What is Head Loss?

Friction loss (also known as head loss) is the loss of pressure in pipe or duct flow due to resistance water meets as it travels through the pipe. This loss in pressure is an important factor to consider in any application involving the transportation of water through a pipe or duct, and will have direct impacts upon the equipment and control settings you will need to use when pumping or transporting water.

Typically, the larger the diameter of the pipe, the lower the friction coefficient — as a smaller percentage of the liquid will have contact with the inside of the pipe. The same can be said for the length of the pipe. Smoother materials again will help reduce friction within, allowing your pumps to work more effectively at moving the water through.

The Head Loss Equation

When dealing with water, we can use the Hazen-Williams equation to calculate how much pressure is lost due to friction. Hazen and Williams developed this formula in 1906 to empirically define head loss for water in pipes, and is accurate while the temperature of the water is between approximately 4° to 24°C (40° and 75°F).

It can be written as hL = 10.67 * L * Q1.852 / C1.852 / d4.87, where hL represents the head loss for water in meters, L represents the length of the pipe in meters, Q represents the flow rate through the pipe in m3/s, and d measures the internal diameter of the pipe. C represents the Hazen-Williams coefficient, which varies based on the surface of the pipe, from higher numbers for smoother materials, and lower for rougher materials.

Altogether this provides a handy formula to quickly calculate the head loss of a system, and plan accordingly.

There are plenty of online calculators for working out the head loss, but be wary that most projects require more careful planning than simply using an online tool!