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Anonymous Poster #1

Process Value PV Not Reach the Setpoint Required. How to Apply PID Tuning

02/10/2019 9:01 PM

My control valve required to open depend on setpoint value. While my controller is in auto mode. Process value is not reach the desired setpoint value applied.

ex: setpoint applied; 20,000 bbl/d

process value; 18,000 bbl/d

valve ouput is 15% open

While in manual mode valve is functioning normal.

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#1

Re: Process Value PV Not Reach the Setpoint Required. How to Apply PID Tuning

02/10/2019 10:25 PM

Has some sort of maximum opening been programmed into the parameters? (Working normally in manual mode suggests no mechanical blockage.)

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#2

Re: Process Value PV Not Reach the Setpoint Required. How to Apply PID Tuning

02/11/2019 12:44 AM
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#3

Re: Process Value PV Not Reach the Setpoint Required. How to Apply PID Tuning

02/11/2019 2:18 AM

Most controllers have an automatic tuning function built-in these days. Invoke it, and report back on what happens, please.

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#4

Re: Process Value PV Not Reach the Setpoint Required. How to Apply PID Tuning

02/11/2019 7:09 AM

What type of "automatic control "is this?

Is is controlled entirely pneumatic,or does it have elecro-pneumatic output,such as an I/P module ?

If using an I/P transducer,check the calibration of the I/P.

Is the valve pneumatically operated,or electrically operated.

The scaling on the controller may be offset.Calibrate the controller using a signal verifiable source.

Rule of thumb for tuning:

Flows are very fast,they need low gain(P),high integral (I),and low derivative (D).

There are several methods to get these variable in the ball park so that you can fine tune them.

I will expound on these at your request.

Good luck.

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#6
In reply to #4

Re: Process Value PV Not Reach the Setpoint Required. How to Apply PID Tuning

02/12/2019 5:05 AM

A method I used before auto-tuning was developed was called sustained oscillation.

Upset the process temporarily,then back to setpoint,and watch the response of the process.

Increase gain (low P) until you get a sustained oscillation when you upset the process.(P is the inverse of Gain).

Record the time interval of one cycle of this oscillation(in repeats-per-minute).

Your reset value will be the reciprocal of this value.

The Derivative will be 1/8 of the reset value.

These are good starting values,and you may fine tune from these values.

However,I do not think tuning is your problem here.

I would re calibrate the entire loop,beginning with the valve,and work my way back to the controller.

20% and 80% are good values for testing,but use 0% and 100% for calibration using a verifiable calibration source.

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#5

Re: Process Value PV Not Reach the Setpoint Required. How to Apply PID Tuning

02/11/2019 10:21 PM

It looks like your Integral or Reset term in your controller is set at such a low value that your process noise drowns out the action that will bump up your output to hit your setpoint..

Does the 15% valve output drop off when you switch from Manual to Auto?

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#11
In reply to #5

Re: Process Value PV Not Reach the Setpoint Required. How to Apply PID Tuning

02/14/2019 8:58 AM

Yes sir. It drop off

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#7

Re: Process Value PV Not Reach the Setpoint Required. How to Apply PID Tuning

02/12/2019 5:09 AM

Two things come to mind, though they are fairly general:

  1. There may be some sort of 'clip' on the control function that is stopping the valve actuator going beyond 15%. If the actuator has end of travel micro-switches, these could be used instead albeit at the expense of more cabling; and/or
  2. The three term controller is over-damped. If auto-tuning is available and works, great; I like the idea of a three term controller having the utility of a flat screen TV! Otherwise try using less integral and more differential control
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#8

Re: Process Value PV Not Reach the Setpoint Required. How to Apply PID Tuning

02/12/2019 8:36 AM

Must have a technical error on feed back.

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#9

Re: Process Value PV Not Reach the Setpoint Required. How to Apply PID Tuning

02/12/2019 12:15 PM

The first thing to check is if there is any integral action programmed in the PID controller. If there is only P (proportional) programmed, then you will get an offset of output from setpoint like you are getting now. If this is the case, correct this by programming some I (integral), which will eliminate the offset ... probably.

That said: the above may not be the answer to your problem. We need to know details about the process you are controlling: from where does flow come from? to where? does anything apart from the valve make it vary? ... things like that.

A diagram would be helpful.

Good luck!

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#10

Re: Process Value PV Not Reach the Setpoint Required. How to Apply PID Tuning

02/13/2019 4:00 AM

Proportional is inverse of gain.

There will ALWAYS be an offset with P only,if the load is not constant from initial starting point.

Integral (Reset) is the factor that repeats the initial output from the controller at a rate(in minutes) according to the offset value,and drives the output back to setpoint.

Derivative(D) is a function of time and reduces the time required to get process back on set point.It responds to how fast the process is changing

When properly tuned,the three factors can handle an upset in the process with minimum overshoot,and minimum deviation.

Therefor I don't think the problem is in your loop tuning,(unless as previously stated is P only).

More info is needed to give a better picture of what is happening.

Do you have a source capable of providing the desired flow?

Can desired set point be achieved manually?

If so,what in position(percent) is the valve opening?

Type of controller.Pneumatic,electronic with pneumatic output?

Foxboro, Fischer,Moore, Omega,Sybron,Leeds/Northrup,Combustion Engineering,etc.?

Type of valve; electronic valve operation,pneumatic?

Any ratio relays,signal multipliers,I/P (current to Pressure transducers),bias relays,etc.?

How is manual testing performed on the valve;is the manual mode selected from the controller to test the valve,or is an external source connected to the valve?

If you need help,you need to provide as many details as possible.

I can make a service call but it will cost you dearly.

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