COMPRESSOR HANDBOOK phần 9 ppsx

19.123 TABLE 19.27 List of Synthetic Lubricants 36 Type Kinematic viscosity, cs 210ЊF 100ЊF Ϫ65ЊF Flash point, ЊF Pour point, ЊF Approximate cost per gallon Typical uses Diester: Turbo Oil 15 3.6 14.2 12,600 430 Ϫ90 $10.00 MIL-L-7808 high load capacity, high temperature jet engine oil. Low volatility aircraft hydraulic and instrument oil. MIL-0-6085 3.5 13.5 10,000 450 Ϫ90 10.00 Aircraft hydraulic fluid for alternator drives. MIL-0-6387 4.6 15.8 5,000 410 ϽϪ80 — Phosphate: Tricresyl phosphate 3.8 30.7 — 465 — 3.60 Low flammability hydraulic fluid for diecasting machines. Nonflammable aircraft hydraulic oil. Skydrole 3.85 15.5 Ͼ20,000 355 70 12.00 Nonflammable hydraulic oil for diecasting machines, punch pressures, etc. Pydraul F-9 c 5.8 54 — 430 ϩ5 3.75 Air compressors. Silicone: SF-96 (40) 16 40 850 600 ϽϪ100 30.0 Low-torque aircraft oil bearings, air craft hydraulic and damping fluid. SF-96 (300) 122 300 7,000 605 ϽϪ55 30.00 Heat transfer, hydraulic, and damping applications. SF-96 (1,000) 401 1,000 20,000 605 ϽϪ55 30.00 Heat transfer, hydraulic, and damping applications. DC-710 40 275 — 575 Ϫ10 40.00 Heat transfer, high-temperature trolley bearings. Silcate: OS-45 3.95 12.4 2,400 — ϽϪ85 20.00 Wide-temperature-range aircraft hydraulic fluid. Orsil BF-1 2.4 6.8 1,400 395 ϽϪ100 19.124 TABLE 19.27 List of Synthetic Lubricants 36 (Continued) Type Kinematic viscosity, cs 210ЊF 100ЊF Ϫ65ЊF Flash point, ЊF Pour point, ЊF Approximate cost per gallon Typical uses Polyglycol: LB-140X 5.7 29.8 — 345 Ϫ50 2.40 Water-insoluble oils used for internal-combustion engines. LB-300X 11.0 65.0 — 490 Ϫ40 2.40 (Prestone Motor Oil), high temp. bearings in ovens and LB-650X 21.9 141.0 — 490 Ϫ20 2.40 furnaces and gears. 50-HB-55 2.4 8.9 — 260 Ϫ85 2.40 Water-soluble oils used in wire drawing, metal forming, 50-HB-280X 11.5 60.6 — 500 Ϫ35 2.40 and some machine tools. 50-HB-2000 72 433 — 545 Ϫ25 3.00 Hydrolube 300N — 666.3 — None Ϫ55 2.50 Water-polyglycol mixture used as non-flammable hydraulic fluid in die-casting and machine tool work. Chlorinated aromatics: Aroclor 1248 3.1 48 — 380 20 2.30 Die-casting machines and high-pressure compressors. Aroclor 1254 6.1 470 — None 50 2.30 Polybutenes: No. 8 7.9 72 — 310 Ϫ40 — Electrical oils, hydraulic and shock absorbing fluids, No. 20 106 3,600 — 410 10 1.05 kilns, and ovens, refrigerator compressors. No. 128 4,000 ——450 70 1.40 High pressure compressors. Fluorolubes: Fluorolubes FS 1.10 3.52 — None — 300.00 Equipment handling liquid oxygen, concentrated Fluorolubes S 4.6 24.1 — None — 300.00 hydrogen perioxide, etc. Density of approximately 1.8 grams/cc. Process and natural gas compressors. 19.125 100.000 50.000 20.000 10.000 5.000 3.000 2.000 1.000 500 300 200 150 100 75 50 40 30 15 10 9.0 8.0 7.0 6.0 5.0 4.0 3.0 20 -20 0 20 40 50 80 100 120 140 160 180 200 220 240 260 280 300 Kinematic Viscosity, Centistokes SAE 50 SAE 40 SAE 30 SAE 20 W SAE 10 W Heavy Steam Cylinder Oil Grade 1010 Jet Engine Oil Light Spindle Oil Light Turbine and Electric Motor Oil Medium Turbine Oil T, F o FIGURE 19.87 Viscosity of petroleum oils. 19.126 CHAPTER NINETEEN TABLE 19.28 Viscosity Conversion Factors Multiply By To obtain Stokes (cm 2 /sec) Density (g/cm 3 ) Poises (gm/cm-sec) Poises 