help for a branch line design

[chemical321]

There is two 3000 m3 storage tanks (two pool of water) for feeding demineralization plant. We have two parallel pumps (2*100%) for demineralization plant. Occasionally we have shortage of water so we want to use a reverse osmosis unit.
Is there any problem to use common header for feeding demineralization and osmosis plant? We want to connect a branch line in the suction line of the pumps for reverse osmosis unit?
Or we must use separate line for feeding osmosis plant and connect it to the pools.
Q(one pump)=160 m3/h
D suction= 6"

Q (for osmosis plant)= 80 m3/h

 

[chemebabak]

Chemical321,

You should consult with the pump vendor. Many pumps have a velocity and pressure drop limit at the pump suction. These limits are to prevent damaging forces on the pump impeller.

At 160 m3/hr (for a 6" schedule 40) the velocity is 7 ft/s and the pressure drop is 1.3 psi/100 ft of pipe. At 240 m3/hr the velocity is 12 ft/s and the pressure drop is 3.4 psi/100 ft of pipe. The higher flow rate may be too much for some pumps.

Nevertheless, according to your description not all of the flow passes to the pump. As long as the piping branches far enough away from the suction to the pumps, the additional 80 m3/hr should not create any additional forces on the pump impeller.

 

[chemebabak]

You should also conduct a hydraulics analysis to ensure that the holding tanks have enough head to move the extra water to the final destination.