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rkm502 (Mechanical) (OP)
27 Dec 08 19:58
Hello - I have been reading the forum for several months and appreciate all of the expert advice.  I purchased and read the GPSA handbooks and Royce Brown's 3rd edition Compressors book, I also read all of the posts in this compression forum.  I am trying to learn as much as possible about well head compression, specifically using screw compressors.  Can you guys point me to additional resources to read.  Also, one of the things I would really like to see is a good P&ID of a screw compression skid.  Any help or advice is greatly appreciated.

Thanks
dcasto (Chemical)
29 Dec 08 1:34
my rule of thumb says, if Pd/Ps is gretaer than 5, NEVER use screw compressors.  If Pd/Ps is <5 and >3  volume capacity and FUEL are the controlling factor and your choice.

The big issue is energy or fuel use.  If you do not care about fuel use them.  If fuel is a direct cost, then use my rules, only use if Pd/Ps < 3.
zdas04 (Mechanical)
29 Dec 08 7:06
DCASTO,
You must be talking about dry screws (which I don't think have any place in wellsite use at all).  For flooded screws I've been very successful running them at 20 ratios.  Below 4 ratios you would get about 4-6% better energy efficiency with a single stage recip (and other considerations might still point you toward a flooded screw), but the screw power usage is actually better than a 2-stage recip.

A recip is a machine that runs best with a constant suction pressure.  A flooded screw does best with a reasonably constant discharge pressure.  I think that when suction pressure is less than about 40 psig, a flooded screw will always out perform a recip for wellsite use because the variability of suction pressure is too great for a recip (recip valves work in a range of about +/- 5% of design conditions, and wellsite transients generally exceed that considerably at low pressure).

David
rkm502 (Mechanical) (OP)
29 Dec 08 22:10
David - thanks for your reply.  I have read all of the threads in the compression forum and you appear to be the authority on well head screw compression.  Can you point me in a direction for another resource to read up on.  I am also interested in your opinion on how to get a better understanding of the subject.  I think what I really need is a good P&ID to study - any help is apprecitated.

thanks
zdas04 (Mechanical)
30 Dec 08 10:06
Being "the authority" on wellhead compression really doesn't mean much--engineers have been avoiding that horrible subject for nearly 100 years.  Most wellhead compressors I see were designed for a traditional plant or pipeline application and then pulled off of a surplus list to supply a field foreman with some hp (e.g., "this application is 3 compression ratios, so you obviously need this 3-stage recip compressor that I've got in my back yard").

P&ID's are typically proprietary to the packager or (rarely) to the purchaser.  I've done a few for clients, but the client owns them at the end of the job.  There really isn't any such thing as a "standard P&ID".  I've been thinking about writing something for Pipeline & Gas Journal about screws that might include a simplified schematic, but I don't know when (if ever) I'll find the space in my schedule to do that.

When you're evaluating designs of flooded screws, the absolute key is oil.  Flooded screws have only been around for 30 years or so and they spent the first 20 years almost exclusively inside plants (either as air conditioning units or air compressors).  In virtually all plant applications, the conditions are very well defined (e.g., the gas might be dehydrated so oil temp is a secondary consideration, or the machine will always have a 50 psi dP so an oil pump is not required).  For wellhead use, an oil pump is always required.  If anyone tells you differently then RUN AWAY from them.  

As I've said many times in this forum, compressor oil is hydrophilic and would make a very good dehydration media if it wasn't so expensive.  You must get the oil hot enough to cook the water out of it or the oil performance will rapidly degrade (i.e., the surface tension will increase which allows progressively larger droplets to avoid coalescence, lubricity will degrade, and viscosity will increase).  The typical plant approach of using a 3-way valve to manage oil temp into the start of the oil system is evaluating the wrong thing and is only effective at exactly one set of operating conditions.  I like to use a secondary cooling loop that has a temp sensor at the outlet of the screw and manages secondary cooling loop flow to keep that temperature constant.  If you can cook off water, the oil will last indefinitely and becomes very inexpensive.

David
dcasto (Chemical)
3 Jan 09 23:11
the reason for the limit on oil screws is ENERGY.  Oil screw compressors are energy hogs so in order to save fuel I limit the ratios.

To give you further insight on oil, we had some real rich gas that we compressed from 15" Hg vacuum to 40 psig in a screw.  the gas composition was so rich that if the presure on the discharge exceed 50 psig, we could not seperate the oil from the condensate and the oil would be diluted and the compressor would suffer severe damage.
zdas04 (Mechanical)
3 Jan 09 23:39
Like I said above, a flooded screw has almost exactly the same skid-edge to skid-edge fuel/MMSCF as a 2 stage recip, and markedly better than a 3 stage recip (if I need 20 ratios, I'll use less fuel with a properly configured flooded screw than I would with a properly configured 3-stage recip, and I wouldn't install a 2-stage recip that had a design condition of 4.47 ratios/stage, temps just get too high).

Your problem with oil contamination is one that everyone is terrified of.  It happens primarily because the particular screw oil selected is mutually soluble with the condensed hydrocarbons.  The solution is often changing oil to one that is incompatible.  I've had mixed success going to an oil that can tolerate a higher temperature and just running the oil at above the dew point of the contaminant in the oil (the times it was successful I had to run the oil temp at 220F and shade the max inlet temp a few degrees).

David
dcasto (Chemical)
5 Jan 09 18:28
the ariel program shoes a 10% increase in a screw versus recip. for a typical package.  The Ajax program says it can get it done with 66 HP versus 72 ariel and 81 screw.
zdas04 (Mechanical)
6 Jan 09 0:27
I ran your conditions in Gardner Denver's RotoSize and got 76.2 hp.  Ariel Performance (a program I really like for recips, not positive I'm ready to love it for screws yet) says you can do it with a 2 stage recip (JGM/2) and I get about 72 hp like you did.  Since Ariel Performance only uses 2.8 hp in the interstage and after coolers (which I don't believe is enough), I'd guess that measurements on real machines in this service would give you pretty close to exactly the same skid-edge to skid-edge hp.

I've always though that the Ajax program was pretty optimistic, and I'd guess that it would be in the ballpark with the other two.

David

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