Steam & gas turbine lubrication – meeting the challenge Peter W.R. Smith, Ph.D. Shell Global Solutions, UK Paper presented at The Ninth European Fluid Machinery Congress The Hague, The Netherlands 23-26 April 2006 Abstract: This paper looks at the lubricant performance requirements of modern industrial steam and gas turbines, which are becoming increasingly exacting, and provides examples of how these are being met by lubricating oil manufacturers such as Shell, using the latest base fluid and additive technologies. It also outlines some of the lubricant related problems that can occur if performance is not adequate. 1 INTRODUCTION Modern steam and gas turbines, increasingly operate under more arduous and extreme operating conditions than ever before, with respect to higher temperatures, and the possibility of contamination by a variety of liquids and/or solids. These place much higher demands on the turbine oil, in terms of achieving higher performance levels, with longer oil lifetimes, and giving trouble-free service. This paper looks at the lubricant performance requirements for such turbines, and provides examples of how these are met by lubricating oil manufacturers such as Shell, using the latest base fluid and additive technologies. Examples are given of what can happen if performance is not sufficient in key performance areas. It also considers the impact and importance of flushing and filling, and oil condition monitoring, in both measuring and optimising a turbine oils performance, service life, reliability, and equipment availability. 2 PERFORMANCE REQUIREMENTS OF HIGH PERFORMANCE TURBINE OILS The main function of a turbine oil is to cool & lubricate the turbine components, preventing metal-to-metal contact, many also act as the electro hydraulic control fluid. Good quality turbine oils enable the turbine to operate at maximum efficiency and with minimum down time, over extended periods of time. Some of the key lubricant requirements are high oxidative and thermal stability, deposit resistance, anti-wear performance (for certain geared turbines), rapid water separation and air release, and foaming and corrosion resistance. Typically turbine oils are made using mineral oils classified by the American Petroleum Institute into groups I to III (see Table 1) depending on their composition. Groups IV and V are for synthetic base oils such as polyalphaolefins and esters.

Table 1

Group I Group II Group III Group IV Group V Saturates

Sulphur

VI

<90% &/or

>0.03% &

=/>80<120

=/>90% &

=/<0.03% &

=/>80<120

=/>90% &

=/<0.03% &

>120

PAO’s

All Basestocks not in Groups I, II, III,

IV

Five to ten years ago most mineral oil based turbine oils were made using group I base oils. Today to meet the more severe performance requirements of the latest generation of turbines, most turbine oils are based on the more hydroprocessed group II and group III base oils. These have reduced aromatic and heterocyclic contents (which are more prone to oxidation and sludge formation than the other base oil constituents) and so tend to be inherently more oxidatively stable (1), they also exhibit greater antioxidant response when blended with the latest generation of synergistic multi-component antioxidants. Base oils on their own tend to have limitations in the key lubricant areas. Therefore specialist performance enhancing additives such as antioxidants, anti-wear, anti-rust, demulsifiers, and antifoams are carefully selected by lubricant formulators to extend their performance and life in these areas, and to ensure that the lubricant meets and exceeds the demanding turbine requirements. The consequences if these demands are not met can be extreme, for example considering oxidative stability alone. In a poor quality turbine oil that is not sufficiently oxidatively stable, in service, rapid and extensive oxidation can occur resulting ultimately in the formation of oil insoluble oxidation products (giving deposits such as sludge and lacquer), and oil soluble organic acids and polymeric species ( giving thickened, potentially corrosive oil), which will both shorten the life of the oil and reduce the service interval of the turbine, and which can give rise to costly and time consuming unplanned turbine shutdowns. All turbine oils will degrade in service with time, but it is the rate at which this occurs that can be controlled, by the lubricant developers knowledge of base oil and additive chemistry. Detailed turbine oil performance limits are specified by the major turbine OEM’s and International bodies (such as ISO, ASTM, DIN, etc). These together with stringent Shell in-house tests and our own exacting performance profiles, enable turbine oils to be fully developed and evaluated in our laboratory prior to successful field trials and full commercialization. The laboratory tests are designed to simulate the field conditions that a turbine oil will experience such as elevated temperatures, catalytic metals, in the presence of water and air, and to prove that the oil is capable of coping with them over its expected service life. Some of the key turbine lubricant requirements, their importance, potential problems and solutions, and how they can be measured are outlined below: 2.1 Laboratory test procedures 2.1.1 Rotating pressure vessel oxidation test (RPVOT, ASTM D 2272-02) 50 g of oil with 5 ml water is placed in a vessel together with a copper coil catalyst, pressurized with oxygen to 620 kPa, placed in an oil bath at 150 °C and rotated at 100 rpm. The time is then measured in minutes for the pressure to drop by more than 175 kPa. 2.1.2 Turbine oil oxidation test (TOST life, ASTM D 943-04a) 300 ml of oil plus 60 ml of water, in the presence of a copper and a steel coil catalyst, is heated to 95 °C, and is blown with oxygen until the acid number of the oil reaches 2.0 mg KOH/g. 2.1.3 Gear load carrying (DIN ISO 14635-1) This uses the FZG spur gear test rig, with special profile gear wheels. It is filled with oil, run at a constant speed of 8.3 m/sec or higher, with a controlled start temperature of 90°C or higher. The loading is raised in stages and the gears inspected at the end of each load stage. The failure load stage is when summation of scoring/scuffing exceeds limits. Results are reported as the first load fail stage 2.1.4 Rust test ASTM D 665-03 To 300ml of oil is added 30 ml of distilled (A) or salt water (B), a cylindrical steel specimen is immersed, and the fluid stirred, whilst being kept at 60°C, for 4 hours (or longer). Rusting of the steel is assessed at the end of test. 2.1.5 Water separation @ 54°C ASTM D 1401-02 40 ml of oil plus 40 ml of water, is stirred for 5 mins (at 82°C for oils >90 cSt at 40°C). The time is then measured in minutes for the separation down to 3ml of emulsion (nearest 5mins) or the Oil/Water/Emulsion heights are measured if this exceeds 30 mins. 2.1.6 Foaming tested by foam sequence I, II, III, ASTM D 892-03 200 ml of oil at 24°C is blown with air at a constant rate for 5 mins, then allowed to settle for 10 mins. The volume of foam is measured at the end of both times (values called tendency and stability respectively). The test is repeated on a second sample at 93.5°C, & then, after collapsing the foam, at 24°C. 2.1.7 Air release ASTM D 3427-03 Compressed air is blown through the oil, which has been heated to 25, 50, or 75°C. The time is measured in minutes to a reduction of entrained gas to 0.2%.

