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Pressure Measurement

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Title: Pressure Measurement


1
Pressure Measurement
  • Mark Murphy, PE
  • Technical Director, Fluor Corp.

2
(No Transcript)
3
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

4
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.

5
Pressure Measurement
  • Pressure Units

6
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
7
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.

8
Pressure Gauge
9
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
10
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.

11
Differential Pressure Gauge
  • Unidirectional and Bidirectional DP Gauges

12
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.
13
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
14
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.

15
Diaphragm Seals
16
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

17
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.

18
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.

19
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

20
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.

21
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.

22
Pressure Transmitter
23
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.

24
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.

25
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

26
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.

27
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.

28
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


29
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.

30
Diaphragm Seal installation
31
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.

32
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.

33
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.

34
QUESTIONS
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