I don't believe the recycle needs to be sized for anything significantly more than the minimum flow of one pump if a suitable setup and control philosophy is adopted.
Say you have 3x50% pumps and each pump has a nominal flow of 100cmh and a minimum flow of 30cmh.
The common recycle just needs to recycle 30cmh and all remaining scenarios will be by common discharge valve throttle and turning off excess pumps.
Flow demand: 0 to 30cmh. 1 pump with recycle 30cmh
Flow demand: 30 to 100cmh. 1 pump with discharge throttle and recycle 0cmh
Flow demand: 100 to 200cmh. 2 pump with discharge throttle and recycle 0cmh. The 2nd pump starts when demand is just above 100cmh so each pump would then be 50cmh so no requirement for recycle.
Lets explore further and see how the scenario works if a third pump is somehow required.
Flow demand: 200 to 300cmh. 3 pump with discharge throttle and recycle 0cmh. The 3rd pump starts when demand is just above 200cmh so each pump would then be 67cmh so no requirement for recycle.
Therefore all that is required is a common recycle valve with control capability, common discharge valve with control capability, common flow meter and recycle flow meter.
The most favourable control which eliminate the extra recycle line and valve is to have all pumps with VFD. The common flow meter controls the speed of all the pumps. The minimum flow will be much lower and in the vast majority of cases, redundant, making the recycle line unnecessary.
If you anticipate that the amount of time requiring full flow: ie: 200cmh based on the above example is not alot, and most of the time partial flow is required, then running at partial speed will be much more favourable for you.
Edited by thorium90, 04 December 2018 - 09:40 AM.