Pressure Measurement

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Pressure Measurement

• Mark Murphy, PE
• Technical Director, Fluor Corp.

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(No Transcript)
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Pressure Measurement

• Pressure is the force exerted per unit area
• Pressure is the action of one force against
  another force. Pressure is force applied to, or
  distributed over, a surface. The pressure P of a
  force F distributed over an area A
  is defined as P F/A

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Pressure Measurement Terms

• Absolute Pressure
• Measured above total vacuum or zero absolute.
  Zero absolute represents total lack of pressure.
• Atmospheric Pressure
• The pressure exerted by the earths atmosphere.
  Atmospheric pressure at sea level is 14.696 psia.
  The value of atmospheric pressure decreases with
  increasing altitude.
• Barometric Pressure
• Same as atmospheric pressure.
• Gauge Pressure
• The pressure above atmospheric pressure.
  Represents positive difference between measured
  pressure and existing atmospheric pressure. Can
  be converted to absolute by adding actual
  atmospheric pressure value.
• Differential Pressure
• The difference in magnitude between some
  pressure value and some reference pressure. In a
  sense, absolute pressure could be considered as a
  differential pressure with total vacuum or zero
  absolute as the reference. Likewise, gauge
  pressure (defined above) could be considered as
  Differential Pressure with atmospheric pressure
  as the reference.

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Pressure Measurement

• Pressure Units

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Pressure Instruments

• Types of Pressure Instruments
• Pressure Gauges (Vacuum, Compound, Absolute,
  Gauge)
• Differential Pressure Gauge
• Pressure Switch (Vacuum, Absolute, Gauge)
• Differential Pressure Switch
• Pressure Transmitter (Vacuum, Absolute, Gauge)
• Differential Pressure Transmitter

DIFFERENTIALPRESSURE TRANSMITTER
PRESSURE SWITCH
PRESSURE GAUGE
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Pressure Gauge

• PRESSURE GAUGES
• A Pressure Gauge is used for measuring the
  pressure of a gas or liquid.
• A Vacuum Gauge is used to measure the pressure in
  a vacuum.
• A Compound Gauge is used for measuring both
  Vacuum and Pressure.
• Pressure Gauges are used for Indication only.

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Pressure Gauge
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Pressure Gauge

• Measuring Principle
• Bourdon tube measuring element is made of a
  thin-walled C-shape tube or spirally wound
  helical or coiled tube. When pressure is applied
  to the measuring system through the pressure port
  (socket), the pressure causes the Bourdon tube to
  straighten itself, thus causing the tip to move.
  The motion of the tip is transmitted via the link
  to the movement which converts the linear motion
  of the bourdon tube to a rotational motion that
  in turn causes the pointer to indicate the
  measured pressure.

Helical Bourdon
Coiled Bourdon
C Type Bourdon
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Differential Pressure Gauge

• Measuring Principle
• Differential pressure gauges have two inlet
  ports, each connected to one of the volumes whose
  pressure is to be monitored.
• In cases where either input can be higher or
  lower than the other, a bi-directional
  differential range should be used.

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Differential Pressure Gauge

• Unidirectional and Bidirectional DP Gauges

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Pressure Gauge - Accessories
Safety Glass Front Safety Glass is normally used
to prevent the glass shattering in the event of
the bourdon tube rupturing.
Liquid Filled Gauge The liquid filling is used to
dampen any vibration/pulsation in the bourdon,
either silicone oil or glycerin is used.
Snubber Used for dampening and filtering and
reducing the damaging effects of pulsation on a
gauge. The snubber has a metal disc available in
standard grades of porosity.
Pulsation Damper (Adjustable Snubber) Threads on
to a gauge socket and provides a restriction by
means of a pin, which may be placed in either of
five different sized holes, and thus allows the
user to vary the amount of dampening to suit
requirements. The pulsating pressure moves the
pin up and down, providing self cleaning action.
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Pressure Gauge - Accessories
Pressure Limit Valve Protects pressure
instruments against surges and pulsations.
Provides automatic positive protection and
accurate, repeatable performance. Automatic
pressure shut-off, built in snubber enhances
instrument protecting performance.
Siphon Tubes Used to dissipate heat by trapping
condensed liquid to keep high temperature steam
or condensing vapor from damaging the pressure
gauge.
COIL PIPE
PIG TAIL
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Diaphragm Seals

