Level Measurements

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• Level Measurements

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Purpose of Level Measurement

• Level Measurement is an integral part of process
  control, and may be used in a wide variety of
  industry.
• The purpose of level measurement is to provide a
  measured variable representing the height or
  material presence within a vessel.

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Purpose of Level Measurement (cont)

• The measured variable is then used in one or more
  of the following ways, where the measured
  variable becomes
• a local level indication,
• a detection of material presence,
• an input signal to a level control scheme,
• or a measured quantity in hydrocarbon inventory
  management.

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Types of Level Measurements

• There are two ways of level measurement
• Point level Measurement
• Continuous level measurement

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Types of Liquid Level Measurement Configuration

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• Point level sensors are used to mark a single
  discrete liquid height a preset level condition.
  Overfill alarm , low alarm

Point level Sensor
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Continuous Level sensors are used when a
complete level monitoring of a system is
required.

Continuous level Sensor
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Point Level Measurements

• Point level process measurements are a type of
  level measurement used when the level measurement
  device or system provides a logical (true or
  false, on-off) representation of material
  presence at some predetermined point (s) within
  the vessel.

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When To Use Point level Measurements

• You can use point level measurement in one of the
  following or in both
• 1.Point Level Process Indication
• 2.Point Level Process Control

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• 1.Point Level Process Indication
• It provides an indication of process material
  level at a point within a vessel. A single point
  level alarm, such as a vessel level high alarm
  limit, is an example of point level process
  indication

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Point Level Indicator

Circuit
Indicator
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• 2.Point Level Process Control
• It provides the control of a material level
  between two points in a vessel.
• To accomplish point level process control, more
  than one level switch senses a liquid presence at
  two (or more) measured point levels in a tank.

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Point level Process Control

High level
Digital Circuit
LT
LT
Low level
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Continuous Level Process Measurement

• In continuous level measurement, a level
  measurement system can provide a numeric
  representation of the current position (height)
  of process material surface.
• The level is measured continuously between lower
  and upper reference level

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When to use Continuous level Measurements

• Continuous level measurement is used in the
  following
• Continuous level process indication
• Continuous level process control

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Terminology and Symbology

• The following are considered when dealing with
  level instrument
• Units of measure
• Drawing symbols

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Units of measure

• Units of measure can be classified according to
  the application as follows
• Distance meters or feet
• Weight Kilograms or Pounds
• Volume Cubic feet, Gallon, Liter, Barrel

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Level measurement can be used to determine
volume and weight of process material as follows

• V volume A vessel area L height of material
• W Weight A vessel area L height of material
  D density

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Drawing Symbols

• Drawing symbols used in instrumentation are

LT
LG
LT
Level Transmitter Diff. Pressure Type
Level Gauge Glass
Level Transmitter Ext. Float Type
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Drawing Symbols

LS
LS
Level Switch Externally Mounted
Multi-position level Level Switch Internally
Mounted
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Instrument Legend

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Level Measurement Principles

• There different ways used to measure a level of
  liquid or solid in a vessel.
• Different sensors with different design
  principles are commercially available.
• Some of theses sensors are contact type and some
  of them are non-contact type.

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Types of level devices

• Sight
• Float type
• Displacement type
• Force type

• Pressure type
• Electrical type
• Ultrasonic Type

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1.Sight

• Sight level measurements gives the operator a
  visual indication of the materials level
• Examples are
• Dipstick
• Tubular Gauge Glass

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a. Dipstick

• Is the simplest level measurement device and its
  accuracy depends on its scale.
• It can be used only for vented and underground
  tanks.
• Source of errors
• Fast insertion
• Insertion with an angle

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b. Tabular Gauge Glass

• The tubular gauge consist of glass pipe which has
  the level of the liquid in the tank.
• Used in low pressure services to ovoid breakage
  .
• Fitted externally , protector are available to
  protect glasses.

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• In conclusion, sight glasses are used for local
  indication and are important for calibration.
• It is important to consider the following
• Operating pressure
• Operating Temperature
• Process material
• Fitting the device externally
  

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Float Level Switch

• Float Level switches is the basic point level
  sensor which moves with the liquid surface.

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• Float device include the following
• Float switch
• Tape float gauge
• Magnetic type

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a. Float Switch

• A float switch is attached to lever that
  activates a relay at a predetermine level

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Float switch (Cont)

• A float resting on the liquid surface can
• activate a magnetic switch
• as shown.

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Float Switch (Cont)

• Spherical float are available from 3 to 7 in
  diameter
• The small diameter switch is used for high
  density liquid
• The larger diameter switch is used for
  liquid-liquid interface detection

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Float Switch (cont)

• Application of float switch includes
• Emergency shutdown
• Alarming
• ON-OFF control application

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b. Tape Float Gauge

• An externally scaled indicator is attached to
  float As shown. When the liquid level is moving
  up or down the float will follow and the
  indicator will point the level.
• Application Used in water
• Storage tank

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c. Magnetic Type

• The float is called magnetic if a magnet is
  inserted in the float.
• The float is magnetically coupled to a tape and
  the tape has a connection to a level indicator as
  shown.
• Application
• In utility service

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Non-Contact Ultrasonic Sensors

• The ultrasonic sensor consists of the
• following
• Sensor element
• Analog signal processor
• Microprocessor
• Binary coded decimal range switch
• And an output drive circuitry

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Ultrasonic Level sensors
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When determining what type of level sensor
should be used for a given application, there are
a series of questions that must be answered

• 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?

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• 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?