Posted 10 February 2011 - 09:10 AM
First of all let me tender my respects to all of the participants in this discussion. It is only through such forums can we explore, discuss and bounce-off ideas and questions. Everybody of you have made such a positive contribution to the thought process.
In my (over) 40 years of design in the O & G industry, I have learnt one thing, and that is no idea, how silly or alien it might be, should be discounted without evaluation.
The configuration of centrifugal compressors in parallel is one of the least and most understood schemes I have yet to come across. The more I talk and hear about it, the more I find there is something more to take into account.
Your main reason to propose the two schemes was motivated by cost savings. Accepted that cost is the denominator in the fraction called engineering. However my question is saving money at what costs?
To illustrate my point, I would like you to consider the following:
1. What happens if there is a problem with one of the compressors and it trips? You will immediately answer that to prevent gas outgress, we would box-in the system and i.e. settle out. But settle out with what?
Or rather would you box in the total circuit i.e.trip the other parallel compressor as well and lose production maybe not only of the gas, but maybe also the associated oil ($$$$$) with it?.
Suppose we isolate at the rogue compressor's suction and discharge. This may require a higher settle out pressure and with it severe problems of dropout of retrogrades during a rapid blowdown at places where you do not want to dropout. Not only have you not considerably increased the settle-out pressure due to no suction buffer volume, you have increased containment risk.
2. If the Compressor Dynamics requires hot gas bypass, admittedly you will get a quicker response with judiciously placed check valves at suction downstream of this common drum, but you are already increasing the number of valves. Cost is biting back at you.
3. In many parts of the world statutory inspection requirements or ability to isolate a boundary for maintenance as the 'tail' (pardon my expression) wags the 'dog'. Would you not prefer to take one train out in its entirety (say to maintain one specific component) rather than have the entire caboodle taken off altogether. Isolation requirements in Industry are getting more and more stringent through incorporation of double block and bleed valves and removable spools. To avoid enormous cost penalty by incorporating DB&Bs around every maintainable component, there is now a positive move towards isolating a discrete boundary with many more things in place. In such a situation, I am sure the shareholders will not thank you to bring down the entire three compressors to attend to a minor or even a major maintenance function.
4. I completely agree with Art Montemayor and would feel well placed to have the permission to re-quote him:
"The flexibility issue cannot be stressed enough. The additional cost of the suction separators are chump change when compared to the ease and flexibility of operation that the production crew has. And this affects the maintenance people as well, since they can shutdown and startup with ease and with total flexibility."
Thank you Art. This is my first post in this ChEResources Forum. I feel privileged to see your writing in this forum.
Ankur, Good luck and do bear in mind MORE IS SOMETIMES LESS!