Throttling valve fluid flow

[ej25]

Hey all,

Just a bit confused on the changes in fluid characteristics when using a valve for throttling.

In the case where we have an incompressible fluid flowing through piping at x gpm and y psi, as it crosses a valve which is throttled, the cross section is basically decreased. Relating that decrease to flow, we can use the continuity equation:

Q=vA

so that..

v1A1 = v2A2
where point 1 is in the piping before the valve, and point 2 is the valve which is throttled.

But sometime z distance away from the valve, couldn't we use the continuity equation to say that v1A1 = v2A2 = v3A3 where A3 = A1 and so v3 = v1? Basically what I am getting at is, common sense tells us that a valve throttled maybe 50$ decreases both pressure and flow, however, shouldn't the flow downstream be the same?

 

[ej25]

Meant to write 50% above.

I realize that there is going to be minimal losses such as friction and non-recoverable pressure losses, but the flow after the valve should generally be close to what it originally is, correct?

 

[LittleInch]

Flow will be exzctly equal. Mass cannot be destroyed or created.

I don't understand your question.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[gerhardl]

To expand on LittleInch's answer:

What you put (measures) into the system is what you get out of the system, once the system is filled and stabilized. Vary input or regulate output and the system will change to stabilize and adapt to the new conditions. In a stbilized condition the mass flow (amount of mass per time unit passing a given area across the pipeline) will be equal for any cross section.