Compressor Staging Calculations 2

[sarahr]

I have to design a system that will take CO2 at 100 psig and 0 F, and run it through a series of compressors and raise the conditions to 4000psig and 140 F. Knowing this, I need to determine how many compressors are required to increase the pressure (without raising the temp. so high that the compressors fail...safe temp of 200 F during each stage) Usually, we just use a rough approximation of 4:1 (as in the gas goes in at 100 psig and is raised to 400 psig and the temp is raised accordingly) however, this design needs to be much more accurate and i need to prove why the ratio is correct. HOwever, i have never had to actually determine the ratio and I'm not really sure where to begin. If anyone has any suggestions or tips I would greatly appreciate it.

 

[zdas04]

I would download the "Ariel Performance" program from

http://iweb.arielcorp.com/ariel/company/products/compressor_software/performance_program.asp

. It is an excellent sizing program that will tell you the specific compressor (in their line) to accomplish your task.

You don't say what your target volume is, but a quick look this morning and you're looking at a 3 stage compressor (3.2 ratios/stage) that gets you to a first interstage cooler inlet temp of 138F. You'll have to cool that to about 45F to stay below 200F out of the second stage. If you come out of the second interstage cooler at 45F, then you'll go into the after cooler at about 200F.

If your goal was to avoid coolers, then it ain't happening. 100 psig to 4,000 psig would raise your temperature from 0F to 549F.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[jcfoley]

sarahr,

I am well familiar with the program that David speaks of. Depending on your accuracy required, you may have to have the entire engineering process completed. By this, I mean all the pulsation vessels with internals, piping studies and the acoustical analysis. Without doing this work, the actual interstage pressure drops will not be accurate. I have seen situations where the pulsation vessel pressure drop required more power than the driver was capable of delivering.

IR & Cooper (Superior) also have programs for this but I think that Ariel's is the best.

 

[Montemayor]

sarahr:

I’ve pumped and compressed a lot of CO2 (I mean a LOT!) in my 45 years as an engineer. My first comment is: Once again, here we go totally confusing and mistreating a really simple, interesting, and practical thread by failing to note the basic data. Are you proposing to apply a reciprocating compressor or a centrifugal compressor? Let’s get this straight up front and avoid a lot of confusion. Both David and JC are assuming a recip (and a very specific and highly fast one, may I add) but you haven’t stated that. I’ll assume you are interested in using a recip as well. Another issue that makes for confusing this thread even more so is that of gas compression and transport as compared with super critical fluid (SCF) compression and transport.

I would not normally use an Ariel reciprocating machine on this application. I’m very familiar with the Ariel since its development and entry into the oil patch field in the early 1980’s. It was a machine specifically designed and made for natural gas applications on a competitive basis. It was not designed as an industrial gas machine. There are BIG differences between compressors for industrial gases and those for natural gas. A lot of the differences are in valve design, piston speeds, and service life. I am not against using an Ariel, but I want to make sure that everyone understands what I mean about the basic scope of design for a reciprocating machine so that a lot of hearts aren’t broken up when expectations are not realized. The Ariel machine is a good machine for what it is intended to do and for the field it is intended to compete in. It was not a custom-designed machine for industrial gases and custom process conditions – however, I could be educated if that has recently changed and they are now designing their machines for industrial gas applications.

Nevertheless, the optimum CO2 compression ratio I’ve used is 3.5 to 4.0 for 2 and 3-stage machines. The CO2 compressors I’m referring to at this point are all GASEOUS compressors. This is a big difference from SCF “compressors” (or pump?). You fail to make mention of the very important basic design point that you are proposing to enter into and continue to “compress” through the Critical Point of CO2 ( 87.8 oF and 1,071 psia). You are not going to get very far with compressor manufacturers without clearly explaining this intent in your scope of work. As a chemical engineer, I presume that you are aware of the importance of this point. Beyond the Critical Point, as we both know, the fluid starts to behave in a totally different manner from that of a gas (or a liquid). It may resemble a gas – but as David would correctly say: “It ain’t!” Special procedures and design must be used to successfully carry out the passage of the fluid from the Critical Point to the 4,000psig and 140 F that you are proposing – which puts the CO2 right smack in the middle of a first-class SCF status.

You may be able to pump liquid CO2 that you produce at 1,200 psig and 70 oF. You would use a 2 or 3-stage recip for this purpose and follow up with a cooler-condenser on the discharge. Once you store this high pressure liquid CO2, you can employ a pump to transport the fluid up to the desired target conditions. Or, as you are inferring, you may also use a special “compressor” design to raise the SCF to a higher pressure. But special mechanical and process design must be incorporated to ensure proper and safe operation of a reciprocating machine while carrying out this very unique unit operation. Piston lubrication and proper valve action are certainly items that have to be carefully looked at – especially since a SCF like CO2 will most like dissolve any liquid lubricant. And this would not be considered a conventional gas compressor at this point.

Like David, I don’t know what the reason is for your concern on the interstage temperatures. On gaseous machines my concern has always been the avoidance of coking or solidifying lubricating oil with temperatures in excess of 300 oF. That’s why my optimum compression ratio for CO2 gas recips is 3.5 – 4.0. At these compression ratios I normally get around 250 oF discharge temperatures and this has proved to be very satisfactory for continuous service of up to 11 months.

