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On/off Valve Cv

on/off valve cv

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#1 ezralh

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Posted 14 June 2015 - 11:51 AM

Hi all,

 

Does on/off valve has Cv? I was told that since this valve only able to fully open and fully close, there is no Cv. As the definition suggest, I still believe that the on/off valve has their own Cv. Appreciate if you can help me on this.



#2 fallah

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Posted 14 June 2015 - 12:07 PM

ezralh,

 

Yes, you're right; on/off valve has its own Cv...


Edited by fallah, 14 June 2015 - 12:08 PM.


#3 Zauberberg

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Posted 14 June 2015 - 12:43 PM

Anything with an opening/area that can pass a fluid, has a Cv or whichever is the equivalent term you want to use.  By definition, valve Cv is the volume (in US gallons) of water at 60°F that will flow per minute through a valve with a pressure drop of 1 psi across the valve. The same criteria can apply for any open area where fluid can flow through.

 

Cv depends on type, size, and manufacturer of the valve. See one example at http://www.cvltech.c...alves_Chart.php



#4 farid.k

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Posted 14 June 2015 - 11:48 PM

Does we need specify CV value for on-off valve specification during purchasing? I don’t think so. I think just enough we specify properties of fluid and the max flow rate isn’t? As the on off valve is just to on when need, off when no need. Not like control valve which is to regulate flow base on process demand. Appreciate to advice



#5 fallah

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Posted 15 June 2015 - 12:48 AM

farid.k,

 

Yes, during on/off valve purchasing you don't need to specify CV value and it's adequate to specify fluid properties and conditions (inlet pressure and temperature) along with the maximum flow passing through the valve...



#6 Zauberberg

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Posted 15 June 2015 - 12:58 AM

Cv is not normally a concern for on/off valves, but there might be applications for which you would have to consider specifying the required flow through an on/off valve:

 

1) Liquid dump valves. They are normally fitted in service where liquid level buildup is very slow (e.g. in certain KO Drum services). For these applications, you want to avoid having a modulating control valve that would continuously operate at very low % opening, thus promoting erosion of valve plug and seat. Employing on/off valve means having the valve fully open once the level inside a vessel reaches a preset value (HL), and having the valve fully closed once the level drops to a preset value (LL). You want to have a valve which will not dump the liquid inventory too fast or too slow, so some sort of Cv or flow specification is needed in this case.

 

2) Certain valves in depressurization service. When no restriction orifice is fitted, you would normally choose an appropriate valve that will give you the required flow for achieving depressurization targets (time, flow). You want to have a valve which will pass sufficient amount of vapor and yet not cause sonic velocities in discharge piping, which can further cause severe vibrations and damage. I have seen globe valves being employed in this service but not very common.






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