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Lubricant Requirements, Properties and Maintenance for Natural Gas Compressors y
G.E. Totten, Totten , G.E. Totten & Associates LLC Roland J. Bishop, Bishop, Dow Chemical Company Tags: compressor lubrication , synthetic lubricants
ompressor or s are are engi engineered neered in a var var iety of types types and and conf igur igur ations. ons. The The f inal desig design n selection Compress depends depend s on numerous merous f actor actor s. s. Gas type and requi required red pressu pressure re are sign signif icant icant f actor actor s on both oth compress ompressor or and compress ompressor or lubr lubr icant icant selection. on. As temper atures res and and pressu pre ssure res s in increas rease e, the stress tress on the lubr lubr icant icant increas rease es as we well. The The f ir st part of t of t his two-pa o-part ser ies appe appea ared i n the May-J May-Ju une issue issue of ML, and and may be acce accessed ssed at www. www.m machi achinery nerylub lubr r icat icati on on.c .com om.. It It address ddre ssed ed compress ompressor or types, types, common oper ating cond condii tions ons and and lubr lubr icant icant volume lume guide guideliline nes. s. In In this se second part, rt , the aut aut hor s address ddre ss issue issues that hat pertai pertain n more speci pecif f ically ically to the lubr lubr icant icant,, wit with some some consi onsider der ation of natur al al gas tr ansmission issi on compress ompressor or and lubr lubr icant icant issue issues.
Compressor Lubricants The The cho choice ice of a compre compress ssor or lubr lubr icant icant depends depend s on the type and cons onstr uct uction of t of the compress ompressor or , the gas be being compre compress ssed ed,, t he deg degree of compressi ompression on and the f i nal outl et temper ature. re. Piston Piston compress ompressor or s provi provide de the highe highest gas pressu pressure res s and and are among mong the mos most diff icult icult from the standpoi ndpo int of cylinder linder lubr lubr icat ication. on. Rot Rota ary compress ompressor or s wit wi th f inal pressu pressure res s be below 1 mega mega Pascal (MPa), approxi pproxi matel tely 145 psi, are are less diff icult icul t to lubr lubr icate icate.. Rot Rota ary vane vane compress ompressor or s requi require re the use use of an antiw ntiwe ear oil 1 because cause an R&O oil is often i nsuff suff icient icient for the cr ankcas nkcase e splash lubr lubr icat ication of a of a reci recipro proca catting compre compress ssor or . The The selection of t of the proper c proper compress ompressor or and applica pplicattion-dependent lubr lubr icant icant wit with the appropr iate iate physicalsical -chem chemical ical properti properties is vit vital to a succe succ essf ssf ul ul proc process.2 ISO 6743 - Part Part 3A provi provide des s a classif classi f icat ication proc procedu edure for compress ompressor or lubr lubr icant ica nts s based based on the type of equi equipment pment and oper ating cond condii tions. ons. Some of t of the mos most commonl ommon l y reported oil-re il -rela lated ted ser vice vice probl problem ems s wit with compre compress ssor or s in include clude::3 y y y y
y y
y y y y y y y y
Increas rease e in oil visco viscosity sity and total total acid acidity Copper corrosi orrosion on (oil turns rns green green)) Slud Sludge deposi depo sitts Subst Subst antial ntial oil entr ainment ainment in dischar ischar ge gas (air (ai r ) due to dec decreas reased ed eff icien icienc cy of the demis demister ter element Oil str str ainer ainer pluggin lugging Bear ing f ailure ailure.. For For succe successf ssf ul ul oper ation, on, compre compress ssor or oil must ust exhibi exhibitt the foll follo owin wing properti properties:4,5,6 Oxidation resis resisttance A wide wide oper ating temper ature r ange (high f lash lash poi poi nt, nt, lo low pou pour poi poi nt, nt, high visco visc osity sity index) ndex) Low vo volat latility ility Super Super ior antiw ntiwe ear performa performance Good demulsibili demulsibility ty Adequa dequate te corrosi orrosion on resis resisttance Therm Thermal/ al/ox oxiidative ive stability ability Rust Rust and corrosi orrosion on i nhibit hibition
y y y y y y
Hydrol ydrol yticall ytically y st able able Mater Mater ial ial comp compa atibility ibility Nonslu onslud dgin ging performa perform ance Min Minimal oil lo loss to the system Nonfoa onfoaming be behavior havi or Nontoxic. ontoxic.
The The succe succ essf ssf ul ul dev devel opment of a compre compress ssor or lubr lubr icant ica nt will depend on how we well the oil meets meet s these tech techn nical requi requirement rements. s.
Gas Solubility in the Lubricant The The solubility lubility of natur al al gas and and oth other hydroca ydrocar r bons ons is much higher high er iin petrol petroleum oils and and pol pol yalp alphao haolef i n (PAO) synt synth hetics etics comp compa ared to oth other c er commonl ommonl y used used synth ynthetic etic base base stoc tocks such as diester s and and pol polyalky alkyllene gly glycols (PAG). That That is expec expected because cause both oth hydro hydroca car r bon gas and and petrol petroleum-bas m-based ed oils are are sim similar ilar mol mol ecule cules con consis sistti ng pr imar ily ily of C -H bond bonds s un unlike like di ester s and and PAGs, which are are rela relattive ively pola polar r . In In f act act, in in a typical typical PAG mol molecule cul e, every third hird atom in the pol polymer back backbone is an an oxyg oxygen atom, tom, which makes kes it it quite uite pola polar r . Therefore Therefore,, hydro hydroca car r bons ons are are less so soluble lubl e in PAGs. In In wet sump sump reci recipro proca catting and and rota rotary scre screw w compre c ompress ssor or s, s, the compress ompressed ed gas and and the lubr lubr icant icant come into contac ontactt wit with each oth other . Hydro Hydroca car r bon gase gase s are are inf initel tely soluble lubl e i n mi ner al al oil and a nd PAOPAO-based based compress ompressor or lubr lubr icant icants, s, while while the solubility lubility of hydroca ydrocar r bon gase gase s in increas rease e s wit with increasi reasin ng pressu pressure re at a con cons stant temper ature in a le less comp compa atible ible f luid luid such as an an ISO 220 pol pol ypropyl ypropy l ene gly glycol as illustr illustr ated in Figure Figure 1. 7
Conv onver sel y, in i ncreasi reasin ng the temper ature at a con cons stant pressu pressure re will resul resultt in lower gas er gas so solubility lubility..7 Because cause increasi reasin ng gas so solubility lubility dec decreas rease es visco visc osity sity,, at a t some poi point the visco visc osity sity reduc reducttion of t of the compress ompressor or lubr lubr icant ica nt may be too much, and and lubr lubr icat ication f ailure ailure may resul resultt because cause of loss of hydrodyna ydrodyn amic lubr lubr icat ication, on, Figure Figure 2.2
y y y y y y
Hydrol ydrol yticall ytically y st able able Mater Mater ial ial comp compa atibility ibility Nonslu onslud dgin ging performa perform ance Min Minimal oil lo loss to the system Nonfoa onfoaming be behavior havi or Nontoxic. ontoxic.
The The succe succ essf ssf ul ul dev devel opment of a compre compress ssor or lubr lubr icant ica nt will depend on how we well the oil meets meet s these tech techn nical requi requirement rements. s.
