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This technical article describes six basic types ofpH control systems. it will help you identify thecorrect application and choice of pHinstrumentation.

Batch Processing (System A)This type of system uses an ON/OFF relaycontroller for batch processing. The systemoperates as follows:1. Process solution is pumped into a tank until it

is full.2. Agitation or mixing commences and chemical

is added until the desired pH is reached. Therelay controller turns on/off the chemicaladdition pump (or solenoid valve).

3. Process then flows out or is pumped out oftank.

In this system, level sensing of some type is verydesirable to signal when the tank is full andempty, and to lock out the mixer and pH controlwhen the solution is not at the proper level. ifmixing is poor, a repeat-cycle timer isrecommended. The OFF time of the cycle willgive the system some time to mix and reduceany eventual overshoot. When sizing the final control element, note thatthere will be some delay between addingchemical and sensing the resulting pH change. Ifthe final control element is oversized, the systemwill have unacceptable overshoot. The fasterthe mixing in the system, the less the delayand/or overshoot will be. This simple system has one disadvantage: itdoes not easily handle a continuous flowingprocess.Continuous with Tank (System B)This type of system is very similar to SYSTEM A, but allows for continuous input. Withcontinuous process input, an ON/OFF relaycontroller with latching or deadband is required.The deadband will hold the final control elementon for a longer time than without deadband.This results in smooth operation without rapidcycling.SYSTEM B does not require as much levelcontrol and monitoring as SYSTEM A, since thereaction tank outlet can be sized large enoughand placed in the tank wall to make tankoverflow unlikely.

CHEMICALTO BE ADDED

ANALYZER/CONTROLLER

PUMP ORSOLENOID VALVE

OUTPUT(DISCONTINUOUS)

REACTION TANK

INPUT(DISCONTINUOUS)

BATCH PROCESSING (System A)

pH SENSOR

CHEMICALTO BE ADDED

ANALYZER/CONTROLLER

PUMP ORSOLENOID VALVE

OUTPUT(CONTINUOUS)

REACTION TANK

INPUT(CONTINUOUS)

CONTINUOUS WITH TANK (System B)

pH SENSOR

The final control element can be a pumpor an ON/OFF valve. Sizing of the finalcontrol element is complicated anddepends on many factors. This is onesituation where a titration curve can beinvaluable. In many cases, it may benecessary to use two final controlelements, each delivering differentamounts of chemical and havingdifferent setpoints. For example, onevalve may deliver 1 GPM of causticbelow 3 pH and another may deliver0.1 GPM of caustic below 4 pH.Good mixing is very important in thesesystems, and the mixer or agitationmethod should not be undersized. Theretention time of the system (tankvolume divided by GPM inflow) shouldbe greater than 10 minutes. if it is muchlonger, a repeat-cycle timer can reduceovershoot.This type of system can be fairlyaccurate, but in general it does notproduce smooth outputs. The pH willtend to cycle between levels.

Continuous with Tank (System C)This type of system uses aPROPORTIONAL GAIN controller withTIME PROPORTIONING (% CYCLE)OUTPUT for applications like those ofSYSTEM B, except that the delay timebetween chemical addition and sensingis at least one minute. This might occurwhere the solution flows through a longtank, a trough or a series of tanks.The time proportioning (% cycle) outputis a switch closure that activates asolenoid valve or pump. The controlleranalog output is fed to an electronicpercent of cycle timer to electricallyadjust the ON time from 0 to 100%.The time base of the cycle timer iselectrically adjustable from a fewseconds to a few minutes. The chemicaldelivery to the system is a series ofshots of chemical. If not enoughchemical is being added, the controlleroutput will affect the cycle timer tolengthen the ON time and shorten theOFF time.

pH Control Systems

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CHEMICALTO BE ADDED

ANALYZER/CONTROLLER

PUMP ORSOLENOID VALVE

OUTPUTREACTION TANK (OR PIPE)

INPUT

CONTINUOUS WITH TANK (System C)

pHSENSORTIME DELAY

pHSENSOR

CHEMICAL TO BE ADDED

4-20 mA

PNEUMATICVALVE OR PUMP

STATIC MIXERIF A TIME DELAY EXISTS BETWEEN CHEMICAL ADDITION POINT ANDSENSOR, SYSTEM E APPLIES

ANALYZER/CONTROLLER

FLOW

CONTINUOUS, ON-LINE CONTROL(System D)

I/P

The cycle time should usually be lessthan the delay time of the system so thata series of shots are in the tank andgradually mixing. By the time chemicalreaches the sensor, it should be mixedenough so that the sensor does notmeasure large variations in pH.The final control element should besized so that it cannot deliver more thanfive times the amount of chemicalrequired at the maximum system load.Continuous, On-line Control(System D)This type of system uses aPROPORTIONAL GAIN controller withan ANALOG OUTPUT for two generaltypes of pH control where the pH is tobe adjusted only slightly or to a valueaway from 7 pH (less than 4 pH or morethan 10 pH). The system consists of thefollowing elements:

1. A pH sensor which senses the pHof the final product.

2. An analyzer/controller whichprovides an analog control signal.

*3. A transducer that produces apneumatic signal proportional tothe analog control signal.

*4. A pneumatic valve which deliversreagent to the process.

5. A mixing device placed betweenthe chemical delivery point andthe pH sensor.

* The transducer and pneumatic valvecan be replaced by an electricallycontrolled pump or valve.

Two very important points of this systemare the mixing and the delay timebetween adding chemicals and sensing

the result of the addition. The mixingmust be thorough and the delay timeshould not be more than a few seconds.A long delay time will result in theprocess pH cycling back and forthabout the desired setpoint.When solution is flowing through a pipe,an excellent mixing device to consideris a static mixer. This device providesgood mixing in a very short time. Byinjecting chemicals at the mixer inputand placing the pH sensor at the mixeroutput, the two most significant problemareas of this type of system areeliminated.An inherent characteristic of this system is that the actual controller pHsetpoint will not be the same as thedesired process pH value. Thedifference may not be large, but therewill be some difference.

System EThis type of system is the same asSYSTEM D except that it uses a TWO-MODE controller in place of theproportional gain controller. The two-mode controller is much more complexthan the proportional gain (one-mode)controller and should be used if there isdoubt about the proportional gaincontrollers performance.The integral (reset) function of the two-mode controller will try to adjust theprocess to the desired setpoint if it isphysically possible to do so.The two-mode controller also has asample/hold feature (transmit time) thatallows it to control a process with asmuch as 8 minutes of delay time fromchemical addition point to actualsensing.System FThis type of system uses the two-modecontroller of SYSTEM E with theON/OFF control element of SYSTEMC. This hybrid system is recommendedfor applications where accuracy ofcontrol is important, process delaytimes of 1 to 8 minutes exist and thechemical to be added is abrasive ortends to clog small openings (limeslurry, for example). For this reason, anON/OFF valve is preferred over theproportional valve, to avoid erosion ofits internal parts and to provide morereliable reagent addition.

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