sistemas de lubricacion

Chapter 16

Steam and Gas Turbines

MECHANICAL DRIVE STEAM TURBINES

Designed for variable speed, steam turbines are used in industry in a multitude ofways to drive compressors, blowers and pumps. As they are both turbo-machines,

turbines and compressors have similar output-to-speed ratios. This is why steam tur-bines are particularly suitable as direct drives for variable-speed compressors. Steamturbines, Figures 16-1 and 16-2, are able to make full use of the economic advantages ofcogeneration by converting the heat produced by a process into drive power and return-ing heat to the process in the form of steam at the right pressure and temperature. Theymust therefore be designed to operate at steam temperatures and pressures which areideally suited to the process in question. This enables them to be used in a wide range ofapplications in the chemical and petrochemical industries.

UTILITY STEAM TURBINES

Utilities use constant-speed, high-pressure steam turbines for power generation. Asis the case with mechanical drive machines, steam turbines can be designed for condens-ing, back-pressure, or combination service (Figure 16-3). Also, a variety of exhaust cas-ing designs, Figure 16-4, are available.

441

Figure 16-1. 3,6 MW packaged backpressure turbine generator. (Source: Siemens Power GenerationErlangen, Germany)

Copyright © 2000 The Fairmont Press, Inc.

442 Practical Lubrication for Industrial Facilities

Figure 16-2. Rea-ction steam turbines.(Source: SiemensPower Generation,Erlangen, Germany)


Steam and Gas Turbines 443

Figure 16-3. Steam turbine design options.(source: General Electric Company,Fitchburg, Massachusetts)



PRESSURE LUBRICATION OF MULTISTAGE STEAM TURBINES*

All multistage turbines require cool, clean oil supplied to the journal bearings whilethe turbine is operating. This oil is supplied by a system provided by one of three parties:the turbine manufacturer, the driven equipment vendor, or the customer/user. If thelubrication system is to be provided by a customer, the turbine manufacturer will speci-fy the applicable flow, pressure, temperatures, and cooler heat load to allow others todesign the system. Normally, the turbine manufacturer will provide either “stub” pipingat the journal areas, or “manifolded” piping for ultimate connection to the customer pro-vided lube system. (Manifolded piping will minimize the field work required.)

A thrust bearing and an auxiliary drive gear are often provided with steam tur-bines, and these must also be lubricated. Driven equipment, whether provided by theturbine manufacturer or others, must also be evaluated in the design of an appropriatelubrication system.

A lubrication system may also be required to provide oil for a trip-and-throttlevalve, or a governor valve power cylinder. When providing oil to a power cylinder, it isnormal practice to provide a combined pressure lubrication and control oil system. Thissegment of our text will address lubrication systems and combined lubrication/controloil systems both as “lube systems.”

Lube systems can be divided into three classifications, each a variant of the others.These classifications, ranging from the simplest (and therefore the least costly) to themore complex are the following:

Figure 16-4. Exhaust casing designs available for steam turbines.

*Source: Murray Turbomachinery Corporation, Burlington, Iowa.


1.) The basic duty lube system (Figure 16-5) is designed for turbines which can be shutdown for maintenance, typically for turbines operating seasonally, such as sugar mill orair conditioning drives. This system includes a single oil filter, and a single oil cooler.While the turbine is operating, the filter element cannot be changed, nor can the coolerbe cleaned.

2.) The continuous duty lube system (Figure 16-6) is designed for units which run 8000hours or more per year, with few scheduled shutdowns. Examples of applicationsfor continuous duty lube systems are boiler feedwater pump drives, or processcompressors.

3.) The turbine-generator lube system (Figure 16-7) is a variation of the continuous dutylube system, and is also designed for units which run continuously. The addition of asecond cooler will allow the lube system to operate for up to three years without shut-down. Since either cooler, or either filter, can be serviced without shutdown, the limit-ing factor on operating time is the turbine, which must be internally inspected at leastonce every three years. An optional oil purifier, which will remove water and lighthydrocarbons, will often be provided to allow years of operation without an oil change.

