I am modelling water flow through valves to find out the throttling characteristics.Basicaly I would like to know what will the mass flow rate for various valve openings.Assuming that the pipe incorporating the valve being connected to a centrifugal pump on one side and open to the atmosphere on the other end, what type of boundary conditions should I give for different valve positions.

Thanking you.

The BC would be for pressures at the upstream of the valve(pump discharge pressure) and the downstream (atmospheric pressure). This in turn gives the delta P across the valve. This and the valve discharge coefficient Cv, will determine the flow as a function of valve opening provided the fluid density is known. Want more, Don't hesitate.

Thanks Noora.I am happy that I had also thought in the same lines.The point to be further decided is what should be the value of the upstream pressure.(1)Whether it is the pressure value obtained when the valve is fully shut or (2)the pressure corresponding to the velocity of water when the valve is fully open or (3) the pressure corresponding to the manometric head of the pump.

Regards and thanks.