Title: Pressure Measurement
1Pressure Measurement
- Mark Murphy, PE
- Technical Director, Fluor Corp.
2(No Transcript)
3Pressure 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
4Pressure 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.
5Pressure Measurement
6Pressure 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
7Pressure 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.
8Pressure Gauge
9Pressure 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
10Differential 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.
11Differential Pressure Gauge
- Unidirectional and Bidirectional DP Gauges
12Pressure 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.
13Pressure 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
14Diaphragm 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.
15Diaphragm Seals
16Pressure 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
17Pressure 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.
18Pressure 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.
19Pressure 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
20Pressure 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.
21Pressure 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.
22Pressure Transmitter
23Pressure 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.
24Transmitter 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.
25Pressure 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
26Diaphragm 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.
27Diaphragm 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.
28Pressure 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
29Pressure 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.
30Diaphragm Seal installation
31Pressure 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.
32Pressure 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.
33Pressure 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.
34QUESTIONS
Any Questions???