2 replies to this topic

[M.Salah]

Hello experts and distinguished contributors:

I am a mechanical guy trying to understand the issue of rotational speed (N) about centrifugal compressors. It is my first time being exposed to this math, so I used GPSA handbook and Royce Brown's - Compressor Selection & Sizing Book. Also, the spreadsheet I set up for myself, gave me numbers that closely match one of the speadhseets I found here on this site by a great contributor on compression issues. (Thank you SIr)

Let's say hypothetically I have an existing (brownfield) multi-stage centrifugal and the vendor info is non-existent except for these few items:
- it is a tandem-driven series dresser-rand D8 R7 B centrifugal (D8 = datum type 8, R7 = 7 stages, B = means back-to-back)
- a solar turbine driver (directly connected = no gear between driver & compressor)

(1) I need to understand the rotational speed N, I made the assumption that each compressor has 7-impellers (and diffusers, and plenum/straightener vane arrangement depending on the model etc.)? Is this correct?
(Caveat: as a mechanical when I hear 7 stages, I am thinking, this machine has 7 impellers)

(2) Someone else, prior to me joining this research, used language in a report to indicate "stage" to mean each representative compressor? Is this correct?
(Caveat: From my understanding he took 1st stage to mean "first stage compressor". I assumed this person is from a process/chemical background person, since his calculations are based on one compressor as a compression unit. Sort of what a positive displacement compressor would behave like.)

(3) I have only understood how to do one calculation at a time for one compressor, has anyone come across calculation methods for tandem-compressors? What are the tricks to do this? (Please don't say ask the vendor!)

(4) Can I use the Nominal Rotational Speed, instead of the actual Rotational Speed when I publish my findings to avert controversy?
(Caveat: I am trying my best to not make this equipment an issue about the mechanical team vs. process team)

I hope you can share some thoughts.

Regards,

Edited by M.Salah, 29 December 2011 - 05:35 PM.

[S.AHMAD]

M. Salah

I can only help you to understand about stages in centrifugal compressor.

1. Stages for centrifugal compressor has different meaning to that of centrifugal pump.

2. If a centrifugal pump has 7 impellers, this means the pump is of 7 stage-pump.

3. For centrifugal compressor, 1 stage can have 7 impellers.

4. To know how many stages for centrifugal compressor, looks at the presence of inter-coolers.

5. Example for 2 stage-compressor. The gas is compressed by 1st stage (nay have 7 impellers) the compressed gas is cooled down by a cooler, then the gas is compressed again by a 2nd stage compressor (maybe of 5 impellers).

6. Speed of a compressor is an important variable in compression. Thus, using the right value is necessary otherwise it may give values that lead to wrong conclusion about the compressor performance.

Edited by S.AHMAD, 29 December 2011 - 09:18 PM.

[ankur2061]

M. Salah,

A commercial centrifugal compressor spreadsheet with the title "Centrifugal Compressor Head & Power Calculations" is available on the "online" store of "Cheresources" where the definition of number of stages is provided and the number of stages for a given polytropic head can be calculated. In this particular spreadsheet stage is defined as follows:

Definition of Stage: For the purpose of this spreadsheet, a stage is defined as an impeller-diffuser pair. A series of stages will comprise a 'section' or 'process stage' between which there is no intercooling or loss or gain in flow (i.e. no sidestreams, bypassing or injection) & whose rotational speeds are in fixed ratio to each other.

Yes it is correct that a single stage in a centrifugal compressor can have multiple impeller-diffuser pairs, however for the purpose of simplification and doing calculations for determining number of stages it is usual to consider one pair of impeller-diffuser as one stage. Remember, that since the design of centrifugal compressors is so much driven by the vendor, most engineers in operations and maintenance follow the terminology provided by the vendor. If the vendor calls a set of 3 pairs of impellers-diffusers as one stage than that becomes one stage for the compressor.

Also, note that the rotational speed of a compressor is basically the function of the impeller tip speed and the impeller diameter. The impeller diameter is a given input whereas the impeller tip speed can be calculated as a function of the calculated nominal polytropic head per stage (one impeller-diffuser pair) and the impeller stage head coefficient (normally this coefficient varies from 0.4 to 0.55). In a nutshell, what you are calculating is the nominal rotational speed since you are assuming some value of the head coefficient and the exact values of head coefficients are available with the vendor only.

Hope this helps.

Regards,
Ankur

Edited by ankur2061, 30 December 2011 - 02:09 AM.