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# Orifice equation for water

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## Orifice equation for water

(OP)
Hello

First time posting.  I have the following question:

How would you calculate flow through an orifice for water?

The situation:  I'm trying to control the fill velocity to 1 ft/s for filling of new water mains.  The fill velocity is controled by operation of a gate valve.  Gate valves vary in size from 4" to 12". Any valves greater than 12" are butterfly valves.  The number of turns on the gate valve will control the amount of open area for water to flow.  In addition the calculation will need to be able to include varing pressure for different parts of our system.

Any help would be appreciated.

Thanks

Tony
Replies continue below

### RE: Orifice equation for water

Hi South town

First thing. I am not sure that the best way to control the velocity is to use a gate valve. I thing that it could be better if you use a seat valve.
If you want to control the velocity with an orifice you have to calculate the necesary head losses. After that you can calculate the pressure drop of your oriffice with the flow requirements to have a 1 ft/s velocity.

The flow and the pressure drop, with the characteristics of the water, sets the inside diameter of the orifice. You could use the next equation:

D=(Q/(1.2636*0.6*(P/1000)^0.5))^0.5  note: only for fresh water!!!

Where D is the inside diameter in mm, Q is the flow in m3/h and P is the pressure drop in bar.

I hope that it will be useful...

bye

Luis

### RE: Orifice equation for water

to further explain the situation.  The gate valve is used to fill a newly installed water main because that is the type of valve we us in our system.

Once the gate valve has been opened, than the flow will go from the pressure side of the valve to the filling side which has no pressure.

This application is for potable water in our distribution system.

thanks

### RE: Orifice equation for water

SouthTown

You really have the wrong type valve for the application.  Gate valves just aren't meant for control.  An orifice will limit the maximum amount of flow, but again won't control the rate of flow.   Rather than trying to install an orifice, why don't you change to a ball or globe valve?  It would give you a lot more "bang for the buck"  Just because you normally use gate valves doesn't mean you have to continue using them.

Patricia Lougheed

### RE: Orifice equation for water

Gate valves can be used for low head throttling though they aren't the first choice.  Still, if that is what is there, you aren't going to change them out.

I'd ask the vendor for some sort of Cv versus % opening for your valves.  Then, based on the % open (eg. so many turns), you know the Cv of the valve and can calculate the flow using  Q = Cv (dP/sp. gr.)^0.5.  For water, the specific gravity is essentially 1 so that can be ignored.  Estimate the dP across the valve and the valve's Cv and you have the flow.

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