Level and Pressure Measurement

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Level and Pressure Measurement

• Christopher Foot
• Patrick Lawlor

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Level Measurement

• Why is it important?

Three Mile Island
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Level Measurement

• Assess the Situation
• Can the level sensor be inserted into the tank or
  should it be completely external?
• Should the sensor detect the level continuously
  or will a point sensor be adequate?
• Can the sensor come in contact with the process
  fluid or must it be located in the vapor space?
• Is direct measurement of the level needed or is
  indirect detection of hydrostatic head (which
  responds to changes in both level and density)
  acceptable?
• Is tank depressurization or process shut-down
  acceptable when sensor removal or maintenance is
  required?

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Level Measurement

• 5 Main Types
• Radar
• Differential Pressure
• Admittance-probe
• Ultrasonic
• Nuclear

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Radar Level Measurement

• Consists of Transmitter, Antenna, receiver/signal
  processor, operator interface.
• Electromagnetic wave sent downward toward surface
  of process fluid, reflected back and collected
• Level of tank based on difference of time from
  emission to collection to EM wave.

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Radar Level Measurement

• Advantages
• Can detect the liquid level a layer of light dust
  or airy foam.
• Least expensive of 5 common types
• Automated calibration.
• Disadvantages
• If foam layer is too thick, level of foam will be
  transmitted, not process fluid
• Interference from rat-holing, angled process
  material surfaces, solid deposition on antenna

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Differential Pressure Level Measurement

• Also know as Hydrostatic Tank Gauging
• Based on density of fluid and pressure changes
• DP h (SG)

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Differential Pressure Level Measurement

• Advantages
• Can be used in either open top or pressurized
  vessels
• Simple theory easy to apply
• Disadvantages
• Bubbles in system (affecting SG) can cause errors
  in measurement
• Error associated with pressure measurement passed
  along to level measurement

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Admittance-probe Level Measurement

• Operate in the low MHz range measuring admittance
  of an AC current circuit that varies with level.
• Based on Dielectric Constant of the material
• Often used as level switches

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Admittance-probe Level Measurement

• Advantages
• Good for level measurement of solids and other
  non-reflective materials
• Sensitivity easily adjusted by varying probe
  parameters
• Disadvantages
• Materials with high conductivity can cause a
  short within the probe
• Temperature has a large effect on the system
• Likelihood of false indications high with fluids

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Ultrasonic Level Measurement

• Also known as SONAR
• Similar to radar issues an electronic pulse
  (sound wave) and measures time until echo received

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Ultrasonic Level Measurement

• Advantages
• Instruments include temperature compensation,
  data filters and self-calibration options
• Can be used to detect the interface level between
  2 liquids
• Disadvantages
• Cannot be used with agitated liquids
• Signal will be absorbed by foam, dust, mist,
  humidity

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Nuclear Level Measurements

• Uses Radiation absorption to directly measure
  level using point source radiation and strip
  detection
• Highest absorption indicates lowest level in
  vessel

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Nuclear Level Measurements

• Advantages
• Can be used to measure either level or density of
  a process material
• Radiation source can be very long-lasting, often
  outliving the process
• Can be installed outside the process avoiding
  process interruptions as radioactivity can see
  through walls
• Disadvantages
• Radiation safety is very involved and can inhibit
  the process
• Measurements can be skewed by density

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Pressure Measurement

• Why is it important?

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Pressure Measurement

• Asses the situation
• What is the range of pressures to be measured?
• Is pressure dynamic or static?
• What is more important absolute accuracy or good
  repeatability?
• How much overpressure protection is required?

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Types of Measurement

• Mechanical
• U-tube manometer, Bourdon tube, Diaphragm and
  Bellows
• Electrical
• Strain Gauge, Capacitive sensor, Potentiometric,
  Resonant Wire, Piezoelectric, Magnetic, Optical

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Mechanical

• Mechanical pressure measurement devices are large
  and cumbersome.
• Not suited for automated control loops typical in
  industry.
• Mechanical devices
• U-tube Manometer
• Bourdon tube
• Diaphragm and Bellows element

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U-tube Manometer

• Measures difference in pressure between two
  points in a pipe.
• Typical in laboratories.

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Bourdon Type

• Flexible element used as sensor.
• Pressure changes cause change in element
  position.
• Element connected to pointer to reference
  pressure.

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Diaphragm and Bellows Element

• Similar concept to Bourdon type.
• Widely used because they require less space and
  can be made from materials that resist corrosion.

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Electrical

• Have become more common with increased reliance
  on computerized control systems.

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Strain Gauge

• Measures deflection of elastic diaphragm due to
  pressure difference across diaphragm.
• Widely used in industry.
• Used for small pressure ranges.
• Measurements tend to drift.

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Capacitive Sensor

• Measures changes in capacitance of electrically
  charged electrodes from movement of metal
  diaphragm due to pressure difference across
  diaphragm.

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Capacitive Sensor, cont.

• Can be operated in balanced or unbalanced mode.
• Balanced always has capacitance of zero. Measures
  pressure indirectly by measuring drift in
  capacitor arms.
• Unbalanced measures ratio between output voltage
  and excitation voltage.
• Widely used in industry.
• Large rangeability.

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Resonant Wire

• Wire is oscillated at resonant frequency by
  oscillator circuit.
• Pressure changes cause change in wire tension
  which changes oscillatory frequency.
• Generates digital signal.
• Very precise, used for low differential pressure
  measurements.
• Sensitive to temperature variation and has
  non-linear output

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Piezoelectric

• Measures the charge developed across quartz
  crystal due to change in pressure.
• Charge decays rapidly making unsuitable for
  static pressure measurements.
• Sensors are very rugged. Pressure can be applied
  longitudinally or transversally.
• Used to measure dynamic pressure changes
  associated with explosions and pulsations .

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Magnetic

• Measures induced current caused by movement of
  magnetic components from pressure changes.
• Used in applications where high resolution in
  small range is desired due to very high output
  signals.
• Sensitive to stray magnetic fields and
  temperature changes.

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Optical

• Detects effects of minute motions due to process
  pressure changes through partial blocking of an
  LED.
• Immune to temperature effects.
• Excellent stability and long-duration capability.

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Questions?
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References

• http//www.omega.com/literature/transactions/volum
  e3/pressure.html
• http//www.omega.com/literature/transactions/volum
  e4/
• http//www.mme.wsu.edu/me305/va.web/Exp16/U-Tube/
  utube_schematic.htm