Centrifugal & Axial Compressor_Construction

8/14/2019 Centrifugal & Axial Compressor_Construction

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CONSTRUCTION OF CENTRIFUGAL AND AXIAL COMPRESSOR

Table of Contents

1.CENTRIFUGAL COMPRESSOR..................................................................................................................................2

1.1.TYPESOFCENTRIFUGALCOMPRESSOR...................................................................................................................................2

1.2.CONSTRUCTIONOFCENTRIFUGALCOMPRESSOR......................................................................................................................3

1.3.IMPELLERANDNOZZLEARRANGEMENTSOFMULTISTAGECENTRIFUGALCOMPRESSOR.................................................................10

2.AXIAL COMPRESSOR.................................................................................................................................................14

2.1.INTRODUCTION................................................................................................................................................................14

2.2.CONSTRUCTIONOFAXIAL COMPRESSOR.............................................................................................................................14

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1. Centrifugal Compressor

1.1. Types of centrifugal compressor

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 horizontalcenter-line are employed for low pressure rating. The suction and deliverynozzles as well as any side stream nozzles, lube oil pipes and all othercompressor-plant connections are located in the lower casing. With thisarrangement all that is necessary to raise the upper casing and gain access to allinternal components, such as the rotor, diaphragms and labyrinth seals is toremove the cover bolts along the horizontal center-line.

Advantages:

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.

Disadvantages:

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: hencethe denotation barrel, used to refer to compressors with these casings. Thesemachines, 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 thecasing, the rotor and diaphragms are essentially the same as those forcompressors with horizontally-split casings. These compressor can withstandhigher pressure than a horizontally split compressor casing. The circular covercan be made gas tight easier than the flat flange of the horizontally splitcompressor. These compressors are used for low molecular gas application andhigh pressure requirements.

Advantages:

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.

Disadvantages:

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:

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These have bell-shaped casings with a single vertical end cover. They aregenerally used for natural gas transportation to take care of the frictional loss inthe pipes. They normally have side suction and delivery nozzles positionedopposite each other to facilitate installation on gas pipelines. These are typicallyused in the gas transportation lines.

1.2. Construction of centrifugal compressorRefer Figure 1, showing the major components / internals of a typical barrel typecentrifugal compressor.

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Figure 1 -Typical barrel type centrifugal compressor

1.2.1. Casing:

Depending on the compressor family the casings can be Horizontal split orvertical 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 rotorformed by a shaft (C), one or more impellers (D), a balance drum (E), and thrustcollar (F). The rotor is driven by means of a hub (G) and is held in position axiallyby a thrust bearing (I), while rotating on journal bearings (H). The rotor is fittedwith interstage labyrinth seals (L) and, suitable end seals (M). Gas is drawn intothe compressor through a suction nozzle and enters an annular chamber (inletvolute), flowing from it towards the center from all directions in a uniform radialpattern. The gas flows into the suction diaphragm and is then picked up by thefirst impeller.

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Suction diaphragm


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Figure 3 -Inlet flow through volute, suction diaphragm and impeller

1.2.2. Diaphragm:

A diaphragm consists of a stationary element which forms half of the diffuser wallof 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 diaphragmis a structural as well as an aerodynamic device. For the last stage or for asingle-stage compressor, the flow leaving the diffuser enters the dischargevolute.

Figure 4 - Diaphragm and Labyrinth seal

Suction (inlet), intermediate and discharge diaphragms create the gas flow pathwithin the stationary components. The suction diaphragm conveys the gas intothe eye of the first impeller and can be fitted with adjustable guide vanes to

optimize the inlet flow angle. Intermediate diaphragms perform the dual functionof 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. Thedischarge diaphragm forms the diffuser for the last impeller as well as thedischarge volute. The diaphragms are usually horizontally-split.

Easily removable labyrinth seals are installed on the diaphragms at impellershrouds, to prevent return flow from discharge to suction and on the shaftsleeves to eliminate interstage leakage.

Interstage labyrinth seal:

Due to the pressure rise across successive compression stages, seals arerequired at the impeller eye and rotor shaft to prevent gas backflow from thedischarge to inlet end of the casing. The condition of these seals directly affectsthe compressor performance.

The simplest and most economical of all shaft seals is the straight labyrinthshown in Figure 5. This seal is commonly utilized between compression stagesand consists of a series of thin strips or fins, which are normally part of astationary assembly mounted in the diaphragms. A close clearance is maintainedbetween the rotor and the tip of the fins. Tight clearance and flow turbulencecreates resistance to leakage flow.

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Figure 5 -Labyrinth in new condition.

The labyrinth seal is equivalent to a series of orifices. Minimizing the size of theopenings is the most effective way of reducing the gas flow. Labyrinths cloggedwith dirt (Figure 6 where turbulence is reduced and leakage flow is increased)

and worn or wiped labyrinths with increased clearances (Figure 7 Whereclearance 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

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In order to reduce or negate the performance effects common with damagedinterstage seals, several improvements have been adopted by compressormanufacturers. Most noteworthy is the use of abradable seals in the impeller eyeand shaft seal areas. The most commonly used material for the abradablelabyrinth is Fluorosint or Nickel-graphite.

