Cv vs Position in a Gate Valve

[aatara]

Hello to everybody

Do you know a Table or Figure where could I calculate the Cv of a standard gate valve vs open or degrees of opening?

My valve is a 10" 150 -lb wedge-shaped Carbon Steel and I don´t have documentation of the valve (20 years old)

Regards,

 

[pennpiper]

Gate valves are made for use as full open or full closed. They are for shut-off. They are not designed or recommended for throttling or control.

Having made this point, I would now like to know and understand why you would want to (or need to) use a gate valve for throttling.

 

[JLSeagull]

Consider the flow characteristics as quick opening. I would expect to see over 80% of the valve capacity within the first 20% of travel from closed toward open.

 

[proinwv2]

Regardless of why you want to do this, and whether you should, basically there are three methods.

1. The valve is lab tested at various openings and the Cv calculated from the test results. The best method. Call the manufacturer.

2. Carefully measure the shape and area of the orifice opening at various positions and calculate Cv which requires knowing the loss coeficient for each geometric shape and it's position in the flow passage (which you probably will not know). This method will give you an idea but will contain errors.

3. Find test data on a similar valve, and establish the percent of flow at various stroke positions, and apply these percentages to your valve. This will be of unknown accuracy.

Good luck.

Paul

http://www.ostand.com

 

[dcasto]

I'd use the open area ratio to calculate the Cv. Its the same formula for finding the volume in a round horizontal tank. At 1/2 open the valve would would have 1/2 Cv. Its a sine function as it goes away from 1/2 open.

 

[proinwv2]

dcasto,

What you say is correct, except that the orifice coefficient is different for different shapes, and in general you will not find these in a book.

There may be software that will perform this calculation, but I am not familiar with it.

Also, I suspect that there are internal entrance and exit losses approaching the orifice due to its location within the body. As the gate moves upward, the location (i.e. centerline) of the orifice also moves upward and the turn required by the flow lessens.

Paul

http://www.ostand.com

 

[JimCasey]

Gate valves are very loosely guided. You may be able to calculate a Cv for an intermediate position, but in reality the disc will be dancing about and the Cv will be dithering back and forth, without commanded movement of the disc.

If, in fact, you intend to throttle continuously with a gate valve, you will find that any guiding surfaces provided will be fretted away and the disc will be found mysteriously, in the middle of the night, downstream, lodged in something that will be expensive to repair.

I will respectfully submit that the reason that the Cv curve is not published for a Gate Valve is because Gate Valve manufacturers know not to use the gate valve for modulating, and don't want to give anybody any information that suggests such a use.

 

[stgrme]

I agree with others above that gate valves are not intended for throttling service. However, I have found information on the resistance of partially open gate valves. The information was published in Section 9 of the "Handbook of Hydraulic Resistance - Coefficients of Local Resistance and of Friction" by I. E. Idel'chik in 1966. This documnent was distrbuted the National Technical Information Service of the U.S. Department of Commerce.

I suggest that you search the web and/or the web site for the U.S. Department of Commerce for this document.

Best of luck!

 

[aatara]

Thanks friends,

I appreciate your helps. I found the Figure "flow resistance vs open" in the Handbook of Hydraulic resistance by Idelchik and I´ve estimated the CV vs %open.

I know that a partially open gate valve is not a good practice, but temporary I have to use it as a throttling valve.

 

[psafety]

Another source for this information on most all valve types % open vs K value is: Internal Flow Systems, by Donald S. Miller.

 

[JimCasey]

>>I found the Figure "flow resistance vs open" in the Handbook of Hydraulic resistance by Idelchik and I´ve estimated the CV vs %open.<<

If you have the curve, then ratio the rated Cv to 100% and the intermediate Cvs to whatever percent they are of full open ....it becomes a simple ratio problem

>>I know that a partially open gate valve is not a good practice, but temporary I have to use it as a throttling valve.<<
Your username is appeded with "Nuclear". "Not Good Practice" can be tolerated if you're irrigating a Golf course, but "not good practice" in a Nuclear plant can make your name appear in the media in a most unfavorable light. And then there's that nagging problem of public safety...Be very careful, and very prudent.

Even if the valve does not malfunction, having an audit catch poor practice (especially now that you are documented as knowing better) can cause unfavorable alteration of your career path.

 

[bheart790]

hello,

anybody who can tell me what's the "NPS" meaning?

alex

 

[lowedogg]

NPS = Nominal Pipe Size