CONSTRUCTION OF CENTRIFUGAL AND AXIAL COMPRESSOR
CONSTRUCTION OF CENTRIFUGAL AND AXIAL COMPRESSOR
Table of Contents
In general the centrifugal compressors are divided in three different types
1.1.1. Horizontally split:
Horizontally-split casings consisting of half casings joined along the horizontal center-line are employed for low pressure rating. The suction and delivery nozzles as well as any side stream nozzles, lube oil pipes and all other compressor-plant connections are located in the lower casing. With this arrangement all that is necessary to raise the upper casing and gain access to all internal components, such as the rotor, diaphragms and labyrinth seals is to remove the cover bolts along the horizontal center-line.
· Typically less expensive to manufacture
· Easier to inspect / maintain in plant and refinery location
· Better for multiple body train.
· Minimum special tooling required.
· Low heaviest part for maintenance.
· Larger sealing surfaces.
· Low pressure ratings.
· Removal of overhead piping for up nozzle configuration.
1.1.2. Vertically split casing (Barrel):
Vertically-split casings are formed by a cylinder closed by two end covers: hence the denotation ‘‘barrel,’’ used to refer to compressors with these casings. These machines, which are generally multistage, are used for high pressure services The barrel compressor has a horizontally split inner casing containing the rotor. This inner casing is inserted into the barrel and closed by an end wall. Inside the casing, the rotor and diaphragms are essentially the same as those for compressors with horizontally-split casings. These compressor can withstand higher pressure than a horizontally split compressor casing. The circular cover can be made gas tight easier than the flat flange of the horizontally split compressor. These compressors are used for low molecular gas application and high pressure requirements.
· Reduced potential for gas leakage.
· Higher pressure rating.
· Can remove rotor and internal components without affecting external piping.
· Removable inner bundle allows easy disassembly / transportation.
· Typically more expensive to manufacture.
· More difficult to maintain if in the middle of a multiple body train.
· Special tooling required for inner bundle removal.
· High “Heaviest part for maintenance”.
1.1.3. Pipeline or booster:
These have bell-shaped casings with a single vertical end cover. They are generally used for natural gas transportation to take care of the frictional loss in the pipes. They normally have side suction and delivery nozzles positioned opposite each other to facilitate installation on gas pipelines. These are typically used in the gas transportation lines.
Refer Figure 1, showing the major components / internals of a typical barrel type centrifugal compressor.
Figure 1-Typical barrel type centrifugal compressor
Depending on the compressor family the casings can be Horizontal split or vertical split. Figure 2 shows a typical cross section of a centrifugal compressor.
Figure 2-Typical cross section of centrifugal compressor
Outer casing contains a stator part, called a diaphragm bundle (B) and rotor formed by a shaft (C), one or more impellers (D), a balance drum (E), and thrust collar (F). The rotor is driven by means of a hub (G) and is held in position axially by a thrust bearing (I), while rotating on journal bearings (H). The rotor is fitted with interstage labyrinth seals (L) and, suitable end seals (M). Gas is drawn into the compressor through a suction nozzle and enters an annular chamber (inlet volute), flowing from it towards the center from all directions in a uniform radial pattern. The gas flows into the suction diaphragm and is then picked up by the first impeller.
Figure 3-Inlet flow through volute, suction diaphragm and impeller
A diaphragm consists of a stationary element which forms half of the diffuser wall of the former stage, part of the return bend, the return channel, and half of the diffuser wall of the later stage. Due to the pressure rise generated, the diaphragm is a structural as well as an aerodynamic device. For the last stage or for a single-stage compressor, the flow leaving the diffuser enters the discharge volute.
Figure 4- Diaphragm and Labyrinth seal
Suction (inlet), intermediate and discharge diaphragms create the gas flow path within the stationary components. The suction diaphragm conveys the gas into the eye of the first impeller and can be fitted with adjustable guide vanes to optimize the inlet flow angle. Intermediate diaphragms perform the dual function of forming the diffuser passage (where gas velocity is transformed into pressure) and the return passage to channel gas to the eye of the next impeller. The discharge diaphragm forms the diffuser for the last impeller as well as the discharge volute. The diaphragms are usually horizontally-split.
Easily removable labyrinth seals are installed on the diaphragms at impeller shrouds, to prevent return flow from discharge to suction and on the shaft sleeves to eliminate interstage leakage.
