Pipeline Hydraulics

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Pipeline Hydraulics
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Importance

• Irrigation hydraulics involves
• The determination of the pressure distribution in
  the system
• The selection of pipe sizes and fittings to
  convey and regulate water delivery
• The determination of the power and energy
  requirements to pressurize and lift water

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Basic Relationships

• Q Vm Af
• Flow rate (velocity) x (cross-sectional area)
• Called the continuity equation
• Units must be consistent
• Maximum recommended V in a pipeline is about 5
  feet/second

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Maximum Flow Rates in Pipelines
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Energy

• Forms of energy available in water
• Kinetic energy due to velocity
• Potential energy due to elevation
• Potential energy due to pressure

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Units

• Energy per unit weight of water "head
• Energy (ft-lb)/Weight (lb) Head (ft)
• Velocity head Elevation head Pressure head
• Length units (e.g., feet, meters) 

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Velocity Head

• Velocity head
•  
• g gravitational constant 32.2 ft/s2
• when V is 5 ft/s, V2/(2g) is only about 0.4 ft
  (usually negligible)

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Elevation Head

• Elevation head (gravitational head) Z
• Height of water above some arbitrary reference
  point (datum)
• Water at a higher elevation has more potential
  energy than water at a lower elevation

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Pressure Head

• Pressure force per unit area (e.g., pounds per
  square inch)
• Pressure head pressure per unit weight of water
• h P / ?
• h pressure head , P pressure
• ? weight of a unit volume of water
• ? 62.4 lb/ft3 0.433 psi/ft
• 1/ ? 2.31ft/psi
• h 2.31P (P is in psi h in ft)

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Calculate P at the Bottom of a Column of Water
When depth of 2 ft is considered V 2 ft3 W 2
ft3 62.4 lb/ft3 124.8 lb A 144 in2 P
W/A 124.8lb / 144 in2 0.866 lb/in2 If
depth is 1ft then V 1 ft3 W 62.4lb P
62.4lb/144in2 0.433lb/in2
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Calculate P at the Bottom of a Column of Water
V 2 ft3 W 124.8 lb A 2ft2 288 in2 P
124.8lb / 288in2 0.433 lb/in2
The area of a pond or tank does not affect
pressure. Pressure is a function of water depth
only.
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Manometer Rising up From a Pipeline
Pressure, P lb/ft2 ? specific weight of
water, (62.4 lb/ft3)
HP/g
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• hydraulic head, H
• Bernoullis equation (conservation of energy)
• H1 H2 hL
• H1 hydraulic head at point 1 in a system
• H2 hydraulic head at point 2 in a system
• hL head loss during flow from point 1 to
  point 2 (hL is due to friction loss)

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Components of Hydraulic Head for Pipeline With
Various Orientations
hL
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Components of Hydraulic Head for Pipeline With
Various Orientations Contd
hL
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Components of Hydraulic Head for Pipeline With
Various Orientations Contd
hL
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Friction Loss

• Description
• energy loss due to flow resistance as a fluid
  moves in a pipeline
• Factors affecting
• flow rate
• pipe diameter
• pipe length
• pipe roughness
• type of fluid

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Ways of Calculating Friction Loss

• Equations
• Hazen-Williams is one of many (eqn 8.8)
• Tables
• for a given pipe material, pipe diameter, and
  flow rate, look up values for friction loss in
  feet per hundred feet of pipe
•  SDR standard dimension ratio pipe
  diameter ? wall thickness

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Dimensional Comparison of Sch. 40, Class 160, and
Class 125 PVC Pipe
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Friction Loss for IPS PVC Pipe
IPS Iron Pipe Size (same dimensions as steel
pipe of same nominal size)
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Friction Loss for IPS PVC Pipe contd
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Example Problem

• A 4-inch nominal diameter PVC pipe has a
  outside diameter of 4.5 inches and a wall
  thickness of 0.173 inches. What is the pipe SDR?
• Solution SDR Diameter/Wall Thickness
• SDR 4.50/0.173 26.0

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Pipes With Multiple Outlets

• lower friction loss because V decreases with
  distance down the pipe
• (Q decreases as flow is lost through the
  outlets VQ/A)
• first calculate friction loss as if there were no
  outlets, and then multiply by the "multiple
  outlet factor", F

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Multiple Outlet Factors for Laterals With Equally
Spaced Outlets of the Same Discharge
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Example Problem

• A 2-inch diameter, SDR 21 PVC pipe carries a
  flow of 60 gpm. The flow is discharged through 15
  sprinklers evenly spread along its 600-ft length.
  What is the total head loss in the pipe?
• Solution Hf 4.62 ft / 100 ft (Table 8.2)
• Hf 4.62 600 ft / 100 ft 27.72 ft
• F 0.379 (Table 8.3 15 outlets)
• Hf 27.72 ft 0.379 10.51 ft

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Minor Losses

• Source of minor losses
• fittings, valves, bends, elbows, etc
• friction, turbulence, change in flow direction,
  etc
• hm head loss in fitting (ft)
• K resistance coefficient for fitting

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Resistance Coefficient H for Use Determining Head
Losses in Fittings and Valves
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Calculation Shortcuts

• V in ft/s
• Q in gpm
• D in inches (INSIDE diameter)

• hm in ft
• Q in gpm
• D in inches (INSIDE diameter)

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Example Problem

• A 4-inch pipe carries a flow of 160 gpm. How
  much head loss occurs when the flow passes
  through a 90o elbow (flanged, regular radius) ?
• Solution K 0.31
• (Table 8.4 4-in, regular 90o elbow)
• D 4.0 inches