Advantages include tighter design operating clearances and minimal efficiencyeffects after a seal rub. As shown in Figure 8, tight clearance and turbulencecreates resistance to leakage flow when the seals are new. Even in the rubbedcondition of abradable seal (Figure 9), tight effective running clearance isunaltered and turbulence continues to create resistance to leakage flow.

Figure 8 -New abradable seal

Figure 9 -Rubbed abradable seal

1.2.3. Rotor:

The rotor consists of shaft, impellers, sleeves, balance drum and thrust collar.

Impellers:

Impellers are shrunk or keyed or combination of shrink and key mounted on theshaft depending on the operating speed and prevailing stress levels. Impellersmay be either of the closed (shrouded) or open (shroud less) design. The bladesare generally back-swept to different angles in accordance with the requiredperformance. The impeller pushes the gas outwards raising its velocity andpressure. On the disc side, the impeller is exposed to discharge pressure (seeFigure 10) and on the other side partly to the same pressure and partly to suctionpressure. Thus a thrust force is created towards suction.

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Figure 10 -Pressure distribution on the impeller

Balance piston seal:

A balance piston (or a center seal) is utilized to compensate for aerodynamicthrust 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 pressuredifferentials to oppose and balance most of these thrust forces. This enables theselection 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 theentry of air into the compressor casing. Depending on the nature of the gas to becompressed and on the degree of sealing to be achieved, different types of sealsmay be used.

1.2.4.1. Labyrinth Seals:

They are used when the properties and pressure of a gas permit aminimal leakage. The labyrinths are made of light alloy or othercorrosion-resistant material and are easily replaceable. The number ofteeth and clearance depend on the operating conditions, as well as thegeometry (plain, step, ring type, honey-comb, etc.). To minimizeleakage, abradable seals are used. In this case the labyrinth teeth arefitted to the rotor and are in contact with an abradable material on thestator. When no leakage whatsoever is permissible (poisonous orexplosive gases, etc.) labyrinth seals are combined with extractionand/or injection systems.

1.2.4.2. Dry Gas Seals:

Sealing is ensured by a gas lock created by the grooves machined intoa rotating seal fitted on the rotor. Depending on the application it ispossible to use gas - taken off the compressor at different levels: firstimpeller diffuser, intermediate or discharge nozzles or an insert gas.

Hydrostatic and hydrodynamic forces balance to maintain a clearanceof 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 toprevent liquid or hydrate formation or for controlling the temperature ofthe seal. Extensive experience has been accumulated on dry gas sealsystems that have been developed to meet specific processrequirements.

1.2.4.3. Liquid Film seal (Oil Film Seal):

For cases where the above described dry type seals are notadaptable, the more elaborate oil seal is used. It will produce a positive

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seal preventing the leakage of gas from or atmospheric air into thecasing. In general, due to its appreciable expense and maintenancerequirements, usage of the oil seal is limited to applications where thepressure level is high, where no leakage is tolerable or where forreasons of unavailability of sealing gas, dilution of the product gas etc.Liquid film seals are available in eight general types:

Labyrinth

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 highspeed machines. The actual seal is accomplished by a thin oil filmsupplied by the seal oil pump to a space between the rotating andstationary seal elements. This oil contacts process gas and must be

degassed before return to the oil reservoir. Contaminated oil must bereconditioned or discarded.When handling hazardous, toxic, or emission-regulated gases, the sealalso must prevent gas leakage to the atmosphere after the compressorhas tripped due to seal oil system failure. Various devices within theseal support system are available to assure that the compressor sealcontains the gas at a standstill, even if no seal oil is being pumped tothe seal. Elevated seal oil tanks can provide for the necessary staticdifferential pressure of the fluid above the sealing pressure for asufficient time to allow the compressor to be depressurized before theelevated tank oil supply is depleted.

1.2.5. Bearings:

1.2.5.1. Hydrodynamic bearings:

Journal bearings:Purpose of journal bearing areSupport and distribute rotor weight and forcesMaintain concentricityProvide stabilizing forceTilting pad bearings are generally used, and are normally equippedwith thermocouples to monitor the bearing temperature.

Thrust bearings: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 thermocouplesfor temperature monitoring and with load cells in high pressureapplications to measure axial thrust.

1.2.5.2. Active magnetic bearings:

In recent years several machines have been equipped with activemagnetic bearings. Operating on the principle of electromagneticsuspension, the active magnetic bearings perform the same functionsas hydrodynamic journal and thrust bearings with two majoradvantages:1. Reduced mechanical losses owing to the absence of friction.2. Adjustable axial and radial position and stiffness of the rotor anddamping characteristics of the bearings.

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1.3. Impeller and Nozzle arrangements of multistage centrifugal compressor

Multistage compressors can be tailored to allow extremely flexible arrangement of theimpellers 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 tomatch 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 orother side streams or for cooling between stages can also be easily incorporated in themost convenient location.