Interstage labyrinth seal:
Due to the pressure rise across successive compression stages, seals are required at the impeller eye and rotor shaft to prevent gas backflow from the discharge to inlet end of the casing. The condition of these seals directly affects the compressor performance.
The simplest and most economical of all shaft seals is the straight labyrinth shown in Figure 5. This seal is commonly utilized between compression stages and consists of a series of thin strips or fins, which are normally part of a stationary assembly mounted in the diaphragms. A close clearance is maintained between the rotor and the tip of the fins. Tight clearance and flow turbulence creates resistance to leakage flow.
Figure 5-Labyrinth in new condition.
The labyrinth seal is equivalent to a series of orifices. Minimizing the size of the openings is the most effective way of reducing the gas flow. Labyrinths clogged with dirt (Figure 6 – where turbulence is reduced and leakage flow is increased) and worn or wiped labyrinths with increased clearances (Figure 7 – Where clearance is increased, turbulence is reduced resulting in increased leakage) allow larger gas leakage. This can affect compressor operation, and therefore the seals should be replaced.
Figure 6-Fouled labyrinth
Figure 7-Rubbed labyrinth
In order to reduce or negate the performance effects common with damaged interstage seals, several improvements have been adopted by compressor manufacturers. Most noteworthy is the use of abradable seals in the impeller eye and shaft seal areas. The most commonly used material for the abradable labyrinth is Fluorosint or Nickel-graphite.
Advantages include tighter design operating clearances and minimal efficiency effects after a seal rub. As shown in Figure 8, tight clearance and turbulence creates resistance to leakage flow when the seals are new. Even in the rubbed condition of abradable seal (Figure 9), tight effective running clearance is unaltered and turbulence continues to create resistance to leakage flow.
Figure 8-New abradable seal
Figure 9-Rubbed abradable seal
The rotor consists of shaft, impellers, sleeves, balance drum and thrust collar.
Impellers are shrunk or keyed or combination of shrink and key mounted on the shaft depending on the operating speed and prevailing stress levels. Impellers may be either of the closed (shrouded) or open (shroud less) design. The blades are generally back-swept to different angles in accordance with the required performance. The impeller pushes the gas outwards raising its velocity and pressure. On the disc side, the impeller is exposed to discharge pressure (see Figure 10) and on the other side partly to the same pressure and partly to suction pressure. Thus a thrust force is created towards suction.
Figure 10-Pressure distribution on the impeller
Balance piston seal:
A balance piston (or a center seal) is utilized to compensate for aerodynamic thrust forces imposed on the rotor due to the pressure rise through a compressor. The purpose of the balance piston is to utilize the readily available pressure differentials to oppose and balance most of these thrust forces. This enables the selection of a smaller thrust bearing, which results in lower horsepower losses.
1.2.4. Shaft end Seals:
Shaft end seals eliminate or minimize the leakage of compressed gas or the entry of air into the compressor casing. Depending on the nature of the gas to be compressed and on the degree of sealing to be achieved, different types of seals may be used.
188.8.131.52. Labyrinth Seals:
They are used when the properties and pressure of a gas permit a minimal leakage. The labyrinths are made of light alloy or other corrosion-resistant material and are easily replaceable. The number of teeth and clearance depend on the operating conditions, as well as the geometry (plain, step, ring type, honey-comb, etc.). To minimize leakage, abradable seals are used. In this case the labyrinth teeth are fitted to the rotor and are in contact with an abradable material on the stator. When no leakage whatsoever is permissible (poisonous or explosive gases, etc.) labyrinth seals are combined with extraction and/or injection systems.
184.108.40.206. Dry Gas Seals:
Sealing is ensured by a gas lock created by the grooves machined into a rotating seal fitted on the rotor. Depending on the application it is possible to use gas - taken off the compressor at different levels: first impeller diffuser, intermediate or discharge nozzles or an insert gas. Hydrostatic and hydrodynamic forces balance to maintain a clearance of a few microns between the rotating seals and the stationary face. This very small clearance reduces gas leakage to a negligible amount. Different patented solutions are available to temper the seals to prevent liquid or hydrate formation or for controlling the temperature of the seal. Extensive experience has been accumulated on dry gas seal systems that have been developed to meet specific process requirements.
220.127.116.11. Liquid Film seal (Oil Film Seal):
For cases where the above described “dry” type seals are not adaptable, the more elaborate oil seal is used. It will produce a positive seal preventing the leakage of gas from or atmospheric air into the casing. In general, due to its appreciable expense and maintenance requirements, usage of the oil seal is limited to applications where the pressure level is high, where no leakage is tolerable or where for reasons of unavailability of sealing gas, dilution of the product gas etc.
