Unit 20 Basic Control loop components Flashcards
When a process variable has no method of sending information to the final control element, the system is known as
open loop
A variable adjusted in order to maintain or achieve a desired value of another variable is termed the
manipulated variable
The apparatus that converts current signals to pneumatic and vice versa is a
Transducer
A device that converts the measurement of a sensing device into a signal, is a
Transmitter
State the most common signal range for pneumatic instrument signals.
20-100kPa
List 2 types of electrical signals used in control systems and give an example of where each might be used
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
A common example of a photoelectric transducer used on boilers is the
flame scanner
An inclined manometer is commonly used to measure
very low pressures or vacuums
In a glass stem thermometer the space above the liquid is usually
evacuated
A chromatoghraph uses electronic devices which
have decreased resistance as temperature rises
The electrical output from a thermocouple is measured by a
millivoltmeter
a device used to measure variations in atmospheric pressure is called a
Barometer
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
spiral or helix
Name 3 types of sensing elements used to measure flow by creating a differential pressure in the pipe
Orifice plate, venture tube, flow nozzle
The pneumatic transmitter nozzle receives a constant air supply pressure of approximately?
140 kPa
When controller output is proportional to the amount of deviation, the controller is
Proportional
when an operator uses CRT or LCD displays, the computer control display
can stretch or compress time periods
A valve that seals on the downstroke is
direct acting
Why is a double seated control valve better suited to throttling action than single seated
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.
What is the difference between and indicator and a recorder
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.
What is the main cause of heating boiler failure
overheating the metal surfaces due to low water level
What are 3 low water fuel cut-off devices
float type; magnetic; electric probe
Automatic boiler programming controls require what kind of control switch
on-off or multi-range
An interrupted pilot of an automatic boiler operates as
ignites, completes a trial, and extinguishes after a main flame trial
liquid fuel manual valves for an automatically controlled boiler should be opened when
prior to pre-purging
During a programmed start-up, if an igniter spark is visible but no pilot flame appears the operator should suspect
a pilot manual fuel valve is closed
State the purpose of purging the boiler furnace before igniting the pilot flame
To clear the furnace of any combustible gases that may have built up
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
The initial low water level or flame failure must be corrected. the operator may need to reset the boiler
List the ASME codes and CSA regulations regarding low water fuel cut-offs
ASME Code, section IV- no shut off valves placed in cut-off piping. Piping will be a min of 1”
CSA -B51 09
A boiler control manual- automatic transfer or selector station may be used to
manually override automatic control loops
Low water fuel cut-off must shut the fuel off Before the water level drops to
25 mm above the bottom of the gauge glass
Modulating combustion control will change
air and fuel flow proportionally to steam pressure change
According to the ASME code, explain the requirements for high pressure limit switches
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
The variable area meter consists of changing
area
The main function of a mainframe computer system is to
rapidly process large amounts of data
An analog control loop output must be put through a ____ before the computer can react to the signal
analog/digital converter
A digital input computer is protected from harmful voltages by means of
isolating circuits
Computer “run time totals” are commonly used for
maintenance management
signals which have an infinite number of values between a minimum and maximum are
analog signals
Digital signals have what values
on or off
a manometer may be filled with what types of fluids
water or mercury
What is the advantage of using an inclined manometer over a conventional U - tube manometer
increased accuracy
How to know if a manometer is correctly calibrated
the level should be the same distance above zero in one leg, as it is below zero in the other
What are 3 things a pressure gauge must be protected against to avoid damage while in operation
High temperatures - water seal
Corrosive or congealing fluids - chemical seal
vibration or pressure pulsation - damper or needle valve
Four types of pressure sensing elements
Bourdon tube, Diaphragm, Bellows, strain gauge
Three types of orifice plates
concentric, eccentric, and segmental
Bernoulli’s Principle
As flow velocity increases static pressure is reduced
state the upper and lower limits of the temperature measuring capabilities for mercury
-39 - 450 C. if the space above the mercury is pressurized with nitrogen or argon, the upper limit may be increased to 550 C
Explain the purpose of the relay used in a pneumatic transmitter
The relay boosts the air pressure signal from the nozzle pressure of 14-28kPa to the output pressure of 20-100kPa
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
Proportional plus derivative (rate) controllers
reset or proportional plus integral controllers
act as the rate controllers except ensure that the setpoint and control point are the same
Describe modulating combustion control as it applies to a steam boiler
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.
Why the low level fuel cut-off on a hot water boiler cannot be tested in the same manner as a steam boiler
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
The CRT is the visual display or monitor used by a computer system to output visual information to the operator.
Cathode Ray tube
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
Modem
Transfer input information to the computer in sequence with the correct timing, to be read into the computers memory and for processing
Input Multiplexer
Bit
one digital piece of information represented by a 1 or a 0 with no other possible value
Byte
Eight bits
Read Only Memory which is burned into a chip and cannot be changed, and is not erased when the power is turned off.
ROM
What are the 3 levels of computer languages
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.
