PIPE FRICTIONAL LOSSES

When a fluid flows through a pipe, the internal roughness of the pipe wall can create local eddy currents within the fluid adding a resistance to flow of the fluid. The velocity profile in a pipe will show that the fluid elements in the center of the pipe will move at a higher speed than those closer to the wall. . Therefore friction will occur between layers within the fluid. This movement of fluid elements relative to each other is associated with pressure drop, called frictional losses. Pipes with smooth walls such as glass, copper, brass and polyethylene have only a small effect on the frictional resistance. Pipes with less smooth walls such as concrete, cast iron and steel will create larger eddy currents which will sometimes have a significant effect on the frictional resistance. Rougher the inner wall of the pipe, more will be the pressure loss due to friction.

As the average velocity increases, pressure losses increase. Velocity is directly related to flow rate.

Velocity=Volumetric flow rate /Cross sectional area of the pipe

An increase or decrease in flow rate will result in a corresponding increase or decrease in velocity. Smaller pipe causes a greater proportion of the liquid to be in contact with the pipe, which creates friction. Pipe size also affects velocity. Given a constant flow rate, decreasing pipe size increases the velocity, which increases friction. The friction losses are cumulative as the fluid travels through the length of pipe. The greater the distance, the greater the friction losses will be. Fluids with a high viscosity will flow more slowly and will generally not support eddy currents and therefore the internal roughness of the pipe will have no effect on the frictional resistance. This condition is known as laminar flow.

Loss of head is incurred by fluid mixing which occurs at fittings such as bends or valves, and by frictional resistance at the pipe wall. Where there are numerous fittings and the pipe is short, the major part of the head loss will be due to the local mixing near the fittings. For a long pipeline, on the other hand, skin friction at the pipe wall will be predominant.

Friction loss has several causes, including:

Frictional losses depend on the conditions of flow and the physical properties of the system. Movement of fluid molecules against each other Movement of fluid molecules against the inside surface of a pipe or the like, particularly if

the inside surface is rough, textured, or otherwise not smooth Bends, kinks, and other sharp turns in hose or piping

The frictional head loss (hf) depends on the type of flow, which can be laminar or turbulent. In laminar flow, fluid flows in layers with orderly movement of fluid particles while in Turbulent flow fluid particles move in a disorderly manner.

Whether the flow is laminar or turbulent is decided by a non-dimensional Reynold’s number Re which is expressed as

Re=ρ vDμ

Where ρ= Fluid density, v = Flow Velocity, D = pipe diameter, μ= Fluid viscosity

In pipes, the flow is laminar when Re < 2000 and turbulent when Re > 4000 with flow transition taking place when 2000 < Re < 4000

For laminar flow,

f=16Re

In pipe flows the losses are due to friction. The former is due to the roughness of the inner part of the pipe where the fluid comes in contact with the pipe material, while the latter is due to obstructions present in the line of flow--perhaps a bend, control valve, or anything that changes the course of motion of the flowing fluid.

Calculating friction loss

One of the accepted methods to calculate friction losses resulting from fluid motion in pipes is by using the Darcy-Weisbach Equation. For a circular pipe

where:

hl = Head Loss due to friction, given in units of lengthfD = Darcy friction factorL = Pipe LengthD = Pipe DiameterV = Flow velocityg = Gravitational acceleration