pH Control Systems by fad10689

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									pH Control Systems
                                                                             BATCH PROCESSING (System A)
                                                                                     CHEMICAL
                                                                                     TO BE ADDED

                                                                                                                                    ANALYZER/
This technical article describes six basic types of                                                                                 CONTROLLER
pH control systems. it will help you identify the                                              PUMP OR
correct application and choice of pH                                                           SOLENOID VALVE
instrumentation.

Batch Processing (System A)                                  INPUT
This type of system uses an ON/OFF relay                (DISCONTINUOUS)
controller for “batch” processing. The system
operates as follows:
                                                                                         pH
1. Process solution is pumped into a tank until it                                     SENSOR
   is full.                                                                                                                 OUTPUT
                                                                                                                       (DISCONTINUOUS)
2. Agitation or mixing commences and chemical
                                                                                     REACTION TANK
   is added until the desired pH is reached. The
   relay controller turns on/off the chemical
   addition pump (or solenoid valve).                 The final control element can be a pump         Continuous with Tank (System C)
3. Process then flows out or is pumped out of         or an ON/OFF valve. Sizing of the final         This type of system uses a
   tank.                                              control element is complicated and              PROPORTIONAL GAIN controller with
                                                      depends on many factors. This is one            TIME PROPORTIONING (% CYCLE)
In this system, level sensing of some type is very    situation where a titration curve can be        OUTPUT for applications like those of
desirable to signal when the tank is full and         invaluable. In many cases, it may be            SYSTEM B, except that the delay time
empty, and to lock out the mixer and pH control       necessary to use two final control              between chemical addition and sensing
when the solution is not at the proper level. if      elements, each delivering different             is at least one minute. This might occur
mixing is poor, a repeat-cycle timer is               amounts of chemical and having                  where the solution flows through a long
recommended. The OFF time of the cycle will           different setpoints. For example, one           tank, a trough or a series of tanks.
give the system some time to mix and reduce           valve may deliver 1 GPM of caustic
any eventual overshoot.                               below 3 pH and another may deliver              The time proportioning (% cycle) output
                                                      0.1 GPM of caustic below 4 pH.                  is a switch closure that activates a
When sizing the final control element, note that                                                      solenoid valve or pump. The controller
there will be some delay between adding               Good mixing is very important in these          analog output is fed to an electronic
chemical and sensing the resulting pH change. If      systems, and the mixer or agitation             “percent of cycle timer” to electrically
the final control element is oversized, the system    method should not be undersized. The            adjust the “ON” time from 0 to 100%.
will have unacceptable overshoot. The faster          retention time of the system (tank              The time base of the cycle timer is
the mixing in the system, the less the delay          volume divided by GPM inflow) should            electrically adjustable from a few
and/or overshoot will be.                             be greater than 10 minutes. if it is much       seconds to a few minutes. The chemical
                                                      longer, a repeat-cycle timer can reduce         delivery to the system is a series of
This simple system has one disadvantage: it           overshoot.
does not easily handle a continuous flowing                                                           “shots” of chemical. If not enough
process.                                              This type of system can be fairly               chemical is being added, the controller
                                                      accurate, but in general it does not            output will affect the cycle timer to
Continuous with Tank (System B)                       produce smooth outputs. The pH will             lengthen the “ON” time and shorten the
This type of system is very similar to                tend to cycle between levels.                   “OFF” time.
SYSTEM A, but allows for continuous input. With
continuous process input, an ON/OFF relay                                 CONTINUOUS WITH TANK (System B)
controller with latching or deadband is required.
The deadband will hold the final control element                                    CHEMICAL
“on” for a longer time than without deadband.                                       TO BE ADDED
This results in smooth operation without rapid                                                                                      ANALYZER/
cycling.                                                                                                                            CONTROLLER

SYSTEM B does not require as much level                                                    PUMP OR
                                                                                           SOLENOID VALVE
control and monitoring as SYSTEM A, since the
reaction tank outlet can be sized large enough
and placed in the tank wall to make tank
                                                             INPUT
overflow unlikely.                                       (CONTINUOUS)


                                                                                                                            OUTPUT
                                                                                                                         (CONTINUOUS)
                                                                                        pH
                                                                                      SENSOR




