Pressure and Pressure Scales

Pressure“A pressure is the ratio of a force to the area on

which the force acts.”

Force

AreaPressure =

Common units include:

Pounds per Square Inch (psi)

Kilo-Pascals (kPa)

Pound force, Kilogram force

Newton, dyne

Square Inches, Square feet

Square Centimeters, Square Meters

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 ScalesPSI 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

27.6806“H20

Applied process

pressure is 1 psi or

6.89 kPa

2.03602“Hg

0.068947 Bar

0.068046 Atmos

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 ChartPressure 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 + PoP=Pressure at bottom of the columnPo=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– 6.64N

In AES (psi)

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)

Pressure at base of fluid column (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 Head• A pressure can also be

expressed as a vertical height of a column of liquid

• The relationship between the pressure at the base of a column of fluid of height h and the pressure at the top is 'particularly simple if these pressures are expressed as heads of the given fluid: if the column is mercury, for example, then

• The height of a hypothetical column would exert the given pressure at its base if the pressure at the top were zero.

• The equivalence between a pressure P (force/area) and the corresponding head Ph (height of a fluid) is given

Absolute and Relative Pressure Scales•Expressed as Absolute or relative.•Depends upon nature of measuring device to make measurementsOpen End would measure Relative pressure

Ref is Atm PClose EndNo Pressure = VacuumAbsolute pressurePrecise value, unchange

AT

Abs 0= Perfect VaccumRelative 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