centrifugal compressors overview

centrifugal compressors overview

1. Centrifugal Compressors A basic overview Presented By : Muhammad Junaid

2. Contents Theory of compression Purpose of compression Understanding of key terms Kinetic theory of gases Compressibility Methods of compression Types of compressors Centrifugal compressors Advantages and disadvantages Basic construction Understanding compressor curves Basic operation Compressor arrangements Maintenance

3. Theory of compression Compression Process of increasing the pressure of a fluid in order to make it travel to another place Compressor Mechanical equipment that is used to increase the pressure of a compressible fluid. Why compressors are required Increase temperature and pressure of gas for certain reactions to take place. Liquefaction of gases for storage purposes Refrigeration effect Pressurized air is used for measuring and control Applications in industry In every field of life. Purpose of compression

4. Absolute Pressure Understanding of key terms Theory of compression

5. Theory of compression Capacity of a compressor Understanding of key terms Volume of gas handled in a unit time . Its units are cubic feet per minute Capacity is always measured at compressor suction flange. SCFM and ACFM

6. Theory of compression Pressure Ratio (R) Understanding of key terms

7. Theory of compression Efficiency Understanding of key terms

8. Theory of compression State and state functions Understanding of key terms

9. Theory of compression The average kinetic energy of the molecules does not change with time. The average kinetic energy of the molecules is proportional to absolute temperature. Kinetic theory of gases

10. Theory of compression Law of Conservation of Energy Energy of a system increases when work is done on it

11. Theory of compression Trap quantities of gas, reduce the volume, push the compressed gas out of the enclosure. Trap gas, carry it without volume change to the discharge opening. Compress the gas by backflow from discharge system, then push the compressed gas out of the enclosure Compress by the mechanical action rotating impellers or bladed rotors that impart velocity and pressure Entrain the gas in a high velocity jet of the same or another gas and convert the high velocity of the mixture into pressure in a diffuser. Methods of compression

12. Theory of compression Methods of compression

13. Theory of compression Methods of compression

14. Theory of compression Methods of compression

15. Types of compressors Continuous and intermittent Compressors can be subdivided into two broad groups based on compression mode. Intermittent This compression is cyclic in nature. A specific quantity of gas is ingested by the compressor, acted upon and discharged, before the cycle is repeated. Continuous The compression mode in which gas is moved into the compressor, acted upon, moved through compressor, and discharged without interruption of the flow at any point in the process.

16. Types of compressors Compressors Positive displacement Dynamic Reciprocating Rotary Centrifugal Axial Single acting Double acting Diaphragm Lobe Screw Vane Liquid ring Scroll Ejector

17. Types of compressors Applications of compressors

18. Centrifugal Compressors

19. Centrifugal Compressors Working Principle The working principle is based upon imparting kinetic energy to a fluid stream and conversion of this dynamic energy into static energy, i.e., from velocity into pressure. The specific compression energy transmitted to the gas by the impeller is called "head." The discharge pressure of a turbo compressor, which is affected by gas inlet conditions, is a function of head.

20. Centrifugal Compressors Less efficient for small volumes. Discharge pressure limitation. Effect of gas density and temperature. Problem of surge phenomenon. Discharge flow is relatively free of pulsation. Mechanical design permits high through puts, capacity limitation is rarely a problem. Centrifugal Compressors are capable efficient performance over a wide range of pressure and capacities even at constant speed operation. These are relatively small, occupy less space, operate with minimum attention and quieter. Less contamination due to lubricants. Disadvantages Advantages

21. Centrifugal Compressors Major components of a centrifugal compressor include: Impeller The part of centrifugal compressor that moves the gas is the impeller. As the impeller rotates, it moves the gas toward the outer rim of the impeller and its velocity increases. Diffuser As the gas leaves impeller, it flows into a passage-way called the diffuser. The diffuser being larger in volume, the velocity of gas decreases and its pressure increases Volute Gas passes from diffuser into the volute. In the volute, the conversion of velocity energy to pressure energy continues. Casing It is the outer cover of a centrifugal compressor which contains inlet and discharge nozzles. Construction

22. Centrifugal Compressors A centrifugal compressor has two main type of parts; Rotor assembly Main shaft Impellers Thrust runner Disc spacer Balance drum Stationary parts Casing Nozzles Volutes Bearings Shaft seals Construction

23. Centrifugal Compressors Rotor shaft Construction

24. Centrifugal Compressors Impellers The impellers add energy (velocity) to the gas. There are three types of impellers Open impeller Semi closed impeller Closed impeller Construction