Liquid film seals are available in eight general types:
· Bushing (Carbon Ring)
· Windback (Reversed Helical Groove Bushing)
· Restricted Bushing (Trapped Bushing)
· Film-Riding Face Seal
· Contacting Face Seal
· Circumferential Seal
· Lip Seal
The liquid film seal or oil film seal is particularly applicable to high speed machines. The actual seal is accomplished by a thin oil film supplied by the seal oil pump to a space between the rotating and stationary seal elements. This oil contacts process gas and must be degassed before return to the oil reservoir. Contaminated oil must be reconditioned or discarded.
When handling hazardous, toxic, or emission-regulated gases, the seal also must prevent gas leakage to the atmosphere after the compressor has tripped due to seal oil system failure. Various devices within the seal support system are available to assure that the compressor seal contains the gas at a standstill, even if no seal oil is being pumped to the seal. Elevated seal oil tanks can provide for the necessary static differential pressure of the fluid above the sealing pressure for a sufficient time to allow the compressor to be depressurized before the elevated tank oil supply is depleted.
18.104.22.168. Hydrodynamic bearings:
Purpose of journal bearing are
Support and distribute rotor weight and forces
Provide stabilizing force
Tilting pad bearings are generally used, and are normally equipped with thermocouples to monitor the bearing temperature.
Purpose of thrust bearing are,
Absorb axial thrust generated by the pressure differentials on the rotor.
Axially position the rotor with respect to stationary parts.
Double-acting, tilting pad bearings with an equalizing device are typically installed. The bearing pads can be fitted with thermocouples for temperature monitoring and with load cells in high pressure applications to measure axial thrust.
22.214.171.124. Active magnetic bearings:
In recent years several machines have been equipped with active magnetic bearings. Operating on the principle of electromagnetic suspension, the active magnetic bearings perform the same functions as hydrodynamic journal and thrust bearings with two major advantages:
1. Reduced mechanical losses owing to the absence of friction.
2. Adjustable axial and radial position and stiffness of the rotor and damping characteristics of the bearings.
Multistage compressors can be tailored to allow extremely flexible arrangement of the impellers and nozzles to meet the process demands of respective users. For example, the number of impellers in the compressor can be varied from two to more than twelve to match the head and flow characteristics. The inlet and exhaust nozzles can be arranged up, down, up and down, or offset at an angle. The additional nozzles for economizers or other side streams or for cooling between stages can also be easily incorporated in the most convenient location.
1.3.1. Straight through flow:
This arrangement may employ 10 or more stages of compression. This arrangement is most often used for low-pressure rise process gas compression.
1.3.2. Double flow:
This arrangement is used to double the maximum flow capability for a compressor frame. Since the number of impellers handling each inlet flow is only half of that of an equivalent straight through machine, the maximum head capability is reduced accordingly. Most commonly used for the first stage of compression for the series compression.
1.3.3. Side stream (side load):
Side stream nozzles permit introducing or extracting gas at selected pressure levels. These flows may be process gas streams or flows from economizers in refrigeration service. Sideloads may be introduced through the diaphragm between two stages (sideload 3), or if the flow is high as in sideloads 1 and 2, the flow may be introduced into mixing section by omitting one or two impellers.
1.3.4. Cooling between the stages (ISO-Cool):
Cooling is required to keep operating temperatures below material or process limits as well as to improve operating efficiency. Iso-cooling nozzles permit the hot gas to be extracted from the compressor and to an external heat exchanger, and then returned to the following stage at reduced temperature for further compression.
The back to back design minimizes thrust when a high pressure rise is to be achieved within a single casing. Note that the thrust forces acting across the two sections act in opposing directions, thus neutralizing one another. The arrangement is the best when gas must not migrate from the first section to the second section in an iso-cool compressor.
1.3.6. First Section double flow:
1.3.7. Multi-iso cool:
1.3.8. Back-to-back with recirculation:
Axial flow compressors are used wherever large volume of gas need to be compressed on the basis of a relatively low intake / discharge pressure ratio (normally upto 1:12). These machines typically find their industrial application in nitric acid plant, Fluid Catalytic cracking unit, LNG facilities, air separation plants and as blast furnace blowers. Their construction is typically extension of the centrifugal compressors.
