Test 2 Flashcards
(44 cards)
1
Q
Ultrasonic Level Detectors (general) Principle of Op
A
- Works on the principle of reflected sounds
- Transmitter generates a high frequency sound directed at the surface of the material in the vessel
2
Q
Ultrasonic Type 1 (Continuous) Principle of Op
A
- Utilizes transmitter-receiver unit which will be mounted on top of a vessel pointed at the surface of material to be measured
- A sound wave is produced and reflected by the material surface back to the unit
- The time taken for the sound to travel from the unit and back is directly proportional to the level in the vessel
- Produces an analog symbol
- Provides contacts that can be used for alarms to turn pumps on or off
3
Q
Ultrasonic Type 2 (On/Off or point level)(Gap Switches) Principle of Op
A
- Utilisez transmitter-receiver unit
- Sound is sent from transmitter to receiver
- A signal is produced if sound wave is broken
- On/off signal only
- Used for high alarms, low alarms, and to count objects
4
Q
Ultrasonic Level Detector Advantages:
A
- Does not contact the process -> can be used to measure solids, acids, etc.
- Easy to install
- Reliable
- Inexpensive
5
Q
Ultrasonic Level Detector Disadvantages:
A
- Can be coated if the process is very dusty or if a heavy mist/vapor is present
- Air can be used to purge the front of the head
- Not used in high temp/pressure
6
Q
Radar Level Application Examples
A
- Fuming acids
- Tars
- Heavy hydrocarbons
- Process reactor levels
7
Q
General Radar P/O
A
- Uses electromagnetic waves to make continuous liquid level measurements
- Micro-waves are at a very low level of power as the typical industrial measuring range is less than 30m therefore there are no special health, safety, licensing or product contamination
- Radar sensor is typically mounted at the top of the vessel and aimed down, perpendicular to the level surface, causing the signal to be reflected from the surface directly to the sensor
8
Q
Pulse Radar Level Detector P/O
A
- As with ultrasonic methods, a radar generator pulses a constant frequency wave toward the surface of the material and it reflects back to the source
- The time it takes for the wave pulse to get to the liquid surface and back is a measure of the level
- Microwaves travel ~speed of light therefore the time must be measured in picoseconds or a trillionth of a second
9
Q
FMCW Acronym
A
Frequency Modulated Continuous Wave
10
Q
FMCW Radar
A
- Accomplished by sending out a continuous signal that repeatedly changes in frequency between a minimum and maximum value
- The microwave signal is sent to the surface of material and reflects back to the emitter
- The reflected signal has a different frequency than the emitted signal
- Instead of measuring the time it takes for the wave to return, it measures the phase difference between the transmitted and received signals
- The phase shift varies based on the distance between the emitter and the surface of the material therefore measures the level
- Can be mounted outside of tank walls of certain material allowing
11
Q
NEMA
A
National Electrical Manufaturer’s Association
12
Q
IEC Acronym
A
International Electrotechnical Comission
13
Q
What does the size labelled on a motor starter determine?
A
Rating of the actual contacts
14
Q
Motor Starter Notes
A
- A motor starter is similar to a contactor except it has built in overload protection
- Overload protection is done by placing overload heaters
- These are placed in series with each phase of line voltage and sense electrical something of current to break electrical continuity of the motor thus shutting it down
- Starters often feature auxiliary contacts
Motor starters are usually physically larger than contactors and control relays
15
Q
Guided Wave Radar Notes
A
- Works on the principle of time-delay reflectometry (TDR)
- An electromagnetic pulse is sent down a probe that usually extends to the bottom of the tank
When the process is in contact with the probe, this creates a reflective point for the electromagnetic pulse - The wave reflects off the point of contact with the process and back to the transmitter
- The transmitter internally measures the time delay and registers a distance which is proportional to the level of the tank
- This method reduces the effect of dust above granular solids as well as turbulent liquids
16
Q
Laser Level Sensor Notes
A
- Transmitter consists of a laser beam generator, timer, and detector
- Method is similar to ultrasonic and pulse type radar in that it measures the time for the light signal to travel to the liquid surface and return
- Used on difficult process applications
- The laser uses infrared radiation with a wave length of 900nm
17
Q
Laser Level Sensor Advantage
A
Unlike ultrasonic and radar, it has a very narrow beam therefore less likely to pick up interference from walls, support beams, and agitators
18
Q
Laser Level Sensor Disadvantages
A
- Cannot measure applications with dust, vapour, or where light may be reflected (off something other than the liquid)
- Laser can damage eye sight so need to follow safety precautions
19
Q
Typical Nuclear Detectors
A
- Geiger Meuller Counter
- Ionization Chamber
- Scintillation Counter
20
Q
General Nuclear Notes
A
- Consists of a natural radioactive source that directs radiation through a vessel to a detector
