Dynamic Compressors

DYNAMIC COMPRESSORS

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Ejector Dynamic

Intermittent flow

COMPRESSORS

Continuous flow

Positive displacement

Radial flow

Mixed flow

Axial flow

Rotary Reciprocating

•Helical lobe

•Straight lobe

•Sliding vans

• Liquid piston

Mechanical piston

DYNAMIC COMPRESSORS


Ejector Dynamic

Intermittent flow

COMPRESSORS

Continuous flow

Positive displacement

Radial flow

Mixed flow

Axial flow

Rotary Reciprocating

•Helical lobe

•Straight lobe

•Sliding vans

• Liquid piston

Mechanical piston

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Centrifugal compressor

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Diffuser

Impeller



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Centrifugal compressors or radial flow compressors are second only to reciprocating compressors in numbers of machines in service.

The centrifugal compressor has been around for quite a long time. Originally it was used in process applications at

relatively low pressure, high volume service .


An Introduction to Pumping Equipment

2

20

200

102 103 104 105 106

Multistage Reciprocating

Single stage Recip.

Flow rate (CFM)

Pre

ssu

re r

atio

Rotary compressors

Multistage Centrifugal

Single S Cen.

Multistage Axial



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Classification

Classification can be established by how the machine casing is constructed:

Horizontal split.

Vertical split.



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Classification

Single stage, vertically split, overhung style centrifugal

compressor.



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Classification

Multistage barrel type compressor.



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Classification

an overhung style machine mounted on a gearbox and uses the gear pinion shaft

extension to mount an impeller .

The casing is also attached to the gearbox.

This style is built in both the single and multistage

configuration.



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Arrangement

The overhung style is, on very rare occasions, constructed in

the multistage form, usually overhanging no more than two

impellers.



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Arrangement

Careful application must be made because the overhung

impeller configuration is more sensitive to unbalance than the

between-bearing design. If impeller fouling is anticipated,

this design may not be acceptable.



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Arrangement

A beam type single-stage is a less common form of the single-stage. In

this form, the impeller is located between two bearings as is the

multistage.

The flow enters and leaves in a tangential direction with the nozzles

located in the horizontal plane.



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Arrangement

The between-bearing, single-stage is found most commonly in pipe line booster service where the inherent

rigidity of the two outboard bearings is desirable.



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Arrangement

The integrally geared compressor. As shown, it consists of three impeller,

the first located on one pinion, which would have a lower speed than the other pinion which has mounted

the remaining two impellers.



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Arrangement

This arrangement is common to the plant air

compressor. Configurations such as this are used in

other air and gas services, with the number of stages

set to match the application.



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Arrangement

The flow path is straight through the compressor,

moving through each impeller in turn. This type of centrifugal compressor

is probably the most common of any found in

process service, with applications ranging from

air to gas.



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Arrangement



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Arrangement



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Arrangement

In the double-flow compressor , the flow enters the case at two points, is compressed by a single impeller at each

end and then enters the double-flow wheel.



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Arrangement

The flow enters the case and is divided to each side of the double flow impeller and then joins at the impeller exit

prior to entering the diffuser.



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Arrangement

If a number of options are open for a given application, the

double-flow option should not be the first choice.

Although it should be evaluated as there are successful

applications in service indicating that with careful design, the

compressor will perform satisfactorily.



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Arrangement

The back-to-back arrangement is normally considered useful in solving difficult thrust balance problems where

the conventional thrust bearing and balance drum size are inadequate or become excessively large.



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Arrangement

The flow is removed part way through the compressor and reintroduced at the opposite end, then allowed to exit at

the center.



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Arrangement

The obvious use is for applications generating high thrusts, higher than can be readily controlled by a normal

size thrust bearing and balance drum.



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Arrangement

An evaluation of the cross leakage between the two discharge nozzles must be made and compared to the

balance drum leakage to determine the desirability of the "back to back".



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Arrangement

The sidestream compressor is the very common compressor design used in the chemical industry,

particularly in large refrigeration systems.



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Arrangement

The properties of the gas stream are modified at the mixing point, as the sidestream is rarely at the same temperature as the stream from the second impeller.



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Construction:

All centrifugal compressors' casings were initially of cast construction, particularly in the smaller sizes.

Casings

Casing materials are, in most cases, cast of iron or nodular iron and steel.

Casings are, on occasion, made of austenitic stainless steel or one of the high nickel alloys.

