subject to change without notice. Please consult your Pall representative to verify that this information remains valid. 


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