Calculating Pressure Losses in Fuel Pipes for Emailing 21373 9 18

8/11/2019 Calculating Pressure Losses in Fuel Pipes for Emailing 21373 9 18

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Slow Speed considerations

Viscosity Chart

Moody Diagram

Relative Roughness Reynolds Number

Kinematic Viscosity

Units of Viscosity Unit conversion

Examples


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Slow Speed FO System

For heavy fuel oil consider the system a ClosedLoop 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 Tanktemperature at say 55C to 120C at the transferpump up to the main FO pump


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15 cts

9 cts

9cts at Main Fuel Pump Pressure 15cts at Main Fuel Pump Pressure

1000Redwood N

From 4 to 3 the

temperature increasesand the viscosity drops


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Consider two plates

Absolute Viscosity =Shear Rate

dydv

A

P

Stress

dv

dy

Load


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Units of Viscosity

Absolute Viscosity =Shear Rate

Stress

Absolute Viscosity (units) = poise

Stress

Shear Rate


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Stress

Shear Rate

Absolute Viscosity

Kinematic Viscosity = Absolute Viscosity / density

Units of Viscosity

Units are called centipoise


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Kinematic Viscosity

Units of Viscosity

We could find the pressure exerted by a fluid by using these formulas and

hence the Pressure drop per metre length

Although the normal units of Kinematic Viscosity are centistokes they are

sometimes in mm/s


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Conversion factors


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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)


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Relative Roughness

4105.2

x

Material Roughness () in metres

Glass or Plastic Smooth

Copper, Brass, lead (tubing)

Cast Iron (uncoated)

Commercial mild steel or welded steel

4105.1

x

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


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For constant Reynolds Number

If is increased as the Friction Factor (f) decreases

If increases as the value of in the turbulent zone increases

Also:

Relative Roughness


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Reynolds Number

Absolute and Kinematic Viscosity is used to

define the Reynolds Number

V= velocity

v = kinematic viscosityD = pipe internal diameter


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Examples

Oil of density 800kg/m has a kinematic

viscosity of 40cts. Calculate the critical velocity

when flowing in a pipe of 50mm diameter

A Reynolds Number of 2000 is normallyselected for find the critical velocity


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Surface Roughness Coefficient

We use more accurate charts when working out

pipe Friction Factors

Mean surface roughness coefficient (k)

Diameter (D)


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Moody Chart

We use a slightly different method

Mean surface roughness coefficient (k)

It gives more accurate results


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More accurate Moody Chart


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Finding the Friction Factor

Friction Factor (f)

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Calculating Pressure Losses in Fuel Pipes for Emailing 21373 9 18