Major Components of a Steam Turbine Lube SystemThe standard main oil pump on direct connected turbines is shaft driven from the

turbine by an auxiliary gear on the governor end of the turbine. Due to physical size


Figure 16-5. Basic duty lube system for steam turbines.


restrictions, a typical flow is 75 GPM at 100 PSIG. This is more than adequate for theturbine requirements, but may not be sufficient for the requirements of the driven equip-ment. If flow greater than 75 GPM is required, or if a preference is specified by thepurchaser, a separate motor driven or auxiliary steam turbine driven main oil pump isgenerally provided.

On a turbine-speed reducer (gear) application, the main oil pump is typically drivenby the blind end of the slow speed shaft of the speed reducer. Larger pumps can bemounted on the speed reducer than on the turbine, but a practical limit on speed reducerdriven pumps, again due to size restrictions, is 100 GPM. Above 100 GPM, a spacer typecoupling becomes unwieldy, and increases the overall length of the train. Therefore,motor driven, or small steam turbine driven main oil pumps are provided for applicationsrequiring more than 100 GPM.

Low oil pressure alarm and trip switches are recommended and will be found in mod-ern systems.

Auxiliary oil pumps for any of the three systems options can be AC motor driven, orDC motor driven, or separate steam turbine driven. (The AC motor driven auxiliary oilpump is used as standard). Auxiliary oil pumps are required on all pressure lubricatedsteam turbines for startup and coast down to ensure lubrication of the bearings. All aux-iliary oil pumps are provided with a pressure switch to start automatically upon reduc-tion of oil pressure from the main oil pump. An auxiliary oil pump running pressure switchis typically wired to the annunciator panel.


Figure 16-6. Continuous duty lube system for steam turbines.


A minimum of one oil cooler is required with each pressure lubrication system.Cooling temperatures and heat loads will vary from system to system, but typical tem-peratures are 140�F oil into the cooler, and 120�F oil from the cooler.

Standard oil coolers are shell-and-tube heat exchangers, with cooling water fedthrough the tubes, and oil flow cascaded over the tubes. Care must be used in specify-ing a proper fouling factor for the site specific cooling water. A fouling factor degradesan overall heat transfer coefficient, thereby increasing the size of the heat exchanger,allowing the heat exchanger to operate for longer periods of time, as the tubes become“fouled,” without losing capacity. Lube systems supplied with one cooler will requirethe turbine and driven equipment to be taken off-line to mechanically clean the tubes.When two coolers are provided, the cooler not in actual operation may be cleaned at anytime, with the turbine and driven equipment in operation.

Stem or dial type thermometers are provided before and after the oil coolers.Operators should periodically read and record these temperatures to ensure that thecoolers are operating satisfactorily, and to establish a basis for cleaning schedules. Highoil cooler outlet temperature alarm and trip switches are recommended and will be foundin most systems.

Prior to about 1980, carbon steel oil reservoirs were provided as standard, with stain-less steel reservoirs available as an option. Since then, stainless steel reservoirs and pip-ing have become the standard. Intermittent duty lube systems incorporate a 1-minute


Figure 16-7. Turbine-generator lube system (typical).


(minimum) retention time, while continuous duty lube systems utilize a 3-minute (mini-mum) retention time. Smaller systems (10 to 20 GPM) often utilize a rectangular reser-voir, with system components mounted on it. Larger units utilize a separate console typeoil reservoir with system components mounted on it. Turbine-gear units, or turbine gen-erators, may incorporate the oil reservoir in the baseplate with all system componentsmounted on the baseplate. Turbine-gear units, or turbine generators, may also be providedwith the reservoir in the baseplate, and filters, coolers, and auxiliary pump(s) on a sep-arate baseplate.

The standard oil reservoir design includes a sloped bottom, an oil level sight glass,clean-out openings, and fill, drain and vent openings. Oil heaters mounted in the oilreservoir are supplied if the oil temperature can go below 60�F, or when specified.

Reservoirs are usually provided with an oil level sight glass, and this level must bechecked periodically. Optional liquid level switches to indicate low and/or high oil levelare found on many systems. Normally a small amount of oil will need to be addedbetween oil changes to maintain the proper oil level. If, however, an elevation in oil levelis noticed when no oil has been added, water is probably collecting in the oil.

