Remaining Useful Life or RULer was introduced a few years back as a great alternative to RPVOT (Rotating Pressure Vessel Oxidation Test – formerly known as RBOT) to establish remaining life of lubricants. Up until this point the traditional method of measuring how much antioxidant was left in the oil was done indirectly by artificially ageing the oil in a rotating pressure vessel which could take up to a day per sample to perform on expensive instruments that required huge investment to provide capacity for clients samples. From a lab perspective if you only had 2 instruments and you received a batch of 20 samples all in good condition this could take 10 days to perform which in condition monitoring terms is a lifetime.
In came RULer to shake everything up with its direct measurement of anti-oxidants rather than indirectly that could measure all those 20 samples on the same instrument in a few hours and with a very small volume of sample. The manufacturers did an excellent job of marketing the product and had all the studies to back up their claims. The price point was also affordable that this was a product not only for high tech labs, but could be used by field engineers too. The product worked by directly measuring the anti-oxidants of the used and new oils with different solutions to highlight the different anti-oxidants. Then you divide one by the other and get a RUL% value of how long the oil had left.
This sounded like the end for RPVOT and I’m sure some labs even started to mothball them as they started to use RULer.
However, there was a problem.
RULer was too dependent on reference oils in that the value without a reference was pretty meaningless. In contrast a RPVOT value was relatively simple, measured in minutes and the lube oil suppliers often quoted a typical new value on their turbine oils to allow comparison. Equally since the RPVOT were all measured in minutes then general rules could be applied such as a value less than 100 minutes was time to consider an oil change. Whereas for RULer a value of 700 could be fantastic or poor and without a reference it was difficult to compare. For laboratories, references are like gold dust and difficult to obtain and when you do get one their usefulness deteriorates. This is because oil companies are always looking at ways to make their oils perform better, so there will be constant tweaks to formulations meaning the product can be significantly different in terms of RULer values over time. Equally as oils are blended to a specification rather than a formulation with several blending plants across the world a product in France with the exact same name may not have the same formulation in the USA for instance. This means a reference is only useful if you also know the supply chain of the customers oil and there is risks the data could be different.
This means RULer data can give values such as 110% life compared to a reference or 75% on a brand new oil. Obviously the oils are all working fine and so is the RULER instrument, but it’s just the situation of formulation differences which can give misleading diagnoses. So for a laboratory this makes the usefulness of RULer limited.
For field use it still is a great device if you use a barrel / IBC of the new oil delivered and put into your turbine and then compare to the in-use turbine oil. However this is great if the oil in your turbine is 100% that oil, which is often not the case in older turbines. This is because oil changes in these large volume systems are very expensive. Hence, a common solution used is to do a 25 to 50% change to freshen but not change the oil, which means different versions of the products will be mixed and also unless you are very loyal a 10 year old turbine may have 3 or 4 lube oil suppliers used all with mixes of these oils. Even if you do change an oil completely you may not have fully flushed the old oil from the system which again with the complex chemistries of anti-oxidants can cause peculiar results. So when using your RULer device you can still end up with wildly different values because the oil in the turbine is a mix of multiple products and formulations.
So is RULer any good? The answer is yes and I will still sell the test to clients as it is quick, an excellent screening test. It does work and can be trended very well with regular use E.g. monthly samples where the oil is not being changed.
However as a snapshot test or using set limits (E.g. change oil at 20% remaining life) is a dangerous strategy as the formation changes can easily change the values and give misleading information. Also don’t be surprised to see a few samples with over 100% rulers if you sample regularly. Please do not think I am suggesting you stop doing RULer testing or don’t enquire with the lab about starting to do so. I strongly advocate you do ask your lab about RULer testing on your turbines, but I suggest going in with your eyes open and don’t believe that it is a miracle test that can be used on its own to make oil change decisions, because it is not. However, when used in conjunction with several other tests including RPVOT and with the help of an experienced diagnostics team where it can be trended regularly it is a powerful tool in identifying the time to change your oil.