Gas Solubility in the Lubricant The The solubility lubility of natur al al gas and and oth other hydroca ydrocar r bons ons is much higher high er iin petrol petroleum oils and and pol pol yalp alphao haolef i n (PAO) synt synth hetics etics comp compa ared to oth other c er commonl ommonl y used used synth ynthetic etic base base stoc tocks such as diester s and and pol polyalky alkyllene gly glycols (PAG). That That is expec expected because cause both oth hydro hydroca car r bon gas and and petrol petroleum-bas m-based ed oils are are sim similar ilar mol mol ecule cules con consis sistti ng pr imar ily ily of C -H bond bonds s un unlike like di ester s and and PAGs, which are are rela relattive ively pola polar r . In In f act act, in in a typical typical PAG mol molecule cul e, every third hird atom in the pol polymer back backbone is an an oxyg oxygen atom, tom, which makes kes it it quite uite pola polar r . Therefore Therefore,, hydro hydroca car r bons ons are are less so soluble lubl e in PAGs. In In wet sump sump reci recipro proca catting and and rota rotary scre screw w compre c ompress ssor or s, s, the compress ompressed ed gas and and the lubr lubr icant icant come into contac ontactt wit with each oth other . Hydro Hydroca car r bon gase gase s are are inf initel tely soluble lubl e i n mi ner al al oil and a nd PAOPAO-based based compress ompressor or lubr lubr icant icants, s, while while the solubility lubility of hydroca ydrocar r bon gase gase s in increas rease e s wit with increasi reasin ng pressu pressure re at a con cons stant temper ature in a le less comp compa atible ible f luid luid such as an an ISO 220 pol pol ypropyl ypropy l ene gly glycol as illustr illustr ated in Figure Figure 1. 7
Conv onver sel y, in i ncreasi reasin ng the temper ature at a con cons stant pressu pressure re will resul resultt in lower gas er gas so solubility lubility..7 Because cause increasi reasin ng gas so solubility lubility dec decreas rease es visco visc osity sity,, at a t some poi point the visco visc osity sity reduc reducttion of t of the compress ompressor or lubr lubr icant ica nt may be too much, and and lubr lubr icat ication f ailure ailure may resul resultt because cause of loss of hydrodyna ydrodyn amic lubr lubr icat ication, on, Figure Figure 2.2
The The solubility lubility of var var i ous gase gases in in lubr lubr icant icants s has been been measu measured red and reported. reported.5,7 ,8 The solubility lubility was ,8 The measu measured red in a f ixed volume lume appa ppar atus. A know known amou mount of gas and and lubr lubr icant icant was allo all ow ed to reach reach equilib equilibr r ium ium at a given given temper ature. re. Gas so solubility lubili ty was calculated calculated usin using the gas laws. The The lubr lubr icant icant was st stirred to f acilit acilitate equilib eq uilibr r ium ium. So Solubility lubility of metha methane ne at pressu pressure res s up up to 5000 psig is comp compa ared at 50ºC for three lubr lubr icant icants: s: PAG, PAO and and petrol petroleum oil in in Figure Figure 3.
The The metha methane ne gas so solubility lubili ty in PAG is roughl roughly y half half that hat for a PAO and and petrol petrol eum oil, and and that hat solubility lubility was nea near ly as high in i n the PAO as in i n the petrol petroleum oil. Gas so solubility lubility exhibi exhibitts a sign sig nif icant icant effec effect on lubr lubr icant icant visco visc osity sity.. The The grea reater the solubility lubili ty of t of the gas in in the oil, the grea reater the visco viscosity sity l oss (visco (viscosity sity dilut ilution). on). A lubr lubr icant icant visco visc osity sity dilut ilution chart chart is sho shown in Figure Figure 4 for metha methane ne at 50ºC.
Similar gas solubility compar isons for nitrogen and ethylene are provided in Figures 5 and 6. 8
Lubricant Solubility in the Gas The lubr icant solubility in the compressed gas should also be minimized to reduce carryover by absorption of the lubr icant in the gas. Matthews, using a constant pressure f low through a load (gr avimetr ic) cell 5 evaluated the absorption of the lubr icant in natur al gas. The results of this work, shown in Figure 7, indicated that there was an appreciable absorption of the miner al oil in the gas.
Compared to the PAG, the lubr icant showed no appreciable loss. Fluid Analysis Procedures Table 1 shows a list of testing procedures that are commonly used to evaluate compressor lubr icants.
Click here to see Table 1.
In this article, the basic pr inciples of natur al gas composition and its compression have been discussed. An over view of var ious compressor designs used for natur al gas, their lubr ication and potential lubr icant-related problems have been provided. Finally, var ious tests that have been reported for use with compressor lubr ication have been summar ized. This information offer s a comprehensive over view of natur al gas compressor lubr ication and f luid maintenance.
References Ar bocus, G. (1977). Synthetic Compressor Lubr icants - State of the Art. Lubr ication Engineer ing, Vol. 34, No. 7, p. 372-374. 2. Patzau, S. and Szchawnicka, E. (1989). Oils for Air and Technical Gas Compressor s. Trybologia, Vol. 20, No. 4, p. 18-21. 3. Sugiur a, K., Miyagawa, T. and Nakano, H. (1982). Labor atory Evaluation and Field Performance of Oil-Flooded Rotary Compressor Oils. Lubr ication Engineer i ng, Vol. 38, No. 8, p. 510-518. 4. Short, G. (1983). Development of Synthetic Lubr icants for Extended Life in Rotary-Screw Compressor s. Lubr ication Engineer ing, Vol. 40, No. 8, p. 463-470. 5. Matthews, P. (1990). Lubr ication of Reciprocating Compressor s. J. Synth. Lub., Vol. 6, No. 4, p. 292-317. 6. Van Ormer , H. (Febr uary 1987). Tr im Compressed Air Cost with Synthetic Lubr icants. Power , p. 43-45. 7. Tolf a, J. (1990). Synthetic Lubr icants Suitable for Use in Process and Hydrocar bon Gas Compressor s. Lubr ication Engineer ing, Vol. 47, No. 4, p. 289-295. 8. Gar g, D. (1991). Polyalkylene Glycol-Based Compressor Lubr icants. Paper presented at the Sixth Annual Reciprocating Compressor Conference, Salt Lake City, UT, September . 9. Mang, T. and Jünemann, H. (1972). Erdöl kohle- Erdgas-Petrochem. Verneigt BrennstoffChemie. Vol. 25, No. 8, p. 459-464. 10. Cohen, S. (1987). Development of a Synthetic Compressor Oil Based on Two-Stage Hydrotreated Petroleum Basestocks, Lubr ication Engineer ing, Vol. 44, No. 3, p. 230-238. 1.
Natural Gas Compressors and Their Lubrication y
G.E. Totten, G.E. Totten & Associates LLC Roland J. Bishop, Dow Chemical Company Tags: compressor lubrication , oil oxidation
Natural gas is widely used to heat homes, generate electricity and as a basic material used in the manufacture of many types of chemicals. Natural gas, like petroleum oil, is found in l arge reservoirs underground and must be extracted from these underground cells and transported to processing plants and then to distribution centers for final delivery to the end user. The gas is moved with the use of many types and sizes of compressors that collect, pressurize and push the gas though the distribution pipes to the various processing centers and points of use. The compressors that move the gas are located in ships and drilling fields, in chemical and process plants, and in the huge maze of pipes that makeup the distribution network, which brings gas to the market in a pure, useable form. This article explains various aspects of gas, gas compressor and compressor lubrication, including compressor lubricants, fluid maintenance and some basic compressor failure analysis guidelines. Natural gas and petroleum oil formed as a result of the decay of plants and animals that lived on earth millions of years ago. The decaying matter was subsequently trapped in huge pockets called gas reservoirs in rock l ayers underground. These pockets may contain predominantly gas or they may exist together. It is estimated that the amount of recoverable natural gas within the United States alone is 900 to 1300 trillion cubic feet (Tcf).1 The composition of natural gas at the well h ead is variable and often contains different compositions of volatile hydrocarbons in addition to contaminants including carbon dioxide, hydrogen sulfide and nitrogen. Commercial pipeline natural gas contains predominantly methane and lesser amounts of ethane, propane and sometimes fractional quantities of butane as shown in Table 1 . 2
Click Here To See Tables 1 and 2.
For transportation and storage, natural gas must be compressed to save space. Gas pressures in pipelines used to transport natural gas are typically maintained at 1000 to 1500 psig. To assure that these pressures are maintained, compressing stations are placed approximately 100 miles apart along the pipeline. This application requires compressors and lubricants specifically designed for this use.
