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Why has my hydraulic fluid darkened?

One of the things that can really worry machinery operators is that moment when they pull a fluid sample from their hydraulic system, only to find that it has become significantly darker since being put into the machinery.

Often when this happens, there will be a mass panic as staff try to work out what could be wrong with their hydraulic oil and if it could have an impact on their machinery, but when they send the oil for analysis, there does not appear to be anything going wrong. What exactly causes this seemingly innocuous change in colour?


If the colour of your hydraulic oil has darkened, but there is no change in its viscosity, oxidation or acid number, then you should check your machinery’s valves. It is likely that you will find a golden residue on them and this signals that varnish is precipitating out of the fluid.

This occurs because varnish s made up of fluid degradation by-products, so you will need to identify the source of fluid deterioration if you want to fix the problem.

Thermal degradation

When a problem like this occurs, it is normally the case that thermal degradation is taking place, and if you do not remedy the problem, you will see a degradation in your valves’ capability to function as they should.

Fixing the problem

It is heat that causes thermal degradation to occur. The two main sources of high temperatures in the average hydraulic system are hot spots emanating from external heat sources close to the lube system and the implosion of entrained bubbles as they move from the reservoir into pumps and valves.

In order to address the issue, you will need to get in touch with the laboratory for a FTIR curve, which looks for a peak in the nitration region. This will tell you whether or not thermal degradation really is the problem.

You will then need to do a root-cause analysis to find out where the heat is coming from and whether a solution can be drawn up to remove the heat source. If it is entrained air causing the problem, for example, you can remove the bubbles from the fluid and the problem will be solved.

If, however, it is a design flaw causing the problem, you will need to use a separate technology to remove the products of degradation from the fluid until you are able to redesign the system.

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