8 replies to this topic

[sam14]

Can anyone have an idea about how to shift the performance curve of a compressor for a new compression ratio with new gas entering.

 

I mean to say that we have 1 existing compressor for which all performance curves are available (Polytropic head vs flow, Power vs flow, Discharge P vs Flow, Effeciency vs Flow) for different rpm's with specified gas properties and compression ratio (4.3) and inlet suction pressure (1.05 bar a).

 

Now we have to check what could be the max. possible flow through this compressor with new gas for which we have gas properties and also we need a different compression ratio (3.8) from which (4.3) the curve is available.

Any idea/ suggestion/ procedure would be of great help.

Gas properties are not much different, but compression ratio and Suction pressure have significant difference.

 

I read somwhere that performance curve (Polytropic head vs Flow) is independent of gas properties except K value. Equation to support this statments are as.

 Hp=1545*Zave*Ts/(MW2*(n2/n2-1))*((Pd/Ps)^((n2-1)/n2)-1)

 

Q = k sqrt ( h*ZRT/MW)*Ps).... (From orifice equation, where k is orifice coefficient and h is delta p across orifice)

so by using both equation and considering pressure profile is similar in compressor and drop in orifice one can develp the Q vs H which is independent of gas properties (Z, Ts, Mw) but i am not very much convienced with this argument.

[PingPong]

This is too vague.

 

What are Ps and Pd of the compressor with the old gas?

What is Ts for the old gas?

What are the Cp, k (=Cp/Cv), Mw and Z of the old gas? Or even better: what is the old gas composition?

 

What are Ps and Pd of the compressor with the new gas?

What is Ts for the new gas?

What are the Cp, k (=Cp/Cv), Mw and Z of the new gas? Or even better: what is the new gas composition?

 

And show us the performance curve(s) and efficiency curve(s) of the compressor as supplied by the vendor, and also the P&ID of the compressor.

Edited by PingPong, 17 July 2014 - 09:01 AM.

[Bobby Strain]

You can find online calculators by using Google search. And you are likely to find a spreadsheet here if you use the search facility or look through the posted spreadsheets.

 

Bobby

[sam14]

This is too vague.

 

What are Ps and Pd of the compressor with the old gas?

What is Ts for the old gas?

What are the Cp, k (=Cp/Cv), Mw and Z of the old gas? Or even better: what is the old gas composition?

 

What are Ps and Pd of the compressor with the new gas?

What is Ts for the new gas?

What are the Cp, k (=Cp/Cv), Mw and Z of the new gas? Or even better: what is the new gas composition?

 

And show us the performance curve(s) and efficiency curve(s) of the compressor as supplied by the vendor, and also the P&ID of the compressor.

Actualy what i was looking a approach to find the flow. but if you need the data to find the more realastic approach , Please see

its a two stage compressor ,

1st stage old Gas, Ps= 1.09 kg/cm2A, Pd= 4.8 kg/cm2A, k = 1.156, Zaverage ((Zs+Zd)/2)= 0.9815, Mw= 36.86, Ts =40 deg C

2nd stage old gas , Ps=4.32 kg/cm2A, Pd= 15.99 kg/cm2A, k = 1.184, Zaverage =0.95, Mw= 34.89, Ts=40 Deg C

 

1st stage new gas, Ps= 1.305 kg/cm2A, Pd= 5.095 kg/cm2A, k=1.1537, Zaverage = 0.982, Mw= 37.956, Ts= 39 degC

2nd Stage new gas, Ps= 4.405 kg/cm2A, Pd= 17.633 kg/cm2A, k= 1.1772, Zaverage =0.949, Mw= 35.80, Ts= 39 degC

 

Please check the Curves as attached for both the stages. Flow is in M3/h

Thanks for your reply.

 

 

 

 

Attached Files

•  Poly head_efeciency Vs Flow 1 st section.png   121.92KB   4 downloads
•  Poly head_efeciency Vs Flow 2nd section.png   154.97KB   1 downloads

[Bobby Strain]

If you have a process simulator, like HYSYS, you can input the curves and simulate the operation. But maybe Ping is nice and will do all the work for you. "Give a man a fish ......."

