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definition: Multistage compressor (Centrifugal)

alwayslearning (Mechanical) (OP)
4 Jun 04 5:02
Hello all,

Just a matter of definition. Can anyone give a strict definition of what a multi-stage compressor is with respect to a centrifugal compression system. There are many loose terms and I was today debating that any compressor having multiple impellers is a multistage compressor, whereas my colleague believes that the no of stages is determined by the no of extraction (process gas discharge) that the system has: for e.g. if a 4 impeller compressor has two extractions (1 discharge after 2nd impeller, and one after the 4th impeller) then it is called a 2 stage, according to him.

To compound the confusion further we have multiple casings as well.

To me, if I have a centrifugal compressor, three casings, and each casing having 4 stages, and total 5 process extraction lines, then I would term it as a 3Casing-12 stages compressor with 5 process extraction lines.

I may be totally wrong. Please guide me giving reference to appropirate literature etc.

Regards
zdas04 (Mechanical)
4 Jun 04 7:39
Your friend is right.  The "stages" terminology comes from recip compressors where the answer is very clear - a six throw recip compressor (i.e., a compressor with six compression pistons) can have anywhere from 1 to 12 stages depending on the piping configuration.  If a centrifugal compressor has 6 implellors it can be anything from 1 to 6 stages of compression.  

The stage count is simply the number of times that the gas is mashed sequentially to a higher pressure between the suction piping and the skid discharge.

David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

quark (Mechanical)
4 Jun 04 7:41
If the compression ratio changes at some point in the fluid flow then presume that you are passing one stage and likewise.

alwayslearning (Mechanical) (OP)
4 Jun 04 22:53
Dear zdas04 and quark

If i understand you correctly, each step where a pressure increase is effected is considered a "stage".

If the above understanding is correct then zdas04, please credit me with the correct answer because my "friend" believes that the no of stages is determined by the no of dishcharge connections, regardless of the no. of impellers that may be on the shaft!

Can you please show me some online authoritative reference giving strict definition of these terms.

Regards
zdas04 (Mechanical)
5 Jun 04 11:13
alwayslearning,
Good luck finding an "authoritative" reference on any compression-related issue.  The language and usage of the field of compression is a tiny part of many other documents and none that I've ever seen get into this sort of discussion.  I hope to write one someday, but it will necessarily start with the scanty "facts", and then evolve into the "compression culture".

Your friend is not quite right.  You could easily have three impellers, two first stage and one second stage and one discharge connection.  It is still a two-stage compressor.

I've seen one four-throw recip that had a different discharge pressure from each cylinder.  One throw sucked on the inlet pipe and discharged all of it's gas to another throw that discharged off the skid.  The third throw sucked on the inlet pipe and sent half it's gas to an intermediate-pressure fuel-gas system and the other half to the fourth throw which was operating at a higher pressure discharge than the first pair.  The fourth throw went to a different gathering system than the first pair.  This mess was created because the well was split connected to two gathering systems operating at different pressures and the working-interest owner that had his gas dedicated to the high-pressure system had a fairly small percentage of the well.  We engineers had fairly spirited discussions over beer about how many stages this machine was - the consensus was that since no part of the stream was compressed three times then it was a two stage machine and the gas that went to fuel just didn't matter.

David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

alwayslearning (Mechanical) (OP)
6 Jun 04 2:04
Dear zdas04,

Thanks again for your post. I wanted to limit my discussion to Centrifugal machines (">>Can anyone give a strict definition of what a multi-stage compressor is with respect to a centrifugal compression system").

I agree that things get complicated when we have compressors especially in the configurations that are employed as in your cited example.


As you have pointed out, I realised that there is scant information available on the language of the compressor terminology.

I am a mechanical engineer and I tend to view things from the machine point of view, and that, in fact shapes my perceptions. For e.g., you gave an example of ">>You could easily have three impellers, two first stage and one second stage and one discharge connection.  It is still a two-stage compressor...<<".

If i understand correctly, you refer to the word stage to imply casing. I understand your described machine as 2 casing machine, with the first casing having two impellers, and the second casing having the third impeller, and then a final discharge line. If I were to define this machine, I would call it a "2 Casing, 3-Stage, centrifugal compressor". Additional information such as 1-discharge, 2 discharge, etc could be used to elaborate the application.

The main problem is with the word "STAGE", and this confusion possibly arises due to the many types of compression configuration that evolved over 1 century for different applications.

In the centrifugal air compression equipment (the integral gear type units); a 3 stage machine would always imply that there are 3 casings and each casing has 1 impeller each (I hope i am correct, it will be interesting to know if any compressor manufacture has employed 2 impellers in a single casing in an air compression application). Thus a "Stage" is equated to a "Casing" in air compression industry.


