Digital Transformation Group Operational Technology
DESIGN STANDARD DS 40-08
Standard for the Control of Chemical Dosing
VERSION 1
REVISION 1
APRIL 2018
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 2 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
FOREWORD
The intent of Design Standards is to specify requirements that assure effective design and delivery of fit for
purpose Water Corporation infrastructure assets for best whole-of-life value with least risk to Corporation
service standards and safety. Design standards are also intended to promote uniformity of approach by asset
designers, drafters and constructors to the design, construction, commissioning and delivery of water
infrastructure and to the compatibility of new infrastructure with existing like infrastructure.
Design Standards draw on the asset design, management and field operational experience gained and
documented by the Corporation and by the water industry generally over time. They are intended for
application by Corporation staff, designers, constructors and land developers to the planning, design,
construction and commissioning of Corporation infrastructure including water services provided by land
developers for takeover by the Corporation.
Nothing in this Design Standard diminishes the responsibility of designers and constructors for applying the
requirements of WA OSH Regulations 1996 (Division 12, Construction Industry β consultation on hazards
and safety management) to the delivery of Corporation assets. Information on these statutory requirements
may be viewed at the following web site location:
https://www.legislation.wa.gov.au/legislation/statutes.nsf/law_s4665.html
Enquiries relating to the technical content of a Design Standard should be directed to the Principal SCADA
Engineer, Operational Technology. Future Design Standard changes, if any, will be issued to registered
Design Standard users as and when published.
Manager, Operational Technology
This document is prepared without the assumption of a duty of care by the Water Corporation. The document is not
intended to be nor should it be relied on as a substitute for professional engineering design expertise or any other
professional advice.
Users should use and reference the current version of this document.
Β© Copyright β Water Corporation: This standard and software is copyright. With the exception of use permitted by the
Copyright Act 1968, no part may be reproduced without the written permission of the Water Corporation.
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 3 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
DISCLAIMER
Water Corporation accepts no liability for any loss or damage that arises from anything in the
Standards/Specifications including any loss or damage that may arise due to the errors and omissions of any person.
Any person or entity which relies upon the Standards/Specifications from the Water Corporation website does so
that their own risk and without any right of recourse to the Water Corporation, including, but not limited to, using
the Standards/Specification for works other than for or on behalf of the Water Corporation.
The Water Corporation shall not be responsible, nor liable, to any person or entity for any loss or damage suffered
as a consequence of the unlawful use of, or reference to, the Standards/Specifications, including but not limited to
the use of any part of the Standards/Specification without first obtaining prior express written permission from the
CEO of the Water Corporation.
Any interpretation of anything in the Standards/Specifications that deviates from specific Water Corporation
Project requirements must be referred to, and resolved by, reference to and for determination by the Water
Corporationβs project manager and/or designer for that particular Project.
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 4 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
REVISION STATUS
The revision status of this standard is shown section by section below:
REVISION STATUS
SECT. VER./
REV.
DATE PAGES
REVISED
REVISION DESCRIPTION
(Section, Clause, Sub-Clause)
RVWD. APRV.
1 1/0 22.01.14 All New standard JB MH
1/1 12.04.18 All Updated to operational
technology
JGB RP
2 1/0 22.01.14 All New standard JB MH
1/1 12.04.18 All Updated to operational
technology
JGB RP
3 1/0 22.01.14 All New standard JB MH
1/1 12.04.18 All Updated to operational
technology
JGB RP
4 1/0 22.01.14 All New standard JB MH
1/1 12.04.18 All Updated to operational
technology
JGB RP
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 5 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
DESIGN STANDARD DS 40-08 STANDARD FOR THE CONTROL OF CHEMICAL
DOSING
CONTENTS Section Page
1 INTRODUCTION ......................................................................................................................... 7
1.1 Purpose .......................................................................................................................................... 7
2 BACKGROUND INFORMATION ............................................................................................. 7
2.1 Scope .............................................................................................................................................. 7
2.2 Standards and Regulations ......................................................................................................... 7
2.3 Bibliography & References ......................................................................................................... 7
2.4 Abbreviations & Definitions ....................................................................................................... 8
3 CHEMICALS AND DOSING REQUIREMENTS .................................................................. 10
3.1 Gas Chemical Dosing ................................................................................................................. 10
3.2 Signal Filter ................................................................................................................................ 13 3.2.1 Analyser PID Controller .............................................................................................................. 13 3.2.2 Multiplication Block .................................................................................................................... 17 3.2.3 High/Low Range Selector ............................................................................................................ 18
3.3 Liquid Chemical Dosing ............................................................................................................ 19 3.3.1 Signal Filter .................................................................................................................................. 21 3.3.2 Analyser PID Controller .............................................................................................................. 21 3.3.3 Multiplication Block .................................................................................................................... 23 3.3.4 Flow PID Controller ..................................................................................................................... 23
3.4 Dry Chemical Dosing ................................................................................................................. 25
4 DESIGN AND PROGRAMMING REQUIREMENTS ........................................................... 26
4.1 Chemical Dosing Systems .......................................................................................................... 26
4.2 Input Processing β All Signals ................................................................................................... 26
4.3 Input Processing β Mains Flow ................................................................................................. 26
4.4 Local/Remote Controls .............................................................................................................. 28
