MICROTSCM (07/09)
Page 15
REV 2.4
7.0 Automatic
Control
This section describes the use of the SCM to control the process. It is recommended that a
review of section 4.1 (sample point) is made to ensure correct installation.
7.1 Optimization of Treatment Process
Prior to turning control over to the SCM, it is crucial to optimize coagulant dosing. The
optimum point is obtained when the minimum coagulant can be fed that produces the
desired results for any particular treatment process. This should be done slowly and in
steps.
Step 1:
Track
the water quality
parameters over the course of several days to establish a
base line of data from which to measure acceptable water quality.
Suggestion: The Installation Evaluation format the back of this manual is a tool that can
be used to track water quality parameters
.
Step 2:
After the base line of acceptable water quality has been reached, reduce the
coagulant dosage by 5% and closely monitor the water quality.
Step 3:
Continue reducing the dosage in 5% increments until there is a detectable
reduction in water quality. Increase the dosage from this point by 5% and
continue monitoring for another hour.
Step 4:
Record the SCU value on the instrument as this will be the optimized set point
(SP) for operating the plant.
Note: There may be a different set point for extreme variations in raw water quality or
demand on the system e.g. winter versus summer.
7.2 PI Control Overview
The MicroTSCM can be incorporated into an existing control scheme using the 4-20mA
or serial outputs. Plant control can also be achieved using the optional Proportional
Integral feature included in the MicroTSCM analyzer (HF Catalog # 19550 only).
When the instrument is used to automatically control coagulant dosing, it monitors the
process value (PV) for a change in charge value and then adjusts the dosage up or down to
achieve the predetermined set point (SP). Using a control algorithm or process calculation,
the analyzer determines the pump speed that is required to keep the PV and SP values the
same, which is the function of any closed loop control system.
When placed under automatic control, the instrument performs the same tasks that an
operator would be required to make. An operator adjusts the dosage level, allows time to
account for mixing and then checks the process for the desired change. Additional changes
to the dosage level are made as required. Under automatic control, the instrument
constantly monitors the process value and makes adjustments as needed to maintain the
process reading at the set point.
In order to put the plant under automatic control, it is required to provide some basic
information to the analyzer that describes how the system responds to changes. The
variables that need to be determined are the Proportional Band and the Integral Time. The
Proportional Band tells the instrument what change in Streaming Current to expect for a