Water in the oil reservoir is attributable to any of several factors. One is simplycondensation from the air within the reservoir and can be minimized by maintainingthe manufacturer’s specified oil level within the reservoir, and good ventilationaround the turbine. Since condensation will contribute only minor amounts to thelubrication systems, any large accumulation should be immediately investigated, theproblem solved, and the oil changed or purified. In some cases a leak in the shell-and-tube oil cooler(s) may allow cooling water into the oil reservoir. An analysis of thewater will determine if the contaminating water is from the cooling source, but acomparison of the operating pressures of the oil and of the cooling water within theoil cooler will determine the necessity of that analysis. If the oil pressure is greaterthan the water pressure, oil will be forced into the cooling water rather than waterinto the oil.

Another possible contributor to water in the oil is steam bypassing the steam seals,Figure 16-8. This is especially prevalent in turbines with high back pressures and/orhigh first-stage pressure, after the seals are worn. It is good practice to minimize thisoccurrence by providing air purge connections on the bearing seals of turbines. Dryinstrument air will provide positive pressure in the oil seal area. This buffers the seal andeliminates the possibility of outside air entering the bearing case. Air purge connectionscan be retrofitted to existing turbines.

Sight flow indicators provide a visual indication of oil flow through the bearings, andare generally recommended.

A minimum of one oil filter is also required with each pressure lubrication system.The standard filter cartridge will remove particles 25 micron and larger. When specified,filters that will remove particles as small as 5 microns and larger are generally provid-ed, but these cartridges will need to be replaced more frequently. Oil pressure gaugesbefore and after the filters are normally supplied, and up-to-date installations incorpo-rate remote sensing and automated data logging as well. A high oil filter differential pres-sure alarm switch is an added safeguard.

Continuous flow transfer valves are provided with systems utilizing dual coolers, ordual filters. When both dual coolers and dual filters are provided, transfer valves may be




provided as common between coolers and filters, or as an individual valve between cool-ers and between filters. These transfer valves have no off position.

Pressure relief or pressure control valves are used to maintain the proper pressure lev-els throughout the lube system, and to provide ultimate protection against overpressurewithin the system.

GAS TURBINES*

Within the context of industrial machinery, the reader is likely to encounter gas tur-bines as drivers for electric power generators and as mechanical drive turbines for largecompressor trains.

Although gas turbines have been on the industrial scene since the late 1920s, large-scale applications had to wait until the 1950s when rapid advancements in aircraft jetengines brought significant improvement and vastly enhanced acceptance of industrialgas turbines. These improvements touched virtually every requirement cited for mod-ern process plants: low initial cost, good efficiency, maintainability, reliability, opera-tional ease, process flexibility, and environmental acceptability.

From a thermodynamic point of view, a gas turbine—or gas turbine engine—is amachine that accepts and rejects heat at different energy levels and, in the process, pro-duces work. While this work is converted to pressure and velocity energy in the aircraftjet engine, the commercial or industrial gas turbine is arranged to convert this work intoshaft rotation or, more correctly, torque.

The gas turbine (Figure 16-9) consists of an air compressor and gas combustion, gasexpansion, and exhaust sections. The gas turbines cycle is composed of four energy ex-

Figure 16-8. Labyrinth-type steam seals for Siemenssteam turbines. An air purge can be introduced intothe space between sealing groups (arrow).

*Source: Bloch, H.P., “Process Plant Machinery,” Butterworth-Heinemann, Woburn, Massachusetts,1988/1998.


change processes: an adiabatic compressor, a constant-pressure heat addition, an adia-batic expansion, and a constant-pressure heat rejection. The four thermodynamicprocesses can be accomplished either in an open-cycle or a closed-cycle system. Theopen-cycle gas turbine takes ambient air into the compressor as the working substancethat, after compression, is passed through a combustion chamber where the temperatureis raised to a suitable level by the combustion of fuel. It is then expanded inside the tur-bine and exhausted back to the atmosphere. Most industrial-type gas turbines work onthis principle, and Figure 16-10 explains their principal components. Enhancements,such as regeneration (Figure 16-11) are employed to increase cycle efficiencies. The useof two or more hot gas expansion stages makes it possible to produce two-shaft turbines,Figure 16-10. This configuration has greater speed flexibility than single-shaft machines.


The closed-cycle gas turbine uses any gas as the working substance. The gas passesthrough the compressor, then through a heat exchanger where energy is added from asource, then expanded through the turbine and finally back to the compressor througha precooler where some energy may be rejected from the cycle.

