6 replies to this topic

[fadzlizainal]

Dear Sir,

I need some guidance on what to look at.

Due to some operational requirement, I need to run the 2 stages contrifugal compressor with much much lower discharge pressure and with the same suction condition (same Ts & Ps).

In normal operations, the compressor discharge pressure is 140 bars to 150 bars.
The new discharge pressure will be around 100 bars.

Please advice.

[Dacs]

You said same suction conditions, but does it include the same flowrate?

If you need to maintain the same flow, then you have to throttle the flow somehow. I think an FV downstream your compressor will do the job. I don't think you need to do anything since a properly tuned FCV will do its job maintaining the same flowrate even if you change your destination pressure (in your case, 40-50 bar lower than normal pressure)

Take note that you're not really changing the discharge pressure per se, you're just adding pressure losses at your discharge piping that will result in a lower destination pressure.

If you're talking about ways to achieve that in your compressor, you have to look for ways to change the compressor performance curve itself, such as inlet guide vane (IGV) that will control the flow going in to your compressor suction.

[fadzlizainal]

Dear Sir,

Thanks for the reply and input. Really appreciate that.

Perhaps my previous explaination was not so clear.

I need to run this compressor at lower discharge pressure (40-50 bars lower than normal pressure) only for a day due to some operational issues.

We will run the compressor like normal operations BUT we will set the lower discharge pressure controller to much lower than normal operations.
My concern:-
1) It is possible for the compressor to achive that lower pressure....because it is 2 stages compressor ?
2) Will it harm the compressor...?

[MYI]

<sub>Fadzlizainali, as per my understanding,

You want to run compressor on the same suction conditon without any change in process parameter, & get the compressed gas pressure lower than the normal opertion (100 bar); so, this is not the big issue if the compressor is design on the said conditions to meet your process requirment; I think you have two option to decrease the discharge pressure of compressor with same flow rate for centrifugal type comprssor.

1- Reduced the comprssore speed (RPM) it will produce less pressure but the same flow rate as per your normal opertion.
2- Installed any pressure lose device (FV or PV), but in this case compressor will generate normal condition pressure & flow rate but control valve will reduce the pressure upto the your test requirment.

In my opinion 1st one option is reliable & suitable, but it is subjected to compressor design that you can check from vendor curve.

Thanks.</sub>

Edited by MYI, 17 August 2012 - 12:09 AM.

[Dacs]

Reducing the RPM is outright out of the question if the compressor motor is not VSD.

Anyway, I'm still having some difficulty imagining the scenario, can you please attach a sketch or P&ID of the said system? You said "Pressure Controller", so I'm wondering how you're controlling your compressor flow.

[Zauberberg]

You haven't provided much information apart from problem description. You might be entering into a potentially dangerous scenario of compressor stonewall and possible driver damage. Before we can give any precise comment, we would need to have a P&ID of the system, or at least a sketch showing process configuration and key control loops.

If you are pumping a condensable gas and the compressor discharge pressure "floats" on the condensing pressure of the compressed gas (directly proportional to the discharge cooler outlet temperature), this can be combated either by reducing the motor RPM (in case you have VFD), or switching off several fin-fan coolers on the compressor discharge. However, your operating pressures are too high and the application looks like something completely different than what I have described. Therefore, posting a sketch and explaining how your system works would be the best way to start.

[fadzlizainal]

Dear Sir,

Thanks for the response and sorry for the late reply.

The driver of this compressor is a gas turbine..... and I'm currently away from office to attach the relevant PID.
My biggest worry in this scenario is not to damage the compressor especially the internal labyrinths and intermediate seal.

Thank You.