2.2 Oxidative stability Excellent oxidative and thermal stability is one of the most important performance requirements for the latest high performance turbine oils whether they are used in gas, steam, combined cycle, or water turbines. This is heavily influenced by both the base oil type and the combination of antioxidants used, as mentioned previously. Most top tier turbine oils use group II or III base oils with an optimised mixed antioxidant system. Table 2 shows some of the latest oxidative stability requirements specified by the major Turbine OEM’s and International bodies in terms of rotating pressure vessel oxidation test (RPVOT) time as measured by ASTM D 2272-02, and oxidation life (TOST life) as measured by ASTM D 943-04a.

Table 2

Specification RPVOT (minutes) TOST life (hours) GEK 32568F (gt) 500 minimum 3,000 minimum GEK 46506D (st) >250 >2,000 GEK 107395 (stag) 1,000 minimum 7,000 minimum Alstom HTGD 90 117 T (st & gt) >300 >2,000 Siemens TLV 9013 04 (st & gt) n.a 2,500 MHI MS04-MA-CL001 (st&gt) 220 minimum 2,000 minimum DIN 51515-1 (st & gt) n.a 2,000 minimum (ISO 32/46) DIN 51515-2 (st & gt) 800 minimum 3,000 minimum * gt = gas turbine, st = steam turbine, stag = combined cycle, steam and gas, n.a=not applicable Modern turbine oils can readily exceed RPVOT times of 1,000 minutes but with equal RPVOT retention capability in service, and TOST lives can far exceed the 10,000 hours maximum quotable, specified according to ASTM D 943-04a (indeed figures of >20,000 hours have been achieved when the test has been allowed to continue past the usual end point of 10,000 hours). Other oxidation screener tests utilised by Shell (2) include TOST 1,000 hours (ASTM D 4310-03), the Mitsubishi Heavy Industries Dry TOST test, and a modified Federal Test Method 791c-5308 (see ASTM D 4636-99, alternative procedure 2). These oxidation tests are amongst the few that are routinely used for turbine oils to assess the deposit forming characteristics of the oil as it degrades in service. This is of key importance to the turbine operator, as significant deposit formation can lead to a cycle of restricted oil flow, hot spot generation, and increased deposition, ultimately resulting in an outage. Maintaining and enhancing oxidative/thermal stability and especially long term deposit control in turbine oils continues to be a fundamental area of Shells turbine oil R&D effort. 2.3 Anti-wear performance Geared turbine oils tend to include extreme pressure/antiwear additives to ensure they can meet the required load carrying capacity for the gearbox, as measured by DIN ISO 14635-1. Turbine OEM’s mainly specify minimum FZG failure load stages of 8 or 9 for such oils. ISO 32 and 46 grade oils without extreme pressure/antiwear additives typically give failure load stages of 6 to 7. 2.4 Anti-rust Rusting and corrosion can reduce oil cleanliness levels, promote abrasive wear and sludge formation, increase oil oxidation and foaming, block filters, and can lead to the failure of components such as bearings, and damaged/blocked control valves. Most turbine oils contain rust and corrosion inhibitors to prevent these issues. However, some are water-soluble and can be washed out by free water leading to the problems above. The remedy other than using only stainless steel pipework for the turbine is to change to a turbine oil whose corrosion inhibitors will not be washed out by water. Typically rust inhibition is measured by ASTM D 665-03 A/B or an equivalent method. Good quality fresh oils should gives passes in both the A (distilled water) and B (salt water) parts of the test, poorer quality fresh oils may give fails in either A &/or B.