I don’t understand what you mean by a “need to determine how many compressors are required to increase the pressure”. You don’t need a multiplicity of compressors to raise the pressure. You need more compression stages which conforms with the title of this thread.

I hope these comments serve to add some help to your cause and clear up a few points.

 

[zdas04]

Welcome back Art.

I mentioned Ariel because their program would tell him (with a pretty short learning curve) that he has a whole list of serious problems. I really like the program for order-of-magnitude compressor sizing. The original post asked about compression options, I answered without asking him if he was trying to drain the swamp or inventory aligators.

The Ariel program is just too easy to get real-world quality scoping numbers that I regularly use it (and point people to it) for applications that I know will not end up with an oil-field compressor. It gives me a feel for magnitude of hp required and expected heat energy (expressed as cooler load and temperature rise). Useful data points when talking to products-compressor manufacturers.

David

 

[jcfoley]

Just FYI,

Just lately, Ariel has been actively pursuing the industrial markets that would normally want an API 618 type compressor. This is an active new market for them. In fact, we are starting to get more CO2 machines though our shop for the petrochemical industry. Ariel definitely wants a larger piece of the compression pie.

By the way, I do not work for Ariel. We use just about any compressor our customer wants. However, Ariel is still better than 70% of our business.

Chris

 

[zdas04]

I think the original poster has moved on with his life and this is just us girls talking amongst the choir. Oh well, maybe this discussion will help someone else someday.

Chris,
Where is your fab shop (I'm always looking for people to put together packages for Oil & Gas applications and you sound like you know what you're talking about)? You can get to my e-mail address by following the link in my signature if you want to tell me about your shop.


David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[Montemayor]

Chris:

I believe David, as usual, has come up with a very practical and useful comment or question. Your engineering position and experience, vis-à-vis gas compressor applications and the engineering that goes into them is a privileged one that I can readily identify as a virtual gold mine for those seeking help or aid in obtaining or justifying compressor applications. You may prefer not to identify your company and its capabilities for fear of being accused as “advertising” on the Forum, but I would like to know this information as it represents your credentials and experience in the field. I may also opt to pass it on to other engineers who consult with me on similar problems. Some of the basic information that is sorely lacking in many engineering forums is the inability to furnish young, upcoming engineers with accurate descriptions of what can and can’t be done with compressors of various types as well as what functions and relationships exist between compressor manufacturers and compressor “packagers” or application designers. We old timers, of course, were brought up during the development of this relationship between the compressor manufacturer and fabbers. But younger engineers are often puzzled by the system of applying gas compressors.

I am pleased to see Ariel taking an active interest in filling the API 618 void left by the evaporation of many stalwart American reciprocating compressor manufacturers. You succinctly defined my thoughts on compressors by mentioning API 618. I don’t believe that every engineer with a compressor application needs to apply API 618 in every detail; however, I earnestly believe that every engineer should be aware, avidly read, and thoroughly understand every principle that API 618 puts forth in its recommendations. It is only by doing this that an engineer can honestly sit down with a manufacturer and/or fabber and accurately and practically discuss his/her compressor specifications and performance requirements. Although I don’t know you personally, I strongly suspect that you hold a lot of that valuable information in the palm of your hand. Are you/your company located in the Panhandle, or further south in the Big Spring area? Does your company have a website we can look at? I look forward to hearing more from you on this compressor forum.

David:
Thanks for the welcome. It's great to breathe again. I agree with you that this section of this thread may be useful for future queries. I, like you, am always looking for others who can sing in our choir.

 

[jcfoley]

Art,

I work for Compressor Systems, Inc. I sent David our website address earlier but it is down and the IT folks are working on it (

http://www.compressor-systems.com)

Our main fab shop is Midland. We have business units all over the gas patch from LA to NM to CO.

Both Hanover and Universal are larger than us but I believe that we are the third largest packager after that. I joined CSI as chief engineer just this past Jan. Before that, I spent the last six with GreenField Compression working in the industrial gas and CNG markets. My work has been NG, Helium, Hydrogen, Air and Nitrogen. ASME vessels, piping codes and instrumentation projects round out the majority of my past experience. The three years before that I worked with liquid and gaseous helium projects all across the US for BOC Gases.

I still feel wet behind the ears most of the time especially when compared to someone of your vast experience!!! I have learned a great deal from you and quite a few others via this site. This is the best engineering forum that I ever used and I can only hope that I can be as much of a help for others as you guys have been for me.

Best Regards,
Chris Foley

PS Hopefully the Eng-Tips folks will bear with me on this post

 

[zdas04]

Chris,
The site management has recommended that people not post their actual e-mail addresses because spiders are crawling all over eng-tips.com looking for people to add to their high-dollar spam lists. They don't complain about the link to my web page in my signature. Listing your company kills anonymity, if that doesn't bother you (as it doesn't bother me) then go for it.

David

 

[jcfoley]

I am not too worried about it. We have decent SPAM software. As Art said, what you have done and what you are doing is a part of your credentials. On hindsight, I guess I should have put one of those extra character strings that needs to be manually removed.