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Reasonable Velocities of fluid in Pipes

Medium Pressure (bar) Service Velocity (m/s) Design dP Notes
Steam (sat) 0 - 1.7 Heating 20 to 30 - + 100mm dia
Steam (sat) over 1.7 Process 30 to 50 - +150mm dia
Steam (sup) over 14 Process 30 to 100 - +150mm dia
Air - Forced Air Flow 5 to 8 - e.g. AC Reheat
Water - General 1 to 3 - -
Water - Concrete Pipe 4.7 - -
Water - Pump Suction 1.2 - -
Water - Horizontal Sewer 0.75 - Minimum
Water - Pump discharge 1.2 to 2.5 - Minimum
Oil - Hydraulic Systems 2.1 to 4.6 - Minimum
Ammonia - Compressor Suction 25 - Max. Permissable
Ammonia - Compressor Discharge 30 - Max. Permissable

Property Identifiers

Identifier Description Units (typical)
a Velocity of sound m /s
cp Specific Heat Capacity at Constant pressure kJ/(kg K)
cv Specific Heat Capacity at Constant Volume kJ/(kg K)
h Specific Enthalpy kJ/kg
k Thermal Conductivity W/(m K)
p Absolute Pressure N / m2
Pr Prantl Number cp. mu / k Dimensionless
t Temperature oC
T Absolute Temperature K
u Specific Internal Energy kJ/kg
W Work Output per unit mass kJ/kg
R Gas Constant = Ro / M kJ /kg.K
Q Heat Input per unit mass kJ /kg
r Density kg /m3
z Elevation m
g gravitationalal accelaration 9.81 m /s2


Fluid Flow Equations

Steady Flow Equation

Q - W = ( u2 + 1/2 V22 + g z2 + p2 / r2) - ( u1 + 1/2 V12 + g z1 + p1 / r1)

If Q = W = 0 and the fluid is incompressible and frictionless then the Bernoulli's equation applies ..:

p2 / r g + V22/ 2 g + z2 = p1 / r g + V12/ 2 g + z1


Simplified Drag Equation

Equation for Drag of any shape in any fluid subject to relative motion

Drag = Cd A r V2 / 2

Consistent units to be used i.e Newtons, kg, m, sec.

Cd = Drag coefficient (very approximate)

  • = 1.42 for an open hemisphere
  • = 1.12 for a circular flat plate
  • = 1.05 for a cube
  • = 1.00 for long cylinder

A = The resisting area or the area facing into the fluid


V = the fluid velocity or the velocity of the body through the fluid


Pipe Flow Calculations

In determining the head loss (pressure drop) along a pipe as a result of friction losses it is necessary to determine the following

  • Pipe Cross Section Area = A
  • Pipe Length = L
  • Volume flow Rate = Q
  • Fluid Velocity = v
  • Fluid Density = r
  • Fluid Viscosity = m
  • Reynolds Number = Re
  • Pipe roughness = r
  • Pipe Friction Factor = f

The equation for the Reynold Number

m Re = v D r

Consistent units to be used i.e Typically r = kg/m3, v = m/s, D= m, m = Ns/m2 ( 1 Ns/m2 = 103cP)

The value for the Reynold number is to be used to evaluate if the flow is laminar or turbulent and can be used to obtain the friction factor "f" from a moody chart. The moody chart plots the friction factor against the Reynold number with a number of different plotted lines for different values of absolute roughnesss/Diameter

Moody Chart

A good moody chart can be downloaded from web2-iastat-Moody Chart
Optionally the friction factor can be obtained using a calculator Moody Chart Calculator

Various typical values of hydraulic roughness (K)

Type of Pipe K (mm)
Cast Iron 0.203
Galvanised Steel 0.152
Steel/Wrought Iron 0.051
Rivetted Steel 0.91 - 9.1
Asphalted Cast Iron 0.12
Wood-Stave 0.18 - 0.91
Concrete 3.0
Spun Concrete 0.203
Drawn Copper, Brass Steel,Glass Smooth
The head loss along the pipe can now be calculated using the Darcy-Weisbach equation

D 2 g hf= f. L. v 2

The losses through fittings are generally evaluated by obtaining equivalent values to the equivalent(L/D ) or equivalent Lengths of piping.

Typical Values of L/D for Fittings

Fitting L/D
Globe Valve 340
Gate Valve 8
Lift Check Valve 600
Swing Check Valve 50
Ball Valve 6
Butterfly Valve 35
Pipe Entrance 0.5
Pipe Exit 1
Tee Through 20
Tee- Branch flow 60
Elbow-90 30
Elbow -45 16
Bend r/D=3 12
Bend r/D=6 17
Bend r/D=12 34
Bend r/D=20 50

The result of the calculation is in units of head of the fluid. This is a very crude calculation and takes not account of head differences between the two ends of the pipe. It also assumes that the pipe is all one dia and the fluid is incompressible

Useful Links

  1. 1MNO eng.. A site containing a large number of Fluid Flow Calculations and Calculators :
    Some of the calculators require registration for a fee;
  2. Fluid Design .. A Site with information and formulas related to Pump calculation :
  3. Monachos.. A greek site with a calculator for fluid flow head losses in piping.
  4. Uengineer.. A site including fluid calculations and various data sheets
  5. The Engineering Page.. A site including various Engineering Calculators - Very good pump calculator
  6. ER_Online.. Useful software downloads - Very good pump calculator
  7. CheResources.. Subscription required $25/a but is probably worth it

    8. ..Page is being constructed..

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    Send Comments to RoyBeardmore@ic24.net

    Last Updated 01/07/2003