PowerPoint-Pr

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Industrial Instrumentation
Dr. Ing. Naveed Ramzan
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Level Sensors
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Level Measurement

• Level is another common process variable that is
  measured in many industries. The method used will
  vary widely depending on the nature of the
  industry, the process, and the application.

Inventory -- a constant supply or storage of
material Control -- continuous, batch,
blending, and mixing control -- stabilize flow to
the next process Alarming -- hi/lo limits,
safety shut down Data Logging -- material
quantities for inventory and billing purposes and
where regulatory requirements are necessary
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What is measured?

• The measured medium can be liquid, gas or solid
  and stored in vessels (open/closed tanks), silos,
  bins and hoppers.
• Units of level can be expressed in
• feet (meters)
• gallons (liters)
• pounds (kilograms)
• cubic volume (ft3, m3)

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Methods ---- Direct or Indirect (inferential)

• Hydrostatic Head
• Float
• Load Cells
• Magnetic Level Gauge
• Capacitance Transmitters
• Magnetostrictive
• Ultrasonic
• Microwave
• Laser
• Radar
• Guided Wave Radar
• Dip Stick
• Vibration

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Direct Methods

• Direct methods sense the surface or interface of
  the liquid and is not affected by changes in
  material density (Specific Gravity)
• Examples
• Dip Stick
• Resistance Tapes
• Sight Glass
• Floats
• Ultrasonic

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Indirect Methods (Inferential)

• Indirect methods infer liquid level by
  measuring some other physical parameter such as
  pressure, weight, or temperature.
• Changing materials means a corrective factor must
  be used or recalibrating the instrument.
• Examples
• Hydrostatic head methods
• Load Cells
• Capacitance
• Conductivity

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Selection Criteria

• When determining the type of level sensor that
  should be used for a given application, there are
  a series of questions that must be answered
• Open tank or closed tank?
• Can the level sensor be inserted into the tank or
  should it be completely external? Contact or
  non-contact?
• Continuous measurement or point measurement?
• Direct or Indirect measurement?
• What type of material is being measured? Liquid
  or Solid? Clean or Slurry?

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Selection Criteria

• For all liquids you will need
• The system operating temperature with max. and
  min. excursions?
• two wide range expensive the sensor
• The system operating pressure?
• Check that system T and P do not conflict
  with the materials of construction?

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Selection Criteria

• For Solids
• Bulk density
• Be careful with very large silos as
  compaction at the bottom can greatly change
  assume bulk densities
• Flow characteristics?
• Expected particle size distribution?
• Is solid abrasive and/or corrosive and what is
  the moisture/solvent content?

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For Liquids
Dip Stick

• Simple and cheap
• Can be used with any wet material and not
  affected by density.
• Can not be used with pressurized tanks
• Visual indication only (electronic versions are
  available)

RodGauge - similar to a dipstick found in a car,
it has weighted line markings to indicate depth
or volume
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For Liquids
Sight Glass

• Another simple direct method of measuring
  liquids.
• Can be used in pressurized tanks (as long as the
  glass or plastic tube can handle the pressure)

Good for applications where non-contact
measurement is needed (like beverages)
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For Liquids
Floats

• Float rides the surface level to provide the
  measurement. Many different styles are available.
  Usually used for pump control, high/low level
  alarms and emergency shut-off

Liquid density does not affect measurement
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For Liquids
Conductivity Level Measurement
Point Level Measurement
Continuous Level Measurement
Advantages and disadvantages Low
Cost Conductive, non-coating liquids
only Insulating coatings can cause problems
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For Liquids
Resistance Tape

• The pressure of the fluid in the tank causes the
  tape to short-circuit, thus changing the total
  resistance of the measuring tape. An electronic
  circuit measures the resistance it's directly
  related to the liquid level in the tank.

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For Liquids
Bubblers

• Bubblers allow the indicator to be located
  anywhere.
• The air pressure in the tube varies with the head
  pressure of the height of the liquid.

Cant be used in closed tanks or where purging a
liquid is not allowed (soap). Very popular in the
paper industry because the air purge keeps the
tube from plugging.
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For Liquids
Bubblers

• Advantages
• -- Easy installation
• -- Continuous reading providing analogue or
  digital signal
• -- No moving parts
• -- Good accuracy and repeatability

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For Liquids
Bubblers

• Limitations
• -- Not suitable for pressurized tanks
• -- Sediments may block tube or probe
• -- Tanks must be freely vented

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Hydrostatic Head Level Sensors

• These methods infer level by measuring the
  hydrostatic head produced by the liquid column.
• A pressure sensing element is installed at the
  bottom of the tank and pressure is converted to
  level.
• Different liquid densities or closed tank
  applications must be accounted for.