4.5 General PID Controller Configuration .................................................................................... 28 4.5.1 PID Controller .............................................................................................................................. 28 4.5.2 Direction of Control ..................................................................................................................... 28 4.5.3 Changing PID Controller Modes ................................................................................................. 29 4.5.4 Output Limits ............................................................................................................................... 29 4.5.5 Process Value Signal Fault........................................................................................................... 29 4.5.6 PV Tracking ................................................................................................................................. 29 4.5.7 Reset Windup ............................................................................................................................... 29 4.5.8 Cycle Time ................................................................................................................................... 30
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 6 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
LIST OF FIGURES
Figure 2-1 Gas Dosing for Chlorine Residual and Ammonia Overview .................................................................... 11
Figure 2-2 Gas Dosing for pH Control Overview ...................................................................................................... 12
Figure 2-3 Residual Chlorine Analyser PID Controller Diagram .............................................................................. 14
Figure 2-4 PID Additional Controls ........................................................................................................................... 16
Figure 2-5 Multiplication Diagram ............................................................................................................................. 17
Figure 2-6 High/Low Range Selector ......................................................................................................................... 18
Figure 2-7 Fluorosilicic Acid, Sodium Hypochlorite and Ammonia Dosing ............................................................. 19
Figure 2-8 Hydrochloric and Sulphuric Acid Dosing ................................................................................................. 20
Figure 2-9 Liquid Dosing mg/L PID Controller ......................................................................................................... 21
Figure 2-10 Liquid Dosing pH PID Controller ........................................................................................................... 22
Figure 2-11 Liquid Dosing Multiplication ................................................................................................................. 23
Figure 2-12 Chemical Flow PID Controller Diagram ................................................................................................ 24
Figure 3-1 Mains Flow Clamped Value Diagram ...................................................................................................... 27
Figure 3-2 Local/Remote Logic ................................................................................................................................. 28
Figure 3-3 Process Response Diagram ....................................................................................................................... 30
LIST OF TABLES
Table 1-1 β Abbreviations and Definitions .................................................................................................................. 8
Table 2-1 Gases used by Water Corporation .............................................................................................................. 10
Table 2-2 Liquid Chemicals used by Water Corporation ........................................................................................... 19
Table 2-3 Solid Chemicals used by Water Corporation ............................................................................................. 25
Table 3-1 Direction of Control ................................................................................................................................... 29
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 7 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
1 INTRODUCTION
1.1 Purpose
The purpose of this standard is to define the control requirements for chemical dosing systems, mainly
for the Water Treatment applications, within the Water Corporation. It defines the basic requirements
for the process control and the desired solutions for different types of chemicals and installations.
It has been written such that the Design Consultant can determine the instrumentation and control
requirements for the control system for chemical dosing plants and to provide some guidance to the
Systems Integrators with programming of the control system.
This standard has been written to achieve a consistent and structured approach to control system
configurations for chemical dosing system.
It is assumed that the process has been designed appropriately for the conditions that are likely to be
encountered. For example, the system has been designed such that there is a reasonable deadtime in
the process and that the equipment has been sized correctly to be able to achieve the flows and dose
rates as required for the expected mains water flow.
2 Background Information This standard has been prepared to provide consistent design of chemical dosing control systems and
to incorporate Water Corporationβs learningβs from many years of building and maintaining chemical
dosing systems. The standard has been developed to complement Water Corporationβs existing
chemical dosing standards and in particular DS78 β Chemical Dosing Standard.
2.1 Scope
This standard covers the control system and software design requirements for various chemical dosing
systems and various configurations of such systems. It provides the basis of design for the control
system as well as the mandatory requirements.
2.2 Standards and Regulations
The following standards and regulations are referenced in this standard:
DS78 Chemical Dosing Standard.
DS40-09 Field Instrumentation
Water Corporation has standard designs for chlorine dosing and sampling systems β see
drawings EO28-60-81.1, EO28-60-81.2, EO28-60-103 EO28-61-8.6 and EO28-61-9.7).
Water Corporation has standard designs for Fluorosilicic Acid dosing and sampling systems β
see drawings GT36-61-83.1
2.3 Bibliography & References
Sources of information used as references for this standard include:
1. Process Dynamics and Control β Seborg Edgar Mellichamp Doyle
2. Siemens SIMATIC Standard PID Control Manual
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 8 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
2.4 Abbreviations & Definitions
The following abbreviations and definitions Table 2-1 are used.
Table 2-1 β Abbreviations and Definitions
Corporation Water Corporation
D Derivative
Dead time (or Time
Delay)
Time taken to first record a change in chemical concentration
measured from the time that the dosing rate is changed
DN Nominal Diameter (Diameter Nominalle)
Dose Rate The required ratio of chemical to mains water flow
Feed-back Feed-back chemical dosing control effects a change in the dosing
rate based on a change that has already occurred in the
concentration value e.g. adjusting the dose rate after the
concentration reading falls outside the specified target range
(sometimes referred to as βresidual trimβ)
Feed-forward Feed-forward chemical dosing control effects a change in the
dosing rate based on a predicted change in concentration e.g.
when the flow rate in the receiving pipe changes. Where the water
flow rate is the feedforward parameter this is referred to as βflow
pacingβ.
