Turbine Oil Selection

August 01, 2013
Greg Livingstone
Online Only Articles
Podcast

 

What: podcast interview transcript for STLE's blog: The STLE Compass | click here to play the full podcast

Topic: Turbine Oil Selection

Interviewee: Greg Livingstone, Chief Marketing Officer for Fluitec International. Contact information can be found in our member directory.

SNIEGOWSKI: Hello, I’m Kara Sniegowski. Welcome to the STLE Compass, brought to you by the Society of Tribologists and Lubrication Engineers. The STLE Compass is your convenient and reliable resource for the latest information and developments in the tribology and lubrication engineering fields. In today’s episode we’ll be talking about turbine oils and some best practices and tips for oil selection.  Our interviewee, Greg Livingstone is the Chief Marketing Officer at Fluitec International based out of Jersey City, New Jersey. He is responsible for Fluitec's marketing and Science & Technology group. Greg has been involved with lubrication contamination control and condition monitoring for the last two decades. He's a Certified Lubrication Specialist and has held industry leadership roles in several committees including ASTM’s Turbine Oil Analysis and Problem Solving committee and STLE’s Power Generation and Wind Turbine technical groups. He has over 40 published papers focused on lubricant condition monitoring and contamination control. And with that introduction, we’ll go ahead and get started with today’s discussion. So, welcome and thanks for joining us today Greg!

LIVINGSTONE: Thank you very much; it’s nice to be here.

SNIEGOWSKI: We’re glad to have you. First, can you give us a little background on the types of oils used in turbines?

LIVINGSTONE: Yes. So, first of all, there are several types of turbines that are used in power generation. Wind turbines, for example, use gear and hydraulic oil. Air derivatives turbines and jet engines use synthetic fluids based on polyol ester chemistry. When most people refer to turbine oils, they refer to rust- and oxidation-inhibited circulating oils. And these are the formulations that are used in steam and gas turbines. Typically, these are mineral-based oils and they have ISO viscosity grades of 32 or 46. Some hydroelectric turbines we see may use up to an ISO viscosity grade of a 68. So, we’ve seen a full-circle in the trend of turbine oil types. Years ago, a lot of the oil manufacturers had a gas turbine oil and a steam turbine oil, then we saw some consolidation of formulas. And so, some manufacturers provided just one formulation that was suitable for both gas and steam turbines and today we’re seeing this trend towards more customized products and more specific formulations for either a gas turbine or a steam turbine, and in some cases, oils that are formulated just for one specific OEM. Mitsubishi is a common one. For example, there is a specific specification for Mitsubishi turbines that require a unique formulation and so, many oil manufacturers will make an oil specifically just for that application. Another trend that we’re seeing, although it’s less common, there are other non-marrow base stocks that can be used in turbine oil applications as well. Occasionally we’ll see synthesized base stocks and we will occasionally see turbines that are using polyalphaolefin or PAG or phosphate ester base stocks and these synthesized chemistries can often be custom-engineered to have some unique characteristics such as fire resistance.

SNIEGOWSKI: You’re discussing many different types of oils. When would someone need to consider selection of those oils for their equipment? For example, when would you have a typical re-evaluation of the products you’re currently using: every year, 3 years, 5 years, etc.? What do you find is the typical time for selection?

LIVINGSTONE: In most cases, what we see is that steam and gas turbines are very large reservoirs, so they’re not changed out very often. They’re not typically based on time. It’s more common to change the turbine oil out based on condition. The approximate life of a turbine oil really depends upon the application, but in general, large frame gas turbines, the average life of an oil in that application is typically between 5-7 years, and the life in a steam turbine is typically about double that. However, we have seen many steam turbines that are still successful using oil – that is, 15, 20, in some cases 30 years old, and they haven’t had a problem. 

SNIEGOWSKI: Wow, that’s a good timeline!

LIVINGSTONE: That’s a long time. Exactly.

SNIEGOWSKI: So, what are some properties of these oils?

LIVINGSTONE: The primary purpose of turbine oils is to hydrodynamically support and cool the bearings. However, many other tasks can also be asked of these fluids and in some of these system designs we see a turbine oil that could be used as a seal oil to keep out gases, it may be used as a lift oil to elevate the shaft upon start-up, it could be used as a hydraulic medium to power all of the turbine controls and in some cases, it can also be used as a gear oil. We have these oils that are in multi-use applications, so there is a wide range of different performance characteristics that we need to have. The most important thing is that turbine oils are oxidatively and thermally robust – they can withstand a lot of heat and thermal conditions. The second thing is that these turbine oils have to be resistant to other foreign contaminants that can get into the fluid. So, some of the contaminants may be air – air is a major contaminant. So, looking at air-release properties, or the foam control properties of the fluid are very important. Another major contaminant that turbine oils may interact with is water – so, understanding how the fluid will separate from water, or the demulsibility characteristics is important. And the rust and corrosion protection of the fluid is very important. Also, the turbine oils need to be maintained very clean, so they have to have good filterability characteristics. One other critical aspect we see with new turbine oil formulations now as well is they have to have good deposit control as the fluid ages.

