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Reciprocating Compressor for beginners.pptx
- 1. Basics and Operation of Reciprocating Compressors INAMULLAH TRAINEE ENGINEER (2010-11) MECHANICAL MAINTENANCE
- 2. Compressors Compressors: Moveair or gas in higher differential pressure ranges from 35 psi to as high as 65,000 psi Blowers: Move large volumes of air or gas at pressures up to 50 psi Fans: Move air or gas at a sufficient pressure to overcome static forces. Discharge pressures range from a few inches of water to about 1 psi Bloch H.P. & Hoefner J. J. (1996). Reciprocating Compressors, Operation and Maintenance. Houston TX: Gulf Publishing Company
- 3. Compressors Ejector Dynamic Intermittent flow COMPRESSORS Continuous flow Positive displacement Radial flow Mixed flow Axial flow Rotary Reciprocating •Helicallobe •Straight lobe •Sliding vans • Liquid piston •Single Acting •Double Acting
- 4. Operating Range ofDifferent Compressors Inlet Flow, CFM
- 5. PD vs DynamicCompressors Positive Displacement (PD) Compressors: Increase the pressure of fluid by reducing its volume Dynamic Compressors: Increase the pressure of fluid by converting momentum into pressure rise, the angular momentum is transferred to the fluid from a continuously rotating member
- 6. Reciprocating Compressors Reciprocatingcompressors consist of a piston working within a cylinder to physically compress the gas contained in that cylinder. a volume of gas is drawn into a cylinder, it is trapped, and compressed by piston and then discharged into the discharge line
- 7. Single Stage VsMulti Stage Single Stage Compression: The Pressure is increased from inlet to discharge pressure in a single step compression. Multi Stage Compression: The pressure is increased from inlet to discharge pressure in several steps of compression. E.g. The two-stage compressor adds a second piston to the process that fires after the first to further compress the air in the tank
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- 9. Advantages of Multi-Staging Higher Pressure ratios Less work input Increase in overall efficiency
- 10. Single Acting VsDouble Acting Single Acting Reciprocating Compressors: It is a compressor that has one discharge stroke per revolution of crankshaft. Double Acting Reciprocating Compressors: It is a compressor that completes two discharge strokes per revolution of crankshaft. Most heavy duty compressors are double acting.
- 11. Working of Reciprocating Compressors First, the intake gas enters the suction manifold, then flows into the compression cylinder where it gets compressed by a piston driven in a reciprocating motion via a crankshaft, and is then discharged. Depending upon the construction of the compressor, a single stroke of a piston may comprise of both the intake and discharge of the gas. A sequence of events describing how compression occurs in a reciprocating compressor can also be represented on a PV-Diagram.
- 12. Thermodynamic Cycle of ReciprocatingCompressors
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- 20. Parts of Reciprocating Compressors Crankshaft and Bearings Connecting Rod and Cross Guide Pistons and Rods Rod Packing Cylinders Valves Distance Piece Variable Volume Clearance Pocket (VVCP)
- 21. CrankShaft May becasted or forged (for Power > 150 hp) Counter weights to minimize rotary unbalance and reciprocating unbalance.
- 22. Connecting Rod and Crossheads Connecting Rod transfers reciprocating motion from the crankshaft throw to the crosshead
- 23. Crosshead Converts rotatingmotion of shaft to reciprocating motion Parts: 1) Crosshead shoe 2) Access 3) Retaining cap screws 4) Crosshead pin 5) Crosshead 6) Shims
- 24. Crosshead Guide Supportsand guides the crosshead Installed, adjusted and doweled to the frame at the factory
- 25. Crosshead bushing Provideprotection to the crosshead Bronze or steel backed aluminum Interference fit Receives lubrication through a bore in the connecting rod
- 26. Shoes and Shims Actual riding surface Jack bolts provided for shim adjustment Aluminum or cast iron Lubricated by oil thrown off crankpins
- 27. Piston Piston transmitenergy from the crankshaft to the gas in the cylinder Piston rings are used for sealing but the wear rate should be minimum Ring material may be metallic or non-metallic Piston rod is a threaded to the piston and transmits the reciprocating motion from cross head to piston. Weight of Piston contributes to compressor shaking forces therefore, it is kept minimum.
