I'm new here, and I have a question that might be easy but really confuses me. Does the volumetric flow rate of incompressible fluid change across a valve?. Based on my analysis, no it can not change since the mass is conserved, and since it's incompressible fluid. I know that the velocity will change within the valve due to change in the area, but at inlet and outlet will be the same. This analysis is even more confusing, if the flow rate is not changing, why would we use a valve? I know that the logic said that the valves are used to control the flow but how?