Confused about Volumetric Flow Rate & Valves?

[Ahm402]

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?

 

[SteamKing]

Valves can be closed to stop the flow entirely. Doesn't that count as controlling the flow?

Obviously, if a valve is opened fully a certain flow will pass thru. If the valve is then partially closed, and no other changes are made to the piping system, can the same flow pass thru the valve? The answer is no; a partially closed valve generates more head losses than a fully opened valve, thus a reduction in flow will result.

 

[russ_watters]

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?

The flow is always constant across the valve under all valve positions, but that doesn't mean it is the same flow at each valve position. For example, fully open, maybe the flow is 10 m^3/hr on both sides of the valve. At half open, it is 5 m^3/hr on both sides of the valve.

 

[Ahm402]

Thanks russ_watters for the explanation, that does make sense.