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- 29. Piston Rod Packing Provide seal around piston rod and across the flat surface of adjacent cup Prevents the leakage of gas into the distance piece Tie studs are for alignment and not for tightening End cup is fitted with gasket to prevent gas leakage around the case. cup Piston rod Tie stud
- 30. Packing Types Fullylubricated: Oil is supplied to the packing from the mechanical lubricator. The cases are drilled so that oil will be carried to the oil cups and then to the piston rod through a drilled hole in the oil cups Mini-lubricated: Same as fully lubricated, except that oil to the packing is decreased approximately 30 percent of normal flow. Micro-flow: There is no oil supplied directly to the packing. The only oil that reaches the packing is carried over on the piston rod from the crankcase. Fully non-lubricated: No oil is supplied to the packing and no oil carryover from the crankcase is allowed to reach the packing. The crankcase oil is prevented from reaching the packing by means of an oil deflector collar attached to the piston rod
- 31. Packing Rings Itseal along the piston rod, between themselves and the packing case Tangential Cut Ring: The tangent ring is cut into three segments so that each cut lies on the side of an equilateral triangle As wear occurs, the ring segments will close radially to compensate while still maintaining sealing contact at the tangential joints
- 32. Packing Rings RadialRings:
- 33. Packing Ring Sets Radial and Tangent Set: An overlapping seal joint that prevents gas passage along the rod from the outside periphery toward the rod bore. It has no through escape The inner radial cuts of this tangent ring are arranged so that they occur approximately between the radial cuts of the first ring in the pair Rings are doweled to maintain this staggered cut arrangement Match marks are always on the pressure side Seals on the compression stroke but allows backflow into the cylinder on suction stroke
- 34. Packing Ring Sets Radial, Tangent and Back-up Set: Used in high pressure compressors The radial packing ring is installed in the groove on the pressure side toward the cylinder. The tangential-cut ring is installed between the radial and the back-up ring Single acting- Seal in only one direction i.e. Seals on the compression stroke but allows backflow into the cylinder on suction stroke Back-up ring is anti extrusion ring and prevents extrusion of radial and tangent rings, It also help in dissipation of heat from the rod Generally radial and tangent rings are non metallic while the backup ring is metallic Used between 700 to 1500 psi
- 35. Packing Ring Sets Tangent and Back-up Ring Set: Single acting- Seal in only one direction i.e. Seals on the compression stroke but allows backflow into the cylinder on suction stroke They are not dowelled together No pressure limitation and has been used to handle thousands psi Double Tangent Ring Set: Double acting therefore, installed with either side facing the pressure Dowelled to prevent the misalignment Used for vacuum application, usually behind the vent to prevent leakage into the distance piece Pressure Breaker Ring: Not intended to seal but to break or slow down gas passage
- 36. Valves Spring Loaded,Gas actuated Permits only one way flow of gas either into or out of the cylinder Components of a Valve: Seat Sealing Element Device to limit travelling of sealing element
- 37. Suction and Discharge Valves Suction Valve: Operates at low pressure Discharge Valve: Operates at high pressure
- 38. Valve Configurations RectangularElement valve Concentric Ring valve Ported Plate valve Disc / Poppet valve
- 39. Valve Configurations RectangularElement valve: Lift is approximately 0.100” to 0.200” The rectangular shape of the elements eliminates the use of a cross ribbing system to provide structural integrity of the valve seat and guard This enables the use of long uninterrupted slots in the seat and guard and provides good flow area. These valves generally make excellent use of available valve port areas to provide flow area at moderate valve lifts Feather Valve Channel Valve Reed Valve
- 40. Valve Configurations ConcentricRing Valve: These valves use one or more relatively narrow metallic or nonmetallic rings arranged concentrically about the center line of the valve Lift is controlled by a flat stop that limits the ring motion wide application range and are used in low- to high- speed application Lifts are as high as 0.16” the top end of application speed is in the area of 1200 rpm The flow efficiency of these valves is not as good as straight element valves, but it is good for the pressure ratios generally used.
- 41. Valve Configurations PortedPlate Valve: Ported plate valves are similar to the concentric ring valves, except the rings are joined together into a single element The number of plate edges available for impact is reduced Spring is generally provided by coil springs that directly contact the plates, eliminating the buttons used in concentric ring valves. Flow efficiency of these valves is roughly equivalent to the concentric ring valves The geometry of the sealing element is not as flexible as the concentric ring valves They are high-speed valve although it can be applied equally well to low-speed machines
- 42. Valve Configurations Disc/ Poppet Valve: They can be found in applications ranging from a few hundred psi to over 50,000 psi Maximum lift is approximately 0.300” Springing is provided by individual coil springs for each poppet. Flow efficiency is excellent because of the high lift and streamlined poppet head This makes an ideal valve for low-pressure ratio, high-gas density applications where valve losses are very important and sufficient pressure drop can be generated to drive the valve open The top end of application speed is in the area of 1200 rpm Poppet valves are ideally suited to the low-speed, medium-pressure, low-ratio efficiency service typical of gas transmission applications.
