Flow rate adjustment with a throttling valve only works with centrifugal pumps. Never try this with a positive displacement pump! Depends on how you're getting the fluid into the mainline. Tr1 assumed you have a pump there, in which case you could bleed pressure (and flow) off your injection line into a tank (or perhaps recirculating it back to pump suction) to reduce the pressure that is driving flow into the mainline, but that may also tend to decrease the flow going to the mainline too. As the pump runs out farther on its curve you gain flow but lose pressue. If the line diameters and tank and the mainline pressures can be adjusted to split the flow as you require as the pump pressure reduces, you may get the balance you need, otherwise you will have to carefully design the takeoff line and the injection line with properly sized diameters to get the flow split. If you need accurate control, best way is to put a real control valve on the pump discharge to control what you need going to the mainline and an offtake line to a tank or to recirculation to take the excess. Take heed of the other's advice too.

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