Globe control valve

[Saad85]

Hi,

If we throttle the globe control valve to be Open 40% what happened in the downstream and upstream of the below:

1/ firiction head loss ( increase or decrease)

2/ velocity head loss ( increase or decrease)

 

[Sheshtawy]

Assuming that the valve was originally fully open, the friction head loss will increase as the the loss coefficient (K) of the valve increases with throttling (h = K *v^2/2*g). The flow rate downstream will decrease causing the velocity to decrease assuming constant cross-sectional area (Q=A*v)

 

[Saad85]

So the friction head loss will increase in downstream and decrease in upstream

and velocity decrease in downstream and increase in upstream 

 

[MikeGlass1969]

friction head and Velocity head both decreases upstream and downstream.  You changed the Cv of the valve and the pressure drop across the valve.

 

[Saad85]

friction head and Velocity head both decreases upstream and downstream.  You changed the Cv of the valve and the pressure drop across the valve.

Are you sure of this statement? I got confused 

 

[MikeGlass1969]

Absolutely certain.  You are decreasing the volumetric flow upstream and downstream of the valve by closing the valve.  Volumetric flow is directly proportional to the velocity.  Velocity head and friction head are both directly proportional to velocity.   

Examine the Bernoulli and Darcy equations.

 

[Saad85]

Thanks 

 

[Sheshtawy]

I can confirm that MikeGlass is correct. I apologize for the confusion!

 

[Saad85]

Thanks all for your feedback 

 

[Saad85]

So if we open the control valve the velocity and friction head increases upstream and downstream 

 

[Sheshtawy]

You betcha!

 

[Slay the P.E.]

We have uploaded a short write-up that further illustrates the issue.

View attachment EngineerBoards Question 001.pdf