7 replies to this topic

[Srikrishna Chaitanya]

Dear All
For Centrifugal Compressors Discharge temperature will be limited to temperatures like (140 – 150 ) Centigrade ,eg: Natural Gas compressor , Based on this Compression ratios are fixed if there are many number of stages . I am not that familiar with compressor conditions please clarify my following doubts
1. What are the reasons for fixing this Compressor discharge temperature
2. Please Mention some code like API , Shell DEP etc for this discharge conditions
3. Is this temperature same for all like Ammonia compressor, Natural Gas compressor that means compressor with different feeds?
4. Is this temperature is same for Centrifugal & reciprocating both?


Thanks in Advance

Satish

[Art Montemayor]

Satish:

What you are stating is not correct. Compressors are not normally limited by their discharge temperatures. The compressor is designed to compress a gas to a certain pressure in each stage of compression. In the process of compressing the gas through that stage, the temperature of the compressed gas product will be higher than it was originally in the suction of that stage. This thermodynamic process is done adiabatically and is affected by the ratio of specific heats, k = Cp/Cv.

Where did you obtain your compressor information? What is the degree of knowledge you have on gas compression or in Thermodynamics? It is necessary to know if we are addressing someone with engineering knowledge, a student, or just a lay person. The subject matter you are trying to discuss is not easily comprehended nor is it readily available to all without some basic preparation in physics and mathematics.

Therefore,

1. There is no “fixing” of the discharge temperature. The compression ratios are fixed in each stage.
2. There are no “codes” involved. There is the basic engineering discipline of thermodynamics.
3. No, the discharge temperature is not the same for all gases. Again, you need to study and dominate the study of thermodynamics.
4. No, the discharge temperatures for centrifugal and reciprocating compressors are not the same; one involves a polytropic process and the other an isentropic process. Again, you have to know your thermo in order to know what I’m talking about.

[Srikrishna Chaitanya]

Dear Art ,
Thank you very much for the reply, I am really sorry since I wasn’t clear , I am a fresh Chemical Engineer
Having one year experience, No problem you can tell me in terms of Thermodynamics .
My problem is that recently I had to do power calculation for a Natural gas compression process (off – shore Platform)
I was given
Suction pressure of 6 kg /cm2 , Suction temperature 54 Centigrade
Discharge pressure of 97 kg/cm2
Number of stages 3 with interstage cooling
After cooling suction temperature to each stage 54 Centigrade

Now thing was that Interstage pressures were not given , Since all that was for proposal engineering we needed to estimate the interstage pressures roughly
Initially I fixed the interstage pressures using Compressor total work minimization, in this case some
Optimum compression ratio which leads to minimum total compressor work is 2. 528 , this leads to discharge temperatures of 138 , 143 , 151 for stages 1 ,2 , 3 respectively . After that I was asked by a senior engineer to reduce the discharge temperature of third stage to 145 by varying compression ratios in the first two stages. I asked the person regarding why we need to limit the discharge temperature to 145 ( I didn’t get a proper answer since he didn’t know it clearly ), He told it is from a standard ( ASME or API) he didn’t tell me even the code because he also didn’t know it clearly , at last I got a answer that it is fixed by Rotating Machinery engineers , I have seen some past projects in that also compressor Vendors , they limited the outlet temperature to 145and a maximum of 150 centigrade .

Now my doubts are those already I asked in the first post , please correct if my fundamental thinking is wrong
Thank you
Satish

[Art Montemayor]

Satish:

First of all, please be aware that if you are going to do any work with compressor calculations, it is important that you designate exactly what pressures you are referring to. You must identify the type of pressure you are describing: is it gauge or absolute pressure? Otherwise, we can't know how to calculate the compression ratios – since these are calculated only in absolute terms. The Polytropic discharge temperatures' calculations also depend on absolute values – both temperature and pressure. You must get in the habit of demanding that other engineers relate to the importance of the absolute values and identities. Otherwise, you are going to make a calculation mistake.

Now that you have finally told us the whole story, I am able to understand a little of what is bothering you. Your 3-stage centrifugal compressors are manufactured with the compression ratios fixed – not the discharge temperatures. The discharge temperatures are a result of the compression ratios. Compression ratios are normally selected in order to balance the machine as to equal horsepower between stages and also to comply (sometimes) with a desired maximum discharge temperature. The reason for this is that the machine may have certain limitations as to metallurgy, thermal expansion, seals, and lubrication. Lubrication oil (cylinders, not crankcase) is a main factor in reciprocating compressors. It usually doesn't affect the operation of centrifugals – except in some shaft seals.