I suspect that over a period of time, the common notion of "stage"  as used in air compression industry is extended to process gas applications too. So, the existence of 3 casings in a process gas centrifugal compressor is considered 3 stages; even though each stage may be comprising multiple impellers, etc.


I think we need to go for that beer again to have a common understanding - and yes, it will be useful to see some of these definitions evolve in your book - at least we have some starting point!

Regards













pmover (Mechanical)
7 Jun 04 12:08
alwayslearning,

from the online Merriam Webster dictionary...

Main Entry: 1stage
Pronunciation: 'stAj
Function: noun
Etymology: Middle English, from Middle French estage, from (assumed) Vulgar Latin staticum, from Latin stare to stand -- more at STAND
1 a : one of a series of positions or stations one above the other : STEP b : the height of the surface of a river above an arbitrary zero point <flood stage>
2 a (1) : a raised platform (2) : the part of a theater on which the acting takes place and which often includes the wings (3) : the acting profession : the theater as an occupation or activity (4) : SOUNDSTAGE b : a center of attention or scene of action
3 a : a scaffold for workmen b : the small platform of a microscope on which an object is placed for examination
4 a : a place of rest formerly provided for those traveling by stagecoach : STATION b : the distance between two stopping places on a road c : STAGECOACH
5 a : a period or step in a progress, activity, or development; especially : one of the distinguishable periods of growth and development of a plant or animal <the larval stage of an insect> b : one passing through a (specified) stage
6 : an element or part of an electronic device (as an amplifier)
7 : one of two or more sections of a rocket that have their own fuel and engine

let's clarify a point, extraction and compression are not the same.  extraction can be considered an extraction stage, but not a compression stage.

the last paragraph in zdas02 original post and in quark's post are sufficient to describe/define compression stage.

yes, it is possible to have multiple impellers for one stage of compression (high-volume, low head units) for a single process stream.

for multiple process streams using a centrifugal compressor, there will likely (with emphasis) be multiple stages of compression, but not always.

hope this clarifies the matter!

good luck!
-pmover
zdas04 (Mechanical)
7 Jun 04 12:17
alwayslearning,
Sorry if I stepped out of your (">>Can anyone give a strict definition of what a multi-stage compressor is with respect to a centrifugal compression system") boundaries.  I thought a real-world example of how difficult (and pointless) it is to make hard-and-fast rules about naming of compression might make the subject clearer.

It doesn't matter how many casings are involved.  The only thing that matters in counting stages is the number times the gas is sequentially compressed.  I don't know how to make it any clearer.

David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

alwayslearning (Mechanical) (OP)
7 Jun 04 13:39
Thanks guys. I guess time now to move on to the next stage :)

banjofive (Mechanical)
10 Jun 04 15:09
Thanks all for the heroic efforts to answer this question for alwayslearning.  I found the discussion very informative as well.  Unfortunately, it seems to land shy of the mark (providing a clear definition).  Not to suggest that I have the answer, but some recent reading of my own suggests that a stage is defined by intercooling.  In general, the reason for multiple stages is to improve compression efficiency by the use of intercooling.  The compression ratio is limited, in part, by the acceptable peak gas temperature.  Beyond that point, the gas must be sent off to inter-stage cooling.  After cooling, it goes to the next stage of compression.  This is consistent with the response by Mr. Simpson, that a stage is one step in a set of sequential compressions.  It just may help to define the end of that step and the reason for multiple steps.
milltir (Mechanical)
23 Aug 04 12:36
Vendors such as GHH Borsig, Dresser-Rand and Solar Turbines define a stage as an impeller and associated guide vanes & diffuser (it gets a little more complicated for parallel flow compressors). When gas is withdrawn, cooled and returned to the casing they will use the term "section", "process stage" or "stage group" where a user might use the term "stage".

API Std 617 uses the term "section" and defines it as "A series of compression or expansion stages between which there is no intercooling or reheating, or loss or gain in flow (i.e., no sidestreams, bypassing or injection), and whose rotational speeds are in fixed relationship (ratio) to each other."  

However, I have seen training material from Nuovo Pignone where they define a stage as "the area of compression between two consecutive nozzles". Whether this is their own internal definition, or something used only in training for the benefit of users I couldn't say.
NEDianME113 (Mechanical)
14 Feb 05 7:17
hey ive gott adesign a centrifugal compressor for natural gas compression............. need help........
dwedel (Mechanical)
14 Feb 05 23:36
Allow me to add my two cents to the mix.  I would define one stage of compression as one mechanical device that causes a single pressure increase.  If two mechanical devices create a single pressure increase with parallel flow, it is a single stage compressor.  If one mechanical device causes two flows at two different pressures, I would call it a single stage, however, I have not seen such a device in my short time in this industry.  I would say that it is the pressure differential or “ratio” that defines a stage.

