Pressure and flow rate control valve, ....

[BORPE]

Dear all,

I'm wondering how a control valve can control pressure and flow rate. I understand that a control valve in a pipe induces a certain pressure drop but I don't understand how this affects flow rate. I'm disturbed with the continuity laws which states that the total flow rate between two points in a systems is the same according to the relation: Av = constant. So for example, in a control valve the area drops, then velocity has to increase in order to fulfill this. This means also according to the Bernoulli theorem that the pressure drops. Furthermore, the pressure energy downstream can't be fully recovered because they're certain energy losses (friction causes some mechanical energy to become thermal energy and so forth). But my question now is how a control valve then controls flow rate? Does the flow rate remains the same? or does it changes because there is a certain pressure drop so that the driving force (pressure difference) for flow downstream is less and thus there is less flow? How works this for a restriction orifice?

 

[russ_watters]

I'm disturbed with the continuity laws which states that the total flow rate between two points in a systems is the same according to the relation: Av = constant. So for example, in a control valve the area drops, then velocity has to increase in order to fulfill this.

That is all true, but when you close a control valve you change your system and the previous continuity relation no longer applies.