13 replies to this topic

[sheiko]

Hello,

One question please:

When you decrease the suction density (by reducing the suction pressure, reducing the molecular weight or increasing the suction temperature) of a variable speed centrifugal compressor, will it run faster? or lower? and why?

Thanks.

[Bobby Strain]

Sheiko,
What provokes this question?
Bobby

[Dacs]

Taking a jab at the question:

Decrease in density -> Increase in vol flow across compressor -> decrease in head (compressor going end of curve) -> decrease in discharge pressure -> decrease in flow across FE (if flow controlled) / decrease in pressure readout (if pressure controlled) -> signal to compressor motor to increase speed

Just a thought (may or may not be right about this)

Edited by Dacs, 25 September 2012 - 08:52 PM.

[sheiko]

Decrease in density -> Increase in vol flow across compressor

Could you please clarify?

Edited by sheiko, 26 September 2012 - 10:11 PM.

[Dacs]

My understanding is for a given power (ie fixed RPM speed), if you decrease the density you need to compensate it for
increase in volumetric flow.

I was thinking if you decrease the density suddenly, the compressor will initially experience a surge in volume (because of lower density) and this change will cause the compressor point to shift right to account for the compressor behavior, and that will cause a decrease in discharge pressure, and the controller will act to correct the upset by increasing the compressor motor RPM.

Well... anyone feel free to correct me on this assumption if I'm wrong

Edited by Dacs, 25 September 2012 - 09:23 PM.

[ankur2061]

Sheiko,

Whichever standard or textbook I look at, the flow for a centrifugal compressor as defined in the problem statement is either at standard volumetric flow conditions(denoted as SCFM/SCFH/SCMH) or in terms of mass flow which means that both these values remain unchanged at the inlet and outlet of the compressor. What changes is the inlet volume flow rate (denoted as ACFM/ACFH/ACMH) defined as the volume flow rate at the inlet flange of the compressor which is a function of the inlet pressure and temperature (pressure and temperature of the gas being compressed at the inlet flange of the compressor) and the molecular weight of the gas being compressed.

Now coming to the density of the gas being compressed. Gas density is a function of pressure, temperature and molecular weight of the gas. A change in either of these changes the gas density which in turn changes the inlet volume flow (the ACFM/ACFH/ACMH) but neither the mass flow nor the standard volume flow.

For a fixed-speed centrifugal compressor there can be essentially no change in either the power drawn (gas horse power or gas kW), polytropic head (ft-lbf/lbm or N-m/kg) and the mass flow rate irrespective of the change in the inlet volume flow conditions. The effect of changing the inlet volume flow rate would manifest itself in the form of change in the differential pressure (Discharge Pressure - Suction Pressure) for the compressor. In other words either the suction pressure conditions or the dicharge pressure conditions will change due to change in the pressure / temperature / molecular weight (gas density change) at the inlet of the centrifugal compressor. This can be proven easily mathematically:

Gas Horse Power or Gas kW = Polytropic Head*Mass flow rate

Neither of these changes for a fixed-speed machine

Polytropic head is related to differential pressure as follows:

Differential Pressure = Polytropic head*Gas Density

Polytropic head remains unchanged for a fixed-speed machine, so a change in density will cause a change in the differential pressure.

For a variable-speed centrifugal compressor you can change the power drawn by changing the frequency of the motor and the affinity laws (speed vs inlet volume flow rate, speeed vs polytropic head, speed vs gas horse power) as applicable to centrifugal pumps can be applied but with limitations compared to centrifugal pumps. Affinity laws can be applied between 80% and 105% of the speed range only beyond which actual performance would be much different from the predicted performance. Also heavier gases (higher molecular weights) will give greater deviation from predicted performance.

Well I hope I haven't gone overboard in explaining basic concepts but then compressors have always interested me.

Regards,
Ankur.

Edited by ankur2061, 27 September 2012 - 12:42 AM.

[Narayanan.u]

Hi,

Developed Head in meters is independent of fluid density.

Pressure increases with increased fluid density.

As RPM increases flow rate increases.


As I understood, so far.


Regards,

Narayanan

[sheiko]

Thanks Ankur,

Do you agree with Dacs concerning the sequence of events leading to an increase of rpm (variable speed)?

Thanks

Edited by sheiko, 27 September 2012 - 07:17 AM.

[ankur2061]

Sheiko,

Discussing first a fixed-speed centrifugal compressor a decrease in gas density (let us say due to decrease in molecular weight with inlet or suction pressure and temperature remaining constant) will cause the inlet volume flow to increase. As mentioned in my earlier post for a fixed-speed centrifugal compressor the polytropic head remains constant but the differential pressure changes due to change in gas density. In this particular case with lower density at the compressor inlet of the gas being compressed the differential pressure will decrease. I have already assumed that the inlet or suction pressure is constant which means that the discharge pressure will decrease.

A typical Volumetric Flow Rate (X-axis) versus Discharge Pressure (Y-axis) curve will show that with increased inlet volume flow the operating point has move to the right of the BEP on the curve. To compensate to bring the discharge pressure back to the original value the rpm of the motor will need to be increased using the variable speed drive. Now your differential pressure has become the same as the earlier value but as mentioned earlier the gas density has decreased. Going by the mathematical formula relating differential pressure to polytropic head and gas density in my earlier post the polytropic head has increased in this case as a direct result of increasing the rpm of the motor. Again going by the mathematical relationship between gas power, polytropic head and mass flow, an increase in polytropic head has resulted in an increase in the gas power.

