2 replies to this topic

[streamt]

Hi,

 

We are installing a 24" branch line on our 72" sea water header. To restrict flow in the new line we need to install a restriction orifice.

 

For flow through restriction orifices, what is the recommended maximum velocity at the orifice throat and why ?

 

Stream T

[Zauberberg]

I didn't hear of velocity being a constraint in orifice sizing. They are designed for whatever the required flow might be, and the resulting fluid velocity will be determined by the specified flow and available pressure differential. That is all.

 

Noise and vibration are important issues. Depending on the total flowrate and pressure drop across the orifice, you might end up with significant noise and vibrations, which further may require from you to consider sequential pressure letdown (i.e. via multiple orifices in series).

 

Depending on the erosion potential of a fluid, some other elements of orifice design should also be carefully evaluated. For streams carrying significant amount of suspended solids, you should try to avoid using square-edge orifices because of tendency of the fluid to erode edges of the orifice over time. This also depends on what levels of accuracy you need in this particular application.

 

Remember that orifices are sized for a single set of process conditions. Anytime there is a change in upstream or downstream pressure, the orifice will pass a different amount of fluid. You need to keep this in mind for future operational requirements. Perhaps it is worthwhile considering a spool piece of reduced size equipped with a butterfly valve, to allow for low modulation for any set of operating conditions in future. Of course, this applies only if you require control of the branch flow - I do not know the details of your application.

[fallah]

Hi,

 

We are installing a 24" branch line on our 72" sea water header. To restrict flow in the new line we need to install a restriction orifice.

 

For flow through restriction orifices, what is the recommended maximum velocity at the orifice throat and why ?

 

Stream T

 

Stream T,

 

The fluid velocity at the orifice throat and subsequently at vena contracta should be restricted to a value such that the pressure at vena contracta wouldn't become equal or higher than the fluid vaporization pressure at flowing system temperature; because otherwise at downstream of the vena contracta in the pressure recovery zone the pressure would rise above vaporization pressure and there would be the risk of experiencing cavitation at downstream...this cavitation risk will act as liquid flow limitation through a restriction orifice and fluid velocity at relevant throat as well...
 

Edited by fallah, 03 May 2015 - 10:11 PM.