The pros and cons of extending oil drain intervals in hydraulics

Over recent years there has been a growing trend within the mining industry to extend oil drainage intervals (ODI) beyond the original equipment manufacturer (OEM)-recommended service intervals.

This can deliver value to mining and power generation industries if carried out correctly — particularly regarding hydraulics systems — but it is important to understand the key risks involved with ODI extension as well as understanding what other tests are required to get a clear idea of the overall health of the oil especially in large volume hydraulic systems.

The current practice within the mining industry is to hook up the hydraulic system to kidney loop filtration units during the scheduled maintenance plan for said equipment, essential for removing particulate particles and even moisture from the system. The removal of such contaminants helps to reduce wear of oil-wetted components. In addition, laboratory test packages associated with routine oil analysis of hydraulic oil samples tend to cover only the following tests:

  • Spectrometric analysis (wear metals)
  • Viscosity
  • Moisture
  • Cleanliness

Some oil analysis laboratories will also carry out Fourier transform infrared spectroscopy (FTIR) on samples, but most would be reluctant to admit being able to accurately predict oxidation and overheating of the hydraulic oil based on the FTIR results.

As such, on receipt of an oil analysis report, many reliability engineers (RE) or maintenance professionals make a call that the lubricant health is satisfactory based on low wear metals, stable viscosity and of course, the ISO cleanliness code. The decision to extend the oil past the oils “use by date” is not based on a holistic picture, as the health of the lubricant in this instance cannot be based on those three indicators.

By this, maintenance teams and reliability engineers review the oil analysis reports placing a large emphasis on cleanliness results. If the ISO cleanliness results are at an acceptable code, then the call to extend the oil is made. In some instances, the hydraulic oil type selected by the company may invariably be a standard mineral hydraulic oil where some OEMs and lubricant companies would only suggest a 2000-hour oil drain interval.

In some instances, companies have been known to extend the ODI of large hydraulic excavator systems well past the capability of the hydraulic lubricant with the decision made based on viscosity and cleanliness of the lubricant. This approach, while initially leading to reduced oil consumption savings, does not take into account the actual health of the oil, which in time will lead to equipment issues resulting in increased maintenance activities and operational cost. A very important test measure not included as a standard measure in routine analysis of hydraulic oil samples is antioxidant content.

When oils are exposed to the process of oxidation, oxidative products are formed. Increases in oxidation could occur as a result of excessive operating temperatures or even from a process known as “microdieseling”, which occurs when entrained air bubbles implode when being passed through high pressure zones within the system. This results in very high localised temperatures of the oil resulting in oxidative by-products being formed.

Lubricants contain antioxidants to combat the process of oxidation but additives are sacrificial in nature and ultimately get “used up”.

When the antioxidants within the lubricant gets too low, then oxidation of the oil accelerates. This, together with an ineffectual method to remove these oxidative by-products, ultimately leads to varnish formation. The formation of varnish does not happen overnight but rather is an accumulation of continued over-extension of the hydraulic oil, sometimes taking more than five years before the presence of varnish within the hydraulic system becomes apparent.

Test results from routine used oil analyses do not detect the level or anti-oxidants in the oil and cannot determine potential formation of varnish. Similarly, routine used oil analysis cannot be used as the main justification for extending oil drain periods. In the case of hydraulic systems using a standard hydraulic oil, it is important for maintenance professionals to liaise with the laboratory to include other tests.

As a minimum, acid number (AN) or total acid number (TAN) must be included in the routine suite of tests. In addition, at 2000 hours operating time on oil and every subsequent 1000 operating hours on the oil thereafter, tests to confirm antioxidant levels left in the lubricant should be carried out in order to determine the overall health of the lubricant.

By closely monitoring physical and chemical characteristics, a call can be made that the oil is at or nearing the end of its useful life. This can be determined by closely monitoring any increasing trend in TAN which is also likely to be accompanied by very small incremental increases in viscosity. This trend, together with decreasing anti-oxidant levels in the oil means that the hydraulic oil must be replaced.

ANAC (ANAlysis Compared) is a service of TOTAL that provides a complete range of analytical monitoring systems for in-service oils, available for any automotive and industrial application. For management and technical monitoring of mining, construction and transportation vehicles as well as industrial equipment, ANAC offers a full range of in-depth analysis processes for in-service oil including Membrane Patch Colorimetry.

Failure to correctly implement the required suite of tests for performing extended oil drains of large hydraulic systems will ultimately result in premature failure or malfunction of the hydraulic system, negating any potential savings initially achieved through the planned oil drain extensions programs. Put in laymen’s terms, if not managed correctly, the process of oil drain extensions provides a short term gain followed by long term pain of the hydraulic system.

For more information, please refer to TOTAL’s Mining Solution website at http://www.miningsolutions.total.com/

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