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

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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

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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
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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 5,000 100,000 125,000 150,000
600,000 800,000-
1,000,000
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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)

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Process Train with Chemical Feed and Process
Control Points
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PID Example