PID&Ratio Control

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Thread Starter

Ravindra

Dear Forum Members,

As we have water treatment plant, where we has to control the water flow to the buffer tank.
The water flow has to be controled by Flow control valve by using PID Controller and Flow indicating Transmitter.

According to the water flow rate we has to control four Chemical dosing Pumps with the set PPM, which will be set by the operatore either by potentimeter or digital Counter.
The four Chemical Dosing pumps were variable speed driven AC Drives , which can accept either 4 - 20 ma signal or 0-10 V signal.
Once the flow of the water changes, the chemical doing pump dosing has to be change to suit the operators set PPM.

i need to built the control system as said above.

I request the forum memers to share their experience and advises.

Thanks & Best Regards,
Ravindra
[email protected]
 
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Richard Wolf

The control requirements you describe are quite common. You need to use a multi loop controller that will ratio the dosing pumps based on water flow. Using on board linearization options will help you optimize the efficiency of the dosing requirement. Your chemists will have the titration data for each chemical plus the overall effect of simultaneous addition. It is a simple matter of adding the data to each pump controller and have the water flow act as the master.
 
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Bill Clemons

Per your description for Chlorinator, there are several basic methods for structuring the controller based on available input from the field. You mentioned you were measuring flow and maybe residual (PPM) content. The first method which relys on Flow Indication only, is Pacing method. The controller measures the water flow rate and adjusts the dosing rate based on a Proportion of the amount of flow to the buffer tank. The suggested control algorithm is Proporitonal Only with a variable lag time placed on the controller update based on water flow rate. The second method is Residual Control and is based on analysis of the buffer tank pH. This information can then be fed back into the controller to adjust the dosing rate. The residual control utilizes integral gain (or Reset) to adjust the magnitude of output to the dosing pump VFD. Since there is an inherent dead time at the analyzer, a fixed lag should be applied between control updates based on residual content. The third method is called Compound control. It is based on an arithmetic combination of both flow pacing and residual control. Usually, the controller is defined as a sum both of the pacing and residual contributions to control the dosing units. Again, both fixed and variable lag are important factors to keep the control responsive to both changes in plant flow and chlorine loading. The compound controller is a specialized case of the more general set of advanced algorithms which include feed-forward sum and feed-forward multiplication.
 
Dear Mr.Richard Wolf,
Thanks for your reply. The question related to the multiloop controler , is it user friendly in operators point of view. As you mention the Flow controller absolutely acts like a Master Controller.And chemical dsing pumps act like as mentioned Mr.Bill Clemons first method. No feed back measurement, just only Flow changes , corrspondigly Dosing pump speed has to change. Could you please Specify the some model numbers or makes of the Multi Loop Controller Whcich are very much user friendly format.
I need a controller in such way that loop 1 flow setpoint, loop 2 chlorine Dosing, loop 3 Alumn Dosing, Loop4 chem dosing, loop 5 membrane clean dosing.
I mean operator press Loop1 , he will change Water flow controller Setpoint, Press Loop 2 change PPM setpoint ETC.
Best Regards,
Ravindra
[email protected]
 
Dear Mr.Bill Clemons,
Thanks for your reply.And also thanks for your Control Philosphy in different methodologies. Here the problem is Factory is Eqipped with ABB Bailey INFI 90 DCS System. But water treatment plant is located one KM away from Site. You Can Advise us Hard wire in this Quite bit easy. The draw Back for hard wiring its the Seasonal Factory, We will Operate the Plant for Six Months only. But we has to operate the Water Treatment plant throughout the year. So i need a stand alone Syetem which are purely related to the your first method. Could you please advise me further more on this Issue. What is the right Controoler make and model number to suit the first method and which are user very much user friendly.

Best Regards,
Ravindra
[email protected]
 
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Bill Clemons

Per more recent responses about your control application, concur on the use of master controller to maintain multiple set points by which gain and ratio settings can be made to provide metered rates of each dopant.

For remotely located operations, you may want to consider one or several 1/4 DIN controller systems. These units are stand alone and can be combined to meet most application requirements. Have a look at http://www.miinet.com and follow links to 535 (Single Loop Controller), 545 (Dual Loop Controller), 555 (Chlorinator). All feature a flexible platform, in terms of hardware and firmware options. These units also feature a Modbus/RTU communications port for remote monitoring and control.
 
Dear Sir,

Thanks for your suggestion. Please give us your E mail ID so that we will be in touch with you.

Regards,
Ravindra Mullapudi
 
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