Gas Compressors Compressors can be classified i nto two basic categories, reciprocating and rotary. 5 Reciprocating compressors are used for compressing natural gases and other process gases when desired pressures are high and gas flow rates are relatively low. They are also used for compressing air. Reciprocating Compressors Reciprocating compressors compress gas by physically reducing the volume of gas contained in a cylinder using a piston. As the gas volume i s decreased, there is a corresponding increase in pressure. This type of compressor is referred to as a positive displacement type. Reciprocating compressors are typically a once-through process. That is, gas compression and lubricant separation occur in a single pass. Reciprocating compressors may be fu rther classified as single-acting or double-acting. Single-acting compressors, also classified as automotive compressors or t runk piston units5, compress gas on one side of the piston, in one direction. Double-acting compressors compress gas on both sides of the piston. To consider the lubrication process, it is convenient to divide the parts that need to be lubricated into two categories, cylinder parts and running parts. Cylinder parts include pistons, piston rings, cylinder liners, cylinder packing and valves. All parts associated with the driving end (the crankcase end), crosshead guides, main bearing and wristpin,
crankpin and crosshead pin bearings are running parts. An equation recommended by Scales for estimating the amount of oil to inject into a cylinder for lubrication is: 4
Q = BxSxNx62.8 / 10,000,000
Where: B is the bore size (inches), S is the stroke (inches), N is the rotational speed (rpm) and Q is the usage rate expressed as quarts of oil per 24-hour day. The lubricant is then fed directly to the cylinders and packings using a mechanical pump and lubricator arrangement. Single-acting machines, whi ch are usually open to the crankcase, utilize splash l ubrication for cylinder lubrication. Compressor valves are lubricated from the atomized gas-lubricant in the system. Compared with cylinder part lubrication, the lubrication of running parts is typically much simpler because there is no contact with the gas. The equipment manufacturer specifies the required viscosity grade. Because gas temperature increases with increasing pressure, if h eat is not removed, the lubricant will be exposed to high temperatures and undergo severe decomposition. Therefore, compressor cylinders are equipped with cooling jackets. One of the most important roles of the compressor cylinder lubricant is as a coolant. The coolant is usually water or a water-glycol refrigerant. Although the same lubricant can be used to cool both the cylinder and the running parts, there are many cases where different lubricants are used because the cylinder lubricant is exposed to compressed gas at high temperatures. Therefore, the lubri cant should also exhibit thermal and oxidative stability. Table 2 compares compressor operating temperatures. 6 Rotary Compressors Rotary compressors are classified as positive displacement or dynamic compressors. A positive displacement compressor utilizes gas volume reduction to increase gas pressure. Examples of this type of compressor include rotary screw, l obe and vane compressors (Figure 1,7,8,9 Figure 2 3 and Figure 3 3).
Figure
1. Screw Compressor
Figure
2. Lobe Compressor
Figure
3. Vane Compressor
The rotary screw compressor illustrated in Figure 1 consists of two intermeshing screws or rotors which trap gas between the rotors and the compressor case. 10 The motor drives the male rotor which in turn drives the female rotor. Both rotors are encased in a housing provided with gas inlet and outlet ports. Gas i s drawn through the inlet port into the voids between the rotors. As the rotors move, the volume of trapped gas is successively reduced and compressed by the rotors coming into mesh. These compressors are available as dry or wet (oil-flooded) screw types. In the dryscrew type, the rotors run in side of a stator without a lubricant (or coolant). The heat of compression i s removed outside of the compressor, li miting it to a single-stage operation. In the oil-flooded screw type compressor, the lu bricant is injected into the gas, which is trapped inside of the stator. In this case, the l ubricant is used for cooling, sealing and lubrication. The gas is removed from the compressed gas-lubricant mixture in a separator. Rotary compressors, such as the screw compressor, continuously recirculate (1 to 8 times per minute) the l ubricant-gas mixture to facilitate gas cooling and separation as opposed to reciprocating compressors, which are once-through processes.10 In a rotary screw compressor, the lu bricant is injected into the compressor housing. The rotors are exposed to a mixture of the gas and lubricant. In addition to providing a thin film on the rotors to prevent metal-to-metal contact, the lubricant also p rovides a sealing function to prevent gas recompression, whi ch occurs when high-pressure, hot gas escapes across the seal between the rotors or other meshing surfaces and is compressed again. Recompression causes gas discharge temperatures to exceed the designed range for the unit. This often leads to l oss of throughput and poor reliability. The lubricant also serves as a coolant by removing heat generated during gas compression. For example, for rotary screw air compressors, the air discharge temperature may be 80 ºC to 110ºC (180ºF to 230ºF), accelerating oxidation due to turbulent mixing of the hot air and l ubricant.6 In addition to these functions, the bearings at the inlet and outlet of the compressor must be lubricated. With rotary screw compressors, the lubricant is i n contact with the
gas being compressed at high temperatures and it experiences high shearing force between the intermeshing rotors. These are demanding use-conditions for the lubricant. A simplified diagram for lubricant flow in a typical rotary screw compressor is shown in Figure 4.8
Figure
4. Lubricant Flow in a Rotary Screw Compressor
The lubricant and gas mixture from the compressor discharge line goes into a gas/lubricant separator where the compressed gas is separated from the lubricant. After separation, the lubricant is co oled and filtered, then pumped back into the compressor housing and bearings. A schematic diagram for a rotary lobe compressor is provided in Figure 2. 3 The principle of operation is analogous to the rotary screw compressor, except that with the lobe compressor the mating lobes are not typically lubricated for air service. As the lobe impellers rotate, gas is trapped between the l obe impellers and the compressor case where the gas is pressurized through the rotation of lobes and then discharged. The bearings and timing gears are lubricated using a pressurized lubricating system or sump. A rotary vane compressor is schematically illustrated in Figure 3 . 3 Rotary vane compressors consist of a rotor with multiple sliding vanes that are mounted eccentrically in a casing. As the rotor rotates, gas is drawn into areas of in creasing volume (A) and discharged as compressed gas from areas of small volume (B). As with reciprocating compressors, lubrication of rotary vane compressors is also a oncethrough operation. The lubricant is injected into the compressor casing and it exits with the compressed gas and is u sually not recirculated. The lu bricant provides a thin film
between the compressor casing and the sliding vanes, while providing lubrication within the slots in the rotor for the vanes. The sliding motion of the vanes along the surface of the compressor housing requires a lubricant that can withstand the high pressures in the compressor system. A dynamic compressor, such as the c entrifugal compressor shown in Figure 5 3, operates on a different principle. Click Here to See
Figure
5.
Energy from a set of blades rotating at high speed is transferred to a gas, which is then discharged to a diffuser where the gas velocity is reduced, and i ts kinetic energy is
converted to static pressure. One of the advantages of this type of compressor is the potential to handle large volumes of gases. In a centrifugal compressor, the lubricant and gas do not come into contact with each other, which is a major distinction from reciprocating, rotary screw and rotary vane compressors. The lu bricant requirements are simpler and usually a good rust and oxidation-inhibited oil will provide satisfactory lubrication of the bearings, gears and seals. The choice of a compressor lubricant depends on the type and construction of the compressor, the gas being compressed, the degree of compression and the final outlet temperature. Piston compressors provide the highest gas pressures and are among the most difficult from the standpoint of cylinder and valve lubrication and equipment reliability. However, R&O (rust and oxidation inhibited) oil i s often sufficient for the crankcase splash lubrication of a reciprocating compressor. Rotary compressors with final pressures below 1 Mpa (approximately 1 45 psi) are less difficult to lubricate. Because of the potential for vane to cylinder or lobe-to-lobe contact, rotary screw and vane compressors require the use of an antiwear (AW) oil. The selection of the proper compressor and application-dependent lubricant with the appropriate physical-chemical properties is vital to a successful process, and will be addressed fully in the second part of this two-part series of gas compressor and compressor lubrication issues. References Estimate obtained from the ³Natural Gas Week´ Web site: www.naturalgas.org/TERMDEF.HTM. 2. ³Unit Course 2: For Natural Gas Compressors.´ Worthington Compression. Corpus Christi, TX. 3. Wills, J. (1980). ³Chapter 14 - Compressors.´ Lubrication Fundamentals. Marcel Dekker Inc., New York, NY, p. 365-394. 4. ³Unit Course 1 - For Natural Gas Compressors - An Introduction to the Basic Function and Components of a Gas Compressor Package.´ Weatherford Compression. Corpus Christi, TX. 5. Scales, W. (1997). ³Chapter 19 - Air Compressor Lubrication.´ Tribology Data Handbook, Ed. E.R. Booser. CRC Press, Boca Raton, FL, p. 242-247. 6. Cohen, S. (1987). ³Development of a Synthetic Compressor Oil Based on TwoStage Hydrotreated Petroleum Basestocks.´ Lubrication Engineering, Vol. 44, N o. 3, p. 230-238. 7. Short, G. (1983). ³Development of Synthetic Lubricants for Extended Life in Rotary-Screw Compressors.´ Lubrication Engineering, Vol. 40, No. 8, p. 463-470. 8. Miller, J. (1989). ³Synthetic and HVI Compressor Lubricants.´ J. Synth. Lubrication Engineering, Vol. 6, No. 2 , p. 107-122. 9. Tolfa, J. (1990). ³Synthetic Lubricants Suitable for Use in Process and Hydrocarbon Gas Compressors.´ Lubrication Engineering, Vol. 47, No. 4, p. 289295. 10. Kist, K., and Doperalski, E. (1979). ³Brief Introduction to the Screw Compressor.´ AIChE 86th National Meeting, Paper 68E. 1.
