Chapter 2 A

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Oct 24, 2013 (3 years and 11 months ago)

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Pressure Head

Chapter 2 A

IMT 124

Spring 2013

Paul Turner

Pressure Head


Pressure head

is a term used in fluid
mechanics to represent the internal energy of
a fluid due to the pressure exerted on its
container.


It may also be called
static pressure head

or
simply
static head

(but not
static head
pressure
).


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Height and Pressure


For each foot in height, a column of oil will
exert .4 psi at the bottom.


A ten foot high column will exert 4 psi at the
bottom.

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Fire Department Uses


Water head is the height of the water column
(lift) due to imposing pressure.


The head pressure is positive (gain) if the hose lay
is downhill because the force of gravity is helping
push the water down, consequently increasing
the pressure.


The head pressure is negative (loss) if the hose
lay is uphill, since the force of gravity is pulling
the water down, when it needs to be pumped up.

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Bernoulli


In fluid dynamics,
Bernoulli's principle

states
that for an flow, an increase in the speed of
the fluid occurs simultaneously with a
decrease in pressure


or a decrease in that fluid’s potential energy.


Bernoulli's principle is named after the Swiss
scientist
Daniel Bernoulli

who published his
principle in his book
Hydrodynamica

in 1738.

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Bernoulli’s Principle

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Bernoulli’s Principle

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Head Loss


In any real moving fluid, energy is dissipated
due to friction; turbulence dissipates even
more energy for high flows.


Head loss is divided into two main categories,
"major losses" associated with energy loss per
length of pipe, and "minor losses" associated
with bends, fittings, valves, etc.


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For relatively short pipe systems, with a
relatively large number of bends and fittings,
minor losses can easily exceed major losses.


In design, minor losses are usually estimated
from tables using coefficients or a simpler and
less accurate reduction of minor losses to
equivalent length of pipe.


Head Loss

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Friction Loss in Pipe


Friction loss

is the loss of energy or “head” that
occurs in pipe flow due to viscous effects
generated by the surface of the pipe.



Friction Loss is considered as a "major loss"
and it is not to be confused with “minor loss”
which includes energy lost due to obstructions.

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Fire Department Uses


As a field rule, the pressure in a line is reduced
by 5 pounds per square inch for each
appliance added to the line.


For example, a hose lay with five wye valves
will result in a 25 pounds per square inch
pressure loss due to the friction introduced by
these fittings.


This approximation is used to simplify calculations and is
not precisely what occurs in the field.

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Friction Loss


This energy drop is dependent on the wall shear stress
between the fluid and pipe surface.


The shear stress of a flow is also dependent on
whether the flow is turbulent or laminar.


For turbulent flow, the pressure drop is dependent on
the roughness of the surface, while in laminar flow, the
roughness effects of the wall are negligible.


This is due to the fact that in turbulent flow, a thin
viscous layer is formed near the pipe surface which
causes a loss in energy, while in laminar flow, this
viscous layer is non
-
existent

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Friction Loss


Friction loss has several causes, including:


Frictional losses depend on the conditions of flow
and the physical properties of the system.


Movement of fluid molecules against each other


Movement of fluid molecules against the inside
surface of a pipe or the like, particularly if the
inside surface is rough, textured, or otherwise
not smooth


Bends, kinks, and other sharp turns in hose or
piping


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Friction Loss


In pipe flows the losses due to friction are of
two kinds: skin
-
friction and form
-
friction.


The former is due to the roughness of the
inner part of the pipe where the fluid comes
in contact with the pipe material, while the
latter is due to obstructions present in the line
of flow
--
perhaps a bend, control valve, or
anything that changes the course of motion of
the flowing fluid.


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Fittings


FL in fittings is calculated or compared to the
friction loss in a straight pipe.


The FL for a 90
°

elbow is equivalent to XX
number of feet of straight pipe.


Example: in 3” PVC pipe, a 90
°

elbow has the
same friction loss of 12’ of 3” pipe.

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FL Chart

Friction Loss in PVC
fittings in Equivalent Feet
of Straight Pipe

Straight Pipe Size (IN.)

1/2”

???´



1 1/2”

2”

3”

4”

90 Elbow, Standard

1.5

2.0

2.25

4.0

6.0

8.0

12.0

45 Elbow, Standard

0.75

1.0

1.4

2.0

2.5

4.0

5.0

Insert Coupling

0.5

0.75

1.0

1.5

2.0

3.0

4.0

Gate Valve

.3

.4

.6

1.0

1.5

2.0

3.0

Male/Female Adapters

1.0

1.5

2.0

3.5

4.5

6.5

9.0

Tee, Flow Thru Run

1.0

1.4

1.7

2.7

4.3

6.3

8.3

Tee, Flow Thru Branch

4.0

5.0

6.0

8.0

12.0

16.0

22.0

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Sources


http://en.wikipedia.org/wiki/Pressure_head


http://en.wikipedia.org/wiki/Hydraulic_head


http://en.wikipedia.org/wiki/Friction_loss




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