The static part of the machine consists of an external fabricated, horizontally split casing, with an inner casing to hold the stator blading. The first section of stator blading may be adjustable by external devices for better performance control. Both rotor and stator blades are designed, for aerodynamic and mechanical behavior. The radial and thrust bearings are normally the tilting-pad type. Shaft-end seals can be labyrinths with extraction or buffer systems, oil film seals or dry seals depending on size and service requirements. All connections such as suction and discharge nozzles, side stream nozzles (if any) and oil piping are normally fitted to the lower half so that the upper half becomes an easily removable cover. Following is the brief description on the major part of a typical axial compressor.
2.2.1. Intake Casing:
The intake casing is fabricated from steel plate and is split horizontally with a flanged joint that corresponds to the horizontally split casing. Air is directed into the annular blade passage by means of the inlet volute that is an integral part of the intake casing. The inlet volute has flow taps that provide flow measurement. Aerodynamic struts are provided between the inner and outer walls to assist in stiffening the assembly. The intake connection is rectangular to permit the use of large, low pressure ducting. The casing assembly is centerline supported through the feet welded on the sides of the casing. The orientation of the inlet flange can be either in the upward or downward position.
2.2.2. Discharge Casing:
The discharge casing is also fabricated from steel plate and is split horizontally with a flanged joint that corresponds to the horizontally split casing. Air exits the blade row through the exhaust diffuser and then discharges through a rectangular flange. The casing assembly is centerline supported through the feet welded on the sides of the casing. The orientation of the discharge flange can be either in the upward or downward position.
2.2.3. Stator Casing:
The stator casing, located in the compressor center section between the intake and discharge casings, is a two piece design manufactured from cast steel. The stator casing is bolted and doweled to the inlet and discharge casings. The inner wall of the stator casing is precision machined to provide the proper clearance between the rotating blades.
2.2.4. Inlet guide vanes:
Inlet guide vanes are located upstream of the first stage of compression and provide for the proper pre-rotation of the air into the rotating blades.
2.2.5. Stator Vanes:
A machine may be supplied with fixed stator vanes throughout or a combination of variable and fixed stator vanes. When a combination is supplied, approximately the first half of the stages, including inlet guide vanes, are variable.
Stator Vanes (Fixed):
The fixed stator vanes are welded into inner and outer shroud rings that are horizontally split in halves to facilitate assembly into the casing. The inner shroud ring supports two sealing strips that reduce interstage leakage. Each stator vane assembly fits into a machined groove in the stator casing.
Stator Vanes (Continuously Variable):
Each variable stator vane is welded to a shank on each end. The inner shank passes through an inner shroud ring that is split into eight sections to facilitate assembly into the casing. The inner shroud ring supports two seal rings that minimize interstage leakage. Each vane is attached to the inner shroud ring with a special locking mechanism that reduces friction and maintains a secure fit.
The shank on the outer end of the vane passes through a stator bearing support insert in the stator casing. The stator bearing support has carbon bushings at each end that provides a bearing surface for continuous variable movement. A drive ring assembly is fastened to this end of the shank.
All of the variable vanes are adjusted simultaneously by each drive ring being linked to a common drive shaft that is automatically adjusted by an Electrohydraulic actuator.
2.2.6. Exit Guide Vanes:
The exit guide vanes are similar in construction to the fixed stator vanes except that there are no seal strips on the inner shroud. These vanes reduce the exit swirl velocity from the rotating blades and provide an axial velocity into the discharge diffuser.
2.2.7. Rotor Assembly:
The rotor assembly consists of the intake and discharge end stub shafts, rotor discs, rotor blades, and tie bolts. The tie bolts pass through fitted holes in the rotor discs and stub shafts. The tie bolts are hydraulically stretched during the rotor assembly. The rotor discs are machined from steel forgings. The single dovetail slots in the disc are manufactured by the broaching process. The rotor blades are made from forged bar and one end of the blade is formed into a dovetail section that fits into the disc. The blades are held in place axially by removable locking tabs that fit between the blade and disc.
2.2.8. Balance Piston:
The balance piston is an integral part of the rotor stub shaft on the discharge end. The stationary labyrinth is supported by the discharge casing.
2.2.9. Shaft Seals:
Labyrinth type shaft seals are provided on each end of the rotor.
2.2.10. Bearing Housings
The bearing housings are horizontally split for access to all of the bearing parts without having to disassemble the top half of the casing. The bearing housing brackets are bolted and doweled to the casing.
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