6 replies to this topic

I posted this at Eng-tips as well, but it appears these forums may get more traffic, so here goes.

I have what I suppose is a basic question about the surge point of a centrifugal compressor.

If I'm given the estimated surge in ICFM of a machine, will the surge flow be different if the inlet conditions change? Note this is a fixed speed machine.

I've always considered a centrifugal compressor as a volumetric device that is not really impacted by inlet composition, temperature or pressure. I thought I only needed actual volumetric flow (inlet) to get what I needed from a supplied curve. I have a vendor completed datasheet and compressor curves that indicate reasonably different surge flows for different inlet conditions as well as different polytropic head for the same ICFM. I'm baffled.

Thanks for any replies.

Mark

[ARAZA]

Hi There,

The performance of the centrifugal compressor is greatly afffected by the inlet composition as well the pressure and temperature. This is due to the simple reason that gas is compressible and its density varies with molecular weight, pressure and temperature. When the inlet compositions changes, M.W changes leading to change in density which changes discharge pressure. This is the reason when selectiing a compressor, attention should be paid in defining the compostion as close as possible.

ARAZA

Thanks for the reply, but I realize the impact that inlet conditions have on the performance or discharge of the machine. My question was with respect to the surge point of the machine. Being a volumetric machine, I believe that the compressor should have one surge flow at a given speed in ICFM. For the same machine, I have varying surge flows listed for cases that have varying compositions or suction pressures. I believe that my inlet conditions do not impact the surge capacity of the machine. My conditions obviously have an impact on reaching that surge point.

I also have curves that show polytropic head vs ICFM. The curves are different for the same fixed speed machine for the different cases. This goes against what I thought I knew or read about centrifugal compressors. For a given ICFM I should have one polytopic head and eficiency (at constant speed) regardless of inlet fluid conditions. I need to know those fluid conditions to use that head to understand the performance of the machine.

I hope this clears up my question more.

Thanks again,
Mark

[ARAZA]

Dear,

Let us re-iterate what a surge is: a point when the compressor cannot add enough energy to overcome the resistance. This causes a rapid flow reversal.

Now in your case, there are diiferent sets of suctions flow conditions inculding compositions and temp. This all affects the D/S pressure, for instance if M.W is suddenly lowered than the discharge would be low and a flow reversal could take place. Hence for a different set of conditions, there would be surge point.

In my experience, a vendor should be able to give one surge point based on the different set of conditions you have provide to him. Can you approach the vendor and ask him about one surge point. Is this a new compressor or you are analysing an old machine?

Another point I want to make here, I think the set of suction conditions which gives you the lowest density is your worst case and the surge control should be designed for that conditions.

ARAZA

[Padmakar Katre]

Dear,
I hope this could be a good piece of information.
http://www.ckdnoveen...000000121_f.pdf

[mishra.anand72@gmail.com]

for incompressible fluid, decrease in flow is always accompanied by increase in pressure where as for compresible fluid, there is a point at which further reduction in flow is accopanied by decrease in pressure. As system pressure will be more, reversal of flow will take place, condition will recover but again as flow requirement itself is low, that point will reach again. That point is surge point. So what u do wn flow requirement is low?? vent, recycle. Maintain the flow more than surge point flow. Wn our anti surge controller were not working, I simply corelated it with amperage. In that case, I instucted them to monitor amperage and maintain amperage more than what I call surge amperage.

Wn that is head v/s flow, why different surge points??

Why that is so for compressible fluids??

Why u r changing inlet condition??

What r those changes??

[fallah]

for compresible fluid, there is a point at which further reduction in flow is accopanied by decrease in pressure.

Are you talking in compressor operation field or as general fluid dynamic?

Please specify.