Date : July, 04, 2008 Lecturer : D. S. KIM
CONTENTS INTRODUCTION PURPOSE & GOVERNING STANDARD TYPES OF RECIPROCATING COMPRESSORS COMPONENTS / AUXILIARY SYSTEMS UNBALANCED FORCES IN RECIPROCATING COMPRESSORS CAPACITY CONTROL (LOADING & UNLOADING) REFERENCE
CONTENTS INTRODUCTION PURPOSE & GOVERNING STANDARD TYPES OF RECIPROCATING COMPRESSORS COMPONENTS / AUXILIARY SYSTEMS UNBALANCED FORCES IN RECIPROCATING COMPRESSORS CAPACITY CONTROL (LOADING & UNLOADING) REFERENCE
INTRODUCTION
COMPRESSOR :Any machine which uses external energy to increase the pressure of a gas/ mixture of gases is called a compressor.
Compressors are broadly classified into the following two categories-
Positive Displacement Type
Dynamic Type
Positi Positive ve Disp Displac laceme ement nt type type compr compress essors ors are those those in in whic which h successive volumes of gas are confined within some type of enclosure (compression chamber) and elevated to a higher
Reciprocating compressors, pressure. Eg. compressors, sliding vane compressors etc.
screw
Dynamic compressors compressors accelerate a continually continually flowing flowing gas stream stream and subsequently subsequently convert the velocity head into pressure. Eg.: Centrifugal Compressors, axial flow compressors, mixed flow compressors etc.
INTRODUCTION Reciprocating compressors are positive INTRODUCTION : Reciprocating displacement machines with a piston compressing the gas in a cylinder. As the piston piston moves forward it compresses the gas into a smaller space, space, thus raising its pressure. There are two types of reciprocating reciprocating compressors, compressors, called "lube" type type with oil injection and "non lube" as oil-free .
INTRODUCTION
PURPOSE & GOVERNING STANDARD PURPOSE: For producing pressurized air/gas at required outlet conditions. (Generally for high pressure applications with capacity as a constraint between Centrifugal & Reciprocating compressors).
Governing standards: API 618 - Specification for Reciprocating compressor.
TYPES OF RECI. COMPRESSOR I. Based on the mounting & cylinder arrangement 1. Horizontal 2. Vertical 3. ‘V’ configuration 4. ‘Y’ configuration 5. ‘L’ configuration 6. Radial 7. Balanced opposed 8. Tandem type
II. Based on no. of cylinders in series 1. Single stage 2. Two stage 3. Multi-stage
TYPES OF RECI. COMPRESSOR III. Based on no. of cylinders per stage 1. Simplex 2. Duplex 3. Triplex and so on
IV. Based on the Lubrication 1. Lubricated R.C 2. Non-lubricated R.C
V. Based on compression sides 1. Single acting 2. Double acting
TYPES OF RECI. COMPRESSOR
Vertical
Horizontal Inlet
Outlet
Single stage
Inlet
V-Type
Y-Type
Radial
Outlet
Double stage
Balanced Opposed
L-Type
Tandem arrangement
TYPES OF RECI. COMPRESSOR Suction
Discharge Suction
Discharge
Single acting
Suction
Discharge
Double acting
P-V Diagram : 3
2
1-2 Adiabatic(Polytropic) compression 2-3 Constant pressure Discharge
P
3-4 Adiabatic (Polytropic) expansion 4
V
1
4-1 Constant pressure Suction
COMPONENTS
Sectional View of simple Reciprocating compressor:
COMPONENTS Sectional View of simple Reciprocating compressor:
COMPONENTS Cylinder: The Cylinders will be one of the following types a. Single acting b. Double acting c. Tandem single or double acting d. Fitted with tail rods ( For high pressure services) The cylinders may be lubricated or non-lubricated, vertical or horizontal. In the case of non-lubricated cylinders, a distance piece is inserted between the cross-head and cylinder to ensure the part of the piston rod adjacent to the cross head (which is lubricated) does not enter the non-lubricated cylinder packing. The distance piece is often fitted with a wiper ring and may have one or more compartments which are pressurized or purged with inert gas and vented if the compressor is used on toxic gas service.