100 Centipoises Centistokes Density (g/cm 3 ) Centipoises Centipoises 1.45 ϫ 10 Ϫ 7 Reynes (lb force-sec/in 2 ) Centipoises 2.42 ϫ 10 Ϫ 9 (lb force-min/in 2 ) Centipoises 5.6 ϫ 10 Ϫ 5 (lb mass in-sec) Reyns (lb forc-sec/m 2 ) 6.895 ϫ 10 3 Pascal-sec (N-sec/m 2 ) Centipoises 10 Ϫ 3 Pascal-sec (N-sec/m 2 ) TABLE 19.29 General Types of Additives with Typical Chemical Compositions 36 Function Typical chemical type Oxidation inhibitor Phenolics Dithiophosphate Detergent Calcium petroleum sulfonate Rust inhibitor Organic acids Sodium petroleum sulfonate Wear preventive Trieresyl phosphate Boundary lubrication Chlorinated naphthalene Sulfurized hydrocarbon Viscosity index improver Polyisobutylene Pour-point depressant Polymethnerylate Defoaming agent Silicone oil carbon-steel back; (2) an intermediate layer of copper or bronze; and (3) an overlay of lead-base babbitt from 0.001 to 0.020 in. thick. The intermediate layers increase the mechanical strength of the babbitt bearing and also provide reasonably good bearing surfaces in cases the thin babbitt surface layer is destroyed in operation. Non-Babbitt Bearing Materials. Other common bearing materials used, when- ever babbitt cannot be employed are: • Bronze. Bearing bronzes may be grouped into lead bronzes, tin bronzes, and high-strength bronzes. The strength and high-temperature properties generally improve as one proceeds from the high-lead to high-tin to various high-strength bronzes. However, there is a loss in the compatibility properties as the amount of lead decreases. For this reason, it is generally advisable to use the highest lead 19.127 TABLE 19.30 Composition and Physical Properties of Babbitts 30 Tin-base babbitts Alloy Specific gravity Composition, % Cu Sn Sb Pb Yield point* psi 66ЊF 212ЊF Ultimate strength* psi 66ЊF 212ЊF Brinell hardness 68ЊF 212ЊF Melting point ЊF Complete liquefaction ЊF 1 7.34 4.56 90.9 4.52 None 4400 2680 12,850 6050 17.0 8.0 433 700 2** 7.39 3.1 39.2 7.6 0.03 6100 3000 14,900 8700 24.5 12.0 466 669 3** 7.46 8.3 83.4 8.3 0.03 6800 3100 17,600 9900 27.0 14.5 464 792 4 7.52 3.0 75.0 11.6 10.2 5550 2150 18,150 8900 34.5 12.0 363 583 5 7.75 2.0 65.5 14.1 18.3 2150 2150 18,060 8750 22.5 10.0 358 565 Lead-base babbitts Alloy Specific gravity Composition, % Cu Sn Sb Pb As (max) Yield point* psi 66ЊF 212ЊF Ultimate strength* psi 66ЊF 212ЊF Brinell hardness 68ЊF 212ЊF Melting point ЊF Complete liquefaction ЊF 6(e) 9.33 1.5 20 15 63.5 0.15 3800 2050 14,550 8060 21.0 10.6 358 581 7(f) 9.73 0.50 10 15 75 0.60 3550 1600 15,650 6150 22.5 10.5 464 514 8 10.04 0.50 5 15 80 0.20 3400 1760 15,600 6150 20.5 9.5 459 522 10 10.07 0.50 5 15 83 0.60 3550 1850 15,450 5450 17.6 9.0 468 507 11 10.28 0.50 — 15 85 0.25 3050 1400 12,800 5100 15.0 7.0 471 504 12 10.67 0.50 — 10 90 0.25 2800 1250 12,900 5100 14.5 6.5 473 498 15(g) 10.05 0.5 1 15 82 1.40 21.0 13.0 479 538 16(f) 9.88 0.5 10 12.5 77 0.20 27.5 13.6 471 495 19 10.50 0.50 5 9 95 0.20 15,600 6100 17.7 8.0 462 495 **In composites. ***Babbitts predominantely used by electric utilities (ASTM alloy B23). 