Fabricated casings are generally made of carbon or alloy steel.



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Construction:

For low temperature, suction applications a low nickel alloy may be used

Casings

API Standard 617 includes material guidelines in its appendix.

The vertically-split, multistage barrel compressor is generally constructed of steel or steel alloy. It may be

cast, fabricated or for very high pressure service it may be forged.



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Construction:

The vertically-split should always be used when the gas contains hydrogen at or above a partial pressure of 200

psig.

Casings

It may also be required in those services where the overall pressure is too high for the horizontal-split joint

compressor.

This occurs when the horizontally-split joint deforms too much at the operating pressure to maintain a gas tight

seal.



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Construction:

They are the stationary members located inside a multistage casing.

Diaphragms

The function of the diaphragm is to act as a diffuser for the impeller and a channel to redirect the gas into the

following stage.

The diaphragm also acts as the carrier for the impeller eye seal and the interstage shaft seal.



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Construction:

The diaphragm are split, located with matching grooves in the upper and lower half casing and pinned to upper half

for maintenance ease.

Diaphragms

Diaphragms are hand fitted to center them to the rotating element.

It is important for the horizontal joint to match well, to keep the joint leakage to a minimum



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Construction:

On barrel compressors, the diaphragm assembly makes

up an inner barrel.

Diaphragms



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Construction:

The assembly and rotor are removed from the barrel casing as a unit using a

special fixture. The diaphragm assembly is split to permit the removal of the

rotor and the diaphragms are generally constructed the

same as those of the horizontally split compressor.

Diaphragms



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Construction:

Casing inlet and outlet nozzles are normally flanged.

Casing Connections

Most of the auxiliary connections on these machines will be screwed fittings.

It is desirable to use standard flanges throughout the connections on the casing. However, for space reasons,

on rare occasions, a nonstandard flange arrangement may become necessary.

It is quite important to have the equipment vender furnish all mating flanges and associated hardware.



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Construction:

The impeller is the most highly stressed compressor component .

Impellers

Impellers are made of low alloy steel for most compressors in process service, either chrome-moly or

chrom-moly-nickel.

Because of the high strength to weight ratio, many of the high head, integrally geared units use aluminum.



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Construction:

Austenitic stainless, Monel and titanium are some of the other materials used for impellers in certain special

applications, generally with corrosive gases involved.

Impellers

Although not universal, on most multistage compressors, the impellers are axially located by shaft sleeves.



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Construction:

Shafts are made of material ranging from medium carbon to low alloy steel and are usually heat-treated.

Shafts

Shafts require the good finish achieved by grinding or honing.

The shaft is the foundation for good mechanical performance to keep the rotor dynamics in control and

maintain good balance.



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Construction:

The requirements are that the shaft must be round and all turns must be concentric to the journals.

Shafts

The tighter the tolerance, the closer to perfection, the more expensive that particular manufacturing step.

However, some added expense at this point will save time in subsequent rotor balancing providing the user with a

rotor, which can be more easily maintained.


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Construction:

Radial bearings or journal bearings are usually pressure lubricated.

Radial Bearings

Most compressors use two bearings on opposite ends of the

rotor assembly.

Or on the overhung design, located adjacent to each other

between the drive coupling and the impeller.


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Construction:

Radial bearings or journal bearings are usually pressure lubricated.

Radial Bearings

On centrifugal compressors the bearings size is not a function of the load but rather it is dictated by critical

speed considerations.


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Construction: Radial Bearings

Straight cylindrical bearings are the most simple in concept.

Because of low resistance to bearing induced problems, this bearing is limited to centrifugal compressors.


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It is found normally in very large compressors with

relatively heavy rotors and low operating speeds.

They are also used in the fluorocarbon refrigerant compressors, where speeds are low because of high

molecular weight and where relatively short rotors are used.


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As a minimum, most compressors with sleeve

bearings use a modified sleeve bearing, such as the dam type


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The tilting pad bearings is the bearing most often found in centrifugal compressors, which is inherently stable. The

bearing also known as the tilting shoe bearing.


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Construction: Thrust Bearings

Centrifugal compressor impellers, with the exception of the open impeller, are thrust unbalanced.

The machine also has a requirement for a location device to maintain axial clearance.

For these reasons, all centrifugal compressors use some form of thrust bearing.

API 617 recognizes the need for the compressor thrust design to take into account peripheral factors such as the

coupling.

Documents

Dynamic Compressors