I Integral
ID Internal Diameter
Integral Windup The condition in which the integral element a PID controller the
output of the controller ramping to extreme positive or negative
values, typically due to persistent errors which are not able to be
corrected by the flow loop. For example the PID controller
continues to ask for more dosing rate even though the dosing
chemical flow has reached a limit
Mode Manual or Automatic β in Manual the operator sets the output, in
Automatic the output is set by the controller algorithm
MV Manipulated Variable (Controller Output)
OC Operations Centre β located in the John Tonkin Centre,
Leederville
OD Outside Diameter
OP Output
Lag time (or Time
Constant)
Time taken after the dead time has elapsed for the chemical
concentration to reach 63.2% of the final chemical concentration
value
P Proportional
PID Proportional integral derivative
Pressure - Absolute Gauge pressure plus atmospheric pressure. Atmospheric pressure
is 101.3 kPa at sea level.
Pressure - Gauge The force per unit area relative to the local atmospheric or
ambient pressure. .
PV Process Variable
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 9 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
PV Tracking When the control loop is in manual with PV tracking turned on,
the controller set point will follow the PV. When the loop is
returned to automatic mode there is no sudden movement of the
process.
Ratio The flow of a chemical additive divided by the flow of the water
SP Set point
WC Water Corporation
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 10 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
3 Chemicals and Dosing Requirements The state of the chemicals that can be used to dose a main line can either be gas, liquid or solids. The
chemical state determines how the chemical is dosed. Typically gas chemicals are dosed directly via a
chlorinator or similar unit and the gas flow is not measured. Liquid chemicals are typically dosed via
a pump and in the case of Fluorosilicic acid and some other chemicals; the liquid chemical flow is also
measured. Thus the pump speed can be controlled via a flow controller. For solids, the chemical is
firstly diluted with water to a set concentration. Once this is completed, the state of the chemical now
becomes a βliquidβ and thus the chemical dosing controls will be similar to liquid controls.
The logic required for a dosing system is also dependent on the final chemical control that is required
and the reason for the chemical use. The chemical control is dependent on the units of measure of the
analyser which has an impact on the detail of calculations used in the controls. The chemical
properties determines the control action required by the controller, that is, increasing or decreasing the
amount of chemical required based on a higher or lower control reading detected.
The following sections outline the controls required for each type of chemical dosing. The sections
have been arranged in chemical state form, with differences shown for different chemical use
requirements and final controls
3.1 Gas Chemical Dosing
Gas chemical dosing is typically used for chlorine injection. The chlorine gas is supplied in either
cylinders or drums. Other gases that are used for dosing into the main water line at the Water
Corporation include:
Table 3-1 Gases used by Water Corporation
Gas Use Chemical Control
Chlorine Disinfection Chlorine Residual (mg/L)
Ammonia Chloramination Total Ammonia (mg/L)
CO2 Reduce pH pH
An overview of the chlorine residual and ammonia chemical control requirements is shown below:
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 11 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
Figure 3-1 Gas Dosing for Chlorine Residual and Ammonia Overview
An overview of the pH chemical control requirements is shown below:
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 12 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
Figure 3-2 Gas Dosing for pH Control Overview
For the chlorination (and chloramination) the water main flow rate is used for feed forward control to
provide rapid response to flow rate changes. The analyser provides feedback βtrimβ via a PID
controller to correct for inaccuracies in chemical dosing equipment, which is particularly relevant to
wide turndown systems. It also corrects for changes in water quality which is mostly slow.
In the above scenarios the chemical sampling system for the analyser draws water from the process,
sends it to the analyser and may recirculate the water back to the process. The analyser output is sent
to the PLC where, in most cases, this signal becomes the Process Variable for a PID (Proportional
Integral Derivative) controller configured in the PLC.
For CO2 addition, the pH feedback loop is not used.
The output of the PID controller is the ratio of gas to mains water flow (βdose rateβ). This part of the
control is typically called the βResidual Trimβ, where the PID controller acts as the feedback loop and
manipulates the dose rate to obtain the required chlorine residual, total ammonia or pH level. The
PLC block shall be configured such that the output of the PID controller is displayed in engineering
units.
If the analyser faults, the PID should maintain the current dose rate. If water flow stops, then the PID
controller action shall be suspended so that the output does not change. The PID controller will
remain suspended until a specified time has elapsed after the flow recovers.
The operator shall be able to manually enter the ratio of chlorine gas to mains water flow, either
locally via the OIP or remotely via ViewX.
The Dose Rate is multiplied by the actual mains water flow to obtain the amount of gas to be added to
the system. Care must be taken when configuring the calculation to ensure that the engineering units
for the gas dosing units are correct when multiplying the dose rate and mains flow signals.
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 13 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
The operator shall not be able to enter a gas flow rate manually via the OIP or SCADA. The operator
can enter a gas flow rate locally at the dosing unit.
For sites with large variations in flow, chlorinators with high and low range limits shall be used. See
Section 3.2.3 for further details. Also, an βadaptive integralβ shall be implemented on the PID
controller to ensure adequate response at high flows and stability at low flows.
On sites where both ammonia and chlorine are added the, designer will need to take into account the
residual ammonia in the feed water when determining the set point for ammonia and chlorine addition.