Managing Lubricant Viscosity to Maintain Compressor Health y
Robert Kasameyer Tags: compressor lubrication , viscosity
If you¶re running one of the approximately 140 working refineries in the United States, the last thing you need is an unplanned shutdown. But a production standstill is exactl y what is at risk if you don¶t keep an eye on the viscosity of the lubricating oil used in any of the rotary compressors in the plant, with the highest risk of these being the gas compressors. One minute all processes are up and running, and the next there¶s a bearing failure and production stops. It¶s not just the cost of l ost production either. A compressor failure in a single part of the refinery can cost tens of thousands of dollars a day in lost revenue, with similar amounts to rebuild a compressor, and hundreds of thousands of dollars for a replacement. There¶s also the cost of maintaining spares. Clearly managing lubricant viscosity is critical to maintaining compressor health, but it is a common practice to monitor lubricant viscosity in each major compressor once a month by sending a sample to a lab for testing. For compressors where lubricant comes in contact with methane and other light hydrocarbon gases, the lubricant¶s viscosity can break down much more quickly, increasing the risk of failure. Through hard luck, refiners also have found that real-time temperature monitoring is inadequate to monitor lu bricant viscosity. A major Gulf Coast refinery claims it has solved the problem by moving to real-time monitoring of lube oil viscosity in critical compressors. ³We recognized that in-line viscometers are the best way to know what i s happening to the lube oil in our large screw compressors,´ says the plant manager. ³Further, we have found in-li ne lubrication viscosity monitoring offers a cost-effective way to keep track of compressor health.´ The true measure of the health of a l ubricant¶s viscosity can only be gauged when measured in i ts natural position with gas vapors dissolved in the lubricant. In addition, monitoring lubricant temperature isn¶t sufficient to protect compressor bearings, especially in a pplications where process starts and stops can occur. What¶s needed is in-line viscosity monitoring to help provide plant operators with real-time data on lubricant viscosity. There is a solution for refinery managers working to keep plants onl ine and producing. New, inexpensive and rugged in-li ne viscometers are able to monitor real-time changes in lubricant viscosity, offering a cost-effective way to keep trac k of compressor health in real time.
Refineries and Compressors Rotary compressors are used throughout oil refineries in applications ranging from vapor recovery to gasprocessing operations. Screw and scroll compressors make u p a significant portion of thi s equipment. Screw compressors use two reciprocal screws to c ompress gases. Gas is fed into the co mpressor by suction and moved through the threads by the rotating screws. Compression takes place as th e clearance between the threads decreases, forcing the compressed gas to exit at the end of the screws. Scroll compressors, often known as spiral compressors, use two interleaved spiral vanes to move an d compress fluids and gases. Typically found in intermediate and end-product applications, scroll compressors are valued for their reli ability and smooth operation.
The Importance of Lubricant Viscosity In both types of compressors, lube oil is used to seal the c ompressor from gas leaks, lubricate moving parts and manage temperature during operation. The condition of lubricant oil is a critical factor in extending a compressor¶s bearing life and overall reliability. Monitoring and managing lubricant viscosity can prevent co stly breakdowns due to bearing failure. Viscosity also plays a role in energy efficiency, as demand for more efficient compressors is driving the use of low er-viscosity lubricants. A range of lube oil s, typically synthetic in composition, i s available for use in compressors. Water resistance, thermal stability, long life, resistance to oxidation and resistance to absorption of process gases are all important characteristics. While the goal is a lubricant with a long and useful li fe, harsh environments, contaminants and even humidity in the refinery¶s external environment can greatly reduce lube oil¶s useable lifespan. Monitoring lube oil viscosity is the best way to prevent bearing wear and compressor failure. While some plants may monitor as infrequently as once a month, rapid changes in viscosity occur, and the results can be severe.
Changes in Viscosity and Consequent Risks Compressor lube oils are formulated to work well and remain stable at high temperatures and pressures. Hydro-treated mineral oils are used for their low gas solubility (1 to 5 percent). Synthetic compressor lubricants are used depending on the process and how much gas dilution is present. PAO (Polyalphaolefin) oils, for example, have excellent water and oxidation resistance. PAG (Polyalkaline Glycol) oils, which do not readily absorb gases, are used in a pplications where process gases are compressed. Many factors can affect lube oil viscosity. These include oxidation, dilution, contamination, bubbles and temperature changes. Oxidation occurs when churning lube oil foams, exposing more oil to surface air and causing oxidation that lowers viscosity and threatens useful lubricant life. Dilution is the result when lubricant oil i s diluted with gas such as methane, dropping viscosity. Bubbles form as foaming oil churns against the screws or vanes of th e compressor, instantly dropping the viscosity of the oil. In contamination, vapors from hydrocarbons being processed can mix wi th lube oil. This light hydrocarbon and methane contamination ± sometimes called ³a wi tches¶ brew´ ± makes measuring viscosity challenging. Significant changes in temperature can occur ± typically at start-up ± that affect the viscosity of the underlying lube oil as well as any contaminants, further aggravating the situati on. A range of compressor failures can result. Bearings, both rotary and thrust, can fail, which in turn cause wear on the rotor assembly. Replacing bearings is less costly than a total rebuild or replacement. Either way, the plant faces downtime. The unpredictability of viscosity changes means monthly checks are not enough to prevent bearing failure and subsequent plant downtime. Some compressor customers are designing in -line viscometers into compressors to monitor real-time viscosity changes that happen between standard oil lab analyses, viewing this ³preventative´ approach as an ideal way to ensure bearing life and minimize the costs associated with unscheduled downtime.
Process Viscometer Approaches
Not all process viscometers are created equal. Several instruments employ an innovative sensor technology that uses an oscill ating piston and electromagnetic sensors. Other process viscometer technology approaches include falling piston, falli ng sphere, glass-capillary, U-tube and vibration designs. In all cases, plant managers should look for certain characteristics for in-li ne lubricant viscosity measurement, such as menu-driven electronic controls, self-cleaning sensors, built-in t emperature detection, multiple output signals, automatic viscosity control, data l ogging, quick-change memory settings, security and alerts. Menu-driven electronic controls can be powerful and easy to use, while a self-cleaning sensor uses the in-line fluid to clean the sensor as it is taking measurements to reduce unscheduled maintenance. With built-in temperature detection, the sensor should show temperature as an analog reading. For automatic viscosity control, look for a sensor that is pre-set but reconfigurable. Th e sensor should be able to ³learn´ how much control is needed for each fluid setting. Security and alerts are designed to prevent unauthorized changes and sound an alarm when set points are reached so operators can take action quickly. With multiple output signals, the sen sors should display temperature and temperature-compensated viscosity readings. For process lines that run more than one fluid, quick-change memory settings simpli fy the process of changing settings. In data logging, the date and time code should be automatically logged, creating an audit trail an d simplifying performance and quality-trend measurement. About the Author Robert Kasameyer is the president and CEO of Cambridge Viscosity Inc., a global leader in fluid viscosity measurement. The company¶s major applications include life sciences and pharmaceuticals as well a s oil and gas exploration, oil analysis, chemical processing and coating. Kasameyer holds a BSME from Tufts University and an MBA from Harvard University.
Oil
Analysis Boosts Compressor Reliability
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Daryl Beatty, Dow Chemical Company Tags: oil analysis , compressor lubrication
There are sever al key elements that must be addressed to ensure an effective compressor lubr icant analysis progr am. Error s or omissions related to these elements can lead to unnecessary expenditures and lower reliability. Knowing what actions should be taken based on compressor lubr icant analysis is important for success. Under standing some common mistakes and misconceptions about water in compressor lubr icants and the effect the environment has on the lubr icants and the compressor is also key to a successf ul lubr icant analysis progr am. Inlet Air Quality For centr if ugal compressor s, the signif icant air quality issues are particulate removal through f iltr ation and the effect of acid gases on inter cooler corrosion. In a centr if ugal compressor , there is only minimal contact between the air and the lubr icant and the sump sizes are typically quite lar ge, resulting in dilution of any contaminants. For these reasons, it is r are for ambient air conditions to signif icantly affect f luid life, or have a detr imental effect on the r unning gear of the compressor . Double-acting reciprocating compressor s are not totally immune to air contaminants, but are less subject to inlet air quality issues because of a continuous inf usion of fresh cylinder lubr icant. The fresh lubr icant ser ves to f lush contaminants through the system with a protective effect, even though these compressor s have a high lubr icant consumption.