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General Theory for Head Measurement
Hydrostatic Head Level Sensors

• The Pressure exerted by the Height of the liquid
  is
• P H x Density
• If the Density of the liquid is known then
• H Pressure
• Density

Note For liquids other than water, use the
density of water 0.0361 lb/in3 as a reference and
multiply by the SG of the liquid.
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Example
A dip stick measurement of the level of these 2
tanks indicates 30 feet of liquid in both tanks.
Calculate the pressure that each gauge will read
if tank 1 contains water (S.G. 1) and tank 2
contains oil (S.G. 0.85)
P ? psi
P ? psi
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Example
A dip stick measurement of the level of these 2
tanks indicates 30 feet of liquid in both tanks.
Calculate the pressure that each gauge will read
if tank 1 contains water (S.G. 1) and tank 2
contains oil (S.G. 0.85)
P ? psi
P H x Density 30 ft x 0.0361 lbs/in3
(30 x 12) x 0.0361 13 psi
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Example
A dip stick measurement of the level of these 2
tanks indicates 30 feet of liquid in both tanks.
Calculate the pressure that each gauge will read
if tank 1 contains water (S.G. 1) and tank 2
contains oil (S.G. 0.85)
P H x Density x SG 30 ft x 0.0361 lbs/in3
x 1 (30 x 12) x 0.0361 13 psi
P H x Density x SG 30 ft x 0.0361 lbs/in3
x 0.85 (30 x 12) x 0.0361 x 0.85 11 psi
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Practical Considerations when using head type
instruments
Hydrostatic Head Level Sensors (Contd)

• The reference point of the tank vs instrument
  input must be considered.

This may not be practical in some applications
where the tank elevation is below grade or where
a remote visual reading is required.
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Tank Elevations
Hydrostatic Head Level Sensors (Contd)

• Vertical rises and drops contribute to the
  overall height and therefore head pressure.
  Horizontal runs have no effect.

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Closed Tank Applications
Hydrostatic Head Level Sensors (Contd)

• Open tanks are vented to atmosphere so the
  pressure at the bottom of the tank is only due to
  the head pressure of the liquid.

Closed tanks are not vented to atmosphere so the
pressure at the bottom of the tank is due to the
head pressure of the liquid the vapour pressure
above the surface.
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Magnetic Level Sensor

• Used where the sight glass level gauge can not be
  used.
• Magneto-resistive types can provide an electrical
  output.

Liquid/liquid interface (such as water and oil)
can be measured by changing the buoyancy of the
magnetic float
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Displacers

• Not the same as a float.
• The displacer is immersed in the tank and the
  buoyant force of the liquid produces a torque
  which is proportional the amount of liquid level.

The output force can be converted to provide a
proportional pneumatic or electrical continuous
output of tank level.
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Displacer side mounted unit
Displacers

• The displacer float size and diameter is selected
  according to the size of tank and the height of
  the level to be measured.
• The output force can be converted to provide a
  proportional pneumatic or electrical continuous
  output of level.
• Advantages and disadvantages
• Very little movement of displacer float
• Affected by different liquid densities
• Limited to smaller tanks sizes

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UltraSonic Level Measurement

• Non-Contact direct level sensor
• Level is a function of the time it takes an
  ultrasonic pulse to hit the surface and return

• Limitations include
• Surface foam absorbs signal, agitation create
  reflections
• High Pressure High Temperatures affect the
  signal speed
• Vapour and condensate create false echos

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Radar Level Sensors (Microwave)

• Similar to ultrasonic but at a much higher
  frequency (6.3 GHz)
• Various designs
• -- Frequency Modulated
• Continuous Wave
• -- Pulsed Wave
• -- Guided Wave

These sensors have better performance in
applications where vapour, dust or uneven
surfaces exist.
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Load Cells

• Tank level is determined by the weight of the
  quantity of material
• Load Cells (strain gauge transducers) placed at
  the bottom of the tank measure the weight and
  then convert it to an electrical signal.

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Capacitance Level Detection (RF Level Probes)
RF Capacitance level sensors are similar in
operation to conductivity sensors and are
typically used when non-conductive liquids are
measured.
The only variable is the dielectric of the tank
material that varies in proportion to the level.
Conductive liquids like mineral water will short
out the probe to the tank wall. Good for solids
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Can be used for non-liquids

• RF capacitance probes are used in a wide range of
  applications and are subjected to certain
  limitations such as
• Electrode Coating
• Low sensitivity
• Temperature
• Different materials
• Shielding noise

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Summary

• Level is measured by locating the boundary
  between two media, called the interface
• Level can be measured directly or indirectly
• Noninvasive devices are preferred when the
  material is corrosive, hazardous, sterile, or at
  a high temperature or pressure

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Batch application using point level measurement
Creative Assignment! HAZOP?????

• Detect if the level is at a predetermined point
• Generally used to detect either a low-level
  condition or high-level condition
• The output of a point level sensor is on-off, or
  1 or 0 state digital signal

A batch process does NOT use a continuous level
measurement device.
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• Discussion Questions?