Calculating percentage change in flow rate through a valve

[gillmitch92]

< Mentor Note -- thread moved to HH from the technical forums, so no HH Template is shown >

The max flow rate through a control valve with an equal percentage characteristic is 10 m3/h. If the valve has a rangeability ratio of 50:1 and is subjected to a constant differential pressure, calculate the % change in flow rate through the valve for valve lifts at 10% intervals.

It asks to show that the vavle is indeed equal % by finding the change in flowrate from one interval to the next.

I am just slightly stuck on which equation to use for this.

 

[davenn]

is this homework ?
looks like it Dave

 

[gillmitch92]

It's similar to a question I have for homework but not the same. I am just needing a bit of help with equations and which one to use as I am stuck?

 

[Dr. Courtney]

http://www.forberg.com/pdf/techSup/Home_Tech%20Support_Valve%20Flow_calc_and_sizing.pdf

 

[jim hardy]

That link appears to size valves for on-off control.

Equal Percentage infers you will use this valve to modulate flow, not just two-state full open or shut

http://www.engineeringtoolbox.com/control-valves-flow-characteristics-d_485.html

Inherent Control Valve Flow Characteristics
The most common characteristics are shown in the figure above. The percent of flow through the valve is plotted against valve stem position. The curves shown are typical of those available from valve manufacturers. These curves are based on constant pressure drop across the valve and are called inherent flow characteristics.

• Linear - flow capacity increases linearly with valve travel.
• Equal percentage - flow capacity increases exponentially with valve trim travel. Equal increments of valve travel produce equal percentage changes in the existing Cv.
• A modified parabolic characteristic is approximately midway between linear and equal-percentage characteristics. It provides fine throttling at low flow capacity and approximately linear characteristics at higher flow capacity.
• Quick opening provides large changes in flow for very small changes in lift. It usually has too high a valve gain for use in modulating control. So it is limited to on-off service, such as sequential operation in either batch or semi-continuous processes.
• Hyperbolic
• Square Root

The majority of control applications are valves with linear, equal-percentage, or modified-flow characteristics.

and is subjected to a constant differential pressure,

^ suggests the remainder of the piping system is ignored.
Sounds like homework to me. How does your reference define 'equal percentage" ?

see also http://www.documentation.emersonprocess.com/groups/public/documents/articles_articlesreprints/headleyrev_spr03_valvemag.pdf

 

[gillmitch92]

It just states that the differential pressure is constant. I'd assume this would mean that with each difference there will be exactly 10% between them. Finding the change in flow rate from one interval to the next. Not sure if that helps at all...

 

[jim hardy]

?
have you yet defined "equal percentage" ?

A quick google search took me to a page that has this example problem worked out in excruciating detail...

Example 6.5.1.
The maximum flowrate through a control valve with an equal percentage characteristic is 10 m/h. If the valve has a turndown of 50:1, and is subjected to a constant differential pressure, by using Equation 6.5.1 what quantity will pass through the valve with lifts of 40%, 50%, and 60% respectively?

we expect you to show some effort.