centrifugal compressor.

[Ambrish_bajpai]

Hey , I am new to centrifugal compressor .compressor will be commissioned next year.
We have a propane centrifugal compressor whose settle out pressure is around 8-9 kg/cm2g .suction pressure is around 3.4 kg2 /cm2 a . In case we start the compressor then settle out pressure will increase the system resistance on discharge side and compressor will not be able to deliver. I am not very clear about starting of compressure at this settle out pressure. Antisurge valve has been provided but how will compressor respond when suction volume will be lower than minimum required flow , during starting load will be less . Can anybody help me on this ??

[NoNickName]

Let me try to decode your problem. You have a centrifugal compressor (I assume a refrigeration compressor) which is designed for a max discharge pressure of 8 bar, and a suction presure of 3.4 bar (I assume bar gauge both).
For propane that would correspond to a condensing pressure of 14°C approx and evaporating temperature of -20°C approx (provided the temperatures corresponding to pressures are saturated).

Now, if this is correct, I assume you have a heat exchanger on the high side, suitable for providing and ensuring the correct heat transfer at 14°C (e.g. a propane to chilled water exchanger).

As for the second question, again I'm not completely confident I fully understand, but anyway if for any reason the mass flow on the suction is restricted, the result is a lowering suction pressure.
In refrigeration, you would input the suction pressure to an inverter drive, for reducing the compressor speed as the mass flow rate reduces.

[piewie]

Becareful of surging.

[NoNickName]

Never mind piewie. They post their question, but never care to feedback or thank.

[Laith Marwin]

Let me try to decode your problem. You have a centrifugal compressor (I assume a refrigeration compressor) which is designed for a max discharge pressure of 8 bar, and a suction presure of 3.4 bar (I assume bar gauge both).
For propane that would correspond to a condensing pressure of 14°C approx and evaporating temperature of -20°C approx (provided the temperatures corresponding to pressures are saturated).

Now, if this is correct, I assume you have a heat exchanger on the high side, suitable for providing and ensuring the correct heat transfer at 14°C (e.g. a propane to chilled water exchanger).

As for the second question, again I'm not completely confident I fully understand, but anyway if for any reason the mass flow on the suction is restricted, the result is a lowering suction pressure.
In refrigeration, you would input the suction pressure to an inverter drive, for reducing the compressor speed as the mass flow rate reduces.

Hello I have a similar question. I am right now in the middle of a plant startup and there is a turbine driven centrifugal propane compressor. I have experience starting up Mycom positive displacement propane compressors but not centrifugals. You seem very knowledgable on this subject so I have some questions that you could help to shed some light on:

The driver is a 3400 HP turbine engine. As such the turbine vendor wants to run the engine through it's paces in recycle (i.e. run the turbine at various speeds and maximum pressure). In a typical turbine/centrifugal gas compressor application this is not an issue. You run the engine at full load and flow gas back to suction via the aftercoolers and anti-surge valve. However a propane system is a whole different animal.

Without a sufficient heat load in the chiller/evaporator how will one be able to achieve sufficient flow to run the compressor at maximum speed and full load (i.e. mass flow). We have anti-surge and quench valves for the stages according to my thinking we will hit a wall if we do not have sufficient cold liquid propane (i.e. quench propane) to cool the anti-surge valve flows. Our liquid propane inventory in the propane accumulator will only last so long right? So one needs to have a heat load in the chiller/evaporator in order to run the system at full load right? Or am I missing something?

Thanks in advance for your input.

[NoNickName]

There is no direct relationship between mass flow and centrifugal compressor speed, just because it's non positive displacement. It is more likely a volume flow to speed relationship.
In other words, if you don't throttle back your compressor, and the evaporator is not giving the ideal mass flow for that volume flow, you will end up with a starving compressor and a depressurisation of the suction.
I'd rather not comment on the results on ancillaries devices, like quench and anti-surge valves (which are not my playground).
Extra mass flow and some cooling may be obtained by injecting liquid in the suction.

[Laith Marwin]

There is no direct relationship between mass flow and centrifugal compressor speed, just because it's non positive displacement. It is more likely a volume flow to speed relationship.
In other words, if you don't throttle back your compressor, and the evaporator is not giving the ideal mass flow for that volume flow, you will end up with a starving compressor and a depressurisation of the suction.
I'd rather not comment on the results on ancillaries devices, like quench and anti-surge valves (which are not my playground).
Extra mass flow and some cooling may be obtained by injecting liquid in the suction.

Hello No Name, thank you for your input I really appreciate it. I guess I was a bit light on the details in my last post so here is some additional information. I apologize if the terminology seems dumbed down but I have discovered that many people have totally different names for the same equipment and I wanted to be as clear as possible.

We are using a liquid quench system for the propane refrigeration process. Liquid propane would be injected from the propane accumulator (i.e storage vessel on outlet of the condensers). Thus, we are injecting liquid propane into the hot gas from the anti-surge system (i.e. anti-surge gas is coming from the compressor discharge). So here is how I see a couple of operating cases occurring and what I believe would occur. I would appreciate your input.

Case 1:
We are at maximum load (i.e. maximum heat load flowrate through our chiller) and suddenly it stops. The anti-surge valves would open to ensure that sufficient mass flow was maintained while preventing compressor surge and liquid propane would be injected to keep the temperatures in check. The liquid injected would provide flash cooling as well as increase the mass flow rate.

Case 2:
We have no load in the chiller and we want to test the compressor at various cooling loads (i.e. push different mass flow rates through the compressor). We can speedup the compressor driver to move more mass through the machine and the anti-surge and liquid quench system would keep everything (i.e. temoeratures) in check. The thing which puzzles me is if there is no heat load (i.e. no process gas to be cooled flowing through the chiller) would we reach a point where there is no liquid propane (coming from the accumulator) in the system for the quenching? To me a balance would be maintained of hot gas with liquid quenching to keep the mass flowrate in check and avoid surging so one could speed the machine all the way to see how it handles moving the full mass flow. Doing this is a waste of energy in terms of using HP to compressor gas but useful for testing the compressor performance right?

Thanks for any insight provided.

[NoNickName]

I think that the whole system falls outside my knowledge and off topic to a refrigeration forum, I'm afraid.
One thing is sure: if you keep compressing propane, without cooling it down, you will end up with a superheated propane and no liquid propane to be injected.
From a thermodynamic point of view, you are doing work on propane without actually removing the energy provided.

[Laith Marwin]

I think that the whole system falls outside my knowledge and off topic to a refrigeration forum, I'm afraid.
One thing is sure: if you keep compressing propane, without cooling it down, you will end up with a superheated propane and no liquid propane to be injected.
From a thermodynamic point of view, you are doing work on propane without actually removing the energy provided.

Hello, thanks for the input. Any suggestions on where I could look for more information on this topic? I have looked at some other locations for information but there seems to be a lack of understanding in terms of the challenges and understanding of the refrigeration process.

For a typical centrifugal gas compressor one just pushes the gas through a air cooler and then back to suction via a anti-surge valve and has no issues.

Thank you.