- 43. Variable Volume Clearance Pocet(VVCP) Control capacity of fix speed compressor Reduce load on driver VVCP open: Greater clearance volume More trapped gases The gas expands on intake stroke and take space in cylinder Effective intake stroke length decreases Consequently capacity of compressor decreases
Editor's Notes
- #13 The Thermodynamic Cycle Compression occurs within the cylinder as a four-part cycle that occurs with each advance and retreat of the piston (two strokes per cycle). The four parts of the cycle are compression, discharge, expansion and intake. At the conclusion of a prior cycle, the piston is fully retreated within the cylinder at V1, the volume of which is filled with process gas at suction conditions (pressure, P1 and temperature, T1), and the suction and discharge valves are all closed. This is represented by point 1 (zero) in the P-V diagram.
- #14 As the piston advances, the volume within the cylinder is reduced. This causes the pressure and temperature of the gas to rise until the pressure within the cylinder reaches the pressure of the discharge header. At this time, the discharge valves begin to open, noted on the diagram by point 2.
- #15 With the discharge valves opening, pressure remains fixed at P2 for the remainder of the advancing stroke as volume continues to decrease for the discharge portion of the cycle. The piston comes to a momentary stop at V2 before reversing direction. Note that some minimal volume remains, known as the clearance volume. It is the space remaining within the cylinder when the piston is at the most advanced position in its travel. Some minimum clearance volume is necessary to prevent piston/head contact, and the manipulation of this volume is a major compressor performance parameter. The cycle is now at point 3. Expansion occurs next as the small volume of gas in the clearance pocket is expanded to slightly below suction pressure, facilitated by the closing of the discharge valves and the retreat of the piston. This is point 4. When P1 is reached, the intake valves open allowing fresh charge to enter the cylinder for the intake and last stage of the cycle. Once again, pressure is held constant as the volume is changed. This marks the return to point 1. Comprehending this cycle is key to diagnosing compressor problems, and to understanding compressor efficiency, power requirements, valve operation, etc. This knowledge can be gained by trending process information and monitoring the effect these items have on the cycle.
- #16 With the discharge valves opening, pressure remains fixed at P2 for the remainder of the advancing stroke as volume continues to decrease for the discharge portion of the cycle. The piston comes to a momentary stop at V2 before reversing direction. Note that some minimal volume remains, known as the clearance volume. It is the space remaining within the cylinder when the piston is at the most advanced position in its travel. Some minimum clearance volume is necessary to prevent piston/head contact, and the manipulation of this volume is a major compressor performance parameter. The cycle is now at point 3. Expansion occurs next as the small volume of gas in the clearance pocket is expanded to slightly below suction pressure, facilitated by the closing of the discharge valves and the retreat of the piston. This is point 4. When P1 is reached, the intake valves open allowing fresh charge to enter the cylinder for the intake and last stage of the cycle. Once again, pressure is held constant as the volume is changed. This marks the return to point 1. Comprehending this cycle is key to diagnosing compressor problems, and to understanding compressor efficiency, power requirements, valve operation, etc. This knowledge can be gained by trending process information and monitoring the effect these items have on the cycle.
- #17 With the discharge valves opening, pressure remains fixed at P2 for the remainder of the advancing stroke as volume continues to decrease for the discharge portion of the cycle. The piston comes to a momentary stop at V2 before reversing direction. Note that some minimal volume remains, known as the clearance volume. It is the space remaining within the cylinder when the piston is at the most advanced position in its travel. Some minimum clearance volume is necessary to prevent piston/head contact, and the manipulation of this volume is a major compressor performance parameter. The cycle is now at point 3. Expansion occurs next as the small volume of gas in the clearance pocket is expanded to slightly below suction pressure, facilitated by the closing of the discharge valves and the retreat of the piston. This is point 4. When P1 is reached, the intake valves open allowing fresh charge to enter the cylinder for the intake and last stage of the cycle. Once again, pressure is held constant as the volume is changed. This marks the return to point 1. Comprehending this cycle is key to diagnosing compressor problems, and to understanding compressor efficiency, power requirements, valve operation, etc. This knowledge can be gained by trending process information and monitoring the effect these items have on the cycle.