COMPONENTS Cylinder: Cylinders may be fitted with air cooling fins, but are usually water cooled. In the case of cryogenic service, the water passages are often filled with a static supply of anti-freeze supplied from a header tank, with the objective of minimizing temperature gradients within the cylinder casting. Cylinders are sometimes fitted with replaceable liners, either of wet or dry type. The material is invariably cast iron or steel depending on the application, but Ni-Resist is sometimes used on non-lubricated service. The bore of the cylinder is honed with PTFE when this material is used for the piston rings. Cylinders for oxygen service are sometimes immersed in water tanks to maintain a low working temperature. Cylinders head can be fitted with clearance pockets to increase the cylinder clearance and thus reduce the volume flow through the cylinder. The pockets can be of fixed OR variable volume and operated manually or pneumatically.
COMPONENTS Typical views of reciprocating compressors:
Fig: Cylinder with wet liner
Fig: Non-lubricated cylinder
COMPONENTS Typical views of reciprocating compressors:
Fig: Double acting Cylinder
Fig: Piston with tail rod
COMPONENTS Typical views of reciprocating compressors:
Fig: Balanced opposed (lubricated) cylinder arrangement
COMPONENTS Typical views of reciprocating compressors:
Fig: Balanced opposed (Oil free) cylinder arrangement
COMPONENTS Typical views for cylinder cooling:
Fig: Valves on sides of cylinder walls
Fig: Valves on cylinder head & tail ends
COMPONENTS Valves: Compressors usually have automatic valves which operate when a set set pressure differential exists across the valve. To function properly a valve must seat uniformly and tightly against its seat, yet must not have snap-action” on opening or closing. Until pressure builds up to the discharge point the valve must remain closed, open at discharge pressure, and then reset as the pressure in the cylinder drops below the discharge value. The same type of action is required for the suction valves. Valves must be made of fatigue resistant carbon or alloy steel or stainless steel, depending upon the service. The stainless steel & carbon steel is often used for corrosive and/or high temperature service. Any springs, as in the plate type valves, are either carbon or nickel steel. Valves passages must be smooth, streamlined and as large as possible. Cylinder efficiency depends to a certain extent upon the proper selection and sizing of the valves. Valves must be adequately cooled, so provision is usually made for water jackets immediately adjacent to the valves, particularly the discharge valves.
COMPONENTS Valves: Valve lift should be kept to a minimum to prolong valve life, but this will of course reduce the flow area and increase the gas velocity and subsequently the losses through the valve. Suction valves are often fitted with un-loaders to facilitate compressor starting. These can take the form of a fork actuated either manually or pneumatically to hold the valve off its seat. Common causes of sudden valve failure are due to compression of wet or dirty gas, and the formation of condensate within the cylinder. Solid or liquid particles settling on the valve seat can cause failure of the plate at that point. Delivery valves should therefore be located at the bottom of the cylinder to prevent accumulation of solids or condensate. In lubricated cylinder service the design of valve pockets should be such that oil cannot collect in the pocket. Compressor discharge temperatures are often above the flash point of the oil used and fire explosion can occur as results of carbon or oil accumulation in the vicinity of the valves. Suction and delivery valve assemblies should be designed to ensure they cannot be interchanged during assembly.
COMPONENTS Types of Valves: 1. Feather type valves 2. Plate type valves 3. Channel type valves 4. Ring channel valves, etc The valves may be channel or reed type, but are usually concentric circular plates or ported plate type. The operations of concentric circular plates are assisted by helical springs bearing on the plate and a dampener plate may be located over the valve plate to reduce flutter. Adjacent concentric plates tend to open out of phase in relation to each other and an alternative ported plate design with an integral central leaf spring gives better performance. The ported plate type can also have a dampener plate and helical springs located around the plate edge.
COMPONENTS Fig: Valves & valves details Plate type valves
Channel type valves
Ring channel valves
COMPONENTS
Fig: Valves & details
COMPONENTS Piston, Piston rod & Packing: Pistons are usually manufactured from cast iron or aluminium in order to balance the masses on successive throws of the crankshaft. Large diameter pistons are often produced in segments bolted together. Single acting pistons are sometimes fitted with cooling fins cast under the crown. First stage large diameter pistons are often of welded construction. Piston rings are normally manufactured from cast iron for lubricated cylinders, but in non-lubricated cylinders, they are either carbon or PTFE. Solid carbon rings are used with segmented pistons, but segmented carbon rings are a common alternative. Carbon relies on moisture for its strength and should therefore not be used on very dry gas service. PTFE has a lower permissible operating temperature than carbon (about 180 C). Above this temperature the wear rate increases rapidly. Split PTFE rings are flexible and easily assembled over solid
COMPONENTS Piston, Piston rod & Packing: pistons, however, because of their flexibility, a steel garter spring ring is sometimes inserted behind the sealing ring. Solid PTFE bearer rings are usually located centrally on the piston to transmit the weight of the piston on to the lower cylinder wall. Running clearances are a function of operating and wall temperatures. When checking the cylinder design ensures that the pressure rings do not span the valve pockets as this causes distortion and uneven wear of the rings, leading to premature failure. Labyrinth pistons do not touch the cylinder wall at any point. The piston rod is carried on a piston guide which runs on lubricated pads above the cross-head. The piston sides are machined with circumferential grooves and there is no contact with the cylinder walls. Leakage is greater with labyrinth pistons than with pressure rings, but by increasing the piston length the loss can be reduced to an acceptable amount. Labyrinth pistons are particularly suitable for oxygen and clean gas service.
