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GAP Fundamentals > Instrumentation & Control > Flashcards

Instrumentation & Control Flashcards

1
Q

Describe the elements of a Control System

A

•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

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2
Q

Describe the difference between open-loop and closed-loop control system

A

•Closed loop system
-Use the system itself as an input to the controller

•Open loop system
-Use an outside reference to control the system

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3
Q

Describe the theory of operation of bimetallic strip thermometer

A

-Two strips of different metals are fit together
-Each metal has a different thermal expansion

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4
Q

Describe theory of operation of pressure measuring devices

A

•Pressure instruments come in contact with the system pressure
-Displayed on local instrument
-Converted into an electrical signal

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5
Q

Describe the theory of operation of level measuring devices

A

•Liquid level detectors measure either:
-Height of a liquid above a reference line
Or
-Hydrostatic head developed by the liquid

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6
Q

Describe the theory of operation of controllers

A

-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

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7
Q

Describe thermocouple

A

-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

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8
Q

Describe Resistance Temp. Device (RTD)

A

-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”

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9
Q

Theory of operation of bourdon tube

A

-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.

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10
Q

Theory operation on diaphragm press. element

A

-Metal disc built into a housing
-One side exposed to system press. And the other side acted by atmospheric press.

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11
Q

The operation theory of bellows press. Element

A

-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

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12
Q

The operation theory of Differential Pressure cell (D/P Cell)

A

-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

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13
Q

Describe the operation theory of ITT Barton (D/P) cell

A

-PVGS MOSTLY USES
-Uses dual liquid filled, rupture proof bellows that withstand over ranging

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14
Q

Different types of level detection

A

-Direct level detection
And
-indirect level detection

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15
Q

Direct level Detection

A

-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

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16
Q

Indirect level detection

A

•Use of hydrostatic head
•Using an open tank design
•Dry reference leg
•Wet reference leg

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17
Q

How does an Open vessel D/P cell lvl detector work

A

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

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18
Q

How does a dry reference leg work

A

-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.

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19
Q

How does a wet reference leg work

A

-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

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20
Q

Describe ultrasonic lvl sensor

A

-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

21
Q

Describe Mangetic float lvl sensor

A

-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

22
Q

Describe float and tape lvl sensor

A

-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

23
Q

Flow measurements

A

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)

24
Q

This flow meter consist of a float and conical glass flow tube

A

Rotameter flow detector

25
Q

-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

A.) Orifice plate

26
Q

Consist of:
•converging conical inlet
•cylindrical throat
•Diverging recovery cone

-Drop in static pressure as its flows through a constricted area.

A

Venturi effect

27
Q

Ultra sonic flow detectors

A

-Use sonic waves
-2 sensors, one sends with the flow and the other sends against it

28
Q

What measures the difference between static pressure and flowing pressure

A

-Annubar flow detector

29
Q

Reed switch

A
  • A sealed switch element actuated by a magnet
    -CEDMEC control rods example
30
Q

-Used for indicating valve position the entire length of the stroke
-slide wire that changes resistance of feedback proportional to valve position

A

-Potentiometers

31
Q

Limit switches

A

-Mechanical switch contacted by indicator on valve stem
-Fully open or closed

32
Q

Linear Variable Differential Transformer (LVDT)

A

-Determines valve position by a core moving through primary and secondary transformers

33
Q

Bistables

A

-Two position switches that are either on or off
-Can have a reset point known as deadband

34
Q

When a sensed value doesn’t match the set point value

A

-Error

35
Q

Gain

A

-Ratio of output and input
-Controller output change over input change

36
Q

Proportional

A

-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)

37
Q

Proportional + Integral (PI)

A

-Eliminates offset error by keeping track of error over time
-Can overshoot the setpoint tho

38
Q

Proportional+Integral+Derivative (PID)

A

-Get to the setpoint quickly without without overshooting

39
Q

Pneumatic Controllers

A

•Receive an input signal and transmit and air signal to a control element

40
Q

Electric Controllers

A

-Receive electrical signals and use algorithms to perform functions

41
Q

Electro-Hydraulic Controller

A

-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

42
Q

Direct acting controller

A

-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

43
Q

Reverse Acting controller

A

-Controller output decreases as the controlled variable increases
-Air to close
-spring to open

Example: All high level control valves

44
Q

Precaution to take during switching controllers between manual and auto

A

-Match the set points with each other
-Verify it’s working right after swap
-Manual mode required constant monitoring

45
Q

Valve actuators

A

-type of control element
-Control valve position
•Hydraulic
•Motor
•Pneumatic
•Solenoid

46
Q

Hydraulic actuators

A

-Control large valves
-Quick and accurate response
-Some times spring loaded
-Sometimes hydraulic on both sides of piston

47
Q

Motor Valve actuator

A

-Remote operation
-Safety related systems because class power and backups
-Has clutch to operate manually
- Fails as-is

48
Q

Pneumatic Valve Actuators

A

-Control small valves low torque
-Diaphragm and piston types
-Spring open and close
-or air on both sides

49
Q

Solenoid Valve actuators

A

-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

GAP Fundamentals (9 decks)

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