Turbine lubrication: Practical guidelines

Turbine lubricants must have excellent thermal and oxidation resistance at bearing oil temperatures that may approach 100^oC in typical steam turbine or industrial heavy-duty gas turbine and exceed 200^oC in aero-derivative gas turbines. Turbine lubricants must control the rust and corrosion that could destroy precision surfaces, resist foaming and air entrainment, which could impair lubrication and lead to equipment breakdown, and have high viscosity indexes that allow more uniform lubricating performance over a wide range of ambient and operating temperatures. They should also be easily filterable without additive depletion.

Turbine lubricants should be versatile, able to serve as both lubricating oil and hydraulic fluid for various turbine systems, generator, gear unit and other auxiliary components. This allows simplifying lubricant inventories to a relatively few multipurpose products, thus minimizing the chances of potentially costly lubricant misapplications. Products of interest in turbine industry are ISO grades 32, 46 and 68. TERESSTIC GT 32 is a good example of super-premium petroleum-based (mineral) turbine lubricant for industrial heavy duty gas turbines and steam turbines. Where the turbine manufacturer specifies higher viscosity oil, ISO grades 46, 68 or 77 also provide excellent service.

Steam and gas turbine oils are expected to provide years of trouble free operation. In service monitoring of turbine oils is a valuable means of assuring optimum oil performance and extended turbine life. Following recommendations are intended as a general guideline:

1- Total acid number increase (warning limit is 0.3 mg KOH/g).

2- Water content (warning limit is 0.2%).

3- Cleanliness: it is necessary to find source of particulates, e.g. make up oils, dust or ash entering system, wear, etc. 

4- Rotary Bomb Oxidation Test - RBOT (warning limit is less than half of test result value of original oil).

STEAM TURBINE LUBRICATION

Steam turbine lube oil system is usually required to provide oil for trip-and-throttle valve, governor system, power cylinder or similar accessories (combined pressure lubrication and control oil unit). Steam turbine lubricants must readily shed the water. Water in the steam turbine train oil reservoir is from one of three following sources. An analysis of water can usually determine source:

1- Simple condensation from the air within the reservoir. It can be minimized by maintaining the manufacturer's specified oil level within the reservoir and good ventilation around the turbine train. 

2- A leak in the shell-and-tube oil cooler(s) may allow cooling water into the oil loop. If the oil pressure is greater than the water pressure, oil will be forced into the cooling water. It is recommended design.

3- Main contributor to water in the oil system is steam bypassing the steam seals. This is particularly prevalent in steam turbines with high back pressure or high first-stage pressure, after the seals are worn. It is good practice to minimize this occurrence by providing air purge connections on the bearing seals of steam turbines. Dry instrument air will provide positive pressure in the lube oil area. This also buffers the seal and eliminates the possibility of outside air entering the bearing case. 

GAS TURBINE LUBRICATION

In general there are two classes of gas turbines:

1- Heavy duty gas turbines. Lubricant selection and lubrication of these gas turbines are similar to steam turbine. Standard components of these turbines are fairly massive and bearings are located at some distance from the heat sources. Usually petroleum-base lubricants perform satisfactory for these gas turbines.

2- Lightweight aero-derivative gas turbines. They are mainly based on aircraft gas turbine engines. Aero-derivative gas turbines are quite compact. Size and weight are extremely important and bearings are located relatively close to sources of heat. Aero-derivative gas turbines require that the oil not only lubricates under more severe thermal and oxidative conditions, but that oil serves as a heat transfer fluid as well, carrying heat away from the bearings and shafts. Additionally aero-derivative gas turbines are more subjected to repeated and rapid starts as well as hot peaks. Extreme operating conditions of aero-derivative gas turbines are usually require a high-quality synthetic-base-oil (usually ester-base). A successful example of synthetic lubricant for aero-derivative gas turbine is ETO 2380.

COMPRESSOR AND PUMP LUBRICATION

Some compressors operate with gas discharge temperatures exceeding 160^oC, therefore, the lubricating oil must have good oxidation and thermal stability. In some compressors or pumps, lubricant is in contact with moisture (from handled fluid), which requires good demulsibility of lubricant. The overwhelming majority of compressors and pumps are best served by premium grade oils with ISO VG 32 or 46 (sometimes ISO VG 68 or 100). However, there are many different types of compressors and pumps and each machine manufacturer is likely to recommend only those lubricants that have been used successfully before. Occasionally compressor lubricants have to be formulated for exceptional severe-service performance.

