DESIGN STANDARD DS 40-08

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


Uncontrolled if Printed Page 2 of 31 Ver 1Rev 1

© 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 – 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.

© Copyright – 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.

https://www.legislation.wa.gov.au/legislation/statutes.nsf/law_s4665.html





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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

Corporation’s project manager and/or designer for that particular Project.





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



technology

JGB RP



technology

JGB RP



technology

JGB RP





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 – All Signals ................................................................................................... 26

4.3 Input Processing – 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





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 – 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





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 Corporation’s learning’s from many years of building and maintaining chemical

dosing systems. The standard has been developed to complement Water Corporation’s existing

chemical dosing standards and in particular DS78 – 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 – 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 –

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 – Seborg Edgar Mellichamp Doyle

2. Siemens SIMATIC Standard PID Control Manual





2.4 Abbreviations & Definitions

The following abbreviations and definitions Table 2-1 are used.

Table 2-1 – 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 “residual trim”)

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 “flow

pacing”.

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 – 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 – 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

http://en.wikipedia.org/wiki/Pascal_(pressure)





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





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 “liquid” 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:





Figure 3-1 Gas Dosing for Chlorine Residual and Ammonia Overview

An overview of the pH chemical control requirements is shown below:





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 “trim” 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 (“dose rate”). This part of the

control is typically called the “Residual Trim”, 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.





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 “adaptive integral” 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 “Trims” 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:





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 “Chlorinator Running” 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 “mg/L” (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:





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 “gain” 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.





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 – 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 “inhibit” 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.





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:

𝑀𝑉 = 𝑎 × 𝑏

That is:

𝐺𝑎𝑠 𝐹𝑙𝑜𝑤 𝑅𝑎𝑡𝑒 (𝑔

ℎ) = 𝐷𝑜𝑠𝑒 𝑅𝑎𝑡𝑒 (

𝑚𝑔

𝐿) × 𝑀𝑎𝑖𝑛𝑠 𝐹𝑙𝑜𝑤 (

𝑘𝐿

ℎ)

Where the “Dose Rate” is the output of the PID controller and the “Mains Flow” 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 “Multiplication” shall be configured as a mathematical equation, in script, or as a “Ratio”

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.





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.





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:





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 (“dose

rate”) 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 –

Flow PID Controller, is used to set the speed of the chemical dosing pump to match the chemical flow

required.





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





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:


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 – 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 “initialisation

mode” 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:

𝐴𝑛𝑎𝑙𝑦𝑠𝑒𝑟 𝑃𝐼𝐷 𝑂𝑢𝑡𝑝𝑢𝑡 (𝑚𝑔

𝐿)

= 𝐶ℎ𝑒𝑚𝑖𝑐𝑎𝑙 𝐹𝑙𝑜𝑤 𝑅𝑎𝑡𝑒 𝑆𝑒𝑡𝑝𝑜𝑖𝑛𝑡 (

𝐿ℎ

) × 𝐶ℎ𝑒𝑚𝑖𝑐𝑎𝑙 𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 (𝑔𝐿)

𝑀𝑎𝑖𝑛𝑠 𝐹𝑙𝑜𝑤 (𝑘𝐿ℎ

)





PV Tracking shall be set to “OFF” 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:

𝑀𝑉 = 𝑎 × 𝑏 × 𝑐

𝑑

That is:

𝐿𝑖𝑞𝑢𝑖𝑑 𝐹𝑙𝑜𝑤 𝑅𝑎𝑡𝑒 (𝐿

ℎ) =

𝐷𝑜𝑠𝑒 𝑅𝑎𝑡𝑒 (𝑚𝑔

𝐿) × 𝑀𝑎𝑖𝑛𝑠 𝐹𝑙𝑜𝑤 (

𝑘𝐿ℎ

) 𝑥 1

𝐶ℎ𝑒𝑚𝑖𝑐𝑎𝑙 𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 (𝑔𝐿

)

Where the “Dose Rate” is the output of the Analyser PID controller and the “Mains Flow” 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 “Multiplication” shall be configured as a mathematical equation, in script, or as a “Ratio”

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.





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:


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 “%” or “rpm” (revolutions

per minute).

The flow controller can be placed in one of three (3) modes – 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 “ON” 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.





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:

𝐶ℎ𝑒𝑚𝑖𝑐𝑎𝑙 𝑊𝑒𝑖𝑔ℎ𝑡 (𝑔)

= 𝐶ℎ𝑒𝑚𝑖𝑐𝑎𝑙 𝐶𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑑 (𝑔

𝐿) 𝑥 𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑊𝑎𝑡𝑒𝑟 𝑖𝑛 𝑇𝑎𝑛𝑘 (𝐿)

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.





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 “Residual Trim” control when the controller is in

automatic mode and the “Flow Pacing” 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 – 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:

𝑉 = 𝑚𝑎𝑥 (𝑀𝑖𝑛 [𝑇𝑠

𝑇𝑠 + 𝑇𝑓 𝑥 𝑉𝑟 +

𝑇𝑓

𝑇𝑠 + 𝑇𝑓 𝑥 𝑉𝑝, 𝑅𝑚𝑎𝑥] , 𝑅𝑚𝑖𝑛)

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 – 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.





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





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 “Remote” 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 “Local” and

“Remote” 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 “Remote” or “Local” 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 “Direct” or “Reverse”. If the error in the PID controller is

calculated as SP – PV, then a “Direct” acting controller will increase the output as the PV decreases.

NOTE: The convention assumed in this document is that “direct” action means the output of the

controller increases as the PV decreases. The designer should check the PLC manufacturer’s

documentation to confirm the sense of the term “direct action” in the manufacturer’s device.





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 “kick” 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 “Upper Value” – Maximum output value and a “Lower Value” – 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 controller’s 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 “OFF” 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 controller’s

calculation of its output can go past 100%. Controllers can have an “anti-reset-windup” feature that

disables integral action.





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 “track” 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 “scan”

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 – 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 – 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.





END OF DOCUMENT

Documents

DESIGN STANDARD DS 40-08