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# Valve Characteristics if Controller Signal is 50%, and Valve Opening is 75%

03/16/2012 9:48 AM

If controller signal is 50%, and valve opening is 75%, what will be valve characteristics? and how we create signal and flow chart.

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#1

### Re: If controller signal is 50%, and valve opening is 75%, valve characteristics is

03/16/2012 10:48 AM

Well you always could do things the old school method of setting the control signal to eleven different evenly spaced positions of 0%-100% and measure the valve opening position and the flow rate for a certain pressure. This will give you directly a control signal to flow chart. You could really impress people by plotting this data on some graph paper and make a guess of the type of curve you've found.

Now go do your homework or you won't get dessert.

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#2

### Re: If controller signal is 50%, and valve opening is 75%, valve characteristics is

03/16/2012 11:22 AM

its not fuuny that'z actully exist q

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#3

### Re: If controller signal is 50%, and valve opening is 75%, valve characteristics is

03/16/2012 11:50 AM

Please re-state this sentence. "its not fuuny that'z actully exist q"

It makes no sense at all. Are you admonishing redfred for giving you a correct answer?

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#4

### Re: If controller signal is 50%, and valve opening is 75%, valve characteristics is

03/16/2012 12:01 PM

When it behaves like that, I believe you have to do calibration/adjustment to synchronize both valve actuator and controller.

Say Signal(50%)= span (50%) = Area open (___?%)

Flow is Q=Area x flow velocity

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#5

### Re: If controller signal is 50%, and valve opening is 75%, valve characteristics is

03/16/2012 12:32 PM

see charts:

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#6

### Re: If controller signal is 50%, and valve opening is 75%, valve characteristics is

03/16/2012 12:46 PM

if condition changed like this

If Equal percentage valve is used and valve opening is 62% what is the controller O/P

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#9

### Re: If controller signal is 50%, and valve opening is 75%, valve characteristics is

03/17/2012 12:29 PM

Is it in open-loop mode (manual) or is it in closed loop (automatic)? If it is in closed loop, it doesn't matter, because it is trying to maintain the process variable - which you haven't specified - at its setpoint.

Just what exactly are you tring to achieve here, Guvnor?

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#10

### Re: If controller signal is 50%, and valve opening is 75%, valve characteristics is

03/17/2012 1:21 PM

A valve's control loop has 3 elements, any of which can be linear or characterized.

The three elements are
- the controller's output
- valve positioner (abbreviated I/P in Bud's table above)
- valve trim

The characterization function (quick opening, or equal percentage or whatever) can be implemented mechanically, like a cam in a positioner; or electronically as functions or look-up tables.

Remember, each element can be linear or characterized.

The reason this is important is that for the posed question, "If Equal percentage valve is used and valve opening is 62% what is the controller output (O/P)?", the output might be linear, and the valve positioner might be quick opening characterized and the valve trim equal percentage characterized.

It's mix and match time, folks. Believe me, mix and match happens, especially in this day and age of smart positioners and PLC/PAC/DCS control logic that might be inaccessible to the instrument guy, which can easily "hide" a characterized controller output function.

Controller output is usually assumed to be linear, but a steel rolling mill was amazed to find its combustion air control output was characterized when they assumed it was linear (discovered by plotting flow against output and looking at the curve).

Valve trim is coded in the 19 digit valve part number, if it wasn't changed out to something else during a rebuild (maybe unknowingly).

Characterization of an electromechanical positioner is a mechanical cam.

Characterization of a smart positioner is a setup parameter.

Gotta know 'em all to answer the question.

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#7

### Re: Valve Characteristics if Controller Signal is 50%, and Valve Opening is 75%

03/17/2012 5:52 AM

In general valve are designed for a continuous control area versus input signal. The curve in relative coordinates is a power function.

For a linear valve the power is 1 if the valve has to open relatively more at start the power is <1 and if the valve has to open less at start the power is >1.

In your case the power is log(0.75)/log(0.5) ≈ 0.41 so that you can compute the other values as Xe= Xi^0.41 where Xi is the input control signal and Xe is the relative valve opening.

In the table which was given as a solution the first part is linear and then progressive this would be for the control very disturbing due to an abrupt change of the area rate.

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#8

### Re: Valve Characteristics if Controller Signal is 50%, and Valve Opening is 75%

03/17/2012 7:03 AM

I forgot to write that usually the exponent 0.5 is used so that it could be possible you made a slight error in the valve opening estimation it could have been 0.71 instead of 0.75. Please do check again.

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