Without a flow transmitter, you cannot accurately know the flow. However, one can calculate the flow using the control valve opening if the following information are available.
[ul]
[li]Inlet and outlet pressure[/li]
[li]Details of pipe between the pressure measurement points and the control valve[/li]
[li]original control valve calculation[/li]
[li]control valve charecteristics[/li]
[li]Fluid properties used in the original control valve calculation[/li]
[/ul]
This is not a simple calculation but if it is required regularly, it is possible to program the necessary calculation in the DCS / BPCS system.