                                                                                    REACTION TANK




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                   CONTINUOUS WITH TANK (System C)
                       CHEMICAL
                       TO BE ADDED


                                                               ANALYZER/
                                                               CONTROLLER

                           PUMP OR
                           SOLENOID VALVE

                                                                                                            System E
                                                                                                            This type of system is the same as
INPUT                                                                                                       SYSTEM D except that it uses a TWO-
                                                                                                            MODE controller in place of the
                                                                                                            proportional gain controller. The two-
                                                    pH                                                      mode controller is much more complex
                                                    SENSOR
                                   TIME DELAY                                                               than the proportional gain (one-mode)
                                                                         OUTPUT                             controller and should be used if there is
                              REACTION TANK (OR PIPE)                                                       doubt about the proportional gain
                                                                                                            controller’s performance.
                                                                                                            The integral (reset) function of the two-
                                                                                                            mode controller will try to adjust the
                                                                                                            process to the desired setpoint if it is
                                                                                                            physically possible to do so.
                                                                                                            The two-mode controller also has a
The cycle time should usually be less                   the result of the addition. The mixing              sample/hold feature (transmit time) that
than the delay time of the system so that               must be thorough and the delay time                 allows it to control a process with as
a series of shots are in the tank and                   should not be more than a few seconds.              much as 8 minutes of delay time from
gradually mixing. By the time chemical                  A long delay time will result in the                chemical addition point to actual
reaches the sensor, it should be mixed                  process pH cycling back and forth                   sensing.
enough so that the sensor does not                      about the desired setpoint.
measure large variations in pH.
                                                        When solution is flowing through a pipe,
                                                                                                            System F
The final control element should be                     an excellent mixing device to consider              This type of system uses the two-mode
sized so that it cannot deliver more than               is a “static mixer.” This device provides           controller of SYSTEM E with the
five times the amount of chemical                       good mixing in a very short time. By                ON/OFF control element of SYSTEM
required at the maximum system load.                    injecting chemicals at the mixer input              C. This hybrid system is recommended
                                                        and placing the pH sensor at the mixer              for applications where accuracy of
Continuous, On-line Control                             output, the two most significant problem            control is important, process delay
(System D)                                              areas of this type of system are                    times of 1 to 8 minutes exist and the
                                                        eliminated.                                         chemical to be added is abrasive or
This type of system uses a                                                                                  tends to clog small openings (lime
PROPORTIONAL GAIN controller with                       An inherent characteristic of this                  slurry, for example). For this reason, an
an ANALOG OUTPUT for two general                        system is that the actual controller pH             ON/OFF valve is preferred over the
types of pH control where the pH is to                  setpoint will not be the same as the                proportional valve, to avoid erosion of
be adjusted only slightly or to a value                 desired process pH value. The                       its internal parts and to provide more
away from 7 pH (less than 4 pH or more                  difference may not be large, but there              reliable reagent addition.
than 10 pH). The system consists of the                 will be some difference.
following elements:
   1. A pH sensor which senses the pH
      of the final product.
   2. An analyzer/controller which
      provides an analog control signal.                                                CONTINUOUS, ON-LINE CONTROL
                                                                                                (System D)
   *3. A transducer that produces a                                                    CHEMICAL
         pneumatic signal proportional to                                              TO BE ADDED
         the analog control signal.
                                                                                                                                           ANALYZER/
   *4. A pneumatic valve which delivers                                                                                                    CONTROLLER
         reagent to the process.
                                                                                                                       4-20 mA
   5. A mixing device placed between                                                                         I/P
      the chemical delivery point and                                                             PNEUMATIC
      the pH sensor.                                                                              VALVE OR PUMP

* The transducer and pneumatic valve                                                                                                     pH
   can be replaced by an electrically                                                                                                    SENSOR
   controlled pump or valve.                                                FLOW
                                                                                                        STATIC MIXER
Two very important points of this system                                                                                         IF A TIME DELAY EXISTS BETWEEN
are the mixing and the delay time                                                                                                CHEMICAL ADDITION POINT AND
                                                                                                                                 SENSOR, SYSTEM E APPLIES
between adding chemicals and sensing

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