The sites where both chemicals are used are generally located in the Goldfields and Agricultural Water
Scheme (G&AWS), in the Mundaring to Kalgoorlie pipeline. Generally chlorine is dosed at a ratio of
4-5:1 of ammonia. However this ratio is dependent on water quality and may have to change to suit
the circumstances.
A detailed explanation of the function blocks is given below:
3.2 Signal Filter
For all analogue inputs from sensors a first order filter of at least 2 seconds shall be applied. The
calculation for the first order filter is as detailed in Section 4.2. This function is either carried out by a
separate controller, or can be part of the Process Variable (PV) processing functions of the controllers
where the input signal is the PV.
3.2.1 Analyser PID Controller
For gas chlorination controls, the PID controller manipulates or βTrimsβ the dose rate to maintain the
target chlorine residual, ammonia or pH value as entered by the Operator. Only the Proportional and
Integral are used for chemical dosing. General considerations for PID controllers are outlined in
Section 4.5.
An overview of the logic associated with the Chlorine Residual Trim PID controller is shown below:
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 14 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
Figure 3-3 Residual Chlorine Analyser PID Controller Diagram
The logic associated with the PID controller for pH control is similar to the above, with the PV and Set
Points measured in pH, rather than in mg/L and the βChlorinator Runningβ substituted with the
appropriate signal.
As pH is a log function then ideally gain scheduling should be used to control pH. Water Corporation
operating experience has shown that some benefit can be gained by enabling the PID square root
function on a single gain PID controller, if gain scheduling is not available.
Further details on the PID controller are given below.
The set point (SP) of the PID controller is from the operator entered value for the chlorine residual or
pH.
In the above diagram an internally calculated register has been used for the input to the set point of the
PID Controller as well as other set points. This is because the operator can enter the values either via
the local OIP or remotely via SCADA. The logic required to select between local and remote set
points is shown in Section 4.4.
The process variable (PV) for the PID controller is the filtered value from the analyser which
represents the actual chlorine residual or pH in the water supply. Appropriate PID range values for the
Process Variable shall be entered by the engineer in engineering units when configuring the PID
controller.
The PID controller attempts to maintain the chlorine residual or the pH from the analyser to the set
point value entered by the operator, by changing the output value. The direction of control for the PID
controllers shall be set as outlined in Section 4.5.2.
The output from the controller is the dose rate, which is the proportion of gas to be injected into the
mains flow. Generally the dose rate is set as βmg/Lβ (mg of gas per litre of mains water). Thus to
make it easier for fault-finding and analysis the PID controller output in the PLC, OIP and SCADA
shall be set in engineering units, not percentage.
The other values that need to be entered by either the operator or the engineer include:
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 15 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
PV Low and High Range Low and high range values in engineering
units for the process variable. Entered by
the engineer during configuration
Proportional Value (P): Entered by the Commissioning or
Maintenance Engineer when tuning the
controller. In some controllers this is
entered as a βgainβ value (i.e. 1/P)
Integral Value (I): Entered by the Commissioning or
Maintenance Engineer when tuning the
controller
For sites with large variations in flow
rates (including dual range chlorination
sites) an adaptive integral shall be
incorporated. See EO28-61-8.6.
Dose Rate (Manual OP): The operator can set the dose rate directly,
if they place the controller into manual
mode. In this mode, the controller is
essentially bypassed.
Maximum Dose Rate (Max
OP)
A maximum dose rate can be set to limit
the possibility of entering too much
chlorine into the water main.
Minimum Dose Rate (Min
OP):
A minimum dose rate can be set so that
there is a minimum dose added to the
water main, regardless of the chlorine
residual reading.
Mode: The operator can set the controller into
Manual Mode at any time. If the operator
sets the controller to manual, then the
controller will remain at its current dose
rate. The operator can then set the dose
rate as required.
The operator can set the controller back to
automatic mode at any time. In automatic
mode the output of the controller will be
changed to achieve the chlorine residual
set point entered by the operator.
Changing the controller from Auto to
Manual and vice versa shall be made
bumpless.
It is possible for the operator to place the PID controller into manual and manually enter the dose rate
in mg/L. The operator can use this function when the analyser is not functioning correctly, or the
scheme needs additional gas added for various reasons. Changing the controller from Auto to Manual
and vice versa shall be made bumpless, thus when in Auto, the manual entered output value shall track
the current PID output value.
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 16 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
The PID controller shall be inhibited from operation under the following circumstances:
1. Site power has been disrupted
2. The Chlorinator or gas injector has been stopped
3. The mains flow has reduced to the point where gas addition should stop
4. Analyser has faulted
5. Analyser has been disabled
6. Downstream controller is not active β Ie it is not in Cascade Mode or is saturated (Only
applicable if PID controller has a secondary loop)
When all the inhibit conditions returned to the healthy state, the PID loop shall remain inhibited for a
period of time prior to being released for operation.
While the PID loop is inhibited, the output of the PID loop shall remain in its last value.
For some PLCs, the βinhibitβ function may need to be programmed by placing the loop into manual
and setting the manual output to zero (0). In this case, once the PID loop is un-inhibited, the mode of
the PID controller shall be set to the mode the controller was in prior to being inhibited.
An example of the additional logic required is shown below:
Figure 3-4 PID Additional Controls
PV Tracking shall be set OFF for the chemical dosing PID controller. This is because the set point of
the controller is specified by the requirements of the Australian Drinking Water Guidelines, and thus
the set point should always remain the same.