API 617 is the principle leading guide and standard for centrifugal compressors and "Centrifugal Compressors: A Basic Guide" by Meherwan P. Boyce is a good start at studying and learning the intricacies and operation of these machines. If you can find an old copy of Lyman Scheel's famous Gas Compression book, you should study it with importance. However, it is out of print.

You don't explain nor show us how you calculated your discharge temperatures for the assumed interstage pressures, so I can't comment on that. The best authority on what compression ratios are used on centrifugals is the manufacturer. For every pre-designed Centrifugal compressor Frame, there is a nominal Polytropic Head per stage employed in the design. The manufacturer usually designs the machine employing one of two methods:

1. The "N" method (so-named because it employs the polytropic exponent, "n"). It is used when (a) the gas approximates a "perfect" gas – such as air, Oxygen, Nitrogen, Hydrogen or (b) when a chart of the gas properties is not available.
2. The Mollier Method – used when there is an available Mollier Diagram to describe the complete thermodynamic properties of the gas.

The Mollier Method is the most accurate – assuming the thermo data is accurate.

I have a copy of Elliott's Bulletin P-25A, published in 1975, and it shows you the calculation methods in order to obtain an approximate sizing of a multi-stage centrifugal compressor. The Bulletin copy that I have does not have a copyright indicated on it and if the Forum Administrator (Chris Haslego) finds it is OK, I would gladly post it on the Forum for downloads and use by others.

[Srikrishna Chaitanya]

Art,
My Pressure units are Absolute
For calculating compressor ratios , I have used a crude method
Optimum compression ratio for 3 stages R = (P3d / P1s ) ^ ( 1/3 )
R = Compression ratio
P3d = Discharge pressure of 3rd satge
P1s = Suction pressure of I st Stage
I did not use any methods specified by you for power calculation , I have assumed a polytropic efficiency of 78 % and Used HYSYS software to calculate discharge temperature , Actually I was asked to do so ,
Actually my concern was more about Discharge temperature , that’s why I didnt write much about power & compressor ratios . Any way thanks for the kind reply , I would be grateful if you can share me the Elliott's Bulletin P-25A
my mail Satishbabu_Garikapati@Lntenc.com
One more doubt , in N method that you stated , I think below formula is used
W = n /(n-1) x m x R x T1 ( (P2/P1)^( (n-1)/n) – 1)
How do we know the N value ?


Thank you
Satish

[pawan]

Satish
Actually when you have intial & final pressures & fixed no of stages, the minimum compression
work comes when each stage is having same pressure ratio. This is true for any no of stages.
Though its a different matter that if U increase the no of stages total work goes down. However,
why only 3 in this case not 4 or 5 or even higher bcoz.....

The optimum economics generally comes when discharge temperatures are in the range of
150 - 200 DegC.

From ur calculations I doubt that the temp are in increasing order for 1,2, & 3 stage whereas
theoretically they should be (Not compulsory) in decreasing order as volumes go high initially
at lower pressure.

[Srikrishna Chaitanya]

Pawan ,


Equal Compression ratio comes for total work minimum , I knew this
Equal Compression ratio is a kind of theoretical value but in practice Compressor vendors may not actually follow that due to Temperature limitation in any stage, You have given a range of temperature from 150 - 200 centigrade, I doubt if some one follow such a large range , In many books I have seen for centrifugal compressor maximum discharge temperature is 350 Fahrenheit ( 176 centigrade ) , I have seen in many projects Vendors limited that to 150 max
Actually my questions were very clear in the beginning

1. What are the reasons for fixing this Compressor discharge temperature?
2. Please mention some code like API, Shell DEP etc for this discharge conditions
3. Is this temperature same for all like Ammonia compressor, Natural Gas compressor that means compressor with different feeds?
4. Is this temperature is same for Centrifugal & reciprocating both?

I had to explain the entire story to make it clear to Art , I need answers for the above four if you can


Thank you

[vinrama]

Hi Satish,
It is almost three years since you asked this question , I hope you have found the answer by now.
Here is somemore further info on this matter.

The maximum operating temperature on a compressor discharge shall be determined as follows:
• when a compressor curve is not available: as 15 °C above the predicted design point temperature to
allow for lower efficiency and higher pressure ratio at compressor surge conditions;
• when compressor curves are available: as the temperature at surge conditions and maximum
compressor speed for normal and start up cases.
The following shall be used to determine the maximum design temperature:
• add 15 °C to the maximum operating temperature to allow for margins in the compressor curves, and for
wear and tear giving lower efficiency over time;
• add 10 °C as an additional margin.

Regards
AR