Pressure
and Pressure Scales
Pressure
“A pressure is the ratio of a force to the area on
which the force acts.”
Pound force, Kilogram
force
Pressure =
Force
Area
Newton, dyne
Square Inches, Square feet
Square Centimeters, Square
Meters
Common units include:
Pounds per Square Inch (psi)
Kilo-Pascals (kPa)
Pressure
• An important operating parameter that is
relevant in many applications
• Pressure applied over a given area can be
used for useful work.e.g
– Steam reforming, Water pressure(energy)
• Pressures can be measured to monitor the
condition of other process parameters.
– Flow,
Pressure units
SI system: Pascal N/m2 (Pa),
• One Newton (1 N) of force
applied to an area of one
square meter (1 m2).
• 1 Pa is equal to 1 N/m2
• small unit, often use
kilopascal 1 kPa=1000 Pa.
AES: psi
• Force of one pound (1lbf)
applied to an area of one
square inch (1 in2).
Metric System : Bar
• One bar (1 bar) is equal to
100 kPa or 14.5 psi.
More Pressure Scales
PSI and kPa are the most common pressure scales but there
a few more:
• Inches* of water /mm Hg
• Inches* of mercury/ ft of H2O
• Bar 100kPa=1bar
• Atmosphere (atm)
• Torr (vacuum)
• dynes/cm2 ,
* or millimeters when using metric
Pressure Scales
Applied
process
pressure is
27.6806
“H20
2.03602
“Hg
0.068947
0.068046
Bar
Atmos
1 psi or
6.89 kPa
The choice of scales will depend on
•
the amount of pressure being measured (high pressure = psi/kPa, low
pressure = inches H20)
•
The type of application ( flow = inches H20, blood pressure = inches of Hg.)
Pressure Conversion Chart
Pressure Units
psi
kPa
inches of Hg
inches of H20
atmospheres
bar
psi
1
6.894
2.036
27.681
.0681
.06895
kPa
0.1450
1
.2953
4.0147
.009669
.01
inches of Hg
0.4912
3.3864
1
13.595
.03342
.03386
inches of H2O
0.03613
.2491
.07355
1
.002458
.002491
atmospheres
14.696
101.33
29.92
406.8
1
1.0133
bar
14.504
100
29.53
401.86
.9869
1
1 psi = 6.89 kPa
1 inch Hg = 0.49 psi
100 inch H20 = 3.61 psi
1 Bar = 14.5 psi = 100 kPa
Conversion Factors
• 1 cubic foot of water that weighs 62.4 lbs acting over an area of
144 in2 produces a pressure of 0.433 pound per square inch
(psi)
• The same volume of water weighs 28.3 Kilograms over an area
of 929 cm2, therefore the pressure is 0.03 kilograms per square
centimeter.
Fluid Pressure and Pressure Head
• Pressure is exerted on the top of the cylinder
of the water by the atmosphere and on the
bottom of the cylinder itself by the water.
• The Pressure at bottom of the static
(nonmoving )column of the water exerted on
the sealing plate is
P
= pgh + Po
P=Pressure at bottom of the column
Po=Pressure at the top of the column
•
•
•
•
Area= 1cm2
Height 50cm
Sp gr at 20C and density of Hg is 13.55g/cm3.
Then force exerting by Hg on the bottom plate with 1cm2
area is
–
In AES (psi)
6.64N
Pressures caused by a fluid:
Fluid pressure
• If a fluid is flowing through a horizontal pipe and a leak
develops, a force must be applied over the area of the hole
that causes the leak.
• The fluid pressure may be defined as the ratio F/ A, where F is
the minimum force that would have to be exerted on a
frictionless plug in the hole to keep the fluid from emerging.
Fluid Pressure on the base of a tank
Pressures caused by a fluid:
Hydrostatic Pressure (HP)
• The pressure at the base of a vertical
column of fluid with density and height
is called the hydrostatic pressure.
• The mass of the fluid will exert a force
on the base of the container i.e. HP.
• F thus equals the force on the top
surface plus weight of fluid.
• P=Po + pgh
• Caused by the mass of a fluid . Area not
matter, so applicable to every system Pressure at base of fluid column (HP)
Pressure Head
• A pressure can also be
• The height of a hypothetical
expressed as a vertical
column would exert the
height of a column of liquid
given pressure at its base if
• The relationship between
the pressure at the top
the pressure at the base of
were zero.
a column of fluid of height h
and the pressure at the top • The equivalence between a
pressure P (force/area) and
is 'particularly simple if
the corresponding head Ph
these pressures are
(height of a fluid) is given
expressed as heads of the
given fluid: if the column is
mercury, for example, then
Absolute and Relative
Pressure Scales
•Expressed as Absolute or relative.
•Depends upon nature of measuring
device to make measurements
Open End would measure Relative
pressure
Ref is Atm P
Close End
No Pressure = Vacuum
Absolute pressure
Precise value, unchange
AT
Abs 0= Perfect Vaccum
Relative 0=Atmospheric P
Types of Pressure
• Atmospheric Pressure
• Is the pressure caused by the weight of the earth’s
atmosphere. Often called Barometric Pressure.
• Absolute Pressure is the Total Pressure. An absolute
pressure of zero is a perfect vaccum. Absolute Pressure
must be used in all calculations unless a pressure
difference is used.
• Gauge Pressure, is the Pressure Relative to
atmospheric pressure.
• Vaccum Pressure, is a guage pressure i.e pressure
below atmospheric pressure.
Suffix ‘a’ and ‘g’
• Psi and atm often carry
• Indicate whether Pressure is absolute or
guage
• psig: Guage Pressure in psi
• psia: Absolute pressure in Psi
• atma: Absolute pressure in atm
• atmg: atmospheric pressure in guage
Standard Atmosphere
• Pressure equivalent to 760mmHg at sea level
and at 0C.
• Unit is atm
• Pressure equivalent to standard atmosphere
are
Figure