19.128 CHAPTER NINETEEN content and the softest bronzes while still retaining the necessary strength and load-carrying capacity. • Silver. Silver bearings normally consist of electro-deposited silver on steel backings with an overlay of 0.001 to 0.005 in. of lead. Indium is usually flashed on top of the lead overlay for corrosion protection. Silver bearings have outstand- ing metallurgical uniformity, excellent fatigue resistance and thermal conductiv- ity, can carry very high loads, and can operated at high temperatures. Although the lead coating helps to relieve problem of poor embeddability and conforma- bility, silver bearings are not recommended for applications where misalignment and dirt are present. • Aluminum. Aluminum bearing alloys offer excellent resistance to corrosion by acidic oils, good load-carrying capacity, superior fatigue resistance, and good thermal conductivity. A smooth machine finish of the running surface is recom- mended along with a clean lubricant, a shaft hardness of 300 Brinell or higher, and a large enough clearance to allow for the high thermal expansion of the aluminum. Sometime the aluminum is overlaid with a thin coating of lead babbitt. This overlay assists in making up for the otherwise poor embeddability and con- formability characteristics of the aluminum. The range of temperatures that these various bearing materials, as well as some other materials, can endure is given in Table 19.31. 19.9 DESIGN CONSIDERATIONS In practice, a designer must obtain quantitative data to ascertain on the one hand whether the bearing will meet his operational requirements, and on the other hand find out what the power losses, flows, temperatures, etc. will be to properly plan the layout of the facility. In Sections 19.3 to 19.7, the graphs and tables offer values for the performance of various bearing designs. These, however, do not exhaust the information required for rational design. What is needed is some orientation how the various geometrical and operational parameters affect bearing operation and how to go about improving or even optimizing a given bearing design. The following paragraphs should offer some guidance as to how to go about approach- ing this task. 19.9.1 Performance Parameters The expressions required for calculating the more important items of bearing per- formance are the following: • Film thickness. For an aligned journal, the film thickness is given by h ϭ (h/C ϭ 1 ϩ ⑀ cos ( ␪ Ϫ ␾ )) (19.58) 19.129 TABLE 19.31 Approximate Temperature Limitations of Various Bearing Materials 36 19.130 CHAPTER NINETEEN The attitude angle ␾ is defined as the angle between the line centers—a line passing the centers of bearing and journal—and the load vector. When the treat- ment is restricted to vertical loads, ␾ denotes the angle between location of h min and the vertical and therefore the importance of ␾ lies in that it determines the location of h min . • Sommerfeld number (load parameter). The Sommerfeld number, given by 2 ␮ NR S ϭ (19.59a) ͩͪ PC has traditionally been the most important parameter. However, a more convenient quantity is the inverse of S, here called the load parameter, given by 22 PC W C W ϭϭ (19.59b) ͩͪ ͩͪ ␮ NR LD ␮ NR where P ϭ (W/LD) is the unit loading. What this parameter says is that any combination of P, ␮ , N, C, and R such as to leave the value of unchanged,W would result in the same bearing eccentricity ratio, ⑀ , and attitude angle, ␾ . • Minimum film thickness. The