Perhaps the most important reasons why process plants use gas turbines are sum-marized as high system reliability and high combined energy system and process effi-ciency. Where the forced outages of a single driver can shut down an entire complex,highest reliability is a must. For projects involving process system modifications of anew process design, choosing the most reliable turbine or energy system rather thanmaintaining an already existing process design can result in significantly higher reliabil-ity and reduced financial loss due to excessive process shutdowns.

Lube Systems for Gas TurbinesThe gas turbine baseplate houses the oil tank and practically all piping connections

ensuring the most attractive arrangement and easy access to the machine. Flexible con-nections are used between the baseplate and the flange-to-flange assembly to allowquick removal of the gas generator or power spool for scheduled maintenance opera-tions. Lube oil pumps, hydraulic oil pumps, filters, pressure control valves and variouscontrol devices are mounted on the lube oil console located near the gas turbine in thebest position for site requirements, or on the turbine baseplate in the accessory area.

Lube oil is fed to the turbine bearings, accessories and load equipment in additionto the hydraulic control devices. High pressure hydraulic oil is used to operate the fuelgas control valves and for the driven compressor seal oil system in turbocompressor

Figure 16-9. General Electric Company Frame 6 gas turbine (35-50 MW range).


Steam and G

as Turbines451Figure 16-10. General Electric/Pignome Model Series 1002 two-shaft gas turbine showing typical nonmenclature.



units. A hydraulic trip system is the primary protection interface between the turbinecontrol and protection system and the components on the turbine which admit or shutoff fuel. An oil-to-air heat exchanger cools oil for the gas turbine lubricating andhydraulic systems. The cooler is sized to meet oil cooling requirements when operatingat the maximum rated temperature.

Lubricants for Steam and Gas TurbinesWhatever their different operating environments, all power generators have a crit-

ical requirement in common: dependable-quality turbine lubricants that will providelong, cost-effective, trouble-free service.

Such lubricants must have excellent thermal and oxidation stability at bearing oiltemperatures that may approach 200�F in a typical steam or gas turbine and exceed 400�Fin modified aircraft (aero-derived) gas turbines. They must readily shed the water thatinfiltrates turbine systems; control the rust and corrosion that could destroy precision sur-faces; resist foaming and air entrainment, which could impair lubrication and lead toequipment breakdown; and filter quickly through bypass or full-flow conditioning filters.

Turbine lubricants should also be versatile, able to serve as both hydraulic fluidand lubricating oil for pumps, compressors, and other auxiliary components.

A handful of premium petroleum-base and synthetic turbine oils easily meet these

Figure 16-11. Gas turbine regenerative cycle diagram. (Source: Nuovo Pignone, Florence, Italy)


demanding requirements. For example, TERESSTIC GT 32, Exxon’s superpremiummineral-oil turbine oil, has served in numerous turbine applications for over 30 yearswithout a changeout. For the higher temperature operation of industrial aero-derivedturbines, Exxon offers ETO 2380 synthetic turbine oil, one of the most widely trusted air-craft turbine engine oils in the world. Table 16-1 summarizes some of these lubricants.

Keep in mind that superior products are generally highly versatile, providing sat-isfactory service in more than one type of plant application. This allows simplifyinglubricant inventories to a relatively few multipurpose products, thus minimizing thechances of potentially costly lubricant misapplication.


Table 16-1. Lubricants for turbines, generators, and industry-associated equipment.(NOTE: The product recommendations given in this table may not apply in all specificinstances. Equipment manufacturer recommendations must always take precedence.When in doubt about a particular application, consult the lube manufacturer’s represen-tative. The product names listed here are trademarks of Exxon Corporation.)


Recall also, from Chapter 4, that TERESSTIC is Exxon’s line of premium circulatingoils. These versatile, multipurpose oils are formulated to provide long service life insteam turbines, land-based gas turbines, hydraulic systems, heat transfer systems, gearcases, friction clutches, and other industrial units for which long, trouble-free service isrequired.

