Unit 20 Basic Control loop components

Question 1

When a process variable has no method of sending information to the final control element, the system is known as

Answer 1

open loop

Question 2

A variable adjusted in order to maintain or achieve a desired value of another variable is termed the

Answer 2

manipulated variable

Question 3

The apparatus that converts current signals to pneumatic and vice versa is a

Answer 3

Transducer

Question 4

A device that converts the measurement of a sensing device into a signal, is a

Answer 4

Transmitter

Question 5

State the most common signal range for pneumatic instrument signals.

Answer 5

20-100kPa

Question 6

List 2 types of electrical signals used in control systems and give an example of where each might be used

Answer 6

Voltage signals - used with switching devices eg. flame scanners, low water fuel cut off, pump controllers
Current signals- used for analog instruments where signals must be sent over long distances

Question 7

A common example of a photoelectric transducer used on boilers is the

Answer 7

flame scanner

Question 8

An inclined manometer is commonly used to measure

Answer 8

very low pressures or vacuums

Question 9

In a glass stem thermometer the space above the liquid is usually

Answer 9

evacuated

Question 10

A chromatoghraph uses electronic devices which

Answer 10

have decreased resistance as temperature rises

Question 11

The electrical output from a thermocouple is measured by a

Answer 11

millivoltmeter

Question 12

a device used to measure variations in atmospheric pressure is called a

Answer 12

Barometer

Question 13

To increase the movement of a Bourdon tube without use of linkage that may cause additional error, the tube can be formed into the shape of a

Answer 13

spiral or helix

Question 14

Name 3 types of sensing elements used to measure flow by creating a differential pressure in the pipe

Answer 14

Orifice plate, venture tube, flow nozzle

Question 15

The pneumatic transmitter nozzle receives a constant air supply pressure of approximately?

Answer 15

140 kPa

Question 16

When controller output is proportional to the amount of deviation, the controller is

Answer 16

Proportional

Question 17

when an operator uses CRT or LCD displays, the computer control display

Answer 17

can stretch or compress time periods

Question 18

A valve that seals on the downstroke is

Answer 18

direct acting

Question 19

Why is a double seated control valve better suited to throttling action than single seated

Answer 19

Less force is required to position the control valve because the pressure of the fluid acts on the top of one seat and the bottom of the other.

Question 20

What is the difference between and indicator and a recorder

Answer 20

An indicator shows only the current value of the process variable, whereas a recorder shows the current value and previous values to allow the operation of the process to be analyzed over time.

Question 21

What is the main cause of heating boiler failure

Answer 21

overheating the metal surfaces due to low water level

Question 22

What are 3 low water fuel cut-off devices

Answer 22

float type; magnetic; electric probe

Question 23

Automatic boiler programming controls require what kind of control switch

Answer 23

on-off or multi-range

Question 24

An interrupted pilot of an automatic boiler operates as

Answer 24

ignites, completes a trial, and extinguishes after a main flame trial

Question 25

liquid fuel manual valves for an automatically controlled boiler should be opened when

Answer 25

prior to pre-purging

Question 26

During a programmed start-up, if an igniter spark is visible but no pilot flame appears the operator should suspect

Answer 26

a pilot manual fuel valve is closed

Question 27

State the purpose of purging the boiler furnace before igniting the pilot flame

Answer 27

To clear the furnace of any combustible gases that may have built up

Question 28

If a boiler trips on low water level or flame failure, the programming controller will shut the boiler down. What steps must the operator take before the boiler will restart

Answer 28

The initial low water level or flame failure must be corrected. the operator may need to reset the boiler

Question 29

List the ASME codes and CSA regulations regarding low water fuel cut-offs

Answer 29

ASME Code, section IV- no shut off valves placed in cut-off piping. Piping will be a min of 1" CSA -B51 09

Question 30

A boiler control manual- automatic transfer or selector station may be used to

Answer 30

manually override automatic control loops

Question 31

Low water fuel cut-off must shut the fuel off Before the water level drops to

Answer 31

25 mm above the bottom of the gauge glass

Question 32

Modulating combustion control will change

Answer 32

air and fuel flow proportionally to steam pressure change

Question 33

According to the ASME code, explain the requirements for high pressure limit switches

