Calculating percentage change in flow rate through a valve

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Discussion Overview

The discussion revolves around calculating the percentage change in flow rate through a control valve characterized by equal percentage flow behavior. Participants explore the equations and concepts necessary to determine flow rates at specified valve lifts, particularly in the context of homework-related queries.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents a problem involving a control valve with a maximum flow rate of 10 m3/h and a rangeability ratio of 50:1, seeking to calculate the percentage change in flow rate at 10% lift intervals.
  • Another participant questions whether the original post is a homework assignment, suggesting that it appears to be so.
  • A different participant notes a similarity to their own homework question, indicating they require assistance with equations.
  • Several links are shared that provide resources on valve sizing and flow characteristics, with one participant emphasizing that equal percentage characteristics imply modulation rather than simple on-off control.
  • Discussion includes the inherent flow characteristics of control valves, detailing how flow capacity can vary with valve travel and the implications of constant differential pressure in the system.
  • One participant expresses uncertainty about the definition of "equal percentage" and suggests that the flow rate changes should reflect a consistent percentage change across intervals.
  • Another participant references an example problem that outlines expected calculations for specific valve lifts, indicating a need for participants to demonstrate effort in their responses.

Areas of Agreement / Disagreement

Participants generally agree that the problem involves homework and that understanding the concept of equal percentage flow characteristics is crucial. However, there is no consensus on the specific equations to use or the interpretation of the problem details, leading to multiple competing views on how to approach the calculations.

Contextual Notes

Some participants note the importance of defining terms like "equal percentage" and the implications of constant differential pressure, suggesting that these definitions may influence the calculations. There is also an acknowledgment that the problem may require further clarification on assumptions and definitions.

gillmitch92
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< Mentor Note -- thread moved to HH from the technical forums, so no HH Template is shown >[/color]

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.
 
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is this homework ?
looks like it Dave
 
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?
 
http://www.forberg.com/pdf/techSup/Home_Tech%20Support_Valve%20Flow_calc_and_sizing.pdf
 
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
upload_2015-7-29_15-51-10.png


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.

gillmitch92 said:
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
 
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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...
 
?
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.
 

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