• Diaphragm seals, also known as chemical seals,
  isolate pressure measuring instruments from the
  process media. The system pressure is
  transmitted to a fill fluid in the upper housing
  of the diaphragm seal, and from there to the
  pressure-measuring instrument itself. The use of
  diaphragm seals should be discussed with and
  approved by the Client.
• Diaphragm sealed gauges should be considered for
• Process fluids that would clog the pressure
  elements.
• Process fluids that are toxic, corrosive,
  slurried and viscous.
• Process fluids that could crystallize or
  polymerize.
• Materials capable of withstanding the process
  fluids that are not available as a pressure
  element, such as high temperature.
• Process fluid that might freeze due to change in
  ambient temperature and damage the element.
• Hydrocarbon services having a Reid vapor pressure
  (RVP) of 18 psig and over. (RVP is the absolute
  vapor pressure exerted by a liquid at 100F. The
  higher this value, the more volatile the sample
  and the more readily it will evaporate).
• Auto-ignitable hydrocarbon services.

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Diaphragm Seals
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Pressure Gauge Selection Guideline

• When selecting a Pressure Gauge, care should be
  given to a number of parameters which have an
  effect on the gauges accuracy, safety, and cost.
• Accuracy required
• Dial size
• Operating pressure range
• Chemical compatibility with gauge construction
  materials
• Operating temperature range
• Vibration, pulsation, and shock
• Pressure fluid composition
• Mounting requirement

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Pressure Gauge Ranges

• Since the accuracy of most pressure gauges is
  better in the middle portion of a gauge, you
  should always select a gauge with a range that is
  about double your normal anticipated pressure.
• The maximum operating pressure should not exceed
  80 of the full pressure range of the gauge.
• Standard pressure ranges are measured in PSI,
  Bar, Pa or kPa and most of the gauges have dual
  PSI/metric scales.
• Very low pressure gauges have scales that measure
  in Inches of H2O, mm H2O, and Inches of Hg. 
• Vacuum gauges have scales in inches of mercury,
  while compound gauges have scales that measure in
  both vacuum and pressure.

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Pressure Gauge Installation

• Top connection preferred for gas installations
  side connection preferred for liquid
  installations.
• The pressure gauge can be connected to the pipe
  by individual block and bleed valves or a two way
  manifold.

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Pressure Switch

• Measuring Principle
• The device contains a micro switch, connected to
  a mechanical lever and set pressure spring. The
  contacts get actuated when process pressure
  reaches the set pressure of the spring.
• It can be used for alarming or interlocking
  purposes, on actuation.
• It can be used for high / high-high or low /
  low-low actuation of pressure in the process .
  The set range can be adjusted within the switch
  range.
• The sensing element may be a Diaphragm or a piston

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Pressure Switch

• Pressure/Vacuum Switch - A device that senses a
  change in pressure/vacuum and opens or closes an
  electrical circuit when the set point is reached.
  
• Pressure switches serve to energize or
  de-energize electrical circuits as a function of
  whether the process pressure is normal or
  abnormal.
• The electric contacts can be configured as single
  pole double throw (SPDT), in which case the
  switch is provided with one normally closed (NC)
  and one normally open (NO) contact.
• Alternately, the switch can be configured as
  double pole double throw (DPDT), in which case
  two SPDT switches are furnished, each of which
  can operate a separate electric circuit.

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Pressure Switch

• The switch housings can meet any of the NEMA
  standards from Type 1 (General Purpose) to Type 7
  (Explosion Proof), or Type 12 (Dust Proof) or
  Type 4 (Water Proof).
• Pressure switches are also available in
  hermetically sealed enclosures.
• Gold plated contacts are available for
  reliability .
• Pressure Switches are not as commonly used today,
  since they contain mechanical moving parts and
  moving parts are significantly more likely to
  fail than transmitters and the failures can go
  undetected.

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Pressure Transmitter
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Pressure Transmitter

• A Pressure Transmitter is used where indication
  and/or record of pressure is required at a
  location not adjacent to the primary element.
• A Pressure Transmitter is used for both
  indication and control of a process.
• A Pressure Transmitter is used where overall high
  performance is mandatory.
• Both Electronic and Pneumatic Transmitters are
  used.
• These can be either Gauge, Absolute or
  Differential Pressure Transmitters.

