The maximum pressure drop through a modulating control valve in a water flow system is designed to be 100 kPa. The flow through the valve is 5 liter/s. The required kvvalue for the valve can be estimated as indicated in the diagram below to approximately 20.
Note - larger valves (with larger Kv) reduces the maximum pressure modulating spans - limiting the ability to provide a stable process control. As a consequence - control valves are often one to two dimensions below the dimensions of the connecting pipes.
Example - with a water flow of 5 liter/s the pipe size would be in the range 3 - 4 inches (80 - 100 mm nominal). A control valve would be in the size kv ~ 20 - or in the range 2 - 3 inches (50 - 80 mm nominal).