Instrumentation & Control Flashcards
Describe the elements of a Control System
•Control systems regulate a process
-Oil temp.
-System pressure
-Tank level
-Water purity
•Detectors (Sensor)
•Tranducers ( ex. I/P 4-20mA to 3-15psi)
•transmitter(sends signal from one device to another) controller, recorder, remote indicator
•Controllers-
-Compare the signal from the transducer to the set point
-generates and output signal
-Sends output to control element
•Control Element
-The component that adjust the process to keep it within the set point
-Control valve
-variable speed
-electric heaters
Describe the difference between open-loop and closed-loop control system
•Closed loop system
-Use the system itself as an input to the controller
•Open loop system
-Use an outside reference to control the system
Describe the theory of operation of bimetallic strip thermometer
-Two strips of different metals are fit together
-Each metal has a different thermal expansion
Describe theory of operation of pressure measuring devices
•Pressure instruments come in contact with the system pressure
-Displayed on local instrument
-Converted into an electrical signal
Describe the theory of operation of level measuring devices
•Liquid level detectors measure either:
-Height of a liquid above a reference line
Or
-Hydrostatic head developed by the liquid
Describe the theory of operation of controllers
-They compare a sensed value to a setpoint value
-They also generate an output based on that and send the signal to a Final control element
Describe thermocouple
-Two different metals joined at one end called the sensing junction
-as the temp. changes it generates a voltage
-measured at the reference junction
-connect to a cold junction to transfer the voltage
Describe Resistance Temp. Device (RTD)
-Works on the known resistance in metals that change proportional to temp. (Ex.-platinum or copper)
-Goes in a thermowell and is spring loaded to ensure contact
-The fractional change is called “Temp. Coefficient of Resistance”
Theory of operation of bourdon tube
-Made of thin tube that’s been flattened and arced
-One end connected to the system and the other end closed
-works on pinion gear, fixed pivot, link to the flat tube
-Counteracted on by atmospheric press.
Theory operation on diaphragm press. element
-Metal disc built into a housing
-One side exposed to system press. And the other side acted by atmospheric press.
The operation theory of bellows press. Element
-More sensitive to low press.
-System press. on one side and atmospheric on the other side
-moving end connected to the indicator
-system press. must over come the spring
The operation theory of Differential Pressure cell (D/P Cell)
-Connected between two points in the system
-Low side connects inside the bellows
-high side connects outside the bellows
-the wall that feels both pressures connects to the indicator
-Have an equalizing to remove the instrument form service
*prevents the bellows from being exposed to only the high side
Describe the operation theory of ITT Barton (D/P) cell
-PVGS MOSTLY USES
-Uses dual liquid filled, rupture proof bellows that withstand over ranging
Different types of level detection
-Direct level detection
And
-indirect level detection
Direct level Detection
-Visual observation of distance on a suitable scale
-Determination of the position of a float riding the liquid surface
-contact of electrode probes with a liquid
Indirect level detection
•Use of hydrostatic head
•Using an open tank design
•Dry reference leg
•Wet reference leg
How does an Open vessel D/P cell lvl detector work
D/P cell:
-HP side->Tank
-LP side->Atmosphere
-Max lvl sensible is the height of the tank
-Min. Lvl sensible is where the cell is tapped into the tank
How does a dry reference leg work
-Closed system
-HP side->Tank (press. of gas + liquid hight)
-LP side-> dry leg
-Fails by building up condensation
-Has a drain valve.
-any gas change is felt on both sides so it cancels, only reads increase/decrease in water lvl.
How does a wet reference leg work
-Closed system
-Prevents condensation error like dry leg by filling the leg with the same liquid as the process
-HP side-> wet leg (Level is high than the tank level more pressure)
-LP side->tank
-Any gas added is felt on both sides so its effect is cancelled, only reads increase/decrease in water lvl
Describe ultrasonic lvl sensor
-Sound waves to measure the level of liquids and/or solids
-Measure the time it takes to come back as it echos off the tank
Describe Mangetic float lvl sensor
-Level chamber connected to the tank
-A float with a magnet is in the level chamber
-The magnet rotates a series of flaps outside the chamber to indicate lvl
Describe float and tape lvl sensor
-Large stainless steel float attached to stainless steel tape to read lvl
-Float is connected to a spring
-Has a knob to test the tape
Flow measurements
Two major categories
•Flow rate=amount of fluid that moves past a given point per unit of time (GPM)
•Total Flow (total quantity)
-The amount of fluid that moves past a given per during a specified time (1,000gal)
This flow meter consist of a float and conical glass flow tube
Rotameter flow detector
-This flow meter has a thin circular metal plate with a sharp-edged hole
-mounted between 2 flanges
-uses a D/P cell to detect pressure difference
A.) Orifice plate
Consist of:
•converging conical inlet
•cylindrical throat
•Diverging recovery cone
-Drop in static pressure as its flows through a constricted area.
Venturi effect
Ultra sonic flow detectors
-Use sonic waves
-2 sensors, one sends with the flow and the other sends against it
What measures the difference between static pressure and flowing pressure
-Annubar flow detector
Reed switch
- A sealed switch element actuated by a magnet
-CEDMEC control rods example
-Used for indicating valve position the entire length of the stroke
-slide wire that changes resistance of feedback proportional to valve position
-Potentiometers
Limit switches
-Mechanical switch contacted by indicator on valve stem
-Fully open or closed
Linear Variable Differential Transformer (LVDT)
-Determines valve position by a core moving through primary and secondary transformers
Bistables
-Two position switches that are either on or off
-Can have a reset point known as deadband
When a sensed value doesn’t match the set point value
-Error
Gain
-Ratio of output and input
-Controller output change over input change
Proportional
-Amount of change to get a 100% change in the output
-Inverses to Gain
-Input change over output change
But can stabilize off the set point (offset)
Proportional + Integral (PI)
-Eliminates offset error by keeping track of error over time
-Can overshoot the setpoint tho
Proportional+Integral+Derivative (PID)
-Get to the setpoint quickly without without overshooting
Pneumatic Controllers
•Receive an input signal and transmit and air signal to a control element
Electric Controllers
-Receive electrical signals and use algorithms to perform functions
Electro-Hydraulic Controller
-Operate solenoids to align hydraulic fluid to open and close large valves
-all or nothing
•Main turbine stop valves
OR
-Operate Servo Valves to control large valves
•Main turbine control valve
Direct acting controller
-Controller output increases as the value of the controlled variable increases
-Air to open
-spring to close
Example: As lvl in the drain tank goes up the valve position goes open
Reverse Acting controller
-Controller output decreases as the controlled variable increases
-Air to close
-spring to open
Example: All high level control valves
Precaution to take during switching controllers between manual and auto
-Match the set points with each other
-Verify it’s working right after swap
-Manual mode required constant monitoring
Valve actuators
-type of control element
-Control valve position
•Hydraulic
•Motor
•Pneumatic
•Solenoid
Hydraulic actuators
-Control large valves
-Quick and accurate response
-Some times spring loaded
-Sometimes hydraulic on both sides of piston
Motor Valve actuator
-Remote operation
-Safety related systems because class power and backups
-Has clutch to operate manually
- Fails as-is
Pneumatic Valve Actuators
-Control small valves low torque
-Diaphragm and piston types
-Spring open and close
-or air on both sides
Solenoid Valve actuators
-Directly control a valve
-Open or close only
-can control the control system for other control valves
-Quick acting and easy to install
-Spring to open or close
