Here’s an interesting design dilemma. I have a contact who is upgrading a produced water system for a central processing platform. The produced water system will have two parallel hydrocyclones in series with a degasser. The hydrocyclones use centrifugal forces to separate produced water such that entrained oil will be removed from the bulk water. The degasser will allow any dissolved gases in the produced water to evolve at near-atmospheric pressure prior to being discharged overboard.
Now, the degasser is sized for processing 3000 kl/d of produced water, while the hydrocyclones are sized for 2000 kl/d each.
What’s strange about this? Well, the process can only handle as much liquid as the equipment with the smallest design throughput. So, in this case the process is limited by the degasser, that is 3000 kl/d. The extra 1000 kl/d processing capacity available at the hydrocyclones is wasted.
It’s also usual to have equipment in parallel so that one unit may be taken out of service without affecting the design throughput of the system. Hence, you may design a train with 3 parallel units, each capable of handling 50% of design flow. This is normally indicated by the statement “3×50%”. Other derivations are “4×33%”, “5×25%”, all allowing for one unit to be taken offline. This type of sparing philosophy is considered when designing pressure relief valves, to ensure there is adequate relief capacity in the event one valve is unavailable due to maintenance or repairs.
What’s strange in the example above? It seems as if it has been designed with “2×67%” hydroclyclones. What gives? If you take one unit down, then the on-line hydrocyclone limits the throughput. If both units are online, there is excess, unused capacity.
Wata - Malaysian Oil and Gas Engineer 
Jabba reckons there’s a good reason.
Let me explain the steps this came about.
1. Some design consultant probably got it wrong, and doubled the design flow, hence instead of 3 x 33%, they got 3 X 67%.
2. EPIC contractor placed order
3. Canceled 3rd unit when they paid for Wata to do a capacity review
4. End up with 2 x 67% unit.
or someone have a hydrocyclone behind his home…..
Which reminds me, I need to check on the degassing vessel behind my house.