Martin: The next thing would be to probably try to understand, what the varnish actually is, and, therefore, how it leads to actual failure modes that it causes. So, in terms of defining what varnish is; varnish is really just oxidized oil molecules that were drifted out the body of the base oil towards the metal surfaces. And that is simply because the oxidized oil molecules have polarity, unlike the base oil itself. But, of course, the metal surfaces of our components also have polarity. And, of course, water has polarity which leads us to sludge.
Haris: Yes, that is what I wanted to ask you. Is that a moment when we start seeing sludge and sediment?
Martin: Correct! So, you know, when you understand what sludge is; by definition sludge is an oxidized oil molecule combined with a water molecule. So, if we are seeing evidence of sludge at the bottom of our tanks or sumps, it is a sure sign that we have got a potential for a problem. And so, this is why I am very keen to stress when doing oil changes; Inspect for sludge! Because sludge, and of course our sediments, are really potentially wear metals that have settled to the bottom of the sump or a tank. And it comes back to the WHAM again. So, the sludge has the water, we’ve got the metals that are catalytic lying at the bottom of the sump. So, the cleaner your sump or tank, the less likely we are to be seeing problems when we put fresh oil in. Ultimately, what happens is that we get this build up of oxidized oil molecules, layer upon layer of this material that we refer to as varnish, I guess because, if you think in terms of woodwork and protecting wood, you would coat it with varnish which is sort of orangy, clear, residue material. And like the varnish when you first put it on to the wood the varnish in you machine is often a sticky residue coating the surfaces. And that can be anywhere in the machine where, obviously, the oxidized oil molecules are drifted out. One of the simplest places I have ever seen is the sight glass, for example. You get sediments actually settling on the sight glass, which is indicative. You know, to jump ahead on the question there, hot to detect for this problem; that’s one of the simplest ways: looking at that, to be honest. But I’ll come back to that. In terms of the failure modes caused by varnish as it builds up, it is often refed to as the cholesterol in the machine. Cause, you and I, Haris, both being health freaks and obviously watching or diets, exercise, we are trying to avoid cholesterol because we know it clogs up our arteries. The same within the machine, we want to avoid varnish because varnish can lead to clogged up oil galleries, it can lead to imperfect surfaces where we are wanting a hydrodynamic oil film, such as up the bearing supporting the shaft on a turbine, so we can get a loss of the film strength, we can get disruption of the oil film and we get metal to metal contact. In fact, it is not uncommon to see that when you are looking at the bearing shell, you’ll see the evidence of varnish, but you may see a section of it that is bare metal which is perhaps where the shaft has actually contacted the bearing and physically wiped the varnish away. Because varnish is fairly easy to wipe and scrape off. In that sense, in terms of failure modes and symptoms, it tends to, sort of, coat on to the surfaces, it is oxidized oil molecules and, of course, when you do put the fresh oil in that has a potential to disrupt fresh oil as well, ultimately. So, flow of oil, things like your oil cooler suffer because oil flow through the cooler can than be reduced and we’re not getting the heat dissipation through the varnish to the actual surfaces of the oil cooler. We’ve got restricted flow of oil going through that oil cooler; because you remember magic number; if you halve the diameter, you basically reduce the flow by the factor of four. So, there are a number of aspects there that are going to be the outcome of varnish. What it would be the most appropriate way to detect for this problem? Well, simple stuff, as I mentioned, looking at the sight glass, when you’re doing oil drain on a pump – check the sight glass because you’ll see the sediment. And, of course, I would recommend cleaning the sight glass, because it can lead to a false sense of level checks, because that sediment looks like oil and therefore make it look like oil is at correct level. Bottom sediment and water balls or bearing oil sight glasses as they are sometimes known, also great for looking to see if there is sludge or sediment at the base of the sump. So, bearing housings fitted with those bearing oil sight glasses, some of them are very, very sophisticated, with magnetic probe inside, stainless steel fittings for offshore applications and corrosive environments, and that kind of stuff. The other thing is taking a sample of oil into a clear glass or clear plastic bottle and then put the bottle on a shelf for 48 hours, leave it to stand, leave it in the dark place, avoid bright sunlight, put it in the back of the cupboard in the darkness away from the fluorescent lighting because actually ultraviolet light, fluorescent lighting can actually affect the colour of the oil. As I said, put it in a dark place and after 48 hours look for any evidence of the sediments at the bottom of the bottle, any evidence of sediment would suggest that there are potentially problems lurking there. And also, simple stuff like when you look at the dipstick, if it is metal dipstick, when you draw it out to check the level look for signs of varnish on that. Some people also hang a strip of steel into the oil and inspect that to see if varnish is settling on that. There are various ways you can detect for it at the work level just purely by inspection. Obviously, any darkening of the oil … so your pumps for example with the constant level of oil bottle on the side, the Denco, Trico, Adams type units, the oil can obviously also start to go dark in those. That can be exposure to sunlight. So, all of those were ways to look for potential for varnish inside the system.
Otherwise, we’re talking laboratory method testing which may include things like, um, techniques to see if we have enough additive left in terms of the antioxidant so instrumentation like the RULER, the Remaining Useful Life Evaluation Routine instrument that can detect for remaining levels of additive. FTIR, Fourier Transform Infra-Red cannot only measure directly the levels of oxidation but also look at the levels of remaining additive but not the greatest tool by comparison to say the RULER and then of course there is our time old favourite, the Rotating Pressure Vessel Oil Oxidation Test or what used to be the Rotating Bomb Oxidation Test (RBOT) – that gives us an indication, at a reasonable level, of the remaining life of the oil by stressing the oil with the heat, air; water, metal…that again is great for the first half of the oil’s life, not so great in the second half so coming back again to things like the RULER.
Ultimately, though, there are some direct techniques for looking for the sediments that actually cause the varnish and so ultra-centrifuge, the oil is being spun at 20,000rpm creating massive G Forces, it’s akin to the bottle and the settling of the sediments but we’re now accelerating that process in the centrifuge so if there is any symptoms of any sediment in the base of the centrifuge test tube clearly there are sediments gathering at the base.
Another technique is an exothermic reaction where they actually heat two samples of the oil; one being fresh, one being used oil sample and then looking at the heat being generated above the oil, obviously oxidation is a process of exothermic reaction so you can get some idea of the progress by looking at that.
But the one that is fairly common and often used is the Membrane Patch Colorimetry. Now this is something I remember seeing many, many years ago with some samples from a customer in terms of filtration when I worked for a filter company. The actual membranes that we were looking at for particle counting were showing discolouration of a yellowish to orange form and what that is; is actually the sediments of the oxidised oil that potentially cause the varnish that are being trapped on the membrane so using something like a 0.5µm rated membrane in the vacuum flask, suck the oil through. Now without going into too much detail here, there are steps in the Ultra…er…Membrane Patch Colorimetry (MPC) test that need to be followed in order to do the test because, as with any test, there has to be a method and that method should be followed consistently because when we start to look at results from different labs, to be able to compare them “like-for-like” the MPC includes a process that needs to be followed and some labs don’t always follow that so you can get labs claiming a reasonably good results whilst others are saying the same sample is giving poor results, so, you know, check with your lab that they are following the MPC test clearly.
Haris: Exactly
Martin: So that kind of concludes on detecting the problem.