is the smallest distance between the journal and bearing surfaces and it is given by: h min h ϭϭ(1 Ϫ ⑀ ) (19.60) min C What is normally referred to as load capacity relates to the load, W, which this h min can support. • Friction coefficient. This is the ratio between the frictional force and bearing load. It is normally expressed in the form of: R (R/C)F ␶ ƒ ϭ ͩͪ CW The general shape of ƒ as a function s is given in Fig. 19.88. The region of sudden rise in ƒ denotes the limit of hydrodynamic lubrication, followed by a regime of ‘‘boundary lubrication’’ characterized by partial contact between the mating surfaces. • Power loss. This, of course, can be obtained from the value of F ␶ , namely H ϭ F ⅐ R ⅐ ␻ ϭ ƒ ⅐ W ⅐ R ⅐ ␻ ␶ HH H ϭϭ (19.61) ͩͪ 323 H [ ␲␮ NLD/C] 0 The quantity by which H is normalized, represents the power loss in an unloaded concentric journal bearing, i.e., one in which ⑀ ϭ 0. It is known as the Petroff equation. PRINCIPLES OF BEARING DESIGN 19.131 FIGURE 19.88 Behavior of friction coefficient in fluid film bearings. • Flow. An amount of lubricant, Q 1 , enters the bearing at the leading edge; an amount, Q s leaks out the two sides of the bearing (one-half Q s at each side), and an amount Q 2 leaves the trailing end of the pad. In most cases, since a journal bearing extends over circumference (2 ␲ ), Q 2 is not discharged outside but reen- ters the next oil groove, so that the net amount of lubricant to be made up from an outside source is Q s . The latter is referred to as side leakage. Clearly we must always have Q ϭ Q ϩ Q (19.62) 1 s 2 All of these flows are given in dimensionless form as: Q Q ϭ (19.63) ␲ NDLC 2 the denominator representing the flow in an unloaded, concentric bearing, i.e., at ⑀ ϭ 0 (for which case Q s ϭ 0 and Q 1 ϭ Q 2 ). The above flows, Q 1 , Q s , and Q 2 are what may be called hydrodynamic flows induced by the shearing action and pressure gradients of the fluid film. Q s is the minimum amount of oil to be delivered to the bearing to maintain a full fluid film with all its potentialities. In practice, designers supply more than this re- quired minimum, using a supply pressure p s Ͼ p a . The effect of the supply pressure, usually of the order of 10 to 30 psig, can be ignored as far as bearing hydrodynamics are concerned. • Temperature rise. A bulk temperature rise can be estimated from the values of power loss and side leakage, namely 19.132 CHAPTER NINETEEN H ⌬T ϭ (T Ϫ T ) ϭ (19.64) av 1 cwQ ps • Dynamic coefficients. The dimensionless stiffness is given by 2 K ϭ (K/2 ␮ NL)(C/R) while the damping coefficient reads 3 B ϭ ( ␲ B/ ␮ L)(C/R) from which the dimensional values of K and B can be obtained. The coefficients ⑀ , ƒ, 1 , 2 , and which serve to evaluate bearing perform-H, QQK B ance are obtained from solutions of the Reynolds equation for the specific geom- etries and operating conditions of the various bearing designs. Many such solutions were given in Sections 19.3 through 19.7. 