12 replies to this topic

[krasygan]

i wank to know how to adjust the PID values in DCS ( the value range and method to calculate the pid values) for pressure, temperature and flow control valves for steady operation

 

thank u

[thorium90]

First of all, are you even authorized to make such modifications? There are many different vendors of DCS systems. All of them are different in one way or another. The fact that you even query stuff like the value range and method to calculate PID values imply you have no knowledge and experience on DCS systems and process control. You should ask your plant engineer or manager if you want to learn some basics on DCS and process control.

Edited by thorium90, 16 March 2013 - 09:04 AM.

[krasygan]

first of all thank u for ur reply, i am basically chemical engineer and i am working as dcs operator, these modification will be done by instrument peoples, i asked them on what basis they change the PID's values, but i havent getting clear answer,i have have ascess of engineering users in DCS. if u can explain plz info

 

thank u

[thorium90]

The controller settings are decided during the initial design of the plant. During commissioning, the values may be further tweaked if necessary. These settings may be from the control engineer's experience on similar plants. The value range and method is really a book size topic. Perhaps you can read books like Process Dynamics and Control by Seborg.

I am curious, so you are a graduate from some university who works as engineer in a plant and also works as an panel operator but have no basic knowledge of process dynamics and process control systems?

[krasygan]

thank u for ur reference, and one thing nobody can work in dcs without basic knowledge,

[Modesty Blaise]

Krasygan,

 

You may find fundamentals of control systems and tuning of PID controllers in these two books:

 

Practical Process Control for Engineers and Technicians by Wolfgang Altmann

 

Good Tuning: A Pocket Guide by Gregory K. McMillan

 

 

Thorium90: first of all I think there is no need for a such posts on this forum like yours #2 and #4 above mentioned. This forum is to enlighten and encourage people not to discourage them.  Please, it is useless to put posts like #2 and #4, because we all very well know that ‘there are many different vendors for DSC system’ , ‘the value range and method is really a book size topic’ and nobody wants to read post about advice to ‘ask your plant engineer or manager’.

 

Regards,

Edited by Modesty Blaise, 17 March 2013 - 12:02 PM.

[thorium90]

thank u for ur reference, and one thing nobody can work in dcs without basic knowledge,

 

 

 KINGMOON,

However the question that remains unanswered is:

Unit operations people informed that the above is not a rare phenomena. Though rarely, but there are instances when pump was started by mistake even when the line is blinded at the other end (by mistake). However, the maximum that happens is vibration and noise and subsequently corrective action is taken by operations.  Nothing like the above (spray of liquid thru' the ruptured opening up to 100 meters) occurred in past due to above.

Can anyone throw some light about the possible causes of the incident?

 because when the line was blinded the pump should be isolated and depressurised from the suction and discharge side up to the blind . 

This incident reminds me about  an editorial i saw  few days ago on HP news letter warning about skilled labor shortage which is jeopardizing the industry safety standards and operation

 

Well, I've been to large, complicated process plants manned by none other than vegetable farmers, school teachers and car mechanics.

In this day and age where cost cutting is a norm, i'm not surprised that anybody can operate a DCS. FYI, the school teachers i've seen operating the DCS panel doesnt even recognize the language it is in. He brings a PDA to translate the words so he knows what to press...

[Steve Hall]

krasygan,

 

Consider this link that takes you to several spreadsheets where you can simulate the tuning parameters and see the results:

http://www.engineers...Description.htm

[krasygan]

THANK U SIR

krasygan,

 

Consider this link that takes you to several spreadsheets where you can simulate the tuning parameters and see the results:

http://www.engineers...Description.htm

[cea]

Dear krasygan-

 

As you said you are DCS panel operator, I will not take you into much of Design aspect & try to help you to meet your day to day requirement.

 

Firstly, please get understanding of PID.

 

P is proportional controller. When this value is 100 or 1, controller will start taking action, only when PV (process value) reaches to SV (set value). By making P as 80 or 0.8, controller will start "reacting" moment PV reaches to 80% of SV. Simillarly if P is 130%, controller will start taking action only when PV exeeds SV by 30%. So, depending on process requirement, you can "awake" controller before or later.

 

Now, coming to I (Integral) controller. Once, controller notes the deviation, I value decides how fast it should act. Lower the value of I, faster is response. Thus low I value allows wide response of controlling element (control valve) to get PV value close to SV as fats as possible, during adjustment. However, there is flaw with low I value. That is the controller will start reacting eccentrically with very low value of I. This is very much true for fast reacting parameters, such as pressure & level.

 

In order to overcome above hurdle, D (Derivative controller) value is used, which is generally set to zero. D controller smoothens the effect of wide response from I controller, so that MV (Manipulated value, such as control valve opening) will not be so wide that controller will not be able to control at any point of time. D controller is generally not used to control Temparature parameter.

 

Hope, above explaination clarifies & helps you as intended.

[krasygan]

thank u sir for ur reply, with this i can get some basic ideas and how to adjust PID values, once again thank u for ur posting

[thorium90]

cea, you have neglected to mention that there are controllers whose I is in terms of time and some are in terms of 1/time. Dont go the wrong way. For those in terms of time, increasing the I values increases the amount of I action.

 

Whether or not to adjust and how to adjust the PID settings really depends on alot of factors and certain considerations for example, noise, hysteresis, deadband, split range, cascade type controllers etc. For example, D action is not used on certain types of cascade controllers.

 

I would definitely suggest you consult with your engineers first. Or, you can test out on spreadsheets like the one by steve or in Matlab.

[engg]

all those are guidelines that can be used only if oyu have experience or initial start. setting right PID is an art that needs to be learnt. consider taking a PID tuning training course online from processcntrl.com (mail@processcntrl.com) and you will also get a PID tuning simulator that simuales a DCS and you adjust PID for various controllers to learn and master it.