Effect of MW on Compressors

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HOME PUBLIC COURSES IN-HOUSE COURSES OPERATOR TRAINING TIP OF THE MONTH REQUEST INFO

NOVEMBER 2008

Effect of gas molecular weight on centrifugalcompressor performance

In this tip of the month (TOTM) we will present the results of several case

studies showing the effect of gas molecular weight on the performance and

efficiencies of centrifugal compressors. We have considered several what if

scenarios such as variation of compressor speed as a function of molecular

weight, while maintaining the same suction and discharge pressures and mass

flow rate. Variation of polytropic head and efficiencies as a function of gas

molecular weight for a given compression ratio, and compressor speed hasalso been studied. In addition, the impact of thermodynamic properties

package has been studied.

Compressors can be generally classified in two categories:

1. Positive displacement; this type of compressor includes reciprocating,

rotary screw, sliding vane, liquid ring and rotary lobe. The compression

principle is volumetric displacement reducing the gas volume increases

pressure.

2. Kinetic or Dynamic: this type of compressor includes centrifugal and

axial compressors. The compression principle is acceleration and

deceleration of the gas kinetic energy is converted to pressure rise.

Reciprocating and centrifugal compressors are the most popular compressors

used in E & P applications. Rotary screw compressors are gaining in popularity

in low to moderate pressure gas boosting service, refrigeration systems and

fuel gas compression for gas turbines. Further detail may be found in reference

[1].

From a calculation viewpoint alone, the power calculation is particularly

sensitive to the specification of flow rate, inlet temperature and pressure, and

outlet pressure. Gas composition is important but a small error here is less

important providing it does not involve the erroneous exclusion of corrosive

components. A compressor is going to operate under varying values of the

variables affecting its performance. Thus the most difficult part of a

compressor calculation is specification of a reasonable range for each variable

and not the calculation itself. Maddox and Lilly [2] emphasize that using a

single value for each variable is not the correct way to evaluate a compressionsystem.

Normally, the thermodynamic calculations are performed for an ideal

(reversible process). The results of a reversible process are then adapted to

the real world through the use of an efficiency. In the compression process

there are three ideal processes that can be visualized: 1) an isothermal

process, 2) an isentropic process and 3) a polytropic process. Any one of these

processes can be suitably used as a basis for evaluating compression power

requirement by either hand or computer calculation. The isothermal process,

however, is seldom used as a basis because the normal industrial compression

process is not even approximately carried out at constant temperature.

Due to practical limitation the compression ratio per stage is often in the range

between 2 and 6. For large overall compression ratio applications multistage

compressors are used. The choice of the interstage pressure is an economicdecision and can be estimated by equal compression ratios for each section but

may be adjusted to minimize total power requirement.

In order to study the effect of feed gas molecular weight on the performance of

centrifugal compressors, several computer simulations using HYSYS [3] were

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Page 1 of 6Effect of gas molecular weight on centrifugal compressor performance

05/11/2008http://www.jmcampbell.com/november--2008?utm_source=Campbell+Tip+of+the+Month...


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performed. The gas mixtures with the composition shown in Table 1 with

molecular weights ranging from 18.2 to 23.17, corresponding to relative

density of 0.63 to 0.80, respectively, were used in this study. The

characteristics curves for the centrifugal compressors used in this study are

shown in Figures 1 and 2. These performance curves were supplied to the

simulation software and used in the course of simulations.

Case 1: Effect of Molecular Weight on Flow Rate for Fixed ?P (Constant

Speed)

For a fixed inlet pressure of 700 kPa, 35 C, and 15000 RPM, the feed gas

relative density was varied from 0.63 to 0.80 with an increment of 0.05. In

order to maintain the outlet pressure, the feed flow rate has to vary. We are

essentially fixing P1 and P2 and wanting to see the effect on the compressor of

varying molecular weight feed. The set up shown in Figure 3 was used to

generate the simulation results. The simulation results for compression ratios

of 2.0 and 2.5 are shown in Figure 4. The PR EOS [4] is used for

thermodynamic properties calculations.




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Figure 4 indicates that as the relative density decreased, the flow rate must

decrease. Note, for the case of compression ratio of 2.5, no convergence could

be achieved for relative density of 0.63 and 0.65 due to the fact the surge limit

had been reached. For the same case, the required power as a function ofrelative density is shown in Figure 5. Since, the flow rate decreased with

decreasing relative density, the required power decreased.

Finally, the variation of polytropic head as a function of inlet actual volumetric

flow rate is shown in Figure 6. Note that the relative densities are identified on

this diagram to show their influence on the performance of the compressor.




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Case 2: Variab le Speed

As in the case 1, for a fixed inlet pressure of 700 kPa, 35 C, and mass flow

rate of 1000 kmol/hr, the feed gas relative density was varied from 0.63 to

0.80 with an increment of 0.05. In this case, the compressor is varying speed

to maintain flow rate at the P speed imposed on it. The schematic setup to

generate simulation results is shown in Figure 7. The simulation results for

compression ratios of 2.0 and 2.5 are shown in Figures 8 and 9. In addition to

the results by the PR EOS, the results obtained by BWRS are shown on these

diagrams. The difference between the results of these two EOS for these cases

is negligible.




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As shown in Figure 8, as the relative density increases, the compressor speed

dropped. However, as relative density or molecular weight increased, the

required power increased, see Figure 9.

As shown in Figures 10 and 11, the polytropic efficiency and head decrease

with relative density. More detail of simulation results can be found in

Reference [5].

Conclusions

The impact of relative density (molecular weight) on the performance of a

centrifugal compressor was studied by performing a series of computer

simulations. Based on the simulation results, it is found that:




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1. For the same feed condition, compression ratio, compressor speed, the

flow rates must decrease as the relative density decreases, and will

eventually approach a surge condition.

2. For the same feed condition, compression ratio, compressor speed, as

the relative density increases, the flow rate increases which results in

more power consumption.

3. For the same feed condition and rate, and compression ratio, the

compressor speed decreases with molecular weight but as expected, the

power requirement increases.

4. The PR EOS and BWRS EOS produced the same simulation results

To learn more about similar cases and how to minimize operational problems,

we suggest attending our ME44 (Overview of Pumps and Compressors in

Oil and Gas Facilities) , ME46 (Compressor Systems - Mechanical

Design and Specification), G4 (Gas Conditioning and Processing) , G5

(Gas Conditioning and Processing - Special) , and G7 (Process

Simulation in Gas Conditioning and Processing) courses.

By: Dr. Mahmood Moshfeghian

Reference:

Campbell, J. M., Gas Conditioning and Processing, Vol. 2, the

Equipment Modules, 8th Ed., Campbell Petroleum Series, Norman,

Oklahoma, 2001

Maddox, R. N. and L. L. Lilly, Gas conditioning and processing,

Volume 3: Advanced Techniques and Applications, John M.

Campbell and Company, 2nd Ed., Norman, Oklahoma, USA, 1990.

ASPENone, Engineering Suite, HYSYS Version 2006, Aspen

Technology, Inc., Cambridge, Massachusetts U.S.A., 2006.

Peng, Y. D., Robinson, D. B., A New Two-Constant Equation of

State, Ind. Eng. Chem. Fund., 15, 59, 1976

Moshfeghian, M., Bothamley, M., and Lilly, L.L., Feed gas

molecular weight affects performance of centrifugal efficiency, Oil

and Gas J., May 10, 2008


Effect of MW on Compressors

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