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Pressure Loss

Pipes

The Darcy Weisbach formula is used to determine the friction loss from water flowing through a pipe. 

 

Pressure Loss (mH) = Friction Factor * (pipe length (m) / pipe diameter (mm)) * (velocity^2/19.62)

 

Friction Factor (until the first decimal places are the same) = 

{f = [1.14 + 2 log10(D/e)]-2} and then iteratively f = {-2*log10[((e/D)/3.7)+(2.51/(Re*(f1/2))]}-2 

 

kPa = mH * 9.8041 

This spreadsheet contains the formula that is used for the H2X calculations.

One pipe size from every pipe material available in H2X has been verified against the above spreadsheet. You can check the verified results here.

 

Note:

 

  • The Darcy Weisbach formula is preferred to the Hazen Williams formula because it is more accurate. For example, the Hazen Williams formula does not consider viscosity (temperature of water) and apparently is not accurate for pipe sizes below 40mm

  • The fluid density of water used in H2X is 997 kg/m^3

  • The viscosity used for water at 20degC is 0.00100005 Ns/m2 

  • The Colebrook White coefficients used are:

    • 0.3 for Cast Iron pipes

    • 0.003 for Copper pipes

    • 0.15 for Galvanised Mild Steel pipes

    • 0.0015 for Stainless Steel, HDPE, and PEX pipes

  • There can be minor differences between the spreadsheet and the H2X result due to the rounding up of multiple decimal places within the calculation. The difference generally occurs within 0.1 kPa.

Vavles + Fittings

The friction loss from water flowing through valves and fittings is dependent on the size of the valve, the k factor of the valve, and the velocity of the water. 

 

Valve / fitting friction loss (mH) = k * (2 * velocity / 9.81*2)

kPa = mH * 9.8041 

 

This spreadsheet contains the formula that is used for the H2X calculations.

Note:

 

  • When a bend is drawn that is <45 or between 45 + 90, the greater size bend will be used for the pressure loss calculation