Acidic Oil Causes Problems…The How, the Why and the Action.
01 March 2021
The next assessment which forms part of the basic oil quality assessment on any insulating medium (along with Moisture & BDV) is the acidity content of the oil.
The acidity content of a mineral oil should be determined in accordance with the IEC 62021: Insulating liquids. Determination of acidity. Automatic potentiometric titration methodology. The standard covers multiple fluid mediums and fluids such as Midel are covered by the IEC 62021-3: Insulating liquids. Determination of acidity. Test methods for non mineral insulating oils methodology.
In brief the oil sample is weighed, broken down with a solution and titrated with potassium hydroxide (KOH) until the acidity of the oil is neutralised (hence sometimes known as neutralisation number) this gives us the acidity content of the oil in mgKOH/g.
The acidity levels within an asset are important as it can cause problems with the oil and the internal components of an asset. When the oil is acidic it can assist in degrading the paper insulation by attacking the insulation layers causing breakdown of the paper.
The higher acidity contents result in degradation of the insulating oil and as a result a by-product of moisture is created. Moisture is also created by the paper insulation degradation and as such causes more degradation to the oil and insulation.
If the acidity and oxidation of the oil continues the oil can breakdown and form sludges within an asset. The sludges will settle on the surfaces of the components in an asset such as the windings and paper insulation in transformers and switch mechanisms in switchgear.
So why is this an issue? All these problems effectively decrease the life of an asset, so ensuring a good low acidity content is critical in maintaining good paper insulation strengths and extending the life or preventing premature failure of an asset.
The reason that a high acidity is an issue is the nature of the oil, an acidic oil is exactly that it is acidic! The acidity is created by the reaction of the oil with oxygen resulting in an oxidation process and formation of compounds such as alcohols, carbonyls, acids and then sludges which are aggressive compounds which will degrade materials and substances.
So what do we do about acidic oil? The first thing to note is to check he oil is acidic and if that acidity is increasing. Generally an acidic oil is represented by a brown colour and the darker the colour of the oil the more acidic it will most likely be, (careful not to be confused between darkening due to degradation and blackening due to carbonisation caused by heating, as those are two very different issues!). A high acidity can also be assessed by the acrid odour that comes with the darkening of the oil, however the only way to truly tell is to get the laboratory to determine the acidity content in mgKOH/g.
The first action step would be to resample the oil and to confirm the oil is acidic. Depending on the results and the severity of the acidic oil, if the paper insulation is in good condition and the oil quality is acceptable except for a slightly elevated acidity content, you may be able to monitor the situation and delay remedial treatment.
If the acidity is high and it is causing issues within the asset or if there are other risk factors then remedial treatment may be your only option. However, when it comes to acidity this isn’t too problematic as a simple oil change can often do the job but when changing oil you need to understand that all other markers will no longer be present. Therefore, this should be considered before any remedial action is taken on an asset. If remedial treatment is carried out then you should discuss this with your laboratory so they know that action has been taken and the next sample should be interpreted accordingly.
Join us in our next blog where we wrap up the trio of Oil Quality assessment analysis and look into Transformer Breakdown Voltage Strength, When to Replace the Oil