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 17 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
3.2.2 Multiplication Block
This block is a multiplication function that calculates the amount of gas to be injected based on the
dose rate set. It is represented diagrammatically below:
Figure 3-5 Multiplication Diagram
Generally the mains flow is measured in kL/h, thus the main multiplication, as shown on the drawing
above will be:
ππ = π Γ π
That is:
πΊππ πΉπππ€ π ππ‘π (π
β) = π·ππ π π ππ‘π (
ππ
πΏ) Γ πππππ πΉπππ€ (
ππΏ
β)
Where the βDose Rateβ is the output of the PID controller and the βMains Flowβ is the value
measuring the flow of the water to be disinfected.
Notes:
1. The calculation above will need to be modified if the Mains Flow is not measured in kL/h
2. The βMultiplicationβ shall be configured as a mathematical equation, in script, or as a βRatioβ
function block - not as a PID loop.
3. The calculations assume that the chlorinator can deliver the gas flow rates required to maintain
the dose rate for the full range of mains flows that are likely to be measured.
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 18 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
3.2.3 High/Low Range Selector
In some sites there is a large variation in the mains water flow, which requires a larger range in
chlorine gas flow. For these sites chlorination with the ability to select from high and low range is
required. This is achieved by having two or more chlorinators. The rest of this section only refers to
dual range systems. Triple range systems shall use the same principles.
For dual range chlorinators, one will operate at the lower flows (low range) and one will operate at the
higher flows (high range).
Note that on sites where duty/standby chlorinators are used and high/low range is also required, there
will be four chlorinators on site: two low range and two high range.
An algorithm will need to be included in the PLC to select the high/low range. The algorithm shall be
configured as shown below:
Figure 3-6 High/Low Range Selector
Thus when the dose rate requested is low (say less than 2 g/h), the low range chlorinator shall be in
use. As the requested dose rate increases above the High Range Start Set Point (typical say at 2.7 g/h),
the high range chlorinator will start and the low range chlorinator will stop. The high range
chlorinator will continue running until the requested dose rate goes below the Low Range Start Set
Point (say 2 g/h), at which point the high range chlorinator will stop and the low range chlorinator will
start. The changeover set points shall be selected so that the High Range Chlorinator is
selected/deselected whilst it is still within its linear range of operation. The turn down ratio for a
chlorinator is typically 10:1.
The local/remote set points are set by the operator via either the local OIP or remotely via SCADA as
show in Section 4.4.
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 19 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
3.3 Liquid Chemical Dosing
Liquid dosing is used for fluoride addition, pH control and disinfection. The types of liquid dosing
chemicals used are listed below:
Table 3-2 Liquid Chemicals used by Water Corporation
Liquid Chemical Use Chemical Control
Hydrochloric acid Lower pH pH
Fluorosilicic acid (FSA) Fluoride addition Fluoride (mg/L)
Sodium hypochlorite Disinfection Free Chlorine Residual
(mg/L)
Sulphuric acid Lower pH pH
Ammonia (liquid) Chloramination Total Ammonia (mg/L)
The liquid chemical is generally distributed to site and placed into a tank. A variable speed pump is
used to inject the chemical into the main water line.
The controls for typical Fluorosilicic Acid, Sodium Hypochlorite and Ammonia Dosing systems are
shown in the diagram below:
Figure 3-7 Fluorosilicic Acid, Sodium Hypochlorite and Ammonia Dosing
The controls for typical Hydrochloric and Sulphuric Acid Dosing are shown below:
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 20 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
Figure 3-8 Hydrochloric and Sulphuric Acid Dosing
The chemical sampling system is similar to the chemical sampling systems described for gas chemical
dosing, in that it draws water from the process, sends it to the analyser and may recirculate the water
back to the process. The analyser value is sent to the PLC where this signal becomes the Process
Variable for a PID (Proportional Integral Derivative) controller configured in the PLC.
For CO2 addition, the pH feedback loop is not used.
The output of the Analyser PID Controller is the ratio of liquid chemical to mains water flow (βdose
rateβ) in mg/L. The Analyser PID controller manipulates the dose rate to meet the set point of the
analysed variable.
If the analyser faults, the PID should maintain the current dose rate. If water flow stops, then the PID
controller action shall be suspended so that the output does not change. The PID controller will
remain suspended until a specified time has elapsed after the flow recovers.
If the analyser has faulted, the operator shall be able to manually enter the ratio of liquid chemical to
mains water flow.
A Multiplication Block is used to multiply the ratio of liquid chemical to mains water flow with the
mains water flow to obtain the amount of chemical to be added to the system. Care must be taken
when configuring the calculation to ensure that the engineering units for the chemical dosing
equipment are correct when multiplying the Analyser PID output and mains flow signals, in particular,
the correct density conversion factor must be used to convert from mg/L to L/h units.
For some chemical additions the liquid chemical flow is measured, and a second PID controller β
Flow PID Controller, is used to set the speed of the chemical dosing pump to match the chemical flow
required.
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 21 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
A detailed explanation of the function blocks is given below:
3.3.1 Signal Filter
For all analogue inputs a first order filter of at least 2 seconds shall be applied. The calculation for the
first order filter is as detailed in Section 4.2. This function is either carried out by a separate
controller, or can be part of the Process Variable (PV) processing functions of the controllers where
the input signal is the PV.
