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Kaw AreaTechnical School
Industrial Technology
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Kaw AreaTechnical School
Richard E. StephensCET Sr.
Industrial ProcessIndustrial Process
ControlControl
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Chapter 10
Pressure Systems
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Pressure Laws
� Pressure is measured as force per unit area(pounds per square inch)
Pressure (P) =Force (F)
Area (A)
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Properties of a Liquid
� Molecules of a liquid are closely attracted to oneanother, giving the property of incompressibility
� The ability of a liquid¶s molecules to flow andtake the shape of its container is called viscosity
� Head is used to describe the height of a liquidabove the measurement point. It is expressed ininches, feet, or other units of distance
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Propertiesof a Liquid
� Density is the weight of a certainvolume of liquid, expressed in pounds
per unit volume
� By multiplying the height of a liquid by its density, hydrostatic pressure
can be calculated� Specific Gravity indicates how much
lighter or heavier a substance iscompared to water. Water has aspecific gravity of 1.0
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Temperature and tmosphericPressure
� The temperature of a liquid affects the pressure it exerts
� Increasing the temperature expands the liquid andincreases the pressure if it is in an enclosed container.In an open vessel the hydrostatic pressure remains the
same� Normal atmospheric pressure is 14.7 psi at sea level.
tmospheric pressure will exert a force on a liquid inan open vessel
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Properties of a Gas
� Gases are another type of fluid� gas can be vapor, air, or steam
� Gas molecules remain separate, unlike moleculesin solids or liquids
� Gases will distribute their molecules evenly withinan enclosed container
� Gases exert equal pressure in all directions
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Temperature of a Gas
� Gas molecules are constantly moving
� Gas molecules collide with one another in an enclosedcontainer; a pressure gage will interpret these collisions asone pressure
� When heat is applied to an enclosed container of gas, themolecules have more collisions and the pressure of the gasincreases
� If the gas is heated in an unenclosed container, the volume
of the gas will increase and the pressure remains constant
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Volume of a Gas Container
� When an area of an enclosedcontainer is decreased, the space between the gas molecules isdecreased
� This action is calledcompression
� Compression can also beachieved by increasing theamount of gas in a container using a compressor
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Gas Removal from a Container
� s gas is removed from a container, the pressuredecreases
� ny reduction of pressure below atmospheric pressure is referred to as a partial vacuum
� If all of the gas is removed, a full vacuum exists� vacuum pump is used to remove gas from a
container
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Pressure Measurement Scales
� There are four basic scales used to measure pressure:
± Gage pressure
± bsolute pressure
± Differential pressure
± Vacuum pressure
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Gage Pressure Scale
� Gage pressure scales use atmospheric pressure as a reference
� If the sensing element is exposed to theatmosphere, it registers zero pressure
� The units of measurement are recorded as psig (pounds per square inch, gage)
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A bsolute Pressure Scale
�A bsolute pressure is referenced to absolute zero,or the complete lack of pressure
� A bsolute pressures are always indicated by positive numbers
� If the sensing element is exposed to theatmosphere, it will register 14.7 psia (pounds per square inch, absolute)
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Differential Pressure Scale
� Differential pressure is used to express thedifference between two measured pressures
� It is determined by subtracting the lower
reading from the higher reading
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Vacuum Pressure Scale
� Vacuum scales use atmospheric pressure as areference point
� Most common vacuum scales use inches of mercury (in Hg ) to express the value
� A vacuum gage will read zero when measuringatmospheric pressure and 29.92 in Hg whenmeasuring a complete vacuum
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Comparison of Pressure Scales
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Pressure Measurement Instruments
� Pressure control is an important process in manyindustrial applications requiring accuratemeasurement and control
� Instruments are often classed by whether theymake d irect or inferred measurements
� Both electronic and non-electronic instruments areused in pressure measurement
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Liquid Column Gauges
� Measuring pressure is possible by monitoring the height of a
liquid in a column
� These gauges are very accurate
and may be used as calibration
tools for other instruments� A barometer is an example of a
liquid column gauge
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Types of Manometers
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Mechanical Gauges
� Mechanical gauges are more rugged than liquid-
filled gauges and are often preferred� They are relatively inexpensive and reliably
accurate
� Major types of mechanical gauges include:
± Bourdon tube
± Diaphragm gauge
± Bellows gauge
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Bourdon Tube Gauge
� The Bourdon tube is a C-
shaped metal tube� As pressure increases, the
tube coil unwinds
� As it unwinds, a needleattached to the end will move
to indicate a higher pressurereading
� Bourdon tubes are availableto measure pressures from 0-15 psi and 0-6000 psi
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Diaphragm Gauge
� Diaphragm gauges are quitecommon in industrial pressuresystems
� Diaphragm gauges aretypically spring-loaded as ameans of setting the range and
sensitivity� Diaphragm gauges can be used
to measure absolute, gage, anddifferential pressures
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Bellows Gauge
� Bellows gauges have wider
ranges of operation than do
diaphragm gauges because of
the pleated sides of the bellows
� Bellows material may be brass,
phosphor bronze, or stainless
steel
� An opposing spring is used to
control range and sensitivity
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Electronic Pressure Sensors
� Electronic pressure sensors are morereliable, more accurate, and less expensivethan many mechanical measuringinstruments
� Two types of pressure sensors are: ± Strain Gauges
± Variable Capacitor Pressure Detector
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Strain Gauge
� Strain gauges measure pressure by detecting the
strain on an object caused by pressure� Strain gauges are typically constructed of
piezoelectric semiconductor material
� Pressure on the elements of a strain gauge causes
the elements to expand or contract� Strain gauges are used as part of a Wheatstone
Bridge to measure pressure
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Strain GaugeConfiguration
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Variable Capacitor PressureDetector
� Uses two conductive platesoriented adjacent to oneanother and separated by air
� One plate is fixed, and theother is attached to a bellows
� The charge of the capacitor isinfluenced by the distance between the plates, andtherefore, the pressure beingexerted on the bellows
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Hydraulic Systems� Hydraulic systems use oil-based fluids to develop high
levels of pressure for many industrial applications� Hydraulic system components include:
± Pump
± ilter
± Valves
± Control elements ± Load
± Reservoir
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Pneumatic Systems� Pneumatic pressure systems are found in nearly
every industrial facility� Operation is very similar to hydraulic systems
except air is compressible and hydraulic fluid is not
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Vacuum Systems� Vacuum systems can be used to provide a number
of functions in industrial applications
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Steam Pressure Systems
� Steam pressure is used
for a variety of purposes
in industry
± Heat source for food
processing ± Refining
± Chemical processing
± Warming the facility