Calculating pressure losses in fuel pipes

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Calculating pressure losses in fuel pipes Powered By Docstoc
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    Two methods for
calculating approximate
pressure losses in pipes
           by
        Bob McNair
                     Menu
•   Slow Speed considerations
•   Viscosity Chart
•   Moody Diagram
•   Relative Roughness
•   Reynolds Number
•   Kinematic Viscosity
•   Units of Viscosity
•   Unit conversion
•   Examples
Slow Speed FO System
      Slow Speed FO System
  For heavy fuel oil consider the system a Closed
  Loop system:
• The viscosity near the main FO pump is a little
  lower than when the fuel is injected
• As the fuel comes from the Service Tank to the
  engine its increased in temperature
• We should therefore calculate the pipe losses at
  a viscosity averaged from Service Tank
  temperature at say 55ºC to 120ºC at the transfer
  pump up to the main FO pump
         1000 Redwood Nº




                                     From 4 to 3 the
                                     temperature increases
                                     and the viscosity drops




15 cts
 9 cts




  9cts at Main Fuel Pump Pressure   15cts at Main Fuel Pump Pressure
      Consider two plates
                Load



                          dv
                                      dy

                                 P
                     Stress
Absolute Viscosity =            A
                     Shear Rate dv
                                 dy
             Units of Viscosity
                       Stress
Absolute Viscosity =
                       Shear Rate

Absolute Viscosity (units) = poise


       Stress


             Shear Rate
          Units of Viscosity

            Stress


       Shear Rate


 Absolute Viscosity           Units are called centipoise




Kinematic Viscosity = Absolute Viscosity / density
               Units of Viscosity

Kinematic Viscosity


Although the normal units of Kinematic Viscosity are centistokes they are
sometimes in mm²/s

We could find the pressure exerted by a fluid by using these formulas and
hence the Pressure drop per metre length
Conversion factors
           Moody Diagram
We use this diagram to find the Friction Factor (f)
and f is used in finding the pressure losses in a
pipe
                                 (metres)

                                       (Pa)

(The tables previously handed out are directly
related to Moody Chart values)
              Relative Roughness
Material                                Roughness () in metres
Glass or Plastic                                 Smooth


Copper, Brass, lead (tubing)
                                              1.5x104

Cast Iron (uncoated)                          2.5 x104

Commercial mild steel or welded steel



Pipes can are of different roughness internally and this affects the
pressure losses. As pipes age roughness factors will increase –
this is particularly true for water carrying pipes
             Relative Roughness

 For constant Reynolds Number


 If      is increased as the Friction Factor (f) decreases


 Also:


If       increases as the value of      in the turbulent zone increases
         Reynolds Number
Absolute and Kinematic Viscosity is used to
define the Reynolds Number




      V = velocity
      v = kinematic viscosity
      D = pipe internal diameter
                   Examples
• Oil of density 800kg/m³ has a kinematic
  viscosity of 40cts. Calculate the critical velocity
  when flowing in a pipe of 50mm diameter
• A Reynolds Number of 2000 is normally
  selected for find the “critical velocity”
Surface Roughness Coefficient
• We use more accurate charts when working out
  pipe Friction Factors
• Mean surface roughness coefficient (k)
• Diameter (D)
             Moody Chart
• We use a slightly different method
• Mean surface roughness coefficient (k)
• It gives more accurate results
More accurate Moody Chart
Finding the Friction Factor




Friction Factor (f)

				
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posted:3/25/2011
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