3.3.2 Analyser PID Controller
For liquid chemical addition controls, the PID controller manipulates the dose rate to maintain the
target mg/L or pH value as appropriate and as entered by the Operator. Only the Proportional and
Integral are used for chemical dosing. General considerations for PID controllers are outlined in
Section 4.5.
An overview of the controls associated with the PID controller for mg/L and pH measurement are the
same as for gas dosing. They are shown below for information:
Figure 3-9 Liquid Dosing mg/L PID Controller
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 22 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
Figure 3-10 Liquid Dosing pH PID Controller
The controls associated with the Analyser PID controller are similar to the controller detailed in
Section 3.2.1 with the following specific requirements.
The Analyser PID controller shall be inhibited from operation under the following circumstances:
1. Site power has been disrupted
2. The chemical dosing pump has been stopped
3. The mains flow has reduced to the point where chemical addition should stop
4. Analyser has faulted
5. Analyser has been disabled
6. Downstream controller is not active β i.e. it is not in Cascade Mode or is saturated (Only
applicable if PID controller has a secondary loop).
For liquid chemical dosing, there is likely to be a downstream controller that can be placed into
manual or can be saturated (that is, the output of the controller has reached limit values). Thus the
Analyser PID controller should be inhibited if the flow Controller is not Active. If an βinitialisation
modeβ is not available in the PID controller (i.e. the PID output does not back calculate if the
secondary controller is not active) then the output will need to be back-calculated manually. That is, if
the flow controller is not active the output of the analyser PID controller shall be set to:
π΄ππππ¦π ππ ππΌπ· ππ’π‘ππ’π‘ (ππ
πΏ)
= πΆβππππππ πΉπππ€ π ππ‘π πππ‘πππππ‘ (
πΏβ
) Γ πΆβππππππ πΆππππππ‘πππ‘πππ (ππΏ)
πππππ πΉπππ€ (ππΏβ
)
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 23 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
PV Tracking shall be set to βOFFβ for the chemical dosing Analyser PID controller, this is because the
set point of the controller is set by the requirements of the Australian Drinking Water Guidelines, and
thus the set point should always remain the same.
3.3.3 Multiplication Block
The Multiplication Block calculates the amount of chemical to be injected based on the dose rate set.
The calculation for the liquid chemical dosing controls will be similar to the controller described in
Section 3.2.2.
Figure 3-11 Liquid Dosing Multiplication
Generally the mains flow is measured in kL/h, thus the main multiplication calculation, as shown on
the drawing above will be:
ππ = π Γ π Γ π
π
That is:
πΏπππ’ππ πΉπππ€ π ππ‘π (πΏ
β) =
π·ππ π π ππ‘π (ππ
πΏ) Γ πππππ πΉπππ€ (
ππΏβ
) π₯ 1
πΆβππππππ πΆππππππ‘πππ‘πππ (ππΏ
)
Where the βDose Rateβ is the output of the Analyser PID controller and the βMains Flowβ is the value
measuring the flow of the water to be disinfected. The chemical concentration is the concentration of
the dosing liquid.
The operator is also able to enter the Chemical Flow Rate manually if required.
Notes:
1. The calculation above will need to be modified if the Mains Flow is not measured in kL/h
2. The βMultiplicationβ shall be configured as a mathematical equation, in script, or as a βRatioβ
function block - not as a PID loop. .
3. The calculations assume that the dosing pump can deliver the flow rates required to maintain the
dose rate for the full range of mains flows that are likely to be measured.
3.3.4 Flow PID Controller
For dosing pump speed controls, the Flow PID controller manipulates the dosing pump speed to
maintain the target dose flow rate. The target dose flow rate can be entered manually by the operator
either locally via the OIP or remotely via SCADA if the PID controller is placed in manual.
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 24 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
An overview of the controls associated with the Flow PID controller is shown below:
Figure 3-12 Chemical Flow PID Controller Diagram
The controls associated with the Flow PID controller are similar to the controller detailed in Section
3.2.1 with the following specific requirements.
The Flow PID controller shall be inhibited from operation under the following circumstances:
1. Site power has been disrupted
2. The chemical dosing pump has been stopped
3. The mains flow has reduced to the point where chemical addition should stop
4. Chemical flow meter has faulted
The process variable (PV) for the Flow PID Controller is the filtered value from the liquid chemical
flow meter.
The direction of control for the Flow PID Controller shall be set as outlined in Section 4.5.2.
The output of the controller is the pump speed, which can be set either as β%β or βrpmβ (revolutions
per minute).
The flow controller can be placed in one of three (3) modes β Manual, Automatic or Cascade. In
Manual mode the operator can enter the pump speed directly. In Automatic mode the operator enters
the liquid chemical flow set point and the controller manipulates the pump speed to maintain the flow
set point. In Cascade mode the liquid chemical flow set point comes from the output of the
Multiplication Calculation.
PV Tracking shall be set to βONβ for the Flow PID controller, this is so that the set point can track the
PV value when the controller is in manual mode, thus when the controller is returned to automatic or
cascade mode there will not be a bump in the process.