 

Bobby

[PingPong]

Actualy what i was looking a approach to find the flow.

The approach depends on the specific situation, which can only be understood by outsiders, if all the data are available.

Unfortunately that is never the case when somebody starts a topic, and it is no different this time.

For example: you did not mention before that this is a two-stage compressor whereby in the old case the mass flowrate of the second stage is 26 % less than of the first stage, presumably due to condensation in the interstage cooler. How much will condense in the new case is still unclear.

Moreover you did not mention before that this compressor has variable speed.

 

Normally a compressor has some sort of capacity control. In other words: the process has a certain flowrate that has to be compressed, and the compressor capacity has to match that flowrate, not the other way around. As this compressor has variable speed, I would expect that the suction pressure controls that speed. In addition to that there could be other control systems at work, such as a control valve in the second stage discharge, or .... (whatever). To understand the capacity control system I therefor asked for a P&ID, which sofar you have not uploaded. If it confidential, then just remove any info that refers to your company, or licensor, including equipment numbers.

 

You give very detailed data for the new gas case, presumably from some simulation you did, yet you imply that you do not know the flowrates and heads for the new case. If so, how did you then determine the interstage pressure, which will be resulting from the compressor operation, and which has a big impact on the condensation, and hence on the flowrate of the second stage versus that of the first stage?

 

In the old case the mass flowrate to the first stage is about 90.5 t/h. In the new case the maximum possible mass flowrate to the first stage will also depend on the amount of interstage condensation as well as the size (max power) of the compressor driver (electric motor, or steam turbine or ... what?).

 

But do you really need to know the maximum capacity? I would expect that the process will produce a certain amount of new gas (vapor) at 1.305 kg/cm2A and you need to know whether the present compressor and its control system can handle that. Right?

 

To determine that requires the P&ID, the desired mass flowrate and composition of the new gas to the first stage. (Composition is required to determine the amount of interstage condensation at different interstage pressures. At the moment it seems to me that your new interstage pressure may not be optimal.)

[sam14]

 

Actualy what i was looking a approach to find the flow.

The approach depends on the specific situation, which can only be understood by outsiders, if all the data are available.

Unfortunately that is never the case when somebody starts a topic, and it is no different this time.

For example: you did not mention before that this is a two-stage compressor whereby in the old case the mass flowrate of the second stage is 26 % less than of the first stage, presumably due to condensation in the interstage cooler. How much will condense in the new case is still unclear.

Moreover you did not mention before that this compressor has variable speed.

 

Normally a compressor has some sort of capacity control. In other words: the process has a certain flowrate that has to be compressed, and the compressor capacity has to match that flowrate, not the other way around. As this compressor has variable speed, I would expect that the suction pressure controls that speed. In addition to that there could be other control systems at work, such as a control valve in the second stage discharge, or .... (whatever). To understand the capacity control system I therefor asked for a P&ID, which sofar you have not uploaded. If it confidential, then just remove any info that refers to your company, or licensor, including equipment numbers.

 

You give very detailed data for the new gas case, presumably from some simulation you did, yet you imply that you do not know the flowrates and heads for the new case. If so, how did you then determine the interstage pressure, which will be resulting from the compressor operation, and which has a big impact on the condensation, and hence on the flowrate of the second stage versus that of the first stage?

 

In the old case the mass flowrate to the first stage is about 90.5 t/h. In the new case the maximum possible mass flowrate to the first stage will also depend on the amount of interstage condensation as well as the size (max power) of the compressor driver (electric motor, or steam turbine or ... what?).

 

But do you really need to know the maximum capacity? I would expect that the process will produce a certain amount of new gas (vapor) at 1.305 kg/cm2A and you need to know whether the present compressor and its control system can handle that. Right?