The ambient air is a more ser ious concern for rotary screw compressor s where the entire f l ow of air through the compressor contacts the f luid, and the f luid is effectively acting as a scr ubber to absor b the acids and contaminants. Even a low concentr ation of acid is signif icant, when the sheer volume of air being handled is considered. Some of this acid will be absor bed by the f luid, which will show up analytically as a lower pH and higher acid number (AN). Lubr icants for rotary screw compressor s are formulated with good corrosion protection, but eventually even that is over whelmed. Once this occur s, f ilter s may plug more frequently due to corrosion particulate. This effect results in signif icantly shortened f luid life. It is not unusual in a contaminated environment to see the life of a nominal 8,000hour f luid reduced to 2,000 hour s. The life of downstream component s, such as after cooler s and dryer s, is also often compromised by corrosion caused by acid gases which pass through the compressor from the environment. These gases then condense with water in the cooler s and dryer s and dr astically increase corrosion r ates. What can be done to extend f luid life and solve these problems? y
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Remote air inlets may be installed to obtain inlet air from a sour ce away from the contamination. This is typically outside the building. Ironically, inside air is r arely of better quality than the makeup air being taken into the building. Air can be tested by suspending corrosivity coupons of copper and silver in the air near the compressor . After a specif ied per iod, labor atory analysis of the resulting compounds on the surf ace of the coupons will reveal the type of contaminant in the air and the degree of contamination. Inlet air scr ubber s may then be prescr ibed based on the degree and type of contamination to remove contaminants from the inlet air . The result is longer f luid life and decreased corrosive attack of compressor bear i ngs, cooler s, dryer s and downstream equipment.
Lubricant Analysis Parameters The key analysis par ameter s vary with the type of lubr icant being used. Most new rotary compressor s are equipped with polyglycol-based lubr icants. With a polyglycol or polyglycol/polyolester-based compressor f luids, the following par ameter s are of great interest: y
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pH - A r apid or excessive decrease in pH indicates ingestion of acid gases or other contaminants from the environment. This will require a f luid change, but also indicates that the sour ce of contamination needs to be eliminated. AN - The acid number is an indication of remaining usef ul f luid life. AN may increase with either oxidative degr adation of the lubr icant or accumulation of contaminants from the environment. Either way, this accumulated acid ref lects the depletion of the corrosion inhibition package. Suggested change points vary, typically from 1.0 to 2.0. The f luid life from the time the AN reaches 1.0 until the time it will reach 2.0 is only 10 per cent to 20 per cent of the over all life span. Due to the diff iculty of removing the last 20 per cent of the f luid from the compressor , it is probable that stretching the change point from 1.0 until 2.0 actually costs more in terms of shortening the life of the next char ge of f luid, than is gained on the f ir st char ge. Stretching the change point here is f alse economy and results only in a greater exposure of the compressor to f luid containing high levels of acid. Viscosity - The viscosity of some or iginal equipment manuf acturer (OEM) compressor f luids are specif ically designed for the needs of that compressor application and do not f it in either the ISO 32 or 46 viscosity r anges. With polyglycol f luids, viscosity will normally increase about 10 per cent with use, then stabilize. If lab per sonnel are not aware of the initial viscosity of a f luid, they often assume it or iginally f it into an ISO r ange and then mistakenly condemn it for high or low viscosity. It is important to always compare viscosity to the specif ication for that f luid, not an ISO r a nge. With polyglycols, it is unusual for f luid to f ail due to viscosity change, because the f luids are resistant to varnish and sludge formation and don¶t have a tendency to gain viscosity. Contaminants - Hydrocar bon contamination is typically monitored to assure that oper ator s are not mixing f luid types. If f luids are mixed, the life of the f luid may be compromised.
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Oxidation - Polyalphaolef in (PAO) or miner al oil change points can be determined by the degree of oxidation of the base f luid. This is not necessary with polyglycols, because AN is a reliable indicator of f luid condition. High Particulate - If the corrosion particles are mostly small particulate, the f ilter s should be changed and measures should be taken to determine what acidic condition is causing the corrosion.
For PAO-based compressor lubr icants, the pH, AN and viscosity must also be monitored. In addition, it is usef ul to monitor oxidation, typically by infr ared spectroscopy. Monitor ing the oxidation level is usef ul in preventing varnish and deposit formation. In cases where the f luid quantity justif ies it, rotating pressure vessel oxidation test (RPVOT) can reveal the remaining usef ul life of a PAO-based f luid. With polyglycols these steps are not necessary, because AN is a reliable indicator of the remaining f luid life.
Table 1. Typical Values for New and Suggested Condemning Limits for Polyglycol Compressor Fluids
Water Content Table 2 compares typical water content of samples from air compressor s with other equipment in a plant. This specif ic plant is located in a humid environment in a southern U.S. state.
Table 2. Water Analysis of L ubricant Samples from Compressors and Other Typical Equipment in the Same Plant The water content of the air compressor s r anges from 0.4 per cent to 0.6 per cent, while other types of equipment in the same plant have about 1/100 the water content. Labs often f lag a sample like this with an alarm, when in f act these levels of water are normal for rotary compressor s, and the compressor f luids that are specially made for them are formulated to f unction in this environment.
Consider this scenar i o: A plant performs per iodic routine analysis on each of these f luids. Each time analysis is done, the water level is reported as shown in Table 2, along with the recommendation that the f luid in the compressor s be changed. What is not considered is that the water level in the new f luid will again reach these levels quickly. Fluid has been wasted with no benef i t. Water levels will also vary with f luid basestock type, because some basestocks are capable of toler ating more water before free water is released into the f luid. The keys are: y
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Know the maximum amount of water that a type of f luid will toler ate before free water is released into the system. For example, polyglycol compressor lubr icants, which are used by sever al compressor OEMs, will toler ate about 0.8 per cent water before free water becomes a problem. With hydrocar bons and synthetic hydrocar bons, free water will typically become an issue at lower levels. Recognize that the water level in a sample is also a char acter istic of the equipment application in which the f luid is used. Rotary air compressor s are higher than near ly any other application because of the contact of the lubr icant with lar ge amounts of humid incoming air . When the air is compressed, water vapor is condensed. That water must either be absor bed by the f luid or allowed to cir culate as free water . Other types of equipment, as demonstr ated in Table 2, tend to be much lower . Analysis alerts should be set accordingly. Changing the f luid does not solve a water problem for long. The water level var ies with humidity, ambient temper ature, duty cycle and machine oper ating temper ature. Water is being continuously ingested. Make sure the lab is aware of f luid type and application, so that water level limits can be set accordingly. Don¶t obsess about water levels in compressor f luids. The water level is high in these units compared to other types of equipment. Histor ically, compressor air ends (also called compressor units) from major OEMs typically last about 10 year s, with a few reaching 20 year s before having to be rebuilt. All of them have had high water levels and ser ved long lives. Only recently have the high levels been noted. The compressor f luid should be specif ically designed for use in rotary compressor s, and contain corrosion protection adequate for this demanding application.
Compressor Condensate The analysis of compressor condensate is a usef ul tool in detecting some corrosive or acid gases in the air that may not be effectively tr apped by the lubr icant. A low pH or high AN in the condensate may reveal a corrosive condition, which if left unchecked, will lead to short after cooler and refr iger ated dryer life. The typical sour ce of these problems is contaminated inlet air and remedies which were previously discussed. Metals analysis of the condensate can also reveal the r ate of corrosion, which may already be occurr ing in after cooler s and dryer s. In addition, the total or ganic car bon (TOC) or total oil and grease (TOG) analysis indicate the carryover r ate of lubr icant from the compressor , which is an indicator of the eff iciency and condition of the air /oil separ ator .