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 25 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
3.4 Dry Chemical Dosing
The Water Corporation purchases several chemicals in their dry form, which are used in the treatment
plants. The main dry chemicals used include:
Table 3-3 Solid Chemicals used by Water Corporation
Dry Chemical Use Chemical Control
Calcium Hydroxide (Hydrated Lime) Raises pH pH
Sodium Carbonate (Soda Ash) Raises pH pH
Sodium Fluoride Fluoride
addition
mg/L
Sodium Metabisulphite Antioxidant ORP (mV)
To inject the chemical into the main process line, the chemical is initially dissolved into a set amount
of water. This produces a concentrated liquid which is injected into the process lines. Once the
chemical dissolves into the liquid form the controls shall be as set out in the Liquid Dosing Section
3.3.
Dissolving of the chemical into the water is done via a batch processing method. The batch controls
are activated when a low level is detected in the tank containing the dissolved chemical. When this
occurs, the tank is initially filled with a set amount of water. The set amount shall be determined by
the process engineer during the design phase of the project. Filling of the tank with water can be
achieved by either: measuring the totalized flow of water into the tank from when the water addition
valve is opened and adding water until a set volume has been added, or by adding water until a set
level is reached on the tank. If the level method is used, the tank volume shall be calculated at that
level during commissioning, with this value then used as the total litres of water in the tank. Thus the
volume of water will be identical for each batch.
The dry chemical is then added to the tank. The amount of dry chemical added is automated to
achieve a set concentration level in the tank and can be calculated as:
πΆβππππππ ππππβπ‘ (π)
= πΆβππππππ πΆππππππ‘πππ‘πππ π πππ’ππππ (π
πΏ) π₯ ππππ’ππ ππ πππ‘ππ ππ ππππ (πΏ)
Where the Chemical Concentration Required is an operator entered value. The operator shall be able
to enter the value either remotely via SCADA or locally via the OIP. The function detailed in Section
4.4 shall be used for the local/remote value selection.
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 26 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
4 Design and Programming Requirements
4.1 Chemical Dosing Systems
The different elements of the control for chemical dosing systems can be summarized as follows:
1. Input processing
Obtaining the values to monitor the process. This includes the instruments themselves as well
as the signal processing within the PLC.
2. Analyser PID controller
The Analyser PID controller provides the βResidual Trimβ control when the controller is in
automatic mode and the βFlow Pacingβ control when the controller is in manual mode. The
characteristics required to be configured for the PID controller are given in the following
sections
3. Multiplication Block
The multiplication calculation is the product of the dose rate and the mains water flow, which
gives the target chemical flow rate.
4. Flow PID controller
The Flow PID controller manipulates the dosing pump speed to maintain the target dose flow
rate.
The following sections outline some of the general requirements of the required control elements.
4.2 Input Processing β All Signals
For all analogue inputs a first order filter time constant of at least 2 seconds shall be applied. When
the PLC contains an inbuilt filter block or input conditioning block then this block shall be used in
preference for the programmer constructing their own block. If this does not exist then a first order
filter calculation shall be used. The calculation for the first order filter is as per below:
π = πππ₯ (πππ [ππ
ππ + ππ π₯ ππ +
ππ
ππ + ππ π₯ ππ, π πππ₯] , π πππ)
Where:
V = Filtered value
Ts = Local controller scan time
Tf = Filter time constant
Vr = Current raw value from instrument
Vp = Previously calculated filtered value
Rmax = Instrument maximum range
Rmin = Instrument minimum range
4.3 Input Processing β Mains Flow
The water mains flow signal analogue input shall also have a first order filter time constant of at least
2 seconds applied as per all other analogue signals. This filtered value shall be the value displayed on
the screen for the operators and logged historically.
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 27 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
In addition to the display value, a second value for the mains flow shall be created. The second signal
will be used as an input to the chemical dosing controllers and calculations. The signal shall be based
on the filtered value with additional clamps applied as per below:
1. If the flow meter has faulted, then the value is clamped at the value monitored 10 seconds
previously.
2. If the value is below the Initiate Chlorine Set Point, then the value is clamped to zero (0.01), as
long as the Minimum Run Time has been exceeded.
3. If value is below zero (0) then value is clamped to zero (0) (Subset of above condition)
The calculations are represented diagrammatically below:
Figure 4-1 Mains Flow Clamped Value Diagram
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 28 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
4.4 Local/Remote Controls
Many of the regional chemical dosing sites will be able to be controlled via either a local OIP at the
site or via the SCADA system. The SCADA system is considered to be the βRemoteβ system and is
either accessed by the operators in the Operations Center (OC) or via the local operators using thin
clients available on their laptops.
Most of the operator entered parameters can be changed either via the local OIP or from the SCADA
system. To determine which parameter is to be used, the operators can select between βLocalβ and
βRemoteβ Operations.
For chemical dosing sites, bumpless transfer is required for operator entered parameters for both the
transfer from local to remote and from remote to local. The typical code for handling local/remote
parameters is given below:
Figure 4-2 Local/Remote Logic
The operator entered values can either be digital (on/off) values or analogue values. The operator
enters the value in registers writen from either SCADA or the local OIP and the parameter that is
selected is based on whether βRemoteβ or βLocalβ is selected. As the value is changed by the
operator, it is written back to both registers, which allows for bumpless transfer when the control
location is changed.