What seems to cause confusion in this thread is the different types of compressors available.  In a pipeline compressor like Solar’s, several impellers are driven by a single shaft.  If it is a six impeller machine and all impellers operate in series, it is called a six stage compressor.  If three of the impellers operate in parallel with the other three impellers, it is called a parallel flow, three stage compressor.  

While cooling does happen between the stages on centrifugal compressors, inter-cooling is not the primary motivation.  Maintaining flows at the desired pressure ratios across the rotating assemblies without causing bad things like surge or choked flow is the primary concern.  Before I make the mistake of categorically stating that inter-cooling is not used in centrifugal compressors, please note the following caveat; “If the flows are higher, or the ratios greater, there will probably be some type of inter-cooling.”  In recips with the higher ratios, inter-cooling and the efficiency gains it brings, is important.

NEDianME113,
Read the the FAQ in zdas signature, then start a new thread and give us some specifics.

dwedel
Hotrod Big Engines!

wedoplus (Mechanical)
6 Mar 05 18:05
-Centrifugal compressors package consists of:
a-Compressor
b-Driver
c-Gearbox
e-Coupling to connect the shafts of major components
f-Gas cooling sytem
 
-The compressor:
The compressor is the largest and most easily identifiable part in the package, because its large casing. Inside the compressor casing there are two gas passages that form the stationary part of the compressor (inlet guide & diffusers). The rotating part of the compressor is called rotor assembly.
The rotor assembly consists of  s shaft, one or more impellers mounted on the shaft. As the rotor assembly rotates, each impeller delivers energy to the gas. The rotor assembly shaft is supported by bearings, the bearings prevent the forward & backward axial movement and radial movement of the shaft and impeller.
The lube oil system provides the necessary lubrication to the bearing of the compressor, the driver, gearbox and coupling.
Seals prevent the compressor gas from leaking out around the shaft. In addition to seal most compressors have  a seal oil system which can be either a part of the lube oil system, or a separate oil system.
-The driver:
The driver supplies the power to the compressor, it can be either an electric compressor or steam turbine or a gas turbine.    
-The gearbox:
The gearbox is often used between the driver and the compressor, which allows the compressor to run at different speed speeds.
The gearbox may be either a part of the compressor casing or it might be in its own housing.The driver connected to the compressor or the gearbox by either a flexible diaphragm coupling or a gear coupling.
-Compressor operation:
When gas enters the compressor several forces are acting on the gas. One of these forces is an aerodynamic force, the aerodynamic force is as the same as in the aero plane, which lifts the plane in to the air. The lifting force helps to move the gas from the eye of the impeller (at the low pressure) to the outer impeller edge, where the (high pressure).


Coupling
Another force acts on the gas, it is the centrifugal force. Gas enters the compressor to the suction port and is directed to the center of the impeller. As the impeller rotates the gas rotates with it. This circular motion generates the force that lift and move the gas away from the center of the impeller as well as creating a suction at the Center of the impeller. Because the out side edge of the impeller travels faster than the center of the impeller, the gas is forced to increase its speed as it moves outward. The two forces the lifting force and the centrifugal force increase the motion energy or kinetic energy of the gas. As the gas leaves the outside edge of the impeller it enters an area of the casing called the diffuser.
 The diffuser is designed so that the flow area increases as the gas leaves the impeller, this increase in the area allows the gas to slow down or decrease its velocity. As the gas slows down the motion energy is converted into increase in the pressure.
Finally when the gas leaves the compressor the compination of all these forces (centrifugal, aerodynamic. Change I velocity) will produce the gas discharge pressure higher than the suction pressure.
-The force generated in the impeller depends on the following characteristics:
a-Impeller rotational speed
b-Impeller diameter
c-Impeller shape
-Multi stage compressor:
The multi-stage centrifugal compressors are performed by joining several impellers together, each is separated from the others by a diaphragm. Some times an impeller and a diaphragm defines a stage, in other cases however a stage is defined from the beginning of the suction port to the end of the discharge port. In this case the stage could have more than impeller in it. In any case the gas leaving one stage will always be at low pressure when it leaves the next stages, In this way pressure is increased in each stage until it reach the final pressure.
In multi-stage compressors the gas leaving an impeller of one stage passes through a diffuser and a diaphragm, and then it is directed by the inlet guide to the eye of the next impeller.

 
Multi-stage compressor

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