# Calculating pressure losses in fuel pipes

Document Sample

```					         21373

Two methods for
calculating approximate
pressure losses in pipes
by
Bob McNair
•   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

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

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)

```
DOCUMENT INFO
Shared By:
Categories:
Stats:
 views: 107 posted: 3/25/2011 language: English pages: 22