The gain and pain in extending oil drain intervals in Hydraulic Systems
Oil drain intervals (ODI) have generally been pre-determined by the Original Equipment Manufacturer (OEM) and designed as such to provide protection to the compartments based on a wide variety of conditions. Over recent years there has been a growing trend within the mining industry to extend ODI beyond the OEM recommended service intervals. Considering hydraulic systems, extending ODI are perceived to be one area where value can be easily delivered to the mining or power generation industries. This can be achieved if carried out correctly 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 particularly within the mining industry tends to be to hook up the hydraulic system to kidney loop filtration units during the scheduled maintenance plan for said equipment. Whilst good practice, the function of kidney looping is to essentially remove particulate particles and even moisture from the system. The removal of such contaminates 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)
Some oil analysis laboratories will also carry out Fourier Transform Infra-red 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 3 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 OEM’s 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, whilst initially leading to reduced oil consumption savings, does not take into cognisance 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 anti-oxidant content.
When oils are exposed to the process of oxidation, oxidative products are formed. Increases in oxidation could be 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 anti-oxidants to combat the process of oxidation but additives are sacrificial in nature and ultimately get “used up”. When the anti-oxidants 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. These varnish deposits will tend to drop out of solution and at the areas that are coolest in the hydraulic or turbine system thus leading to equipment unreliability, for example, sticking of servo valves. 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 5 years before the presence of varnish within the hydraulic system becomes apparent.
Test results from routine used oil analysis 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 anti-oxidant 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.
In addition and as a supplement to the above, laboratories can perform Membrane Patch Colorimetry (MPC). Here, a set volume of oil is passed through a filter membrane, also referred to as a patch. The patch is then compared against known reference patches and a number assigned to the patch. The higher the assigned number, the higher the amount of varnish precursors present in the oil.
ANAC (ANAlysis Compared) is a service of TotalEnergies 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. This in turn, can lead to increased component change outs and reduced oil drain intervals thereby 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.
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