 

To determine that requires the P&ID, the desired mass flowrate and composition of the new gas to the first stage. (Composition is required to determine the amount of interstage condensation at different interstage pressures. At the moment it seems to me that your new interstage pressure may not be optimal.)

 

Dear Ping,

Thanks for your time and efforts you put on this but this is the available data i have, i don;t have any P&ID's of existing compressor control.

Yes you are correct the new data is from simulation, actually the problem is that i have existing steam driven compressor available with me which curves i already uploaded I have fixed process from which new gas rate and other properties are defined i want to check how much the existing compressor can handle the new gas and for rest of the amount i would install a new compressor.

But installing a new compressor with reduced capacity or with full capacity is depends on the how much this existing compressor can handle with existing system available.

I hope this makes the problem more clear and you can give me the approximate way to find the reduced capacity. Optimization of interstage pressure and condensation i will take care if you please let me know the correct way to find the flow through that existing compressor even in 1st stage only.

[PingPong]

You can't determine a two stage compressor capacity by looking at 1st stage only.

 

The approach is:
Both stages run at the same speed, thereby delivering a certain Head/Flow relation (the curves), so you need to find those flowrates and heads that match the curves.
Therefor you need to do simulations for several 1st stage flowrates (say: 60000 , 65000 , 70000 m3/h) and for each of those flowrates at several different interstage pressures (the one you already used for the new case, plus one 0.5 kg/cm2 higher, and one 0.5 kg/cm2 lower), to obtain 2nd stage volumetric flows for a each 1st stage volumetric flow at each interstage pressure.
After that you need to find out, using the compressor curves, what combination of 1st stage and 2nd stage flowrates and Heads correspond with the same (interpolated) RPM curve for both stages. Moreover you need to verify whether the new required shaft power can be handled by the current driver. Also the old intercooler and old aftercooler of the compressor may not be big enough to handle the new, higher, duties.

(Without a gas composition I did some spreadsheet calculations, assuming that the mass flow to the 2nd stage is 74 % of the mass flow to the 1st stage, like in the old case. It is a very arbitrary assumption but that's all one can do without more info. Based on that I expect that the 1st stage volumetric flowrate will be in the order of 65000 m3/h (124 mt/h) at about 6600 RPM. Due to the high mass flowrate the new required shaft power will be about 35 % higher than the old shaft power, which may not be possible with the current driver.)

Edited by PingPong, 21 July 2014 - 08:19 AM.

[sam14]

You can't determine a two stage compressor capacity by looking at 1st stage only.

 

The approach is:
Both stages run at the same speed, thereby delivering a certain Head/Flow relation (the curves), so you need to find those flowrates and heads that match the curves.
Therefor you need to do simulations for several 1st stage flowrates (say: 60000 , 65000 , 70000 m3/h) and for each of those flowrates at several different interstage pressures (the one you already used for the new case, plus one 0.5 kg/cm2 higher, and one 0.5 kg/cm2 lower), to obtain 2nd stage volumetric flows for a each 1st stage volumetric flow at each interstage pressure.
After that you need to find out, using the compressor curves, what combination of 1st stage and 2nd stage flowrates and Heads correspond with the same (interpolated) RPM curve for both stages. Moreover you need to verify whether the new required shaft power can be handled by the current driver. Also the old intercooler and old aftercooler of the compressor may not be big enough to handle the new, higher, duties.

(Without a gas composition I did some spreadsheet calculations, assuming that the mass flow to the 2nd stage is 74 % of the mass flow to the 1st stage, like in the old case. It is a very arbitrary assumption but that's all one can do without more info. Based on that I expect that the 1st stage volumetric flowrate will be in the order of 65000 m3/h (124 mt/h) at about 6600 RPM. Due to the high mass flowrate the new required shaft power will be about 35 % higher than the old shaft power, which may not be possible with the current driver.)

Thanks for your suggestion / efforts.

I am also getting the similar results. and conclude that even if driver is changed to new power requirements or adjusted the new gas capacity it can handle only 30% at max. operational rpm available. So it would be better to give a new compressor of full capacity.

thanks once again for your help