25. Centrifugal Compressors Thrust collar Construction

26. Centrifugal Compressors Construction Thrust collar

27. Centrifugal Compressors

28. Centrifugal Compressors Balance drum Construction

29. Centrifugal Compressors Casing Construction

30. Centrifugal Compressors

31. Centrifugal Compressors Volute and diffuser Construction

32. Centrifugal Compressors Bearings Construction

33. Centrifugal Compressors Radial bearings Construction

34. Centrifugal Compressors Radial bearings Construction

35. Centrifugal Compressors Radial bearing lubrication Construction

36. Centrifugal Compressors

37. Centrifugal Compressors Thrust bearing Construction

38. Centrifugal Compressors Thrust bearing lubrication Construction

39. Centrifugal Compressors

40. Centrifugal Compressors Shaft end seals Construction

41. Centrifugal Compressors Non contact type mechanical seal Construction

42. Centrifugal Compressors Mechanical oil seals Construction

43. Centrifugal Compressors Dry gas seal Construction

44. Centrifugal Compressors

45. Centrifugal Compressors Inter stage seals Construction

46. Centrifugal Compressors Guide vanes Construction

47. Centrifugal Compressors Coupling Construction

48. Centrifugal Compressors Understanding Compressor Curves Single speed compressor drive with suction and discharge pressure control P1 Source P3 Destination C

49. Centrifugal Compressors Understanding Compressor Curves Flow Head Stone wall Surge Compressor curve System curve Operating point

50. Centrifugal Compressors Incoming flow to compressor decreases Understanding Compressor Curves Flow Head Stone wall Surge

51. Centrifugal Compressors Incoming flow to compressor decreases below surge limit Understanding Compressor Curves Flow Head Surge control point Surge Stone wall

52. Centrifugal Compressors Gas Demand Increases Understanding Compressor Curves Flow Head Stone wall Surge

53. Centrifugal Compressors Understanding Compressor Curves Variable speed compressor drive with speed controller

54. Flow Head minimum speed maximum speed surge limit stonewall or choke limit Centrifugal Compressors Understanding Compressor curves

55. Flow Head Centrifugal Compressors Understanding Compressor curves Incoming flow to compressor decreases Surge Line Stone wall N 1 N 2 N 3 N 4 N min

56. Flow Head Centrifugal Compressors Understanding Compressor curves Incoming flow to compressor decreases below surge limit Surge Line Stone wall N 1 N 2 N 3 N 4 N min SCL

57. Flow Head Centrifugal Compressors Understanding Compressor curves Gas demand increases Surge Line Stone wall N 1 N 2 N 3 N 4 N min

58. Flow Head Centrifugal Compressors Understanding Compressor curves Gas MW increases Surge Line Stone wall N max N min N max N min

59. Centrifugal Compressors Start up Sequence All jobs completed and blinds removed All instrument calibrated and in service System Purging Lube oil Circulation Cooling water circulation System pressurization Sealing system Start up Basic Operation

60. Centrifugal Compressors Proper Oil lubrication Basic Operation

61. Centrifugal Compressors Bad Oil viscosity Basic Operation

62. Centrifugal Compressors Lube Oil system Basic Operation

63. Centrifugal Compressors Lube Oil tank Basic Operation

64. Centrifugal Compressors Instrumentation on LO tank Basic Operation

65. Centrifugal Compressors Lube Oil Pump Basic Operation

66. Centrifugal Compressors Lube Oil Cooler Basic Operation

67. Centrifugal Compressors Lube Oil Filter Basic Operation

68. Centrifugal Compressors Lube Oil Circulation Basic Operation

69. Centrifugal Compressors Start up Procedure Open suction valve Open discharge valve (discharge check valve closed). Open bypass or vent valve Start and bring up to speed Close bypass or vent Place bypass or vent on automatic control Basic Operation

70. Centrifugal Compressors Basic Operation Start up Procedure

71. Centrifugal Compressors Thermodynamic condition A compressor's efficiency is reflected by its thermodynamic performance Evaluation of performance requires acquisition of the following data. Suction and discharge pressures of each stage Suction and discharge. temperatures of each stage Volume flow Density Operating speed Power consumption Cooling water Flow rate, Fouling Basic Operation

72. Centrifugal Compressors Mechanical condition The most critical components of a compressor are the rotor, its bearings and seals. Monitoring of mechanical compressor functions, therefore, will center around the behavior of these components. The following data allow assessment of the mechanical condition: Radial vibration Shaft position Bearing temperature Lube-oil supply pressure and temperature Seal-oil supply temperature, and differential pressure between oil supply and reference gas Seal buffer-gas pressures and differential pressures Compressor speed (with variable speed driver) Basic Operation