19.9.2 Bearing Configuration The behavior of a bearing is naturally a function of its geometry. However, even for a given design there are a number of variables that will affect its performance. Among the more known parameters are the L/D and C/R ratios and the degree of preload. Of the less familiar ones one can cite load orientation, the geometry of the oil grooves or the relative proportions of a bearing’s geometrical elements. Journal Bearings. Although one often hears about the use of full, that is, 360 Њ arc bearings, it is very rarely that such sleeves are employed in machinery. Most journal bearings consist of two or more pads separated by horizontal oil grooves making them in fact partial bearings, used either singly or in tandem. The number and distribution of these angular pads on bearing performance is one of the more important considerations in bearing design. Partial Bearings. Whenever a single pad of an angular extent ␤ Ͻ 2 ␲ is used, it is called a partial bearing. When ␤ is very small, its load capacity is low, as illustrated in Figs. 19.89 and 19.90. However, soon a limit is reached at about ␤ ϭ 140Њ beyond which no further gains are registered. The reason for this asymptotic behavior is due to oil cavitation at the trailing end of the pad where the pressures decrease close to or even below ambient pressure. Thus, if a partial bearing is used there is no need to go beyond a 140 Њ arc. The effect of temperature in partial bearings is a combination of two phenomena. The higher the arc the longer the dissipation path and the higher the temperatures; however, a longer arc produces thicker films and thus less heating. Consequently, as shown in Fig. 19.91, a cross- over point occurs; at high loads low values of ␤ are preferred, if low ⌬T’s are desired; at low loads a longer arc is preferred. Grooved Bearings. Partial bearings are not used extensively. The most common designs are grooved bearings which consist of a number of pads arranged in tandem by cutting axial oil grooves around the 360 Њ circumference. There is a great variety [...]... 19. 36 The additional merit of this design is that upon starting and stopping, the runner rides on a flat surface reducing wear TABLE 19. 32 Relative Load Capacity 3- and 5-Pad Bearings 29 On-pivot load Load between pivots W 3 pads 5 pads 3 pads 5 pads 20 40 60 80 100 0.72 0. 79 0.80 0.82 0.83 0.82 0.87 0 .90 0 .92 0 .93 1.42 1.58 1.60 1.61 1.62 0 .97 1.05 1. 09 1.10 1.11 19. 140 CHAPTER NINETEEN FIGURE 19. 96... 0.175 0.25 0.17 19. 143 19. 144 CHAPTER NINETEEN FIGURE 19. 99 Hydrostatic forces and films under misalignment PRINCIPLES OF BEARING DESIGN TABLE 19. 35 Optimum Pad Arrangement25 L/R2 h1 / ␦␪ ␤, deg Number of pads 1/3 1 1/2 1/4 1/8 1 1/2 1/4 1/8 1 1/2 1/4 1/3 Ͻ30 Ͻ30 Ͼ10 Ͼ10 35 40 40 45 50 60 50 60 80 Ͼ80 9 8 8 7 6 5 6 5 4 4 1/2 2/3 FIGURE 19. 100 Effect of extent of taper (or flat).13 19. 145 19. 146 CHAPTER... 5–8, 199 5, Houston, Texas, 95 GT180 6 Compressor Handbook, Gulf Publishing Co., Book Division 7 Gross, W A., ‘‘Gas Film Lubrication,’’ John Wiley, 196 2 8 Heshmat, H., J A Walowit, and O Pinkus, ‘‘Analysis of Gas-Lubricated Compliant Thrust Bearings,’’ ASME Paper 82-LUB- 39, 198 2 9 Heshmat, H., J A Walowit, and O Pinkus, ‘‘Analysis of Gas-Lubricated Foil Journal Bearings,’’ ASME Paper 82-LUB-40, 198 2 10... Propulsion Conference, June 24–26, 199 1, Sacramento, CA, Paper No AIAA -91 -2103 13 Heshmat, H., and P Hermel, ‘‘Compliant Foil Bearing Technology and Their Application to High Speed Turbomachinery,’’ 19th Leeds-Lyon Symposium on Thin