Figure
1. Two-stage Tandem Compressor
Fluid Analysis Certain actions should be taken based on elemental analysis or particulate in the f luid. The sour ces and the importance of particulate and tr ace elements in a rotary compressor should be considered f ir st. A rotary compressor is unique in that the metals and particulate in the f luid can or iginate from sever al sour ces. Pr imary sour ces include: y y y
Ingestion with the inlet air , either through or bypassing the inlet f ilter Corrosion particles, pr imar ily from the upper portion of the receiver tank Wear debr is from rotor s, housing, gear s and bear ings
The key to determining the type and sour ce is analysis. When particulate is the concern, analytical ferrogr aphy is one simple and usef ul technique for differentiating between these three sour ces. Once determined, any of these problems can be readily resolved. Figure 2 demonstr ates the var ious types of ferrous particulate that can readily be distinguished by analytical ferrogr aphy. In this case, wear debr is and corrosion particles are present.
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2. Analytical Ferrograph from a Rotary Compressor Showing a Variety of Particulate Types
Figure 3 shows a var iety of particulate types identif ied by this technique. Of particular interest in rotary compressor s is the ability to distinguish ferrous particles or iginating as wear debr is, which is a ser ious concern, from corrosion particles that typically or iginate from the surf ace of the receiver tank, and should be tr apped by the bear ing f ilter before reaching the bear ings.
Figure 3. A Variety of Particulate Types Distinguished by Analytical Ferrography
In addition, a var iety of particulate is ingested with the air from the environment. While it is not usually necessary to specif ically identify the items that are ingested, it does indicate an inlet air f iltr ation problem, which can then be diagnosed and remedied. Identif ication of particulate into these three categor ies - wear , r ust/corrosion and ingested mater ial - will determine the action needed to alleviate these problems, thereby enhancing compressor life and reliability. Tr ace element analysis is also usef ul for the ear ly detection of potential problems in compressor s. Table 3 offer s some possibilities for explaining the presence of var ious elements in compressor f luids and the signif icance of those elements. Click Here to See Table 3. It is important to note that many of the elements are usef ul for resolving specif ic issues, but are not tr acked on a routine basis. Of the elements listed in Table 3, iron is the most usef ul wear metal for helping determine the compressor¶s specif ic condition. In addition to spectroscopy, one of the particle size analysis methods will provide more usef ul data. An increasing trend in particulate levels should be investigated.
Figure
4. Lubricated Compressors
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5. Two-stage Tandem Compressor
Good Observation Skills Analytical techniques are only part of the story. There are many simple oper ational issues related to the f luid and f luid system that the lab has no way of detecting or resolving. There is no substitute for effective obser vation of oper ating conditions, trends and the environment in which the compressor is oper ating. For centr if ugal compressor s, trending of vibr ation readings on each stage will reveal the formation of bear ing deposits or other subtle changes that can then be remedied before signif icant damage occur s. The following items should be considered for rotary screw air compressor s: Inlet
air temperature, humidity and contamination. Contaminated air contaminates the compressor f luid, resulting in equipment f ailures and downtime. Avoid ingestion of air from sour ces containing acid gases. These would include boiler exhaust, diesel exhaust, any oper ations dischar ging acid and other s. Dr aeger tubes used for per sonnel air monitor ing are usually not sensitive enough to detect the concentr ations of contaminants that will cause a problem. Air quality test kits, utilizing silver and copper coupons are usef ul in evaluating the quality of the air that is in contact with the lubr icant. Armed with this knowledge, a remote air inlet may be installed to obtain air from a cleaner area, or an air scr ubber may be installed to protect both the lubr icant life and the compressor . Low operating temperature. Proper dischar ge air temper ature is important to maximizing f luid and compressor life. As a gener al r ule of thumb for a humid environment, the dischar ge temper ature should be 100°F (55°C) higher than the ambient temper ature of the inlet air to prevent accumulation of water in the lubr icant. Air-cooled compressor s typically have cooler s sized to prevent the compressor from r unning below this temper ature and thus automatically avoid this problem. A watercooled compressor may cool too eff iciently and condense water in the f luid. If the water separ ates from the f luid and collects in the receiver tank, the free water may not be detected by lab per sonnel, because the sample will not be representative of what is tr uly occurr ing in the compressor . One solution is to obser ve the temper atures and make ad justments as necessary. Depending on f luid type, temper ature and humidity, water levels of up to 0.7 per cent in a rotary screw compressor are normal. Levels above that amount indicate free water in the system and require inter vention. High operating temperature. High oper ating temper ature presents a different set of problems. The f luid will easily remain below 0.5 per cent water content with no free water , but high temper ature is detr imental to f luid life. Every 18°F (10°C) increase in temper ature will reduce f luid life by approximately half . The temper ature at which a f luid is r ated for its nominal life, typically 8,000 hour s for most polyglycol, ester or PAO compressor f luids, will vary. A f luid r ated for 8,000 hour s at 200°F would be expected to signif icantly outlast a f luid r ated for 8,000 hour s at 180°F. Load and unload performance. Load and unload performance affects carryover and ener g y savings. A compressor , which is allowed to r un unloaded with a minimal air demand, will typically exper ience high lubr icant consumption and more internal condensation and corrosion. In addition, it will use much more electr icity, resulting in massive ener gy waste and increased expense. This cycle and loading should be obser ved and the compressor s¶ usage should be ad justed to match the air demand. Compressor OEMs have developed computer ized control systems to continually monitor and ad just to maximize savings.
Leaks and condition of all couplings. Minor f luid leaks may be a warning of an impending f ailure of a coupling, gasket or seal. They should not be ignored. The amount of makeup fluid. The volume of makeup f luid added to each compressor should be logged for frequency and amount. Attempt to do a ³mater ial balance,´ or accounting of the f luid; how much f luid is lost to leaks ver sus what is carr i ed over into the condensate tr aps or plant air . A sudden increase in f luid makeup r ate might correspond with the change to an ineff icient, defective or improper ly installed air /coolant separ ator . Finding this quickly can result in f luid savings and avoid the contamination of downstream components. OEM separ ator s are typically closely matched to the air f lows and other char acter istics of specif ic compressor s. Also, the mater ials of constr uction are compatible with the OEM f luids, to prevent compatibility f ailures. Savings in lubr icant and ener gy can greatly exceed the cost of separ ator elements. Separator and bearing filter monitoring. It is important to tr ack separ ator and bear i ng f ilter differential pressures and frequency of changes. These are usef ul trends for sever al reasons. Fir st, the depletion of corrosion protection packages in the lubr icant may result in shortened f ilter element life, due to blinding with corrosion particles. Progressively shorter inter vals between f ilter changes, while on the same char ge of lubr icant, are strong indicator s of this condition. The ingestion of particulates in the air may also result in short separ ator life. Particulate levels and element life may relate to the quality of the inlet f ilter element, an air leak or improper installation. Also, changing separ ator elements at the OEM-prescr ibed differential pressure (usually about 10 psi) results in ener gy savings and avoids the potential collapse of a separ ator , which results in massive amounts of f luid being dischar ged downstream into the air system. These are a few examples of lubr icant-related obser vations which are easily made and may be consistently monitored to improve compressor reliability.
A Strategy for Compressor and Compressor Fluid Health A str ategy should focus on leading r ather than tr ailing indicator s. In trying to anticipate compressor reliability issues, the focus should be on the indicator s discussed above, which can anticipate and prevent problems. Fluid condition, in terms of pH, AN and contamination should be monitored before corrosion results in high metallic content. The delivery of clean inlet air to the compressor to prevent contamination of the lubr icant and corrosion of the system should be a main focus. Analysis is also cr itical, but only if the proper par ameter s are assessed and action is taken to prevent and resolve f uture problems. http://www.machinerylubrication.com/View/28594/compressor-oil-loss
PAGs are Rising to the Top of the Synthetic Market y
Daryl Beatty, Dow Chemical Company Martin Greaves, Dow Chemical Company Tags: synthetic lubricants
For centuries, lubricants have been utilized as a way to reduce friction and wear on
moving parts. In 2005, 40 million tons of lubricants were produced. While natural mineral oil-based fluids represent the majority of the market demand, many technological advances in equipment and machinery would not be possible without the benefits offered by improvements in synthetic lubricants, which currently make up only two percent of the market. While polyalphaolefins (PAOs) fill some of these needs, a growing number of applications are demanding higher performance requirements, or require unique specifications that are not met by traditional lubricants.
One of the most versatile types of synthetics is polyalkylene glycol (PAG) lubricants. PAGs are generally known as compressor lubricants, and their use in industry has increased since the 19 80s. Increasing performance standards in the automotive and industrial markets peg these sectors as areas that show promise for growth. This article offers an overview of the main synthetic base stock chemistries and an indepth analysis of the benefits and uses of PAGs.