COMPONENTS Labyrinth piston
Piston
Cylinder
COMPONENTS Piston, Piston rod & Packing: A compromise between the labyrinth and self-lubricating ring design is the captive ring and piston. This arrangement is fitted with PTFE rings and a piston guide. The inner edge of the ring is retained by a flange. After an initial wear period, no contact exists between the rings and cylinder. Clearances are smaller than can be accomplished with the labyrinth design and momentary contact is harmless. The piston rod, where it enters the cylinder, is sealed by spring loaded segmented sealing rings. These are PTFE or carbon in nonlubricated machines, but can be metallic in lubricated cylinders. The packing box is sometimes water-cooled and can be vented at some point along its length. Purging with inert gas or high-pressure gas is used for special applications. The pressure seal between the cylinder pressure and the crank case or atmosphere is maintained by a packing gland.
COMPONENTS Pulsation dampers: Pulsations generated by reciprocating pistons cause vibration and pulsations in the gas lines on both inlet and outlet sides of the compressor. These have an adverse effect on the valves and on any measuring instruments fitted to the gas lines, and cause vibration in the piping and support structures. These effects can be reduced by fitting a large volume bottle or vessel as close to the cylinder as possible. Pulsations can thus be reduced to 2% of the line pressure. In large installations, it is necessary to conduct a study of the piping in the vicinity of the compressor to ensure that harmful vibrations or pulsations do not exists. This study is conducted by an analog computer programmed to produce a model of the piping and indicate where pulsations might occur. The study should be performed at all expected part load flow rates.
AUXILIARY SYSTEM Auxiliary Systems 1. Filters 2. Coolers 3. Drivers 4. Piping, instrumentation & controls
Filters: Filters will be used to avoid entering of dust particles, moisture etc into the compressor cylinder, in order to get better efficiency and to meet the system output requirements. Depending upon the moisture content of the fluid, separators are provided before some compression stages to ensure that no moisture is entrained in the gas flow to the cylinders. Coolers: Where the overall compression ratio is so high that the discharge gas temperature is unacceptable, it is necessary to effect the compression in stages. Delivery from the first stage is cooled to somewhere near the initial suction temperature before passing to the next stage. Inter-coolers can be mounted directly on the cylinders, along with
AUXILIARY SYSTEM Drivers: Drivers for Reciprocating compressors are Electric motors Diesel engines or Gas engines Turbines
The driver may be connected to reciprocating compressors by means of Coupling Rigid Flexible
Constant or Variable speed arrangement Belt Gear box Fluid coupling, etc.
AUXILIARY SYSTEM Fluid (Hydraulic) Coupling: The fluid or hydraulic coupling is a device used for transmitting power from driving shaft to driven shaft with the help of fluid. There is no mechanical connection between the two shafts. It consists of a radial pump impeller mounted on a driving shaft and a radial flow reaction turbine mounted on the driven shaft. Both the impeller and runner are identical in shape and they together form a casing which is completely enclosed and filled with oil. In the beginning, both the shafts are at rest. When the driving shaft rotates, the oil starts moving from the inner radius to the outer radius of the pump impeller . The pressure and kinetic energy of the oil increases at the outer radius of the pump impeller. This oil of increased energy enters the runner of the reaction turbine at the outer radius of the turbine runner and flows inwardly to the inner radius of the turbine runner. The oil, while flowing through the runner, transfers its energy to the blades of the runner and makes the runner to rotate. The oil from the runner then flows back into the pump impeller, thus having a continuous circulation.