All the TERESSTIC grades have superb thermal and oxidation stability and excellentrust-preventive, antifoam, and water-shedding properties. Their high viscosity indexesallow more uniform lubricating performance over a wide range of ambient and operat-ing temperatures. They are also easily filterable without additive depletion. TERESSTIC GT32, Table 16-2, is Exxon’s first recommendation for steam and industrial gas turbineapplications. It has a potent antioxidant incorporated in a carefully selected and refinedbase oil. This assures exceptionally long life in demanding high-temperature turbineoperations. In many cases TERESSTIC GT 32 has lasted more than 30 years in such appli-cations without a changeout.

The extraordinary thermal and oxidation stability of TERESSTIC GT 32 was con-firmed in severe laboratory tests in which it was compared with seven premium ISO 32competitive turbine oils.

The results are shown in Figure 16-13 and Table 16-3.While most of the oils performed well in at least one test, TERESSTIC GT 32 was the

only one that achieved excellent performance across the board.These laboratory results, combined with many years of proven dependability in the

field, provide strong assurance of reliable, long-life performance under a wide range ofoperating conditions.

Where the turbine manufacturer specifies a higher viscosity oil, TERESSTIC GT 46and TERESSTIC 68 and 77 also provide excellent service in turbine operations.

Lubricants for Geared TurbinesGeared steam and gas turbines are subject to shock loads and occasional over-

loading. The resulting extreme pressure can force the lubricating film out frombetween meshing gear teeth, causing metal-to-metal contact and excessive wear. Tomeet these extreme conditions, Exxon offers TERESSTIC GT EP and TERESSTIC SHP anti-wear turbine oils.

TERESSTIC GT EP, a premium antiwear turbine oil, is formulated to meet the specialrequirements of geared turbines, while offering the same high-quality performance asthe other TERESSTIC products. Under shock conditions, the non-zinc antiwear additive inTERESSTIC GT EP reacts with the metal surfaces to form a protective boundary layer, thusminimizing wear. For typical inspections see Table 16-4.

TERESSTIC SHP, a synthetic-base oil incorporates polyalphaolefin (PAO) basestocksand carefully selected zinc-free, ashless additives. It offers outstanding antiwear per-formance and mild EP characteristics. Compared with conventional petroleum-base oils,TERESSTIC SHP has superior oxidation control and thermal stability, better lubricity, andlower carbon-forming tendency. These qualities can reduce unscheduled downtime,extend drain intervals, and maximize the life of bearings and other critical components.

The superior lubricity of TERESSTIC SHP versus comparable petroleum-base oils canreduce energy consumption. Laboratory and field tests on synthetic-base lubricants have




Table 16-2. Typical inspections for super-premium mineral-oil-base turbine oils.

Table 16-3. Competitive survey of seven turbine oils.



shown that energy savings of 3.5-8.5% are achievable, compared with a petroleum-baseoil. In some cases, because of the higher viscosity index of synthetic-base oils, theseenergy savings were achieved using a lower ISO viscosity grade synthetic lubricant. Itshould be noted, however, that switching to a lower viscosity grade should be done onlywith the concurrence of the equipment manufacturer.

Figure 16-12. Simplified lube and hydraulic oil systems diagrams for industrial gas turbines. (Source:Nuovo Pignone, Florence, Italy)

Figure 16-13. Thermal and oxidation stability of TERESSTIC GT vs. premium ISO grade 32 turbine oils.


Lubricants for Aero-derived Gas Turbine EnginesIn general, there are two classes of gas turbines used in industrial applications:

• Heavy-duty gas turbines based on steam turbine technology (Figures 16-9 and 16-10)..

• Lightweight gas turbines derived from aircraft gas turbine engines (Figure 16-14).

Heavy-duty gas turbine designs are not restricted by size and weight. Standardcomponents are fairly massive and bearings are located at some distance from heatsources. Petroleum-base lubricants like TERESSTIC oils perform satisfactorily under theseoperating conditions.

By contrast, size and weight are extremely important design considerations in aero-derived gas turbine engines. Equipment is quite compact, with bearings locatedrelatively close to sources of heat. Aero-derived gas engines require that the oil not onlylubricate under more severe thermal and oxidative conditions, but that it serve as a heattransfer fluid as well, carrying heat away from the bearings and shafts. Additionally,aero-derived gas turbine engines subjected to repeated, rapid start-ups during peakpower demand typically carry higher loads than conventional heavy-duty turbines.