2.5 Water separation This is most relevant to steam turbines and combined cycle turbines with a common oil system, where steam leaks can become entrained in the oil. If the oil does not readily separate from water, this can lead to the formation of stable emulsions which may:

a. Restrict the turbine oil circulation. b. Reduce the effectiveness of filtration. c. Lower the oils viscosity and film forming properties.

These can give problems such as sludge formation, and increased oxidation, rusting, corrosion, foaming and air entrainment. The risk of bacterial growth may also be increased. Poor demulsibility can result from particulate contamination (this can be improved by filtration), excessive water content (improve by centrifugation/vacuum dehydration), contamination with an inappropriate lubricant (e.g. engine or hydraulic oil), or age related degradation of the oil. Typically it is measured by ASTM D 1401-02. Values for good quality oils are generally 15 minutes or under to reach 3 ml or less of emulsion, while poor quality oils can exceed 30 minutes for a similar separation. 2.6 Foaming and air entrainment Performance problems associated with excessive foaming and air entrainment include, reduced oil filterability and flow, lower fluid film lubrication, accelerated oxidation, cavitation (which can form sludges and deposits and result in viscosity increase and associated oil pump and bearing damage) and loss of efficiency in hydraulic governor systems. Mechanical problems such as a low oil level in the reservoir, or a discharge line too high above the reservoir, can contribute to this problem. As can chemical problems such as additive depletion, oil contamination, and oil degradation. Air release is typically measured by ASTM D 3427-03, and foaming by ASTM D 892-03. Antifoam additives are incorporated into turbine oils to control foaming. While air release is generally a function of the base oil and cannot be improved with additives, indeed some additives can cause air release to get worse. Foaming values for good quality ISO grade 32-46 fresh oils would typically have foaming tendency values of 30 ml or less and air release values =/<4 minutes, while poorer quality oils can have foaming tendency values exceeding 500 ml, and air release values significantly >5 minutes. 2.7 Cleanliness The presence of high levels of oil insoluble material can result in additive depletion, sludge formation, foaming, poor water separation, air entrainment, filter blocking and increased abrasive wear. These particles can be caused by oxidation of the oil, chemical contamination (solid, liquid or gaseous contaminants reacting with the lubricant additives), and particulate contaminants (such as rust, soot, dust, wear debris). Improving filtration can control these issues. Typically cleanliness is measured by ISO 4406: 1999 or NAS 1638. The major OEM’s tend to specify oil supply cleanliness levels from --/18/15 (NAS 9) or better, while in service target specifications can be of the order of --/16/13 (NAS 7). 3 FLUSHING AND FILLING TURBINES After a turbine has been manufactured it can contain material debris (such as metal swarf, weld spatter, etc.) and chemical contaminants (such as anticorrosion materials, cleaners, assembly compounds, etc.). Turbine oil performance can be significantly reduced by such materials especially surface properties such as air release, foaming, and water separation. Proper flushing and filling at the turbine commissioning step to remove these materials, will maximise the turbine oils performance and lifetime, your oil company can advise you on suitable procedures or refer to OEM or ASTM requirements. Alternatively if an aged turbine oil is to be changed to either a fresh fill of the same oil or an entirely different turbine oil then a number of questions need to be addressed to clarify whether flushing is required before the change over. These include: What is the condition of the oil (as residual aged turbine oil can accelerate the oxidative degradation of a fresh oil fill)? Are the oils compatible (not all turbine oils are compatible, this should be checked)? Some of the symptoms seen from the field where inadequate flushing and filling has taken place are: Foaming. Deposit formation (in the reservoir, bearings, filters) due to additive incompatibility between different lubricants. Poor water separation (chemical cleaners present). Reduced oil life.

4 OIL CONDITION MONITORING OF TURBINE OILS IN THE FIELD

Regular condition monitoring of turbine oils in the field is important in ensuring that the turbine oil properties and condition remains within operational limits, and to reduce the risk of unplanned turbine down time. Table 3 gives a range of turbine oil condition monitoring tests for steam and gas turbines broadley based upon ASTM D 4378-03 (In-Service Monitoring of Mineral Turbine Oils for Steam and Gas Turbines).