SNIEGOWSKI: When or why might you use one oil over another?

LIVINGSTONE: The most common selection criteria that we see today is probably not the best – it’s price. So, in the mindset of many people, they will see that the turbine oils are formulated very, very similarly. The majority of the turbine oil chemistry is base oil (98-99% of a turbine oil formulation may be the base oil), and then you have about 1-2% additive (the majority of which are antioxidants), and the fluids are meeting the OEM specs. On paper these fluids all look the same, so it’s understandable why price is sometimes a criteria. People commoditize them and say that all the turbine oils meet spec, so what is the cheapest oil I can get? We prefer the mindset, instead of turbine oil being something that is a consumable, maintenance expense and something that’s accounted for on your P&L statement (profit and loss) purely as an expense, we prefer to consider turbine oils as an investment and maybe something to be accounted for on your balance sheet. Something, if you make a good investment and buy the best fluid for your application, and if you treat this turbine oil as an asset, then you will yield much, much higher performance and results from the fluid. So, selecting the best turbine oil for your application really can result in some good long-term savings. Most of the available turbine oils on the market, as I said, are meeting the OEM specs. There are also industry guidelines for turbine oil types – ASTM, DIN, ISO, all provide guidelines for turbine oils. In reality, however, we see a wide range of performance differences among the commercially-available turbine oils. One of the reasons for this is that the tests that are used to qualify a turbine oil (and all of the tests that you see on the specification sheet), are not really indicative of the performance that we see of the turbine oil in the field. Oxidative stress tests for example, like the rotating pressure vessel oxidation test, a well-known test, ASTM D2272, there’s also the turbine oil stability test, ASTM D943. These tests are critical tests that are on all OEM specs and you’ll see these test values on all new oil spec sheets. However, there’s not really a relationship between the values of these results and in-field performance. For example, you can have a turbine oil that has an initial RPVOT value of 2500 minutes – very, very high. However, it may generate lots of deposits within the first 6 months of operation. So, we’ve seen a positive trend toward developing new tests that are more indicative of field performance. Ideally in the future, we’ll be able to derive some value out of the spec sheet. As of today, it doesn’t provide a lot of value other than to indicate that it does meet the OEM spec. But an example of this is Mitsubishi – they have developed a unique specification that’s based on sludge control of the fluid as it ages. So, we’re starting to see more tests that are being developed for new oil specs that are more indicative of field performance. Recently in ASTM we approved a membrane patch colorimetry test, ASTM D7843. It’s a test to look at the varnish potential of a fluid. So, I believe that we’ll start to see tests like this be incorporated into new oil tests at some point. The other challenging thing that we see, is we don’t really see a correlation of the price of the oil and the performance, which is a little counter-intuitive. You would expect that if you paid more for a premium brand that you would get higher performance – that’s not necessarily the case. That being said, all turbine oils can perform well in the field if they’re properly maintained and monitored. A key part of this is the plant has to have the knowledge to be able to know what tests to run, when, and what technologies to augment and use to help maintain the fluid, therefore having a strong technical and customer support aspect form your oil vendor/supplier is a valid criterion when selecting turbine oils. That’s a valid thing; I’d say much more so than price. Another criterion that people look at is field experience. If you have had good success running with a particular turbine oil for years, and there are other people in your community that are running similar turbines that have similar success with a certain brand of turbine oil – that’s some great data to take into account in the decision-making and selection process. So, we see message boards and user groups are now commonly used as one aspect in selecting turbine oils. Ultimately, a power plant wants to select the best turbine oil for their application that provides the longest life without performance problems that provides them clear indicators of when the fluid needs to be changed, and is also provided by a vendor that can give them some dependable, reliable information.

SNIEGOWSKI: Great! Some great advice in there. Any other ideas or best practices listeners should take into account when choosing a turbine oil?

LIVINGSTONE: I think the most important thing is to realize that there’s nothing on the spec sheet that will indicate the performance of the fluid. Tests such as RPVOT and other oxidative tests are really not going to give you any indication as to how long the fluid will last and whether or not you’ll have any performance problems. That being said, there is a big benefit in gathering some data and trying to select the best turbine oil for your application.

SNIEGOWSKI: On that note, you mentioned some information you look for on the data sheets and you’ve referenced some standards. When selecting an oil, what kind of tests/data would you look for, or would you run before purchasing an oil?

