Machinery Matters: by Larry D. Rumbol “Condition monitoring” is one of those phrases one hears in conferences and learned corners of the dock. Those really…
Machinery Matters: by Larry D. Rumbol
Oil analysis is one of those tasks that should be de rigueur on a superyacht. Like brushing one’s teeth, there is no immediate benefit yet the consequence of not doing it will be painful with the (often very short) passage of time.
To continue with the analogy, if a dentist shared the x-rays and a lot of dental code on a clipboard, one would be in possession of important information yet be bewildered as to what to do with it. So it is with oil analysis lab reports. Despite the cost benefit, often only the color code (green = OK, file it; red = ring someone) is regarded by too many. Not only is this a waste of money but it seriously misses the point
It is nobody’s fault really. We all warm to our subject and can spout what we know as a mother tongue when to others it is confusing at best. So let’s break it down.
The purpose of an oil analysis report is to decide what action, if any, should be taken to maintain the mechanical integrity of the equipment, be it engine gearbox, crane, stabilizer or Jet Ski. The report will not pinpoint specific problems but it will identify where to start looking to find the cause of an anomaly.
Firstly, the right data must be on the report, and it is essential that this vital information comes from the yacht. Specifically: vessel name; date sample taken; and machinery description including manufacturer, type, model, serial number, oil grade, unit life, oil life, sample point.
This is a minimum for the yacht to provide in order to get something viable back.
Also, if there is a suspected problem, it should be highlighted. No doctor likes an uncommunicative patient.
The lab’s responsibility from a basic admin point of view relates to sample receipt date and other information to identify the sample. The scientific analysis part is where it gets complicated. A typical report would be split into four parts:
A wrongly specified viscosity oil or one where the viscosity is impaired cannot perform its duty. The oil film at a given load is finitely calculated by the OEM and that is related to viscosity. Poor viscosity can lead to contamination, overloading, poor heat dispersion and, of course, accelerated wear with an extreme end point at machinery failure. One of the most common causes of abnormally low viscosity is fuel dilution; for high viscosity, it’s a high soot content.
With each property, there are set parameters that the report must detail:
Flash Point: A sure indicator of fuel dilution is a reduced flash point in the oil. The latest equipment can give a precise value and this is of far greater use than a go/no-go value.
Water: Free water in oil is an obvious dilemma, especially its source. Water is a poor lubricant and can result in corrosion of metal surfaces, many of which have high tolerances that will not endure this. Free water is not always visible. Dissolved water catalyzes oxidation, is detrimental to the oil’s performance, and can precipitate the additive package, which can be the essential ‘ingredient’ for that specific oil’s intended service. Dissolved water, if left un-resolved, can cause rapid increases in friction with attendant high temperatures that could ultimately result in machinery failure. Water should never exceed 500ppm, and even 200ppm is cause for concern. While 500ppm might sound a lot, it’s just 5ml in 10 litres of oil. How many drops of rain in 5ml? Not many.
Oxidation and TAN (Total Acid Number): When oil oxidizes, it has started to break down, generally as a result of air, heat and, of course, overheating. This produces weak organic acids that over time increase in concentration. Oil specifically has antioxidant additives to cope with this but these will eventually become depleted. When a TAN test is conducted, this quantifies the acid concentration in the oil by measurement of the amount of alkaline re-agent (potassium hydroxide) that is required to neutralize that acid. As the TAN increases, TBN (Total Base Number) decreases. In marine crankcase oil analysis, often TBN is all that is required.
Nitration: Heat can cause atmospheric nitrogen (N2) and oxygen (O2) to react, forming nitrous oxides (NOx). These in turn react with a lubricant creating organic nitrates or soluble/insoluble nitrous compounds. This will adversely affect the viscosity of the oil causing thickening and the attendant problems that will cause. There are a number of root causes for this and they are all in the area of poor/inefficient combustion.
Insolubles: This relates to all of the material in the oil that is not dissolved. These insolubles are centrifuged out and a weight calculation performed. Should there be a high concentration, then further investigation should result to a) identify the material and b) the source.
TBN (Total Base Number): This test measures the reserve alkalinity in the oil and is reserved for engine crankcase oils. The alkalinity factor of the oil is its ability to neutralize acids formed as a result of combustion. Both TBN and TAN tests are directly related to the acidity of the oil and each gives a counterpoint value. TBN the level of alkalinity left in the oil and thereby the amount of remaining oil additive left to neutralise the acids and TAN which gives a similarly useful figure of the actual acid concentration in the oil. Generally the TBN analysis only is performed as an adequate indication and TAN only if specifically requested.
PQ Index: Particle Quantification is a measure of the ferrous particles (Fe) in the oil. It does not measure the size but the concentration. It is useful to relate the PQ index with the Fe identified in the spectrographic analysis. High PQ and low Fe is a major cause of concern. It is likely the majority of wear is above 10 microns and could be component breakdown, which can rapidly accelerate. If both PQ and Fe are high, this points toward particles less than 10 microns and would likely be the result of rubbing wear. If PQ is low and Fe is high, this indicates wear particles are less than 10 microns and could be the result of rubbing or acidic wear from organic acids formed in the oil. (Check TBN.) A situation indicating normal rubbing wear without cause for concern would be a low PQ index and a low Fe level. PQ analysis is useful to identify wear particles of 6-10 microns and bigger, which are above that able to be identified as their actual weight is too great to be suspended in the mist that is introduced to the plasma source of the ICP (see below).
The following elements are typical in a spectrographic lab report, each with its potential and likely (though not exhaustive) source.
A random sample will tell a lot but absolutely not indicate if a situation is worsening or improving and at what rate. “Can we wait until the refit?” is a common question. Only a carefully monitored trend analysis can say.
It is also important not to look at values in isolation. Like a blood test, an oil analysis will give a clue where to look for an anomaly. It is also worth remembering that once said anomaly has been identified and dealt with, to ensure that was the case, take another sample, as there could be a downstream effect that had not yet presented itself fully.
The above is a rough guide to understanding a lab report. Surveys say that between 33% and 50% of marine professionals struggle to understand one fully. That is no criticism as it is a complex subject, and that is why there are highly qualified laboratories to deal with it.
Not every doctor can read an x-ray. Specialists specialize; generalists keep the thing going until they need a specialist. That’s an equitable solution in my world.
Larry D. Rumbol has 40 years of expertise in marine condition monitoring and is marine business development manager with Spectro | Jet-Care in the United Kingdom, United States and Switzerland. Comments are welcome below.Topics: