My hydraulic steering system for the Thruster (marine propulsion) is now 5 years old. The technical department tells me to replace the oil Mobil DTE 25. The oil analysis reports are always good and we use (apart from the designed filters) a CJC 3 micron offline filtering unit and a Kleenoil filtering cart that filters the each (4 systems) for two weeks every 6 weeks.There are no leaks so the systems do not get fresh oil added.It is my belief that the existing oil is cleaner than new oil and oil change is not necessary.I will attach the latest oil report.Should the additives have been consumed, surely the report would have detected that? However I do believe that our oil analysis are the cheapest possible (not a very thorough analysis) and does not test all that much?Is the fact that the oil is 5 years old, reason enough to change it?What tests should I request in order to establish if the oil is 100%, which is no already done? (See report attached)
Hydraulic_oil_report.pdf (11 Kb, 331 downloads) Oil report
I would at least ask for a Total Acid Number (Tan) - If it is above 4, the oil should be changed.
I responded to this post on eng-tips_dot_com already. Guess he really wants an answer.
I am very familiar with Mobil DTE 25. It is a fairly high end non-synthetic hydraulic oil for non-aerospace use. One very good thing about it is that it resists varnish build-up, so much so that it is the only oil MTS recommends for their servo-hydraulic systems.
"It is my belief that the existing oil is cleaner than new oil and oil change is not necessary."
There are two reasons to change oil - one is particle contamination, the other is additive depletion. (and if you have really abused oil the viscosity will go away.) As Ed and Nahum said you should be checking the TAN. You should also monitor the water content, especially in a marine environment.
At the end of the day it really comes down to maintenance costs vs. potential repair/downtime costs. What are you comfortable with?
ISZ, Ed and Nahum
Thanks for your input. Didnt see your latest reply at Eng tips.
That answered all my questions.
Since I am not paying for the oil, and the fact that we work in the oil buisness (where money is not an issue) I'll go ahead and change it anyway.
Have to keep them poor Oil companies alive.
Eng Tips is a good Furum for most technical questions, but a bit slow on the Hydraulic and Maritime subjects.
Total Acid Number, is the measure of acidity in oil,new oil is about o.2mgKOH/grm of oil sample, if you do not have TAN number on your analysis, I concur with the comments do another sample and request a ACIDITY TEST, a TAN number of 2 and above change the oil. I would suggest that all future oil tests you have a TAN number, tou will then be able to monitor the rate of change I would say that this is more important than its absolute number.
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