Synthetic Lubricant Base Stocks There are six major base stock types used i n the development of synthetic lubricants, with each offering its own set of unique properties and applications. Silicones are valued for their low volatility, inertness to most chemical contaminants and thermal stability in severe applications, as well as their performance in lowtemperature environments. These qualities make them an excellent candidate for use as heat transfer fluids, specialty grease applications and DOT Type 5 automotive brake fluids. However, there are two limitations of silicones that must be considered for lubricating applications. First, they cannot be used in the cylinder lubrication of internal combustion engines because the combustion by-product is sili con dioxide. Second, extreme pressure performance is limited, and common extreme pressure additives are incompatible with them. In their proper applications, the fluid life and hydrolytic stability of silicones is unsurpassed.
Diesters, or dibasic acid esters, were developed during World War II and are the reaction product of long-chain alcohols and carboxylic acids. Historically, they have been effective as reciprocating compressor lubricants due to their low coking tendency at temperatures of 400°F or hi gher. They also provide excellent solvency and detergency. The aggressiveness of diesters toward elastomers, seals and hoses has limited the usefulness of these fluids. Newer fluids, such as polyol esters, meet the needs of many applications formerly filled by diesters. Polyol esters, or Neopentyl poly esters, have largely replaced diesters in hi ghtemperature applications where oxi dative stability is critical. Common applications include their use as lubricants in aircraft engines, high-temperature gas turbines,
hydraulic fluids, and as heat exchange fluids. They can also be used as a co-blended basestock with PAOs to enhance additive solubility and reduce the tendency of PAOs to shrink and harden elastomers. PAOs are hydrocarbon polymers manufactured by the catalytic oligomerization of linear alpha olefins like alpha-decene. They are considered high-performance lubricants and provide a high viscosity index and hydrolytic stabili ty. PAOs are the most commonly used, and are generally l ess expensive than other synthetic lu bricants. They have been used in passenger car motor oils, as well as numerous industrial lubricant applications. Phosphate Esters are valued in applications where safety and fire resistance are critical considerations, whi ch include fire-resistant hydraulic fluids and aircraft fluids. High flash points and fire points enhance their resistance to ignition, and their low h eat of combustion makes them excellent self-extinguishing fluids. However, they do have several weaknesses including poor hydrolytic stability, which can lead to the formation of aggressive acidic by-products. Care must be taken when used because they can also react and degrade a variety of commonly used sealants and paints. PAGs offer quality lubricity, high natural viscosity index and goo d temperature stability. PAG base fluids are available in both water soluble and in soluble forms, and in a wide range of viscosity grades. Th ey offer low volatility in high-temperature applications and can be used in high- and low-temperature environments. They are commonly used as quenchants, metalworking fluids, food-grade lubricants and as lubricants in hydraulic and compressor equipment. However, the water soluble PAGs are incompatible with petroleum oil, and care must be taken in transitioning equipment from hydrocarbon oils to PAGs.
The Development of PAGs PAGs were one of the first synthetic lubricants to be developed and commercialized. They were created under mandate from the U.S. Navy in response to hydraulic fluid fires on ships resulting from ordnance strikes during World War II. In 1942, and for the next 30 years, the Navy b egan to exclusively use PAG-based water glycol hydraulic fluids that were fire-resistant and could operate over a wide temperature range. Later, PAGs began to see extensive use as textile lu bricants and as quenchants in metal heat treating. PAGs are classified by their weight percent composition of oxypropylene versus oxyethylene units in the polymer chain. PAGs with 100 weight percent oxypropylene groups are water insoluble; whereas those with 50 to 75 weight percent oxyethylene are water soluble at ambient temperatures. Although PAGs have long been used as in dustrial lubricants, recent work has l ed to the development of PAG lubricants for use in equipment in the food processing industry. These products are known as food-grade approved lubricants. In these applications, they offer excellent lubricity, increased oxidative stability, a high viscosity index (180 to 280) and low pour points. They are one of the few synthetic substances identified in the FDA¶s food additive regulation for food-grade lubricant base stocks, 21 CFR § 178.3570, for use in industrial machinery when incidental food contact with a lu bricant may occur.
PAG Applications and Benefits Because of the properties that make up PAG lubricants, they are uniquely sui ted for a number of industrial and manufacturing applications. Their water solubility allows for easy clean-up of equipment. PAG lubricants offer high viscosity indexes, and are shear stable. PAGs are also valued for their low volatility in high-temperature applications, and for resistance to formation of residue and deposits. Their biodegradability makes them ideal for environmentally sensitive applications.
PAGs are best known as compressor lubricants. PAGs are also the lubricant of choice in high-pressure natural gas and ethylene compression, where the viscosity stability of hydrocarbon-based lubricants is adversely affected due to solubility of the gas in the fluid. In refrigeration compression, PAG and polyol ester-type lubricants are used almost exclusively with the current generation of environmentally friendly HFC refrigerants such as R-134a and R-152a. The two largest U.S. air compressor OEMs have used PAG lubricants as the standard factory fill in rotary screw air compressors for almost 20 years. More recently, a third compressor OEM has begun to offer PAG as an optional fluid. From the laboratory perspective, the condition of PAG fluids i s relatively easy to monitor. In most applications, as the end o f the useful life approaches, the only significant change is the increase in acid number (AN) from fluid oxidation. Depending on the additive package, fresh PAGs will typically have an AN of 0.1 to 0.5 mg KOH/g. An increase of 1.0 from the new fluid specification is a good condemning limit. Viscosity remains fairly stable, even during the latter stages of fluid life. Water li mits may be set higher for PAGs than hydrocarbon fluids because they are more water tolerant than other fluid types. Even a ³water insoluble´ PAG will tolerate as much as 0.7 percent water contamination before allowing free water to exist in the fluid. PAGs are also useful in i ndustrial equipment operating year-round without seasonal changes. Their superior heat transfer characteristics and thermal and oxidation stability make them ideal for use as heat transfer flui ds in large, open vented systems and for process fluids in the production of plastics, elastomers, threads or fabricated parts where compatibility of the fluid with the processed part is i mportant. Textile fiber production is another industry that benefits from the use of PAGs. These lubricants do not stain or discolor fibers, and are easily removed during the scouring process. PAGs are also the lubricant of choice for many high-speed, high-temperature fiber processes where shear stability is a requirement. In addition, they are often used as lubricants in textile manufacturing equipment as extreme-pressure gear lubricants. A renewed emphasis on energy conservation has increased interest in energy-efficient gear lubricants. For example, the extreme demands of gear lubrication in wind turbines are being met by PAGs. The l ow velocities and high surface loadings on the gears in these units have resulted in micropitting problems with conventional hydrocarbon oils that have been overcome with PAG-based fluids. In other gearbox applications, especially worm gears, the naturally low coefficient of friction found in PAG flui ds results in energy savings, lower temperatures and lower wear rates.
Versatility Meets Performance For more than 60 years, synthetic l ubricants have provided a viable alternative to traditional hydrocarbon lubricants. Each type serves unique rol es, with PAGs performing in both high- and low-temperature environments, in areas of extreme pressure and where water solubility i s desired. PAGs can be designed to form a wide variety of polymers. The design of the polymer can be tailored to the lubricant application to provide, for example, the desired viscosity, pour point, solubility and other attributes. This versatility and the applications in which they are used shows that PAGs account for about 24 percent of the entire synthetic lubricant market. Low pour points, a wide range of viscosities, resi stance to varnish formation, increased solvency and a wide range of solubility all add to PAG lu bricants¶ reputation as a high-performance synthetic lubricant on the market. With continuing emphasis on environmentally acceptable lubricants in industry, these qualities will continue to push PAGs to the forefront of the synthetic market.
Bearing basics for gas-industry screw compressors Tags: Cylindrical roller thrust bearings | Machinery and equipment | Reliability 16 September, 2005 · Technology ·
Optimum bearing selection can extend bearing life and improve compressor performance. Screw compressorusage in natural gas applications has risen steadily over the past decade and continues to grow. This is chiefly due to lower gas pressures in many older natural-gas fields, which make reciprocating compressors uneconomical to install a nd operate. Today screw compressors are found in both sweet-gas (not containing H 2S) and sour-gas (containing H 2S) applications. They are employed at the wellhead t o draw natural gas from the ground and boost the pressure to feed gas into pipelines.