- Examples: Cesium 137 (used most often because it decays slowly), Cobalt 60 (used when tank walls are thick)
- A stronger source is more accurate, but requires more shielding
21
Q
Point Level Measurement Nuclear Transmitter Notes
A
- Used only for alarms or on/off control
- Only determines whether there is radiation or not. Therefore, the detector sees full field intensity or little to no field intensity
- Geiger Meuller (GM) tubes are most commonly used for level switch designs
- GM tubes have a serious draw back, in that they can fail causing receiver to apparently see a level that is too high
22
Q
Point Level Measurement Nuclear Transmitter Configurations:
A
1 source and 2 detectors
2 sources and 2 detectors
Both configurations give a high and low alarm
23
Q
Continuous Level Meassurement Nuclear Transmitter Notes
A
- Gas ionization chambers and scintillation counters are used for continuous measurement
- Measures the amount of radiation travelling through the vessel
24
Q
Continuous Level Meassurement Nuclear Transmitter Configurations
A
- 2 sources and a strip detector
- A strip source and a strip detector
25
Continuous Level Meassurement Nuclear Transmitter Advantages
- Source and detector are mounted on the outside of the vessel, therefore, can measure any material (slurries, liquids, corrosives, solids, etc)
- Not affected by temperature or pressure
26
Methods of solid level measurement:
- Ultrasonic
- Capacitance
- Radiation
- Radar
- Rotating Paddle
- Diaghpragm Box
- Float Type
- Tuning Fork
- Weighing System
27
Continuous Level Meassurement Nuclear Transmitter Disadvantages
- Very expensive due to shielding, safety hazards, licensing, etc
- Less accurate than most other methods therefore only used when other methods will not work
28
Rotating Paddle Switch Notes
- Used to detect presence or absence of solids in a silo
- A low torque, slow speed motor keeps a paddle in motion at a very low speed
- When solids are absent, there is no torque on the paddle drive assembly
- When level reaches the device, the motor stalls, activating a switch that can be used for alarms or for activation of emptying / filling equipment
- Larger paddle areas can be used for low density materials and wire designs for heavier materials
- Mounting location should be carefully selected, giving special consideration to possible variations in angle of repose and how inlet and outlet will affect average level
29
Rotating Paddle Switch Advantage
Low cost
30
Rotating Paddle Switch Disadvantage
Needs constant power to run motor
31
Rotating Paddle Switch Operation
- When no solids present -> entire drive assembly rotates at the speed of gear clockwise
- With solids present, the paddle stops, torque develops in the tube, and torque rod is forced clockwise relative to the rest of the assembly
32
# D
Diaphragm Switch Operation
- As pressure on the diaphragm increases caused by material level, a switch is activated
- In newer designs, the diaphragm itself vibrates
- Vibrating diaphragms are smaller, lighter, and more sensitive than earlier designs
- When solids rise up to the diaphragm level, the load on the diaphragm decreases the amplitude of vibration, triggering the level switch
33
Diaphragm Switch Advantage
- Inexpensive
- Less maintenance than a rotating paddle (no moving parts)
34
Diaphragm switch notes
- On/off device
- Can be mounted internally or externally
- Pressure on diaphragm caused by material will activate a switch
35
3 Ways to Install Diaphragm Switch
- Suspended on a support pipe
- Mounted inside wall of thick walled silos
- Most commonly on the outside of thin wall bins/silos
36
Diaphragm Switch Installation
- Mounting location must always be selected to guarantee free flow of solids to and from the diaphragm area
- Switch 1 protects against overfilling
- Switch 2 signals a low supply
- Switch 3 indicates choke-up on the screw conveyor
- Analog measuring device 4 detects overfeeding of the elevator boot
- Analog measuring device 5 detects plugging of the elevator discharge spout
- 6 and 7 in the storage silo signal extreme level conditions (6 high, 7 low)
37
When measuring level...
Consider an ultrasonic or radar first
38
Consideration when choosing a level measuring device
- Accuracy
- Cost/Budget (including maintenance cost)
- On/off or continuous output required
- Environment - whether dust or vapor is present; if process is corrosive
- Contact or non-contact measurements
- Rangeability
- Temperature
- Pressure
- Changes in SG of process
- Open or closed tank
- Reliability (mechanical moving parts?)
- Compatability with existing systems
- Turbulence
39
Float-Type Tape Gauge nickname
Yo-Yo
40
Float-Type Tape Gauge Notes
- High maintenance device as the bob often gets caught in the material being measured
- Can work at timed intervals or on-demand remotely
41
Float-Type Tape Gauge Disavantages
- Relatively high maintenance
- Expensive
- Error-prone
42
Classes for temperature filled systems
1. Liquid
2. Vapour actuated
3. Gas expansion
5. Mercury
43
Thermometer Accessories
- Colored back
- Metal well and holder
- Can be shaped to magnify reading
- Can have a restriction to slow down change in indication
44
Methods of temperature measurement:
- Thermometers
- Filled systems
- Bimetallic
- RTD (resistance temperature detector)
- Thermistor
- Thermocouples
- Pyrometer
