Instrumentation Overview
What is Instrumentation?
- Instrumentation is art and science of measurement and control.
- It makes available the necessary process information [indication / trending / status] at a predetermined destination in a predetermined form.
- It also controls the parameter within a specified limit / at specified value.
- Instrumentation provides a means to the plant operator to operate the plant safely, continuously, and consistently with optimum productivity.
- It minimizes the human element in the plant operation
- Instrumentation provides handle for monitoring:
- Health of the process,
- Health and performance of Equipments
- Draws attention to the exceptional conditions
In fact instrumentation acts as eyes and ears for engineers of all the disciplines in general and chemical engineers in particular.
Parameters Measured
The parameters encountered in a process industry are:
· Pressure
· Level
· Flow
· Temperature
· Quality of intermediate and finished products
o Physical
o Chemical
· Speed/vibration/displacements for rotary machines
- Every possible law of physics and chemistry has been exploited to achieve accurate measurement of the above parameters
- All the field instruments/devices adhere to Ingress Protection (IPnn) & Area Classification norms
Evolution of Instrument Engg Field
Instrumentation and Automation” was recognized as a separate branch of engineering in between the two world wars as the concept of mass production came into vogue due to war efforts.
Changing Technologies
1. Indication local
2. Indication remote
- Pneumatic
- Analog electronics
- Digital electronics
- Microprocessors (Configurable)
- DCS & PLC (Programmable)
3. Recording
- Paper-pen
- Thermal charts
- Trending
4. Exception reporting/Alarms
- Local
- Remote audiovisual – annuciators
- Alarm logs
5. Control
- Local instruments
- Local panels
- Plant wise control room
- Centralized control room for number of plants
Transmission media
- Pneumatic air pressure 0.2 to 1.0 kg/cm2 through tubes
- Electrical 4 to 20 ma through copper conductor
- Digital signals through twisted / shielded pairs
- Light signals through Fiber optic cables
- Air waves – wireless
Connectivity
- One to one
- Multiplexing
- Multidrop – bus topology
- Networks
Logics
- Switches
- Relay
- PLC
- ESD
· Hardware Base
· Configuration Base
· Software Base
CPUs have shifted approach from individual instruments to systems.
Networking capabilities have shifted approach towards large integrated systems.
System Components
- Power supply ®
- Central Processing Unit ®
- Memory
- Input Modules
- Output Modules
- Communication Modules ®
- Bus/network ®
- Programming terminal
- Human Machine Interface
System Features
Redundancy
Fall back – primary/main – secondary/standby
Sharing – 50/50 – 100/0
- Cold standby: If primary system fails, the changeover to secondary is manual. Interruption in operation. Moderate cost. Almost obsolete.
- Hot standby: Both primary and secondary are running continuously & processing I/Os in parallel. Only the primary has control on outputs. If primary fails, secondary immediately takes charge bump-lessly. Widely used. Moderate cost.
Ø Easy troubleshooting
Ø Easy expandability
Ø High availability – redundancy
Ø High reliability – fail safe design
Ø Self-diagnostic capability
Ø Alarm handling & Sequence Of Events (SOE)
Ø System security from unauthorised changes.
Ø Hierarchical Presentation like Complex, Plant, Unit, Tag.
Ø Historisation & data logging for ease of maintenance.
Each system has the capability to communicate with another system in general and DCS in particular
Objectives of Control Systems
- Operating personnel and Plant safety.
- Simplified and Efficient Plant operation.
- Automated stepwise start – loading – unloading – S/D
- High Plant availability.
- Easy and guided maintenance/diagnostics.
- User friendliness for Engineering, Operation and Monitoring functions.
- Fast and accurate analysis.
Instrumentation Systems
- DCS
- PLC
- ESD
- F&G
- MCMS
- ATG
- CTM
- BMS
- AMS – lab / online analyzers
- CCTV & Security Automation
Distributed Control System
- The distribution can be
o Functional (most commonly used)
o Functional + Physical
- Operator window for interface with plant
- Integrated in to a single site wide system.
- Single window to operations makes plant more manageable by reducing operator training by supporting information from all the other control systems.
- Used for monitoring & control of regulatory level, advance control & Optimisation.
- Self documenting features like loop diagrams.
- Redundancy is provided in all control processors & communications.
- Two to three operator console monitors are provided per operator.
- Provides detail History of plant with Alarm summary, Trend displays, detail displays etc.
PLC System
- PLCs are used for control & monitoring of packaged equipment like compressors, fired heaters, water treatment plants, back wash filters, etc.
- Stand alone control system & also having considerable facilities for centralized configuration & data monitoring.
- Systems are redundant in processors, power supplies & communications.
- Each PLC system has serial interface with DCS for centralized data & status monitoring by control room operators.
- Both CPUs run the application programmer. Primary CPU only controls the I/O & secondary CPU monitors the I/O.
- Windows based HMI software packages can be used for standalone control of the PLC.
Emergency Shutdown System
- Monitors & controls start-up & shutdown of entire plant.
- These systems are fault tolerant, Triple Modular Redundant (TMR) with a high availability, self test & self diagnostic facilities. Three CPUs solve logic parallel and output is based on 2oo3 voting.
- Sequence of events recording facility.
- All ESD systems are connected to plant wide DCS.
- Fail safe philosophy is adopted.
- 2oo3 logic is utilized in critical areas.
- ESD devices are separate from process instruments & will be used only for the the purpose of shutdown.
- Start ups are automated as far as possible.
- Soft/Key lock bypass switches with status indication.
- All bypasses are serially interfaced with DCS for monitoring.