Answer 33

The code states that all automatically fired steam heating boilers must have a high limit pressure activated switch that shuts off the fuel to the burner when steam pressure exceeds normal ranges

Question 34

The variable area meter consists of changing

Answer 34

area

Question 35

The main function of a mainframe computer system is to

Answer 35

rapidly process large amounts of data

Question 36

An analog control loop output must be put through a ____ before the computer can react to the signal

Answer 36

analog/digital converter

Question 37

A digital input computer is protected from harmful voltages by means of

Answer 37

isolating circuits

Question 38

Computer "run time totals" are commonly used for

Answer 38

maintenance management

Question 39

signals which have an infinite number of values between a minimum and maximum are

Answer 39

analog signals

Question 40

Digital signals have what values

Answer 40

on or off

Question 41

a manometer may be filled with what types of fluids

Answer 41

water or mercury

Question 42

What is the advantage of using an inclined manometer over a conventional U - tube manometer

Answer 42

increased accuracy

Question 43

How to know if a manometer is correctly calibrated

Answer 43

the level should be the same distance above zero in one leg, as it is below zero in the other

Question 44

What are 3 things a pressure gauge must be protected against to avoid damage while in operation

Answer 44

High temperatures - water seal Corrosive or congealing fluids - chemical seal vibration or pressure pulsation - damper or needle valve

Question 45

Four types of pressure sensing elements

Answer 45

Bourdon tube, Diaphragm, Bellows, strain gauge

Question 46

Three types of orifice plates

Answer 46

concentric, eccentric, and segmental

Question 47

Bernoulli's Principle

Answer 47

As flow velocity increases static pressure is reduced

Question 48

state the upper and lower limits of the temperature measuring capabilities for mercury

Answer 48

-39 - 450 C. if the space above the mercury is pressurized with nitrogen or argon, the upper limit may be increased to 550 C

Question 49

Explain the purpose of the relay used in a pneumatic transmitter

Answer 49

The relay boosts the air pressure signal from the nozzle pressure of 14-28kPa to the output pressure of 20-100kPa

Question 50

What type of controller compares actual conditions to setpoint; and sends a variable signal to the control valve, causing it to be open, closed or somewhere in between. In addition they include action that forces the final control element to move more quickly for rapid changes in the process variable

Answer 50

Proportional plus derivative (rate) controllers

Question 51

reset or proportional plus integral controllers

Answer 51

act as the rate controllers except ensure that the setpoint and control point are the same

Question 52

Describe modulating combustion control as it applies to a steam boiler

Answer 52

occurs when a burner remains lit at all times but the flame size is increased or reduced by controlling the fuel and airflow to the burner according to steam pressure.

Question 53

Why the low level fuel cut-off on a hot water boiler cannot be tested in the same manner as a steam boiler

Answer 53

Because the boiler is completely filled with water along with the piping systems, it is difficult to lower the water level below the operation level for the low water cut-off device. If the drain valve is opened, the water will not drain out because the upper connection is also filled with water

Question 54

The CRT is the visual display or monitor used by a computer system to output visual information to the operator.

Answer 54

Cathode Ray tube

Question 55

A modulator-Demodulator that converts digital signals generated by a computer to analog signals for transmission over telephone lines, and then back to digital when received by the receiving computer

Answer 55

Modem

Question 56

Transfer input information to the computer in sequence with the correct timing, to be read into the computers memory and for processing

Answer 56

Input Multiplexer

Question 57

Bit

Answer 57

one digital piece of information represented by a 1 or a 0 with no other possible value

Question 58

Byte

Answer 58

Eight bits

Question 59

Read Only Memory which is burned into a chip and cannot be changed, and is not erased when the power is turned off.

Answer 59

ROM

Question 60

What are the 3 levels of computer languages

Answer 60

Machine language - consists of ones and zeros at the memory chip level. Used by manufacturers to program microcomputers. Assembler language - a set of standard commands which the computer can interpret and convert to machine language. High Level language - English like commands which can be used by the programmer to program the computer to perform any desired function within its capacity.