- #18 With the discharge valves opening, pressure remains fixed at P2 for the remainder of the advancing stroke as volume continues to decrease for the discharge portion of the cycle. The piston comes to a momentary stop at V2 before reversing direction. Note that some minimal volume remains, known as the clearance volume. It is the space remaining within the cylinder when the piston is at the most advanced position in its travel. Some minimum clearance volume is necessary to prevent piston/head contact, and the manipulation of this volume is a major compressor performance parameter. The cycle is now at point 3. Expansion occurs next as the small volume of gas in the clearance pocket is expanded to slightly below suction pressure, facilitated by the closing of the discharge valves and the retreat of the piston. This is point 4. When P1 is reached, the intake valves open allowing fresh charge to enter the cylinder for the intake and last stage of the cycle. Once again, pressure is held constant as the volume is changed. This marks the return to point 1. Comprehending this cycle is key to diagnosing compressor problems, and to understanding compressor efficiency, power requirements, valve operation, etc. This knowledge can be gained by trending process information and monitoring the effect these items have on the cycle.
- #19 With the discharge valves opening, pressure remains fixed at P2 for the remainder of the advancing stroke as volume continues to decrease for the discharge portion of the cycle. The piston comes to a momentary stop at V2 before reversing direction. Note that some minimal volume remains, known as the clearance volume. It is the space remaining within the cylinder when the piston is at the most advanced position in its travel. Some minimum clearance volume is necessary to prevent piston/head contact, and the manipulation of this volume is a major compressor performance parameter. The cycle is now at point 3. Expansion occurs next as the small volume of gas in the clearance pocket is expanded to slightly below suction pressure, facilitated by the closing of the discharge valves and the retreat of the piston. This is point 4. When P1 is reached, the intake valves open allowing fresh charge to enter the cylinder for the intake and last stage of the cycle. Once again, pressure is held constant as the volume is changed. This marks the return to point 1. Comprehending this cycle is key to diagnosing compressor problems, and to understanding compressor efficiency, power requirements, valve operation, etc. This knowledge can be gained by trending process information and monitoring the effect these items have on the cycle.
- #28 Metallic Piston Rings: CAST IRON*Dirty or corrosive gases BRONZE*High pressure*High temperature Non Metallic Piston Rings: PTFE –TEFLON *Carbon filled -Non-lube and lubricated *Glass andMolyfilled -Lubricated *Bronze filled -Non-lube and lubricated (mostly air service) High temperature, non-corrosive *Ryton •THERMOPLASTICS-(Lubricated)
- #41 The concentric ring valve uses one or more relatively narrow rings arranged concentrically about the centerline of the valve These valves have the advantage of a low stress level due to the lack of stress concentration points. The disadvantage is that it is difficult to maintain uniform flow control with the independent rings.
- #42 The ported plate valves, are similar to the concentric ring valve except that the rings are joined into a single element. The advantage is that the valve has a single element making flow control somewhat easier. Because of the single element, the number of edges available for impact is reduced. The valve may be mechanically damped, as this design permits the use of damping plates. It has the disadvantage that because of the geometry used, the stress is higher due to the potential of high stress concentration.
- #43 The poppet valve consists of multiple, same-size ports and sealing elements. The advantage of the valve is that has a high flow efficiency due to the high lift used and the streamlined shape of the sealing element. The disadvantage is that the valve is not tolerant of uneven flow distribution. The valve is most commonly used in gas transmission service and in low speed, low-to-medium compression ratio compressors. There appears to be an increase in the use of poppet valves in hydrocarbon process service because of the ease of maintenance.
- #44 Reciprocating compressor always have a certain amount of clearance or un-swept volume at the end of the cylinder. Some of this comes from the valve pockets and the rest from the small distance between the piston head and the cylinder head. This clearance space effects the capacity of the compressor because the vapor tapped in this volume doesn't leave the cylinder at the end of the compression stroke. It just re-expands on the suction stroke and takes up space that would other wise be used by low pressure vapors entering the cylinder. Designers take advantage of this condition to help control the capacity of fixed speed compressors. It can be done by adding a fixed volume to the compressor cylinder's head or by adding a variable volume device with a handwheel or pneumatic operator.