LIVINGSTONE: It’s always good to gather as much information and data as possible before you make a decision. Since we know that there really isn’t sufficient data from the spec sheet and we know that there may be limited data you can get from the field and field experience, one thing we’ve found a lot of value in is actually taking the oils and stressing them and understanding what happens when they’re stress in a controlled environment. We’ve developed a test – we just refer to it as a turbine oil performance prediction test – it’s a fairly easy, straightforward test that any commercial laboratory can run for you. The purpose of the test is to have an accelerated way that we can basically beat up the oil and measure some key things that happen while the oil is in service. Two of the leading causes of in-service turbine oil degradation are oxidation and additive depletion. Then, the physical changes that occur when a turbine oil is used, is that we start to see deposits. This test methodology that we’ve come up with involves placing an oil in a beaker at 120 degrees with an iron catalyst in the fluid and we put it in a stressful environment for about a month. Each week, we draw some tests to look at the amount of oxidation, the antioxidant health and the varnish potential. The interesting thing that we’ve seen from this – we’ve run this test many, many times on new turbine oil formulations – there is a very large difference between some of the different brands and types of turbine oils. In some cases, you’ll see an oil degrade and produce a tremendous amount of deposits, and in other cases, you will see an oil degrade and produce no deposits, or at the end of the life or stress test, you may have lots of antioxidants left in the fluid, or no antioxidants. Stressing two new (or multiple) formulations in such a way can provide some very valuable information that can go into your selection criteria.

SNIEGOWSKI: Once you’ve gotten into that selection and someone has purchased an oil or are in the selection process – what are some issues you’ve encountered in the field and how have you solved them?

LIVINGSTONE: Well, the most common issue that we hear is that people have been running a turbine that may be 20 or 30 years old and have had no problems whatsoever and suddenly they start to have high bearing temperatures, or their valves are starting to stick. Nothing has positively changed mechanically, but now they’re starting to have more problems. This is often a result of the introduction of new oil chemistry. We have certainly seen a shift in turbine oils over the years – 10, 20 years ago, the majority of turbine oils were Group I base stock with a fairly simple and common antioxidant system. Today, we’re seeing more complex formulations with really high quality Group II, some cases Group III base stocks, with more complex antioxidant chemistries. In general, these oils will provide a much higher life and performance, however, they have to be monitored and maintained a little bit differently than some of the oils from a couple decades ago. If you are running the same oil monitoring tests that you’ve been doing for the last 40 years, you may, with this new oil chemistry, start to experience problems that your oil analysis tests don’t pick up, for example. Another consequence or issue that we see a lot in the field is that plants may put new formulations on top of their existing ones. We often see a Group II formulation being added on to, as a top-up fluid, on top of a Group I formulation. Group I and Group II base stocks are certainly compatible, and even their additive chemistries may be compatible, but one of the challenges we see is that Group II base stocks have a lower solubility – they aren’t able to hold as much in the fluid. If you mix that into a Group I oil, that has a much higher solubility, then you’re changing the solubility of the whole mixture and as a result, we start to see a lot of deposit issues. A third challenge that we see in the field is that some of the new formulations in the field have a lower tolerance for contaminants. As a result, we may see the demulsibility retention, or the fluid’s ability to separate rapidly from water, may diminish much more rapidly. It’s important to make sure that as you migrate to modern and more advanced turbine oil formulations that your maintenance program and monitoring program advances along with it.

SNIEGOWSKI: What would be an example of some of the changes you might see in a condition monitoring program?

LIVINGSTONE: The key thing is that the oils from 20 or 30 years ago degraded differently and produced different byproducts. So, you would have, for example, acidic byproducts that could be measured with the total acid number (TAN), or you could have changes in viscosity over time (an increase over time). We don’t see that now. It’s very, very rare to see a turbine oil with a very high acid number, or so much stress that the viscosity is now out of specification. These tests have been run for decades on turbine oils, but they’re much less valuable today than they were because the degradation products are different. Looking at the health of the antioxidant system, looking at the potential for the fluid to generate deposits are important and key.

SNIEGOWSKI: That was a great nugget of information. Given what we’ve discussed today, what are some key takeaways for listeners?

LIVINGSTONE: I think the first thing is that there is a large, measureable benefit in selecting the best turbine oil for your application and there is a wide range of performance. So, you can, in fact, maybe with the same amount of money on a particular oil (or in some cases, you could spend less money), and have a higher performing product for your application. Second, there is nothing really on the spec sheet that will suggest how a turbine oil will perform in the field. Price is counterintuitive – it’s not usually indicative of performance. And the final thing is in order to make the best decision in selecting turbine oils, you want to collect as much data as possible. So, in addition to field experience, a key piece of data that we find really valuable in selecting better turbine oils is to stress new turbine oils in similar conditions and monitor and see what happens in the field. We refer to this as a turbine oil performance predictor test, but there are many different stress tests that you can do, or you can work with your laboratory to develop one. And this data will go a long way to help you make a much better decision on what turbine oil is best for your application.

SNIEGOWSKI: Great advice. Greg, thank you for joining us today and for your insight. For more news, information and research on power generation and turbines, please visit our website. Thanks for joining us today. This has been another episode of The STLE Compass, pointing you in the right direction.

 
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