73. Centrifugal Compressors Common Problems Dirty Intercoolers - Increased gas temperature reduces the density into the compression stage. Hot Coolant - Increased gas temperature reduces the gas density into the compression stage. Dirty Inlet Filter - Reduces the pressure into the first stage . Driver Input Speed - The ability  for a centrifugal compressor stage to make pressure falls as the square of the stage speed. A reduction in the driver input speed can dramatically affect the total machine ratio. Discharge valve closure - Discharge valve closure decreased the flow through the compressor hence increase the pressure and causes surging . Suction valve closure - Suction valve closure or suction vent opening causes pressure drop in suction reducing flow and surging the machine. Inter stage vent malfunction - In a multistage compressor if inter stage vent valve malfunction and opens the down stream stage will surge. Change in gas composition . If the gas composition change it effects the pressure thus the compressor can go into surge at the same speed Recycle valve malfunction .  Recycle valves are normally open during reduced flow operation. if closes by accident or malfunction it will surge the compressor.  Basic Operation

74. Centrifugal Compressors Basic Operation What word strikes fear in the heart of centrifugal compressor operators?  SURGE!

75. Centrifugal Compressors Surging Flow reverses in 20 to 50 milliseconds Surge cycles at a rate of 0.3 s to 3 s per cycle Compressor vibrates Temperature rises “Whooshing” noise or “Clanking” noise Trips may occur Conventional instruments and human operators may fail to recognize surge Basic Operation

76. Rapid flow oscillations Thrust reversals Potential damage Rapid pressure oscillations with process instability Rising temperatures inside compressor FLOW PRESSURE TIME (sec.) 1 2 3 TEMPERATURE TIME (sec.) 1 2 3 TIME (sec.) 1 2 3 Centrifugal Compressors Trend of major process parameters during s urge

77. Q s, vol P d Machine shutdown no flow, no pressure Electro motor is started Machine accelerates to nominal speed Compressor reaches performance curve Note: Flow goes up faster because pressure is the integral of flow Pressure builds Resistance goes up Compressor “rides” the curve P d = P v + R losses A Compressor reaches surge point A Compressor looses its ability to make pressure Suddenly P d drops and thus P v > P d Plane goes to stall - Compressor surges B Because P v > P d the flow reverses Compressor operating point goes to point B C Result of flow reversal is that pressure goes down Pressure goes down => less negative flow Operating point goes to point C D System pressure is going down Compressor is again able to overcome P v Compressor “jumps” back to performance curve and goes to point D Forward flow is re-established P d P v R losses P d = Compressor discharge pressure P v = Vessel pressure R losses = Resistance losses over pipe Compressor starts to build pressure Compressor “rides” curve towards surge Point A is reached The surge cycle is complete From A to B 20 - 50 ms Drop into surge From C to D 20 - 120 ms Jump out of surge A-B-C-D-A 0.3 - 3 seconds Surge cycle Centrifugal Compressors Surge representation on compressor curves

78. Centrifugal Compressors Unstable flow and pressure Damage in sequence with increasing severity to seals, bearings, impellers, shaft Increased seal clearances and leakage Lower energy efficiency Reduced compressor life Some surge consequences

79. Centrifugal Compressors Surge Factors leading to onset of surge Startup Shutdown Operation at reduced throughput Operation at heavy throughput with: - Trips - Power loss - Operator errors - Process upsets - Load changes - Gas composition changes - Cooler problems - Filter or strainer problems - Driver problems Surge is not limited to times of reduced throughput. Surge can occur at full operation

80. Q s, vol R c stable zone of operation adding control margins Actual available operating zone Centrifugal Compressors Anti surge control

81. OUTPUT INPUT Suction Flow Anti-Surge Valve Suction Temperature Suction Pressure Discharge Pressure Anti-surge Controller Discharge Temperature

82. OUTPUT INPUT Suction Flow Anti-Surge Valve Suction Temperature Suction Pressure Discharge Pressure Anti-surge Controller Discharge Temperature

83. Centrifugal Compressors Single stage compressor Compressor arrangements

84. Centrifugal Compressors Single stage compressor with internally split flow Compressor arrangements

85. Centrifugal Compressors Multistage compressor Compressor arrangements

86. Centrifugal Compressors Multistage compressor with single intercooler Compressor arrangements

87. Centrifugal Compressors Multistage compressor with double intercooler Compressor arrangements

88. Centrifugal Compressors Multistage compressor with double inlet Compressor arrangements

89. Centrifugal Compressors Multistage compressor with side streams Compressor arrangements

90. Centrifugal Compressors Commonly used drivers for a centrifugal compressor are: Steam Turbine Electric motor Induction Synchronous Gas Turbine Expander Compressor drivers

91. Centrifugal Compressors