Instrumentation and Process Control

1
Instrumentation and Process Control

• Due to stringent drinking water standards
• More sophisticated processes and equipment are
  required.
• A modern supervisory control data acquisition
  (SCADA) system is needed.
• Advantage provide timely and accurate
  information
• Effective process control
• Process variables
• Raw water quality plant flow rate chemical
  feed rate finished water quality

2
Benefits of Instrument System

• Process
• Improved process results efficient use of
  energy and chemicals automation of process
  adjustment greater ability to control complex
  processes and timely detection of process
  changes
• Personnel
• Timely and accurate process information safer
  operation increased security efficient use of
  labor easy overview of plant operations
  decreased manual paperwork and more complete
  records
• Equipment
• Increased running time known status of
  equipment and automatic shutdown to prevent
  major damage

3
Considerations

• Local conditions availability of equipment and
  qualified operators follow-up services weather
  conditions and the living standard of the region
• Benefits of the system slow sand filtration vs.
  ozonation system R/O pumping plant filter
  process
• Selection of the system
• Todays most advanced technology may be obsolete
  in several years anticipate future needs
• Difficulty of obtaining parts
• Use the same type of system and control signals
  for a few neighboring plants .

4
Considerations - continued

• Needs of the System
• Essential control
• Flow rate pump rate ozonation system
  chemical feed rate filter control filter
  residual Cl2 Cl2 gas leakage etc.
• Useful check
• Data logging programmable filter backwashing
  pH and coagulant dosage check by closed-loop
  control
• Require extensive custom designed software
• Luxury items

5
Type and Purpose of System

• Operational purposes
• Inform current activities correct or prevent
  problems provide base control
• Maintenance purposes
• Warn for temp. change vibration abnormal power
  consumption
• Safety purposes
• Warn for leakage of chlorine gas ozone etc.
• Documentation purposes
• Identify the control modes running time
  analyze the history of repair work provide
  inventory

6
Availability of the System

• Digital on/off open/closed high/low
  alarm/normal
• Analog range of values Flow rates liquid
  level pressure pH
• Control instruments on/off or programmable
  controller with a computer backup
• Well organized training course
• System maintenance internal vs. service contract

Understanding and Commitment
7
Common Measurements

• Level float operated transmitters pressure
  sensors capacitance probes sonic or ultrasonic
  units or a bubbler tube system
• Pressure absolute pressure (vacuum - zero
  pressure) gauge pressure (atmospheric pressure -
  zero) differential pressure U-tube manometers
  tubogauges bellow gauges or pressure or strain
  gauge type transmitters
• Flow open channels weirs and flumes pipes
  differential flow meters Do not over design the
  meter size based on future max. flow rate due to
  a narrow range of turndown ratios (51 to 101).

8
Common Measurements - continued

• Turbidity
• Nephelometric turbidimeters Surface scatter type turbidimeters 30 NTU use
  in a vibration-free environment
• No air bubbles in water sample (install an air
  bubble trap)
• pH pH sensing electrode a reference electrode
  a temp. compensating electrode
• Free and combined chlorine residuals residual
  chlorine analyzers (amperometric titration
  through oxidation-reduction titration procedures).

9
Basic Controls

• Manual
• Semiautomatic require limit switches times
  analytical instruments controllers and
  programmable logic controllers (PLCs)
• Supervisory remote-controlled either in-plant or
  remote from the plant

10
Control Mode

• Proportional
• Control a device (e.g. valve) continuously to
  balance the input with the process demand
• 2. Cascade
• Output of one controller adjust the set point
  of another controller e.g. pumping control
• 3. Ratio
• Maintain a constant ratio between two
  variables e.g. ammonia/chlorine dosage

11
Basic Controls - continued

• Open loop
• Closed loop

Control panel
Operator
Motor
Valve
Position indicator
Control panel
Operator
Valve
Meter
Controller
Motor
Set point
Feed back
12
Computer-Based Monitoring Control

• Uses digital signals by a remote terminal unit
  (RTU) which can receive analog or digital signals
• Programmable controllers (PCs) and programmable
  logic controllers (PLCs)
• Distributed control systems (DCSs)
• Supervisory control data acquisition

Keyboard
RTU
CPU
CRT
Printer
13
Schematic of Monitoring Control System
14
Levels of Computer-BasedMonitoring and Control

• 1. Report generation
• 2. Data acquisition and logging/report
  generation/alarm indication
• 3. Data acquisition and logging/report
  generation/alarm indication/plant graphic display
• 4. Data acquisition and logging/report
  generation/alarm indication/plant graphic
  display/analog variable display
• 5. Data acquisition and logging/report
  generation/alarm indication/plant graphic
  display/analog variable display/manual plant
  control
• 6. Data acquisition and logging/report
  generation/alarm indication/plant graphic
  display/analog variable display/automatic plant
  control

15
Levels of Computer-BasedMonitoring and Control
- continued

• The estimated costs (1989) for the six
  alternative systems for a 2 m3/s (50 mgd)
  ordinary conventional process water treatment
  plant at ENR index of 5000.

System 1 System 2 System 3 System 4 System 5
System 6 5000 100000 125000 150000
600000 800000-
1000000
16
Design

• Process and instrumentation diagram (PID) based
  on the process flowsheets (PFS)
• Process control diagram (PCD)
• Instrumentation and input-output summaries
  (IIOSs)
• Instrumentation specification sheets (ISSs)
• Logic diagrams
• Panel layout drawings
• Loop interconnection drawings (LIDs)
• Instrument installation details (IIDs)
• Basic design philosophy KISS (keep it simple
  system)

17
Process Train with Chemical Feed and Process
Control Points
18
PID Example