Conservation of fluid momentum

I have a valve inside which fluid flows past an open 'poppet' at 1,000LPM. Downstream of the valve there is a back pressure of approx 1.39bar caused by a pipe and head pressure (this is a calculated value, not measured...yet). Unfotunately I have no data on how quickly the poppet closes so I will have to assume it is instantaneous. In my head conservation of fluid momentum would imply a low pressure would develop in the pipe downstream of the valve. How do i calculate what this pressure will fall to? And following the pressure fall, the fluid will 'snap back' causing a pressure spike - how can i calculate what the maximum value of that spike will be?