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.

 

[alphy]

Hello, and welcome! Your question is a common one among those new to fluid mechanics. Let's break it down and explore the concept of volumetric flow rate and valves.

Firstly, the volumetric flow rate is the measure of the volume of fluid that passes through a given point in a specific amount of time. It is typically measured in units of cubic meters per second (m^3/s) or liters per minute (L/min). This is different from mass flow rate, which measures the mass of fluid passing through a point in a given time.

Now, onto your question about incompressible fluids and valves. You are correct in saying that the volumetric flow rate of an incompressible fluid does not change across a valve. This is because, as you stated, the mass of the fluid is conserved. In other words, the same amount of fluid that enters the valve must also exit the valve.

But as you mentioned, the velocity of the fluid can change within the valve due to a change in the area. This is where the purpose of valves comes into play. Valves are used to control the flow of fluid by changing the area through which the fluid can flow.

For example, if you have a valve that is partially closed, the area through which the fluid can flow is reduced, causing the velocity of the fluid to increase. This increase in velocity can be used to control the flow rate of the fluid.

So, while the volumetric flow rate of an incompressible fluid may not change across a valve, the valve itself is used to control the flow rate by manipulating the velocity of the fluid.

I hope this explanation has helped to clear up any confusion you may have had about volumetric flow rate and valves. Keep asking questions and exploring the fascinating world of fluid mechanics!