Sir, I work in a water treatment plant. Plant is based on Reverse osmosis. Where one pump feeding water to the R.O membrane by two inch pvc pipe we know the pressure it is 58 psi but still we don't know about water flow. Please give any formula to find out water flow by calculation.
Thank you

You haven't given us much iinformatio to go on. The length of the line. Friction loss in that line. Friction loss doe to fittings. Elevation, (will the line run up hill). etc. All of which factors in but I'll try to give you a "bare bones" answer to your question.
For 2" sch. 40 PVC, a flow of 50 GPM would see a pressure loss over a 100 foot length, of 1.73 PSI, and the water velocity would be 4.77 feet per minute. Both of these are reasonable numbers, so if you needed 50 GPM through 100 feet of pipe, 2" would be an appropriate size.
Hope this helps you out. Tom

It as accurate a possible without additional info.