# Homework Help: Calculating percentage change in flow rate through a valve

1. Jul 29, 2015

### 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.

Last edited by a moderator: Jul 29, 2015
2. Jul 29, 2015

### davenn

is this homework ?
looks like it

Dave

3. Jul 29, 2015

### 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?

4. Jul 29, 2015

### Dr. Courtney

5. Jul 29, 2015

### 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

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

6. Jul 30, 2015

### 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...

7. Jul 30, 2015

### 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...

we expect you to show some effort.