As the usageof screw compressors increases, so does the focus on a major screw compressor component ± rolling bearings. Bearings play a critical role in screw compressor performance. Bearing-related problems, however, can impair compressor reliability. In the worst case, they can lead to catastrophic failure, interrupting gas production. Wells and scrubbing stations are often located in remote, hard-to-reach areas, making repairs difficult and expensive. Although standby compressors are sometimes available, they must be maintained to avoid potential problems at start-up. Accordingly, r eliability-conscious gas-industry professionals and well operators seek t o avert bearing-related problems before they occur. A big first step is becoming familiar with the basics of bearing operation in screw compressors and learning how to specify the correct bearing arrangement, bearing types and materials for compressor applications.
In twin screw compressorstwo meshing rotors turn in opposite directions inside the compressor housing. On the suction side, gas is drawn into a cavity produced between the housing wall and the two rotors. As the rotors turn, the cavity decreases in size, compressing
the gas and then discharging it. Gas field compressors are usually driven by a gas-fuelled engine, either directly or though a gearbox. The function of rolling bearings in screw compressors is to provide accurate radial and axial positioning of the compressor rotors and to properly support rotor loads. Bearings are employed at the suction and discharge ends; their number and configuration vary depending on load and application requirements. Typically, there are two cylindrical r oller bearings at the suction end, one supporting each compressor rotor. A second pair of cylindrical roller bearings is used at the discharge end. Some compressors use journal bearings for radial support. The discharge end is equipped with two or more thrust bearings, which are usually single-row angular contact ball bearings or four-point contact ball bearings (QJ bearings). These bearings support pure axial loads.
As the gasundergoes compression, the temperature increases. Oil injected into the compression cavity lubricates the screws, seals leakage paths and cools the gas. After being discharged with the compressed gas and separated out, the same oil lubricates and removes heat from the bearings. Operators should recognize the risks of injecting too much oil into screw compressors. This as natural gas contains a mix of gases and is usually saturated with water vapor and sometimes acids and hydrogen sulfide are present. Injecting too much oil can lead to excessive cooling, causing condensation of water and acids. The condensates will mix with the oil and impair bearing lubrication, cause corrosion, and can lead to bearing failure. The oil used should have the proper viscosity and should not contain additives that cause water emulsification. Engine crankcase oils are typically not suitable as compressor oils. Oil samples should be taken periodically and analyzed for degradation, viscosity and water content. Natural gas also contains solid particles that can da mage bearings and other components. To prevent damage, compressors should have effective oil filters.
Sour natural gas, which contains hydrogen sulfide (H 2S), poses a particularly difficult challenge. Hardened steel and materials containing residual stresses are susceptible to sulfide stress cracking. This is a brittle crack phenomenon that occurs at low stress levels. In addition, hydrogen sulfide can mix with water and form sulfuric acid (H 2SO4) leading to corrosion. Bearing cages should be made of stress-free materials to avoid damage from sulfide stress cracking. Well operators should carefully consider cage materials when specifying compressor bearings.
Polyamide cages,which are made of an injection-moulded glass-fiber reinforced polymer, also offer a possible solution. These cages have proved succ essful in sour-gas applications with operating temperatures up to 70°C (158°F). Higher temperatures, however, can cause polyamide to age prematurely when exposed to aggressive gases and acids. A new solution is
now available with cages made of PEEK, a polymeric material with superior chemical, temperature and aging resistance. PEEK also has superior properties as a bearing material with low friction and wear rate, making it suitable as a cage material in bearing applications with marginal lubrication. Traditionally, the natural-gas industry has refrained from using brass and other ³yellow´ metals in compressor and pipeline applications because of potential sulfide str ess cracking. Residual-stress-free brass, however, is not subject to stress cracking. Machined brass cages made from centrifugally cast tubing are free from residual stresses. A large number of single row angular contact ball bearings, four-point contact ball bearings and cylindrical roller bearings are available with this type of cage. Although somewhat contradictory to industry practice, several screw compressor manufacturers have successfully employed such brass cages in sour natural gas compressor bearings since the mid-l980s.
Pressed brass cages, made from brass sheet, are not suitable in sour-gas compressors, since the pressing oper-ation leaves residual stresses in the material. Machined steel cages are commonly asked for by compressor users. Although there is no stress cracking with machined steel, there is a greater risk of metal smearing between the cage pockets and the rolling elements, especially in marginal lubrication. With the availability of PEEK and stress-free machined brass cage materials, there is no longer any reason to use machined steel cages. element bearings must be made of hardened steel to function reliably. Hardened steel components, however, can suffer hydrogen sulfide damage. Rolling
SKF has investigated the use of special coatings to protect bearing steel in sour-gas applications. At the SKF Engineering and Research Center in The Netherlands, SKF scientists have identified critical properties for coating materials. The coating hardness must bond securely to the bearing steel and resist flaking under load. It must also also be non porous so there is no diffusion of hydrogen sulfide through the coating. Another even more promising solution is to use a new grade of a super tough stainless bearing grade steel that is resistant to sulfide stress cracking. SKF is developing and testing such steels with very promising field test results in sour gas screw compressor applications. In these new bearings the balls or rollers are made of ceramic materials. Alternative solutions include SKF NoWear coated balls and rollers made of high nitrogen stainless steel.
The SKF NoWearcoating provides very low friction, protection against smearing and reduced wear, but NoWear is not dense enough to prevent diffusion of hydrogen sulfide. NoWear coated bearings provide a solution in difficult applications working under poor lubrication or low loads. SKF Explorer performance class bearings feature improved steel quality and heat treatment. SKF Explorer single row angular contact ball b earings, as standard manufactured for universal matching, four point-contact ball bearings and cylindrical roller bearings yield longer service life than conventional bearings in demanding screw compressor applications.
Once compressor bearings have been selected and installed, they should be monitored. Handheld monitoring devices for periodically monitoring or permanently installed monitoring devices for online surveillance can provide trending data and detect eventual bearing damage in the early stages and enable well operators to schedule maintenance and repairs for periods of planned shutdown.
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Miscibility Miscibility refers to the ability of a gas or liquid to dissolve uniformly in another gas or liquid.Gases mix with each other in all proportions, but this may or may not apply to liquids, where miscibility depends on chemical affinity. Ethanol, an alcohol, and water are miscible because they are chemically similar, but benzene and water are only slightly miscible because of the very large differences in their chemical properties. Some liquids are essentially insoluble in other liquids, e.g., gasoline in water (and water in gasoline);such liquids are said to be immiscible. Other liquids are only soluble up to a point. In the case of water and ethyl ether (CH3CH2OCH2CH 3), it is possible to dissolve up to about 4 g of ethyl ether in 100 g of water, but the addition of more ethyl ether results in the production of separate layers of the less dense diethyl ether above the denser water layer. And some liquids are completely soluble in other liquids, regardless of the amounts combined; such liquids are said to be miscible in each other in all proportions. (Traditionally, the term solubility has been used interchangeably for miscibility in reference to liquids, even though it should strictly only be applied to solids.) There are several fundamental rules governing the miscibility of liquids in other liquids. First, the solubility of liquids in l iquids increases with increasing temperature. Second, the more similar two compounds are in terms of polarity, the more likely that one is soluble in the other, i.e., polar compounds dissolve in polar compounds, and non-polar compounds dissolve in non-polar compounds. (Polar molecules dissolve in polar molecules because the dipole of one attracts the dipole end of the other.) Thus, benzene and carbon tetrachloride, being both non-polar, dissolve in each other, but neither will appreciably dissolve in water, which is polar. Both alcohols and ethers with up to three or four carbons are miscible in water because the OH groups in these molecules form hydrogen bonds with the water molecules. Alcohols and ethers with higher molecular weights are not miscible in water, however, because the water molecules can not completely surround those molecules. The molecule 1-heptanol, for example, consists of an alkyl chain of seven carbons and an OH group. The OH group forms hydrogen bonds with water molecules, but the alkyl portion of the molecule exerts no attraction on the water molecules. This part of the molecule is called hydrophobic, meaning water-hating. Because this part of the molecule cannot be surrounded by water, 1-heptanol is immiscible in water. In aqueous solutions, globular proteins usually turn their polar groups outward toward the aqueous solvent, and their non-polar groups inward, away from the polar molecules. The nonpolar groups prefer to interact with each other, and exclude water from these regions, leading to immiscibility. This type of interaction is usually weaker than hydrogen bonding, and usually acts over large surface areas.
