Understanding the Flow Control Principle of Valves

[fonz]

How exactly would you describe the principle of a valve for flow control?

In a globe valve for example the fluid flows through valve seat and generally leaves the valve with the outlet diameter being the same as the inlet diameter.

My original assumptions were that Bernoulli's principle had to be somehow related but if you ignore friction losses etc. then surely the energy is conserved and the dynamic pressure at the outlet would equal the inlet dynamic pressure due to the diameters being the same. In this case there would be no change in velocity or static pressure.

Energy must be lost somehow for the volumetric flow to change but how?

 

[fonz]

Some more thoughts...

An decrease in flow area would cause a static pressure drop which would, again assuming no friction losses, have the effect of increasing flow velocity. This I can understand would happen through the inlet and seat of the valve however fundamentally as the fluid leaves the valve the opposite should occur and the velocity should return to its original value.

By this logic there should be no net static or dynamic pressure change across the valve.

 

[Jano L.]

In reality the valve restricts the cross-section of the flow, thus increasing the power of friction resisting the flow and also supporting the development of turbulent flow. As a result of these processes, for a given pressure drop the mean velocity of water gets lower.

 

[fonz]

In response to Jano L.

So what you are saying is that energy is lost through friction as heat? I guess this would make modelling the flow through a valve fairly difficult.


I also refer to the image below, theoretically assuming no losses in the pipe to friction and incompressibility etc. if we could somehow control the area of A₂ this would have no effect on the net volumetric flow through this section? Therefore the concept of a valve solely relies on the increase in friction?

 

[Jano L.]

Friction and also turbulence. The valve may increase the whirling motion of the water which also leads to decrease of pressure, and the friction then leads to loss of even this whirling motion into heat. The picture you posted is very simplified picture of what happens to water. In practice the flow is usually not so simple.

 

[gmax137]

... I guess this would make modelling the flow through a valve fairly difficult.

yes. In practice it is done empirically. The result is a curve provided by the valve vendor, Cv vs. position, that allows you to include a head loss term on the RHS of your Bernoulli equation.