Scan Time comparison for DCS, ESD & PLC
- DCS - 500 msecs
- PLC - 20 TO 50 msecs
- ESD - 1 msec
Hazards in Refinery
- Fire
- Leakage of Toxic Gases
- Leakage of Combustible Gases
- Rise in Temperature
Fire and Gas Monitoring System
- Detection of LEL, Toxic gas, Fire, Smoke, Heat.
- Also displays wind speed & Direction.
- Universal viewing at all strategic points like Fire stations, Security offices, Plant Control Rooms, Site in-charge’s offices etc.
F&G Indicators
- Stand alone indicator
- Addressable LCD display
- Dedicated F&G Work Station
- Integrated DCS F&G Work Station
F&G Devices
Machine Condition Monitoring System
The system is used for monitoring health and protection of the rotary machines.The following machine parameters is normally measured:
- the vibration levels of the casing and / or the shaft
- the shaft RPM
- axial shift of the shaft
- temperature of the thrust bearing pads / bearing housing
- Measure "Rider band” wear and tear (Rod drop measurement) in case of Reciprocating Compressor.
Following sensors are used in general to take vibration and temperature data of the rotary machines:
- Displacement probes
- Velomitors
- Accelerometers
- RTDs
ATG types of tanks
- Cone Roof Tank (With or W/O N2 purging)
- Open Floating Roof Tank
- Internal Floating Roof Tank. (With N2 Purging)
- Dome Roof Tank
- Sphere
- Horizontal ( Bullet )
ATG Objective
Raw parameter measurements in tank
- Level
- Temperature – multi spot / single spot
- Hydrostatic Pressure
- Static Pressure
- Water bottom
Calculated Tank Inventory
- Gross volume @ amb.
- Net Volume @ amb.
- Net standard volume @ 15 deg C.
- Mass
- Ullage
The above values are either calculated in TIS/DCS
Types of Tank gauging
- Servo
- Hydrostatic
- Radar
Custody Transfer Metering System
- Used For Custody transfer of raw materials & finished products
- N+1 meter run configuration for providing standby & master meter.
- Individual meter runs are corrected for pressure, temp. & density in the flow computers.
- Provers are provided for calibration of flow meters.
- Accuracy of the system should be 0.25% or better and repeatability should be 0.02% or better.
- Integrated with DCS.
- Used For Custody transfer of raw materials & finished products
- N+1 meter run configuration for providing standby & master meter.
- Individual meter runs are corrected for pressure, temp. & density in the flow computers.
- Provers are provided for calibration of flow meters.
- Accuracy of the system should be 0.25% or better and repeatability should be 0.02% or better.
- Integrated with DCS.
Burner Management System
- BMS is required for controlling the Fired Heaters light up sequence and also to monitor the safe operation of the furnaces.
- Relays/PLCs are used for the implementation of the BMS logic.
- A local panel is provided near fuel skid for operator interface during heater start up operation.
- For the detection of individual flames, ionization rods are provided for the pilot flame & IR scanners are provided for the main flame.
- Automatic shut off valves in double block & bleed arrangement are provided for fuel lines in fuel skid.
- Start up sequence can be typically divided in to leak test, purging, pilot firing & main fuel firing.
- Hard wired interface with ESD system for shutdown.
- Serial interface with DCS for monitoring by control room operator.
Heater trip logic is in general based on following philosophy:
Event Pilot Fuel Trip Main Fuel trip
Loss of pilot pressure/flame Y Y
Loss of fuel pressure/flame N Y
Rise in furnace pressure N Y
Analyzer Management System
Various types of Analysers include:
- Steam & water quality analyzer – pH, Conductivity, silica, oil in water
- Gas Analyzers – CO, O2, H2S, SOX/NOX, SO2, H2
- Physical property analyzers - Flash point, Pour Point, Distillation, Moisture, density, Smoke/Opacity, Viscosity
- Gas Cinematography – It can analyze multiple components based on retention & evolution in/from an adsorption media.
Sampling: It is the extraction of the process fluid and its delivery to the analyzer.
Sample conditioning: It consists of assembly of components which remove moisture, particulate and regulates sample pressure, temperature & flow at acceptable limits to the analyzer
Sampling Delay: It is the time taken by the sample in reaching the analyzer from the process.
Fast Loop: Part of the sample is recycled to the process to avoid dead volume in the sampling system.
Cycle Time: It is the total analysis time taken by the analyzer for analyzing desired components in the
Process stream.
- Analyzer Houses are located at strategic locations to collect various samples required for analysis & reduce the length of sample lines.
- All analyzers are connected to analyzer management system for collection of data.
- The system
o monitors the healthiness of sampling system
o monitors the healthiness of analyser electronics
o trends the parameters
CCTV & Security Automation
CCTV system
- Is used for monitoring of flare stacks, General surveillance (loading /unloading, major equipment's monitoring), fire prone areas etc.
- Time lapse video recording is provided for reviewing history.
Security system
- Used for access control
- Automatic movement of gates
Controllers
One of the basic functions executed by instrumentation system in process industry is ‘controlling the process parameter’. There may be two types of controls:
- Open Control Loop
- Close Control Loop (Automatic Feedback Control)
PID/Regulatory Control
- Proportional Action
o Output = K1 * e
- Proportional + Integral Action
o Output = K1 * e + K2òedt
- Proportional + Integral + Derivative Action
o Output = K1 * e + K2òedt + K3 de/dt
Where
e = error (difference between set point and measured variable)
K1 = Gain
K2, K3 = constants
Instrumentation Controls
- Application specific
o Compressors – Anti-surge, performance, speed control
o Furnace/Boiler – Lead-Lag control
o Boiler – 3-Term Control
- On-off / PID controls
- Complex loops / traditional advanced control
o Ratio, feed forward, cascade, adaptive
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