3 replies to this topic

[Kondepudi]

I am a process engineer involved with a CO2 compression project for a Middle Eastern client. The project is in EPC (Engineering, Procurement and Construction) phase.

 

CO2 as part of this project is compressed in 2 stage reciprocating machine from 40 barg, 60 °C to 238 barg and 140°C. 1st stage discharge is at 94 barg and 145°C. Interstage cooler and after cooler is provided to cool the gas to 60°C.  CO2 is 99% pure. It goes to supercritical (dense phase) above 72 bar and 31degC. The recip has 50% step control. Project requirement is 25% of maximum flow (100%) is needed. There is a discharge pressure control over ride (only acts when the pressure goes higher than 245 barg).

 

CO2 gas fed to recip compressor is dry to the tune of 20 lb/mmscf. The drying (noisture removal) was done by mol sieve unit before it goes to recip compressor. Due to this no separators are required interstage or after 2nd stage. The recip sends the gas to a 45 km long 8" pipeline on a continuous basis.

 

I have been talking to manufacturers of recip machines. They said that recip machines are well suited to this application. The interstage pressure and temperature figures are slightly different to what I mentioned above. Each manufacturer has a different figure.

 

When the machine is shut down, it is blown down (depressurised) before restarting. A manual valve is proposed to help restart the machine to achieve flow circulation.

 

FEED (Front End Engineering) engineer proposed recycle for each stage. Suction pressure of each stage is controlled by the corresponding recycle valve. Sheme-1 shows this scheme.

 

I want to propose a simplified scheme where suction pressure of 1st stage is controlled by one recycle valve which is taken from discharge of 2nd stage. The dense phase CO2 from 2nd stage discharge at 238 bar and 140°C when dropped to 40 bar reaches a temperature of 48°C which has no problems at all.

My simplified scheme is shown in Sheme-2.

 

My questions are two:

 

1. Will Scheme-2 work?

2. What is the best way to have a planned shut down of compressor and restart? Do we need the manual valve?

Attached Files

•  scheme-1.pdf   335.49KB   93 downloads
•  Scheme-2.pdf   319.14KB   71 downloads

[ankur2061]

Kondepudi,

 

1. The scheme-2 does not give the bias or ratio control for the split in the signal to the PC at the 1st stage and 2nd stage of the compressor. How do you propose to set up a combination of 4 bias or ratio control which includes:

 

a. 1st stage PC to 1st stage suction unloader

b. 2nd stage PC to 2nd stage suction unloader

c. 1st stage PC to 2nd stage suction unloader

d. 2nd stage PC to 1st stage suction unloader

e. 1st stage PC to 2nd stage PC

f. 1st stage suction unloader to 2nd stage unloader

 

When you have numerous bias or ratio controls from a single signal source it will be difficult to minimize the lag in signal causing controllability issues. Have you discussed this with your instrumentation engineer on how the signal ratios or bias will be decided and whether there will be some signal transmission lag causing controllability issues? IMHO, the scheme-2 would have controllability issues and in deciding the bias or ratio controls for both the stages simultaneously. I think that Scheme-1 provdes better controllability of stage pressures and flexibility in terms of independent pressure control of the 2-stages. Having said that, if the compressor package supplier guarantees desired performance including inter-stage pressure control as per your proposed Scheme-2, then there is no debate about the merits and demerits of the two schemes. 

 

2. The start-up line connecting from the outlet of the after-cooler to the inlet of the 1st stage is correctly depicted in scheme-2. I might think of putting a globe valve on the star-up line for throttling purpose during start-up.

 

Hope this helps.

 

Regards,

Ankur.

[Art Montemayor]

 

Kondepudi:

 

Just because you don’t have much moisture in your gas doesn’t mean you don’t have a need for interstage separators.  Separators not only mitigate a lot of the pulsations inherent in reciprocating compressors, they also serve as a security item in ensuring no other undesirable contaminants will enter any of the stages or inlet valves – such as rust, solids, etc..  I would normally not operate a reciprocating compressor without inlet separators – especially on the 1^st stage.

 

What KIND of capacity control are you proposing?  “Step control” is not a type of compressor control.  Clearance pockets, valve lifters, plug suction valves, recycle valves, speed are all types of capacity control.  Which one do you mean?  Clearance pockets can be fixed or variable.

 

A reciprocating manufacturer has to balance his machine.  The type of frame and configuration affects his design.  A horizontal, balanced opposed design – depending on the size and type of driver – is a usual selection.  Normally, the interstage pressures are selected to equalize the work being done in each stage – if possible.  This means your machine would have approximately 98 barg as your 1^st stage discharge pressure.  Manufacturers will determine the exact interstage pressures required by their machines because of the balance required, the size of the cylinders selected, the speed, and other factors.

 

All reciprocating compressors must be started up and shut down while they are 100% unloaded.  The usual way to quickly unload the machine is to vent to atmosphere.  A usual way is to block off the suction and discharge while opening vents to the suction and discharge.  You can also unload the compressor 100% and shut it down.  The manufacturer has a dominant role in determining which method is best for the application.  The most important factor in your type of project is selecting a proper, experienced, and recognized compressor manufacturer very early-on.  This step cannot be over-emphasized enough.  The success of your project will depend to a large degree on the decision(s) that you take with respect to the compressor manufacturer.

 

Do not dictate process and mechanical design to the manufacturer or you will suffer later on.  Your front end engineering persons are demonstrating their complete ignorance and lack of experience in this field when they propose a recycle for each stage.  This is NOT the way to control the capacity of a reciprocating compressor and will cause not only a waste of time and effort, but will transfer compressor performance responsibilities to your company.  This is a very silly way to do engineering.

 

It is normal to use a recycle valve from the last stage discharge back to the suction of the 1^st stage on a multi-stage compressor - but on relatively small compressors.  However, for your machine (judging by the size of the 8” discharge) I would not depend on this method because it is very power inefficient and wasteful.  Speed is the most efficient method with Clearance pockets as the next most efficient capacity control on a recip.  However, speed control is costly and subject to slow response and maintenance issues.  What size of recycle capacity control valve have you calculated?  It might work, but I doubt if that type of control is what would be recommended.  Clearance pockets would be my recommendation, based on what little I know of your project.

 

[Kondepudi]

Mr Montemayor,

 

Thanks very much for your reply.

 

We have clearance pockets (fixed volume) that would achieve 50% turndown and some vendors can also give 75% turndown through port valve, valve lifters too. We have selected horizontal, balanced opposed design.

 

I am wondering if based on suction pressure control, 50% or 75% turndown can be achieved with the help of clearance pockets, valve lifters etc. One suggestion I got is that based on the recycle valve position, this can be done. I will surely ask this question to vendors, but wondering what you and the forum can suggest.