4.5 General PID Controller Configuration
4.5.1 PID Controller
A PID controller attempts to maintain the Process Variable (PV) to the set point value entered by the
operator by changing the output value. This is achieved via a Proportional Integral Derivative (PID)
algorithm. Note that different PLCs use slightly different versions of the PID algorithm. The
Designer shall check the PLC documentation for details on the PID algorithm used by the PLC. (see
also DS40-09 paragraph 3.15)
Since the deadtimes in the pipeline dosing systems can be long, the PID algorithm shall have the
capability for integral times up to 30 minutes.
For chemical dosing loops the derivative value shall not be used.
The general requirements for a PID controller are summarised below:
4.5.2 Direction of Control
The direction of control can be set as either βDirectβ or βReverseβ. If the error in the PID controller is
calculated as SP β PV, then a βDirectβ acting controller will increase the output as the PV decreases.
NOTE: The convention assumed in this document is that βdirectβ action means the output of the
controller increases as the PV decreases. The designer should check the PLC manufacturerβs
documentation to confirm the sense of the term βdirect actionβ in the manufacturerβs device.
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 29 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
The direction of control is dependent on what the chemical is being used for. An overview of the
direction of control for the different PID controllers used for chemical dosing, based on a SP - PV
error calculation, is shown below:
Table 4-1 Direction of Control
Use Direction of Control
Disinfection Direct
Reduce pH Reverse
Increase pH Direct
Flouride addition Direct
Flow control (using pump speed) Direct
The Designer and Commissioning Engineer shall ensure that this value is selected correctly.
4.5.3 Changing PID Controller Modes
The operator may change the PID controller mode from manual to auto and vice versa. Placing the
PID loop into manual will allow the operator to enter an output value manually. The operator may
wish to do this if there is an issue with the instrumentation or if a set output is required for operational
reasons.
All mode changes shall be bumpless, that is, if the operator has the PID control loop in manual and
switches the loop into auto mode, the output of the loop will start moving from the last position with
no βkickβ in position on mode change. Likewise, if the operator has the PID control loop in auto and
switches the mode into manual, the output of the loop will again remain in the last position.
4.5.4 Output Limits
For each PID controller an βUpper Valueβ β Maximum output value and a βLower Valueβ β Minimum
output value shall be configured. Note that in the case of the standard ADWG logic the output limits
have been configured separately to the PID block.
The output limits shall apply in automatic mode only.
An alarm shall be generated in the UWSS if the controllerβs output remains at either limit for longer
than fifteen (15) minutes.
4.5.5 Process Value Signal Fault
If a fault is detected on the PID PV (process variable input), then an alarm shall be generated in the
UWSS for the operator and the PID controller output shall remain at the last value. This may be
achieved by placing the controller into manual (without a change to the output).
4.5.6 PV Tracking
The PID controllers have an option to enable or disable PV Tracking. With PV Tracking on, when the
control loop is in manual, the controller setpoint will follow the PV. Thus when the loop is returned to
automatic mode there is no sudden movement of the process. However, PV tracking will normally be
set βOFFβ for the chemical dosing controllers.
4.5.7 Reset Windup
For PID controllers, the integral action will continue to change the controller output value even after
the output reaches its limit. This is called Reset or Integral Windup. For example, if the controller is
connected to a pump which is at 100% speed, the pump cannot go any faster, however the controllerβs
calculation of its output can go past 100%. Controllers can have an βanti-reset-windupβ feature that
disables integral action.
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 30 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
Similarly, if the PID controller is not controlling the final element in the system, for example, the
output of the PID controller may be an input to a Multiplication Block or other downstream
controllers/calculations. In this instance, Reset Windup may occur if the downstream controller is
placed in manual, or if downstream controller itself has reached its output limits.
Under these circumstances the primary PID controller shall be inhibited from increasing and the
output of the controller set to βtrackβ the downstream controller such that a bump will not occur in the
process when the downstream controller is placed into Cascade, or if the outputs of the downstream
controllers have returned from the maximum or minimum value.
4.5.8 Cycle Time
For some PLC Controllers a Cycle Time may need to be configured. This is essentially the βscanβ
time of the PID controller. To be able to set the Cycle Time, some knowledge of the process response
is required. The figure below depicts a typical process response:
Figure 4-3 Process Response Diagram
The dead time is defined as the time taken to record a change in process variable from the time that the
output is changed i.e. Tdead = (T1 β T0).
The lag time, is defined as the time taken from when a change is first recorded in the process variable
to the process variable reaching 63.2% of the final value i.e. Tlag = (T2 β T1)
The Cycle Time of the PID controller shall be set to:
Cycle Time (s) = 0.1 x (T1 + T2)
The cycle time calculation above should be the slowest cycle time allowed. Faster sample rates
provide a smoother control output and more accurate PV performance, but may use more CPU time.
For any particular control loops there is no single perfect cycle time to use. A good cycle time is a
compromise that simultaneously satisfies various guidelines.
Multiple PID loops within one controller generally require a slower cycle time.
Design Standard No. DS 40.08
Standard for the Control of Chemical Dosing
Uncontrolled if Printed Page 31 of 31 Ver 1Rev 1
Β© Copyright Water Corporation 2014 - 2018
END OF DOCUMENT