DYNAMIC COMPRESSOR AND DYNAMIC PUMP LUBRICATION

Superior dynamic compressor and pump lubricants are usually premium extreme-pressure (EP), multi-purpose oils designed for dependable performance over a wide range of temperatures and operating conditions. Usually gear unit and dynamic machine are lubricated with same oil and selected lubricant should be well suited for gear units, bearings, etc. Synthetic lubricant formulated from PAG base-stock dedicated for anti-wear, severe-service and long-life are usually specified for dynamic machines.

Successful dynamic machine lubricant should have: 1- Superb oxidative and thermal stability. 2- High viscosity indexes 2- Low pour points for easy cold temperature startup. 4- Excellent lubricity for enhanced resistance to friction and wear. 5- Extreme pressure lubrication. 6- Mechanical breakdown resistance. 7- High resistance to sludge and varnish formation. 8- Non-corrosively and stain resistant. 9- Compatibility with elastomers and coating (particularly seal system components, gear unit internal paint, etc). GLYCOLUBE is an example of successful dynamic machine synthetic lubricant particularly recommended for centrifugal compressor. Lubricants with ISO VG 32, 46 and 68 are commonly used in dynamic compressors as well as dynamic pumps.

GEAR UNIT LUBRICATION

Lubricants in gear units are subject to shock loads and associated overloading. This creates extreme pressure (EP) requirements for gear unit lube oils. Gears should be continuously lubricated, at the same time, lube oil must be kept clean. Gear unit lubricants should perform satisfactory for gear systems as well as gear unit bearings. Viscosity is probably the single most important factor in gear unit lubricant selection. Viscosity relates to load, speed and temperature. Other important factors are: EP additives (relates to load and speed), viscosity index (relates to temperature), and oxidation stability (relates to temperature and contamination). Lube oil film thickness is mainly function of operating speed. Based on experience, high speed gear units (above 5000 rpm) often require heaver oil (for example sometimes heaver than ISO-grade 100).

MINERAL OILS FOR GEAR UNITS

Mineral oils are still in common use for gear units. Extreme pressure (EP) additives of the lead-napthenate, sulphur-phosphate or similar type are recommended for gear drives when a higher load capacity lubricant is required. As a general rule, this type of oil should be used in low speed, highly loaded gear drives, with medium operating temperature. EP oils are more expensive compare to straight mineral oils. Some of EP oils have a very short life above 75^oC.

Compounded oils are available for gear units using many different additives. The most commonly available additive is a molybdenum disulfide compound that has been successfully used in some gear applications. It is difficult for a gear manufacturer to recommend these oils since some of these additives have a tendency to separate from the base-stock. Viscosity improvers in gear drives should be used with great care. These polymer additives do great textbook things for the viscosity index and extend the operating temperature range of oil. What must be remembered is that polymers are non-Newtonian fluids (shearing reduces viscosity). A gear unit is a very heavy shear application, and as a result, the viscosity reduces rapidly if too much polymer is used.

SYNTHESIZED HYDROCARBON LUBRICANTS

Synthesized hydrocarbon lubricants are gaining more wide-spread acceptance in gear unit applications. Properly formulated synthesized hydrocarbon lubricants based on diesters-PAO can result in significantly improve gear unit (gears and bearings) reliability. SPARTAN EP is an example of successful synthesized hydrocarbon (PAO base) lubricant for gear units.

Not to be confused with the highly desirable synthesized hydrocarbons (typically diesters and PAOs), true synthetic lubricants are not recommended for general gear applications due to cost, availability, and lack of knowledge of their properties (in gear unit applications). In extreme applications of higher or lower temperature or fire protection, true synthetic lubricants may be used for gear units. The user must be careful when selecting these lubricants since some of them remove paint and attack rubber seals. The more recent synthesized hydrocarbons have many desirable features such as compatibility with mineral oils and excellent high and low temperature properties. They should be an excellent selection when EP lubricants are required along with high temperature operation.  

(Amin Almasi is a rotating machine consultant based in Brisbane, Australia)