POWER February 2013 - 43

INSTRUMENTATION & CONTROL
tion point should wind and/or personnel safety
be a concern. The barrier pipe diameter is large
enough to ensure no reagent contacts it. The new
layout prevents draining and refilling issues and
dramatically reduces process dead time.
Efficient Mixing of the Reagent
Uniform mixing of the reagent into the inlet
stream of the pond is essential to obtaining
acceptable pH measurement. Consideration
is given to highest inlet flow potential to
ensure the mixing apparatus effectively provides
a near-homogeneous solution for the
control measurement pH cell.
In one project, the " as found " installed mixing
system was an air blower with a single pipe
submerged in the inlet stream. An imaginary
matrix plane placed perpendicular to the flow
through the inlet trough was used to locate the
measurement cell during pH tests to determine
mixing effectiveness (Figure 4). Each section
measurement is obtained by using a portable pH
meter. With a constant inlet flow and a locked
control valve, tests indicated considerable pH
variation across the measurement plane.
Minor changes to the air blower piping
improved the mixing results. Piping was teed
under the water level and moved close to the
bottom of the trough. The ends of the tee
were positioned approximately one-third and
two-thirds of the distance across the trough.
Design of a pH Controller
Sediment pond influent is made up of many
converging sources. Demineralizer effluent triggered
by regeneration of deionizing beds is one
major pH control difficulty. This effluent has
sulfuric acid as a prevalent component. The reagent
used for the control of acidic influent to a
pond is caustic soda, which is diluted to approximately
20% by weight. The lowest observed
sediment pond inlet pH value is 1.4. The limits
of the outfall pH value are 6.0 and 9.0. Control
" soft " limit goals are 6.5 and 8.0. Note that tuning
this control system loop is performed for
system loading upset, as setpoint changes are
either minimal or not utilized.
PI controllers have limitations in working with
nonlinear processes, though a certain amount of
robustness can be tuned in. There is a tradeoff,
however. The controller that is tuned to maintain
minimum error is not robust for changes in the
system or process. The controller that is tuned
to be overly robust will be too sluggish at many
points of control to be considered effective.
System nonlinearity results from three factors:
the end-element, control-valve characteristic;
head pressure change, as the reagent is
gravity fed from a diluted caustic storage tank
with a level that is not automatically maintained;
and the nature of pH itself.
The accepted practice is to linearize any
control loop such that tuning settings of the
February 2013 | POWER
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43
controller produce acceptable results by
minimizing error from the setpoint-by being
robust and by being able to perform well
over the entire system load range. If the end
element is the cause of the nonlinearity, then
that is where the characterization should take
place. If the measurement has nonlinearity,
then the characterization should take place on
the measurement. As a result, characterization
of the input, output, or both is dependent on
which areas are developing the nonlinearity.
However, in the case of pH control, the process
itself is nonlinear, and this nonlinear nature
presents a major challenge for the series PI controller.
The gain of the process is represented by
the pH titration curve. Figure 5 is an actual titration
curve from a plant site. Notice the caustic
solution was found to be 21.1%. This illustrates
that the reagent solution is another variable, as
the plant dilutes the delivered 50% caustic soda,
so minor variances can be expected.
The " S " shape of the curve represents gain
change of the process that the controller must
handle. Actual measured low pH inlet value to
the pond inlet is 1.4. To illustrate this further,
consider the following examples:
■ Situation 1. To raise 1.4 pH to 2.0 pH requires
that 2.0 ml of reagent be added to a
100 ml volume (14.60 ml reagent - 12.60 ml
reagent = 2.0 ml reagent added). Therefore,
2.0 - 1.4 = 0.6 pH change, with the addition
of 2.0 ml of reagent, for a 0.6/2.0 ratio.
■ Situation 2. To raise 2.0 pH to 7.0 pH requires
an addition of 0.77 ml of reagent to the
100 ml volume (15.37 ml reagent - 14.60 ml
reagent = 0.77 ml reagent added). Therefore,
7.0 - 2.0 = 5.0 pH change, with the addition
of 0.77 ml of reagent, for a 5/0.77 ratio.
■ Situation 3. Notice that an addition of 0.03
ml reagent raises the pH value from 7.0 to
9.0, a required pH limit boundary (15.40 ml
3. Delivery point. Reagent delivery piping modifications. Courtesy: NRG Energy
Caustic
reagent tank
Elevation
Long piping length
Pond inlet
4. Test plane matrix. An imaginary matrix plane in the inlet trough across the point where
the measurement cell is located is utilized for testing. Courtesy: NRG Energy
Reagent delivery
Air mix blower
Original piping
Added piping " TEE "
Sample point matrix
pH cell normal location
pH cell normal location
Influent flow
Plane of samples

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POWER February 2013

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