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Transmitter Measuring Principle

• The diagram shows an electronic differential
  pressure sensor. This particular type utilizes a
  two-wire capacitance technique.
• Another common measuring technique is a strain
  gauge.
• Process pressure is transmitted through isolating
  diaphragms and silicone oil fill fluid to a
  sensing diaphragm.
• The sensing diaphragm is a stretched spring
  element that deflects in response to the
  differential pressure across it.
• The displacement of the sensing diaphragm is
  proportional to the differential pressure.
• The position of the sensing diaphragm is detected
  by capacitor plates on both sides of the sensing
  diaphragm.
• The differential capacitance between the sensing
  diaphragm and the capacitor plates is converted
  electronically to a 420 mA or 1-5 VDC signal.
• For a gauge pressure transmitter, the low
  pressure side is referenced to atmospheric
  pressure.

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Pressure Transmitter

• Typical Outputs
• 4 to 20 milliamp (mA). analog signal
• Smart HART digital signal (superimposed on analog
  signal)
• Fieldbus digital signal
• 3 to15 psi pneumatic signal

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Diaphragm Seal System

• A diaphragm seal system consists of a pressure
  transmitter, diaphragm seals, a fill fluid, and
  either a direct mount or capillary style
  connection.
• During operation, the thin, flexible diaphragm
  and fill fluid separate the pressure sensitive
  element of the transmitter from the process
  medium. The capillary tubing or direct mount
  flange connects the diaphragm to the transmitter.
• When process pressure is applied, the diaphragm
  transfers the measured pressure through the
  filled system and capillary tubing to the
  transmitter element.
• This transferred pressure displaces the sensing
  diaphragm in the pressure-sensitive element of
  the transmitter.
• The displacement is proportional to the process
  pressure and is electronically converted to an
  appropriate current, voltage, or digital HART
  output signal.

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Diaphragm Seal System

• WHY USE DIAPHRAGM SEALS?
• Diaphragm Seal systems provide a reliable process
  pressure measurement and prevent the process
  medium from contacting the transmitter diaphragm.
• Transmitter/ Diaphragm Seal systems shall be used
  for
• For process fluid that would clog the pressure
  elements.
• For process fluids that are toxic, corrosive,
  slurry and viscous.
• For process fluids that could freeze or solidify.
• For process temperatures outside the normal
  operating range and cannot be brought to those
  limits by impulse piping.
• For process that needs frequent cleaning.
• For processes that need replacement of wet legs,
  to reduce maintenance.

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Pressure Transmitter Selection Guideline

• When selecting a pressure transmitter care
  should be given to a number of parameters which
  have an effect on transmitter accuracy, safety,
  and utility.
• Accuracy required
• Power supply
• Operating pressure range
• Operating temperature range
• Body Material
• Pressure fluid composition and Internal parts
• Mounting requirement
• Process connection size

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Pressure Transmitter installation

• Mounting above tap is typical for gas service and
  mounting below tap typical for liquid and steam
  services.
• Direct mount is possible for low temperature
  services.

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Diaphragm Seal installation
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Pressure Instruments

• Selection of Pressure Instruments Rules of
  Thumb
• Application Understand your application. Examine
  the particulars of your application. Is it
  necessary to know if the pressure is negative or
  positive? Do you need to know the difference in
  pressure between two points? Answering these
  questions about your application will go a long
  way in helping select the right pressure
  transmitter.
• Wetted Parts Selecting the transmitter with
  wetted parts that are compatible with the medium
  to be measured helps to ensure a long-lasting
  measurement solution.

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Pressure Instruments

• Accuracy From an accuracy point of view, the
  range of a transmitter should be low (normal
  operating pressure at around the middle of the
  range), so that error, usually a percentage of
  full scale, is minimized. On the other hand, one
  must always consider the consequences of
  overpressure damage due to operating errors,
  faulty design, or failure to isolate the
  instrument during pressure-testing and start-up.
  Therefore, it is important to specify not only
  the required range, but also the amount of
  overpressure protection needed.
• Output Required Pressure transmitters can send
  the process pressure of interest using an analog
  pneumatic (3-15 psig), analog electronic (4-20 mA
  dc), or digital electronic signal.

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Pressure Instruments

• Protection Do you need special protection from
  the elements? Many applications require special
  protection, such as, corrosive environment, or an
  outdoor environment. Pressure transmitters are
  available in various NEMA ratings or can be
  assembled in special NEMA rated housings that
  help protect them from harsh environments.

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QUESTIONS
Any Questions???