AUXILIARY SYSTEM Fluid (Hydraulic) Coupling: In usual practice the speed of the driven shaft will be 2% less than the speed of the driver shaft. Power output Efficiency of the hydraulic coupling:
Efficiency
=
Power input
UNBALABCED FORCES Unbalanced forces in Reciprocating Compressors: In a single cylinder, single crank arrangement, the primary forces set up by the out-of-balance weight of the piston can be reduced to half the maximum value by placing a counter weight on the crank throws, though the secondary force, having twice the frequency of the primary force, will remain unbalanced. With a two crank vertical arrangement the primary forces are balanced, but the secondary forces are twice those produced in a single crank machine and cannot be economically balanced. With this arrangement, a primary couple, due to the distance between piston centerlines (d) and equal to half the primary force multiplied by the distance (d), if the machine is fitted with counterweights, then is also present. Two cylinder horizontally opposed arrangements have no out-ofbalance forces or couples and are preferred where space permits. ‘V’ and ‘L’ cylinder arrangements have balanced primary forces, but the secondary forces are not balanced.
CAPACITY CONTROL Capacity control (Loading & Unloading) in Reciprocating Compressor:
Types: Variable speed Suction throttling Bypass control Receiver pressure Start/Stop method Adjusting (Screwing) the piston rod further into or out of the cross
head, for some single acting units Suction valve un-loaders Clearance pocket un-loaders Fixed clearance pocket Variable clearance pocket
Combination of Suction valve & Clearance pocket un-loaders
CAPACITY CONTROL Variable speed
Capacity control can be achieved with the help of Variable speed drives like variable speed motors, Hydraulic converter, etc. Suction throttling
Capacity control can be achieved by throttling the flow in the suction line. Bypass control
With the help of bypassing the amount of air (which is not exactly required or which is a excess of flow at that particular operating point) back to the suction line capacity control can be obtained, but the bypassing air needs to be cooled before passing into the suction line.
CAPACITY CONTROL Receiver pressure Start/Stop method
In this method the compressor will be loaded OR unloaded depending on the receiver pressure control switches. But the compressor will be subjected to frequent start & stop. By Adjusting (Screwing) the piston rod further into or out of the cross
head (only in case of single acting units) capacity control can be achieved. Suction valve un-loaders
In this method when compressor un-loading is required then the suction valve will be kept open for complete cycle, that means with in the cylinder there will not be any compression. What ever the air its sucks into the cylinder will be discharged through the suction valve without compressing it to a higher valve. In this type depending on the cylinder arrangement capacity can be control in steps of
CAPACITY CONTROL a) 0%, 100% (for single stage) b) 0%, 50% & 100%. ( for double acting ) c) 0%, 25%, 50%, 75% & 100% (for combination of double acting & Duplex cylinder arrangement) Clearance pocket un-loaders By increasing the clearance volume at the cylinder head end capacity can be controlled. There are two types of clearance pockets Fixed clearance pocket Variable clearance pocket
Fixed clearance pocket : In this type of clearance pocket capacity can be controlled in steps depending on the cylinder arrangement. The steps of control may be a) 0%, 100% (for single stage) b) 0%, 50% & 100%. ( for double acting ) c) 0%, 25%, 50%, 75% & 100% (for combination of double acting & Duplex cylinder arrangement)
CAPACITY CONTROL Variable clearance pocket : In this type of clearance pocket capacity can be controlled in a wide range depending on the area of the clearance pocket. If the area of the clearance pocket is sized for
1) The complete piston displacement in case of single acting cylinder arrangement 2) Half the piston displacement in case of double acting or duplex cylinder arrangement 3)1/4th of the piston displacement in case of combination of double acting & duplex arrangement then the capacity control range will be from 0% to 100% of compressor discharge capacity. Combination of Suction valve un-loaders & Clearance pocket un-
loaders
By using combination of suction valve un-loaders and clearance pocket un-loaders the capacity can be controlled to a great extent with out providing excessive clearance pocket area.
CAPACITY CONTROL Figures: Capacity control
Using two clearance pockets each having 50% area of piston displacement Using single clearance pockets with 100% area of piston displacement
CAPACITY CONTROL Pneumatic control schematic Fixed clearance pocket
CAPACITY CONTROL 5-Step capacity control:
Using clearance pockets only
Combination of suction valve un-loaders & clearance
REFERENCE API 618 - Specification for Reciprocating Compressors: This specification covers the minimum requirements for reciprocating compressors and their drivers used in petroleum, chemical and gas industry services for handling process gas or air with wither lubricated or non-lubricated cylinders. This specification covers Compressors which are of moderate to low speed and in critical services. This specification also covers the related lubricating systems, controls, instrumentation, inter-coolers, after coolers, pulsation suppression devices and other auxiliary equipment.