Figure 16-4. Typical inspections, EP-grade super premium turbine oils.The values shown here are representative of current production. Some are controlled bymanufacturing specifications, while others are not. All may vary within modest ranges.



Figure 16-14. Rolls Royce aeroderivative gas turbine—generator and applications.


ETO 2380—Unsurpassed Performance in Aero-derived Gas Turbine Engines

These extreme operating conditions usually require a high-quality synthetic-baseoil—an oil like Exxon’s ETO 2380. This ester-base synthetic oil supplies approximately50% of the free world’s commercial airline requirements for 5-cSt turbo oil. Over 360 air-lines entrust the safety of their passengers to ETO 2380. Evidently, the “syntheticsoption” deserves closer examination. Accordingly, the reader is referred to Chapter 7 ofthis text.

To summarize, in most applications petroleum-base lubricants provide excellentlubrication. However, modern industrial machinery design is placing unprecedented,severe demands on lubricants. Newer machines are designed for faster speeds and higherunit loads, resulting in higher operating temperatures. Older equipment is being runharder to maximize output. These punishing operating environments have placed a pre-mium on lubricants that can ensure machine reliability and efficiency in severe opera-tions. Additionally, safety and environmental concerns increasingly are dictating the useof long-life, low-volatility lubricants.

In many cases, these demands have pushed petroleum-base lubricants to the limitsof their capabilities, necessitating the development of a new generation of lubricants:synthetics.

Although their initial cost may be higher, synthetic lubricants can offer numerousadvantages over conventional petroleum-base lubricants in severe-service applica-tions—advantages such as longer lubricant life, superior wear protection, greater ther-mal stability, and lower carbon-forming tendencies (Figure 16-15). These qualities cansignificantly reduce long-term costs by extending equipment life and minimizingdowntime.

Here are the lubricants that merit consideration:SPARTAN Synthetic EP (polyalphaolefin base)—a line of seven long-life, extreme-

pressure industrial gear and bearing lubricants, particularly recommended for geartrains and worm gears.

SYNESSTIC (diester base)—a versatile line of five industrial lubricants for compres-sors, hydraulic systems, mist lubrication systems, air-cooled heat exchanger drives, andbearings in pumps and electric motors.

SGO (polyalphaolefin base)—a line of three automotive gear oils specially formu-lated for longer gear life and improved operating economies.

TERESSTIC SHP (polyalphaolefin base)—superpremium circulating, gear, andhydraulic oil, with applications in gear reducers, pumps, marine centrifuge gear boxes,and work gears containing copper alloys where mild EP performance is required.

POLYREX (polyurea soap)—a high-temperature, long-life, multipurpose grease forall types of bearings.

UNIREX SHP (polyalphaolefin base)—a line of five lithium-complex synthetic baseoil greases for automotive and industrial applications.

UNIREX S 2 (polyolester base)—high-viscosity, low-volatility lithium complexgrease that provides excellent high-temperature performance where frequent relubrica-tion is impractical.

SUPERFLO Synthetic, SUPERFLO Synthetic Blend, XD-3 Elite (polyalphaolefin base)—passenger car and heavy-duty automotive oils.




In-service Monitoring of Turbine Oil QualityA well-ordered method of surveillance of the lubrication system is essential for

trouble-free operation. Several resources are available to the lubrication engineer.

ASTM Recommendations

Steam and gas turbine oils are expected to provide years of trouble-free service. In-service monitoring of turbine oils is a valuable means of assuring optimum oil perform-ance and extended equipment life. ASTM D 4378, “Standard Practice for In-serviceMonitoring of Mineral Turbine Oils for Steam and Gas Turbines,” can be used by the tur-bine oil user as a basis for developing a monitoring program and interpreting the testresults. The essential tests and recommendations of ASTM D 4378 are summarized inTable 16-5. This summary is intended only as a general guide; consult an application spe-cialist for assistance in implementing a monitoring program and in interpreting test results.

Figure 16-15. Features and benefits ofsynthetic lubricants summarized.


Steam and G

as Turbines461

Table 16-5. In-service monitoring of turbine oil performance.


462P

ractical Lubrication for Industrial Facilities

Table 16-5. (Continued)


sistemas de lubricacion

Documents

exhaust casing

stainless

main oil pump

pressure control

generator

auxiliary

system components

pressure lubrication