To conclude, gas density decrease (due to decrease in molecular weight, increase in inlet temperature or both) at constant suction pressure will result in decrease of discharge pressure. To make-up the discharge pressure a variable speed compressor will increase the speed of the compressor. The speed increase will lead to a higher polytropic head and thus higher gas horse power requirement for the compressor.

Regards,
Ankur.

[sheiko]

Ankur, thanks.

To conclude, gas density decrease (due to decrease in molecular weight, increase in inlet temperature or both) at constant suction pressure will result in decrease of discharge pressure. To make-up the discharge pressure a variable speed compressor will increase the speed of the compressor. The speed increase will lead to a higher polytropic head and thus higher gas horse power requirement for the compressor.Regards,Ankur.

That's completely answers my initial question.
FYI, I have shown on the attached pdf the evolution of the density, speed, power, ... of one of our steam turbine driven centrifugal compressor during 2010 (good reliability period in my refinery).

As mentioned in my earlier post for a fixed-speed centrifugal compressor the polytropic head remains constant but the differential pressure changes due to change in gas density. In this particular case with lower density at the compressor inlet of the gas being compressed the differential pressure will decrease.

Just to fix ideas, here is my understanding:
When the density decreases, the system curve changes, its heads becoming less for any given flowrate. The initial high head from the compressor causes the fluid in the pipe to accelerate, now that the system resistance is lower. That is what causes the flow to increase in the piping and only after the fluid starts moving faster down the pipe will the compressor drop its head, thus decreasing its differential pressure (@ the new lower density).
Do you agree with this interpretation?

A typical Volumetric Flow Rate (X-axis) versus Discharge Pressure (Y-axis) curve will show that with increased inlet volume flow the operating point has move to the right of the BEP on the curve.

The problem is that every manufacturer seems to have its typical curve and every curve has some underlying assumptions. I have seen: pressure ratio versus mass flowrate, head versus volume flowrate, deltaP versus volume flowrate, discharge pressure versus volume flowrate...That is, in my opinion, a source of confusion...

Attached Files

•  Sheiko - 2012 (sigma).pdf   120.04KB   103 downloads
•  Sheiko - 2012 (other parameters).pdf   97.69KB   88 downloads

Edited by sheiko, 27 September 2012 - 10:53 PM.

[ankur2061]

Ankur, thanks.

To conclude, gas density decrease (due to decrease in molecular weight, increase in inlet temperature or both) at constant suction pressure will result in decrease of discharge pressure. To make-up the discharge pressure a variable speed compressor will increase the speed of the compressor. The speed increase will lead to a higher polytropic head and thus higher gas horse power requirement for the compressor.Regards,Ankur.

That's completely answers my initial question.
FYI, I have shown on the attached pdf the evolution of the density, speed, power, ... of one of our steam turbine driven centrifugal compressor during 2010 (good reliability period in my refinery).

Just to fix ideas, here is my understanding:
When the density decreases, the system curve changes, its heads becoming less for any given flowrate. The initial high head from the compressor causes the fluid in the pipe to accelerate, now that the system resistance is lower. That is what causes the flow to increase in the piping and only after the fluid starts moving faster down the pipe will the compressor drop its head, thus decreasing its differential pressure (@ the new lower density).
Do you agree with this interpretation?

Yes, your interpretation is correct. The behaviour is similar to a centrifugal pump that as the volume flow rate increases the operating point on the volume flow rate (x-axis) vs the differential pressure (y-axis) curve shifts to the right of the earlier operating point which indicates a higher volume flow rate but a lower differential pressure.

A typical Volumetric Flow Rate (X-axis) versus Discharge Pressure (Y-axis) curve will show that with increased inlet volume flow the operating point has move to the right of the BEP on the curve.

The problem is that every manufacturer seems to have its typical curve and every curve has some underlying assumptions. I have seen: pressure ratio versus mass flowrate, head versus volume flowrate, deltaP versus volume flowrate, discharge pressure versus volume flowrate...That is, in my opinion, a source of confusion...

You are also right on the confusion created by centrifugal compressor manufacturers by providing centrifugal curves showing volume flow rate against pressure ratio and the others that you have mentioned and I wish they generate a proper convention for providing compressor curves. IMHO, the representative curve for compressor performance should be based on Inlet Volume Flow Rate (x-axis) versus the differential pressure (discharge pressure - suction pressure) (y-axis). Again in my opinion, the concept of showing a curve of a centrifugal compressor in terms of volume flow rate versus "head" in ft or m is confusing and misleading for compressors and should be only applicable to pumps.

I thank you for raising this very interesting debate since it has helped me re-visit my understanding on how centrifugal compressors operate with changing inlet conditions.

Regards,
Ankur.

[Bobby Strain]

Ankur,
Performance curves are presented with polytropic head for a good reason. No matter the gas or conditions, the compressor will produce the polytropic head shown on the curve as a function of inlet volume. Knowing this, all engineers should have a quick means at their disposal to translate differential pressure to polytropic head, or vise-versa. This should not be confused with pump head which can be derived in a simple manner. And it helps to know that compressor manufacturers will typically design for a maximum polytropic head of about 12,000 ft. per wheel. If you go to Dresser's website, they have software to do this. Including a component library. Or, better yet, use one of the commercial process simulators.

Bobby

[benabed]

Sheiko,
Compressor vendor often provide performance curves as differential pressure versus inlet flow rate or discharge pressure versus inlet flow rate. These curves represents only a specific sets of inlet conditions. In order to evaluate the performance of a centrifugal compressor we need to have the performance curve as polytropis head versus the inlet flow rate which represents the whole range of operating conditions.

Regards
Benabed

[sheiko]

Thanks Dacs, Ankur and al.