Film in Tribology—From Micro Meters to Nano Meters, Leeds, U.K., Sept 199 3, D Dowson, et al (eds) (Elsevier Science Publishers B.V., 199 3), pp 5 59 575 14 Heshmat, H., and O Pinkus,... Conference Held at NASA / MSFC May 17– 19, 199 4, NASA CP3282, vol 1, Sept 19, 199 4, pp 372–381 36 Wilcock, D F., and Booser, E R., ‘‘Bearing Design and Application’’ (New York: McGraw Hill, 195 7) CHAPTER 20 COMPRESSOR VALVES Walter J Tuymer Hoerbiger Corporation of America, Inc Dr Erich H Machu Consulting Mechanical Engineer Hoerbiger Corporation of America, Inc 20.1 PURPOSE Compressor valves are check valves... Bearings, July 199 3, Alexandria, VA 18 Jones, G J., and F A Martin, ‘‘Geometry Effects in Tilting-Pad Journal Bearings,’’ ASLE Paper No 78-AM-@A-2, 197 8  19. 152 CHAPTER NINETEEN 19 Ku, C.-P R., and H Heshmat, ‘‘Compliant Foil Bearing Structural Stiffness Analysis: Part I—Theoretical Model Including Strip and Variable Bump Foil Geometry,’’ Journal of Tribology, Trans ASME, vol 114, no 2 ( 199 2): 394 –400 20... but also to the higher linear velocities of the runner at the outer radius of the pad  19. 138 FIGURE 19. 94 Effect of a slot and a hole on hydrostatic pressure PRINCIPLES OF BEARING DESIGN FIGURE 19. 95 19. 1 39 A tilting 3-pad journal bearing Tapered Land Bearings A conventional tapered land bearing was shown in Fig 19. 35 There are three parameters here; the taper (h1 Ϫ h2), the pad arc ␤ and the (L/R2)... Bearings,’’ MTI Report Nos 82TR42, 82TR43, April 198 2 23 Pinkus, O., ‘‘Analysis of Elliptical Bearings,’’ Trans ASME, vol 78, 195 6, pp 96 5– 97 3 24 Pinkus, O., ‘‘Analysis and Characteristics of the Three-Lobe Bearing,’’ Trans ASME, Ser.D., vol 81, March 195 9 25 Pinkus, O., ‘‘Solution of the Tapered-Land Sector Thrust Bearing,’’ Trans ASME, vol 80, Oct 195 8 26 Pinkus, O., ‘‘Analysis of Non-circular Gas...PRINCIPLES OF BEARING DESIGN FIGURE 19. 89 19. 133 Effect of bearing arc on load capacity.21 of such designs, the most common being a 2-pad bearing with two grooves at the horizontal split Others may have 3, 4 or 6 grooves forming the same number of individual pads The more grooves the lower the load capacity, as shown in Figs 19. 92 and 19. 93 Thus, if load capacity is the primary objective,... cutting a short oil supply channel on the outside of the bearing shell PRINCIPLES OF BEARING DESIGN 19. 137 FIGURE 19. 93 Comparisons of 2- and 4-axial groove bearings Misalignment It was pointed out in an earlier section that an overhung impeller will cause bearing misalignment As shown in Fig 19. 99, in severe misalignment the journal at one end may find itself in the upper half of the bearing even . 4.56 90 .9 4.52 None 4400 2680 12,850 6050 17.0 8.0 433 700 2** 7. 39 3.1 39. 2 7.6 0.03 6100 3000 14 ,90 0 8700 24.5 12.0 466 6 69 3** 7.46 8.3 83.4 8.3 0.03 6800 3100 17,600 99 00 27.0 14.5 464 792 4. 12 ,90 0 5100 14.5 6.5 473 498 15(g) 10.05 0.5 1 15 82 1.40 21.0 13.0 4 79 538 16(f) 9. 88 0.5 10 12.5 77 0.20 27.5 13.6 471 495 19 10.50 0.50 5 9 95 0.20 15,600 6100 17.7 8.0 462 495 **In composites. ***Babbitts. radius of the pad. 19. 138 FIGURE 19. 94 Effect of a slot and a hole on hydrostatic pressure. PRINCIPLES OF BEARING DESIGN 19. 1 39 FIGURE 19. 95 A tilting 3-pad journal bearing. TABLE 19. 32 Relative Load