This specification does not cover a. “Integral gas engine driven compressors with single acting trunk type (automotive type) pistons that also serve as crossheads” and b. “Either plant or instrument-air compressors that discharge at a gauge pressure of 9 bar (125 pounds per square inch) or less”. Requirements for packaged high speed reciprocating compressors for
oil and gas production services are covered in “API-11P”. Requirements for packaged reciprocating plant and instrument-air
REFERENCE API 618 - Specification for Reciprocating Compressors:
Following are the minimum requirements to be complied as per “API-618 Specification for Reciprocating compressors”: Design requirements: General : 1. Minimum service life shall be 20 years and uninterrupted operation shall at least be 3 years. 2. The capacity at the normal operating point shall have no negative tolerance. 3. The maximum predicted discharge temperature shall not exceed 1500C(3000F) 4. In case of high pressure hydrogen (or those with molecular weight of 12 or less) OR applications requiring non lubricated cylinders , the predicted discharge temperature shall not exceed 1350C (2750F).
REFERENCE API 618 - Specification for Reciprocating Compressors:
5. The Maximum Allowable Working Pressure (MAWP) shall be at least 10% more than rated discharge pressure or 1.7 bar (25pounds per square inch), which ever is greater. 6. The cylinder shall have replaceable, dry type liner, not contacted by the coolant. The liner shall be at least 9.5 millimeters thick for piston diameters up to & including 254 millimeters (10 inches). For pistons diameters larger than 254 millimeters, the minimum liner thickness shall be 12 millimeters. 7. Replaceable, precision-bored shell (sleeve) crank-pin bearings and main bearings shall be used; however tapered roller type antifriction bearings are acceptable for main bearings in compressors with nominal frame ratings of 150 kW or less. Cylindrical or roller or ball bearings are unacceptable. 8. All tapered roller-type anti-friction bearings shall be suitable for belt drives and shall give an L10 -rated life of either 50,000 hours with continuous operation at rated conditions or 25,000 hours at maximum axial & radial load & rated speed.
REFERENCE API 618 - Specification for Reciprocating Compressors:
9. The frame lubrication system shall be a pressurized system; however, splash lubrication systems may be used on horizontal compressors with anti-friction journal bearings when the compressor’s nominal frame rating is 150 kW or less. 10. The crankcase oil temperature shall not exceed 700C for pressurized oil systems and 800C for splash systems. 11. Cooling coils shall not be used in crankcase or oil reservoirs. 12. If not specified the capacity shall not have negative tolerance. 13. The driver nameplate power should be selected to be a minimum of 110% (for electric motor) or 120% (for turbines) of the greatest total power required. 14. The power required by the compressor at the normal operating point shall not exceed the stated power by more than 3 percent. 15. Air cooled cylinders shall not be furnished.
REFERENCE API 618 - Specification for Reciprocating Compressors:
16. The walls of cylinders without liners shall be thick enough to provide for re-boring to a total of 3.2 mm (1/8 inch) increase over the original diameter without encroaching on either the MAWP, or the maximum allowable continuous gas load, or the maximum allowable combined rod loading. 17. To prevent the gas condensation in the cylinder Coolant inlet temperature shall be at a minimum of 6K (10 0F) above the inlet gas temperature. 18. Coolant exit temperature should not exceed 16K (300F) above the gas inlet temperature to avoid capacity reduction. 19. The coolant circulated shall be controlled to maintain a rise in coolant temperature across any individual cylinder, including the cylinder heads between 5K (100F) & 10K (200F). 20. Pneumatically operated un-loaders shall be used for capacity control.
REFERENCE API 618 - Specification for Reciprocating Compressors:
21. If hollow pistons are used, they shall be continuously self-venting; that is, they shall de-pressure when the cylinder is de-pressured. 22. All piston rods, regardless of base material, shall be continuously coated from the piston-rod packing to the oil wiper packing travel areas with a wear resistant material. 23. Cross-head packing boxes shall employ wiper packing to effectively minimize oil leakage from the crankcase. 24. Lubricators shall have provisions for pre-lubrication of the compressor start-up. 25. Lubrication oil reservoir shall be adequate for 30 hours of operation at normal flow.
REFERENCE P & ID – HAND OUT
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