1.3.1. Straight through flow:

This arrangement may employ 10 or more stages of compression. Thisarrangement is most often used for low-pressure rise process gas compression.

1.3.2. Double flow:

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This arrangement is used to double the maximum flow capability for acompressor frame. Since the number of impellers handling each inlet flow is onlyhalf of that of an equivalent straight through machine, the maximum headcapability is reduced accordingly. Most commonly used for the first stage ofcompression for the series compression.

1.3.3. Side stream (side load):

Side stream nozzles permit introducing or extracting gas at selected pressurelevels. These flows may be process gas streams or flows from economizers inrefrigeration service. Sideloads may be introduced through the diaphragmbetween 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 processlimits as well as to improve operating efficiency. Iso-cooling nozzles permit thehot gas to be extracted from the compressor and to an external heat exchanger,and then returned to the following stage at reduced temperature for furthercompression.

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1.3.5. Back-to-back:

The back to back design minimizes thrust when a high pressure rise is to beachieved within a single casing. Note that the thrust forces acting across the twosections act in opposing directions, thus neutralizing one another. Thearrangement 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:

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1.3.8. Back-to-back with recirculation:

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2. Axial Compressor

2.1. Introduction

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 Catalyticcracking unit, LNG facilities, air separation plants and as blast furnace blowers. Theirconstruction is typically extension of the centrifugal compressors.

2.2. Construction of Axial Compressor

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 beadjustable by external devices for better performance control. Both rotor and statorblades are designed, for aerodynamic and mechanical behavior. The radial and thrustbearings are normally the tilting-pad type. Shaft-end seals can be labyrinths withextraction or buffer systems, oil film seals or dry seals depending on size and servicerequirements. All connections such as suction and discharge nozzles, side streamnozzles (if any) and oil piping are normally fitted to the lower half so that the upper halfbecomes 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 aflanged joint that corresponds to the horizontally split casing. Air is directed intothe annular blade passage by means of the inlet volute that is an integral part ofthe intake casing. The inlet volute has flow taps that provide flow measurement.Aerodynamic struts are provided between the inner and outer walls to assist instiffening the assembly. The intake connection is rectangular to permit the use oflarge, low pressure ducting. The casing assembly is centerline supported throughthe feet welded on the sides of the casing. The orientation of the inlet flange canbe either in the upward or downward position.

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2.2.2. Discharge Casing:

The discharge casing is also fabricated from steel plate and is split horizontallywith a flanged joint that corresponds to the horizontally split casing. Air exits theblade row through the exhaust diffuser and then discharges through arectangular flange. The casing assembly is centerline supported through the feet

welded on the sides of the casing. The orientation of the discharge flange can beeither in the upward or downward position.

2.2.3. Stator Casing:

The stator casing, located in the compressor center section between the intakeand discharge casings, is a two piece design manufactured from cast steel. Thestator casing is bolted and doweled to the inlet and discharge casings. The innerwall of the stator casing is precision machined to provide the proper clearancebetween the rotating blades.

2.2.4. Inlet guide vanes:

Inlet guide vanes are located upstream of the first stage of compression andprovide 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 combinationof variable and fixed stator vanes. When a combination is supplied,approximately the first half of the stages, including inlet guide vanes, arevariable.

Stator Vanes (Fixed):

The fixed stator vanes are welded into inner and outer shroud rings that arehorizontally split in halves to facilitate assembly into the casing. The inner shroudring supports two sealing strips that reduce interstage leakage. Each stator vaneassembly 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 shankpasses 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 thatminimize interstage leakage. Each vane is attached to the inner shroud ring witha 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 supportinsert in the stator casing. The stator bearing support has carbon bushings ateach end that provides a bearing surface for continuous variable movement. Adrive ring assembly is fastened to this end of the shank.

Stator Vane Drive Mechanism:

All of the variable vanes are adjusted simultaneously by each drive ring beinglinked to a common drive shaft that is automatically adjusted by anElectrohydraulic actuator.

2.2.6. Exit Guide Vanes:

The exit guide vanes are similar in construction to the fixed stator vanes exceptthat there are no seal strips on the inner shroud. These vanes reduce the exitswirl velocity from the rotating blades and provide an axial velocity into thedischarge diffuser.

2.2.7. Rotor Assembly:

The rotor assembly consists of the intake and discharge end stub shafts, rotordiscs, rotor blades, and tie bolts. The tie bolts pass through fitted holes in therotor discs and stub shafts. The tie bolts are hydraulically stretched during therotor assembly. The rotor discs are machined from steel forgings. The singledovetail slots in the disc are manufactured by the broaching process. The rotorblades are made from forged bar and one end of the blade is formed into a

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dovetail section that fits into the disc. The blades are held in place axially byremovable 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 dischargeend. 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 partswithout having to disassemble the top half of the casing. The bearing housingbrackets are bolted and doweled to the casing.

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Centrifugal & Axial Compressor_Construction