Table 3

Test Steam Turbine Frequency Gas Turbine Frequency (based on service hours)

1. Appearance/colour X Daily/weekly X 100 hrs

2. Metals ICP X 1-3 mths X 500-1000 hrs

3. TAN X mthly X 500-1000 hrs

4. Viscosity at 40°C X 3-6 mths X 500 hrs

5. Water content X 1-3 mths X 500-1000 hrs

6. Millipore sludge X 1-3 mths X 500-1000 hrs

7. Cleanliness X 1-3 mths X 1000 hrs

8. Rust test X 6-12 mths 1000 hrs

9. Water separation X 1-3 mths 1000 hrs

10. Foaming X 1-3 mths X 1000 hrs

11. RPVOT X 6-12 mths X 1500-2000 hrs

RPVOT is used to measure the remaining antioxidancy of the lubricant. ASTM D 4378-03 suggests considering an oil change once the oils initial value falls to 25% and a high acid value is found. Generally speaking for a turbine oil to perform well in the field, and give a long and trouble free life, it should have a slow and gradual decline in antioxidancy as the oil degrades normally in service. It is therefore the rate of decline of the RPVOT that is more important than the absolute magnitude of the starting RPVOT (3). This rate can vary due to a number of factors such as, the severity of the application, the duty cycle, the frequency and quantity of oil top-up, type and amount of contamination, etc. Figure 1 shows the results of RPVOT oil condition monitoring from a long term field trial of a top tier group II Shell combined cycle turbine oil.

0

500

1000

1500

2000

0 5000 10000 15000 20000 25000 30000 35000 40000

Gas turbine oil service hours

RPV

OT,

min

utes

Figure 1 RPVOT data for a Shell combined cycle oil running in an Alstom MS6001 Frame 6B gas turbine.

The 25% level of RPVOT degradation was developed around older group I turbine oils that typically had low starting RPVOT values e.g. ~400 mins, and so at a 25% residual level of ~100 mins (predicted to be reached at 10,000 hrs or less under these same field conditions as the group II oil) would have very little remaining antioxidancy left, and so be vulnerable to rapid degradation. Figure 1 shows that after more than 40,000 operating hours in service, even though the RPVOT value is approaching 25% of its initial value, it still has an RPVOT value in excess of 500 mins (well in excess of the starting RPVOT of many of the older generation of turbine oils) and therefore with high remaining antioxidancy. As the other oil condition parameters monitored, such as acid number change, viscosity change, etc. are acceptable, the oil has been approved for continued use, although with an increased frequency of oil condition monitoring (OCM). When assessing the condition of a turbine oil with regard to its continued use it is important to assess all the criteria that have been measured.

4 CONCLUSIONS Modern steam, gas, and water turbines require good quality, high performance turbine oils, to give optimum lubrication with minimum risk of oil related unplanned turbine down time, over the lifetime of the oil. Key lubricant performance properties include, high oxidative and thermal stability, deposit control, anti-wear performance (for certain geared turbines), rapid water separation and air release, low foaming, and good resistance to corrosion. To achieve these high performance levels the latest generation of good quality turbine oils increasingly use group II and group III base oils, with optimised, synergistic, multi-component additive systems. Sub-optimal turbine oil performance can give rise to a variety of field problems, which can lead to costly and time consuming unplanned turbine down time. To ensure that turbine oil performance is not compromised either on filling new turbines, or when replacing an aged turbine oil with a fresh oil, suitable flush and fill procedures should be employed. Regular oil condition monitoring of turbine oils in the field is important in ensuring that turbine oil properties and condition remain within operational limits, and to facilitate the early identification of operational problems.

REFERENCE LIST

(1) Schwager, B. P ., Hardy, B. J., and Aguilar, G. A., “Improved Response of Turbine Oils Based on Group II Hydrocracked Base Oils Compared with Those Based on Solvent Refined Base Oils”, Turbine Lubrication in the 21st Century, ASTM STP 1407, W. R. Herguth and T. M. Warne, Eds., American Society for Testing and Materials, West Conshohocken, PA, 2001, pp. 71-78.

(2) Guerzoni, F. N., Graham, J., “The relationship Between Oxidative Stability and Field Behaviour of High Performance Turbine Lubricants”. The Society of Tribologists and Lubrication Engineers, 56th Annual Meeting, Orlando, FL, 2001

(3) Smith, P. W. R., “Modern Turbine Oil Oxidation Performance Limits – Meeting & Measuring Them. A Shell Perspective”. To be published in the online Journal of ASTM International (JAI)