Discharge Piping Velocity

Friction loss increases, load on motor increases.

If there is grit in the pumpage, velocities > 2 m/s or so may lead to erosion of the piping. High velocities → high pipe friction losses → higher required pump head → higher pumping costs.

Thanks for reply,Ok if there is no check valve on the discharge piping and the flow going firstly by the recycle piping to the Tank on a velocity 8m/s in 4"pipe then open the main line with a 35m3/hr on 10"pipe that mean the velocity will be around 0.192 m/s and close the recyle line block valve is any possibility for back pressure,water hammer?

I'm not sure about these numbers.

8 m/s in 4" (100mm) pipe ≈ 226 m³/h. (This is a very high velocity.)

For 35 m³/h in 10" (250mm) pipe, I get ~0.198 m/s velocity, which is close to your number. (But this is a very low velocity.)

What is the m³/h actually supposed to be?

If valves are opened/closed slowly, water hammer should not be a problem.

The rated velocity is 1480rpm,so it will be about 7.8 m/s ,but frankly i am not sure this the velocity, but as per operation procedure it suppose to start with 10HZ up to 50 HZ(1480 RPM ) I don't think they follow the correct procedure since after open the main line and closing the recycle the flow 35 m3/hr almost 30% of the rated flow 120m3/hr ,this pump shut down and scratch mark and pitting found in the impeller

Well, these things always bring trouble so while connecting to it make sure use rubber hand.