Head Loss

INTRODUCTION

Head loss is a measure of the reduction in the total head (sum of elevation head, velocity head and pressure head) of the fluid as it moves through a fluid system. Head loss is unavoidable in real fluids.

There are two categories of head loss in pipe. One of them is due to viscous resistance extending throughout the total length of the circuit. Next is due to localized effects such as valves, sudden changes in area of flow and bends. Many factors affect the head loss in pipes, the viscosity of the fluid being handled, the sizes of the pipes, the roughness of the internal surface of the pipes, the changes in elevation within the system and the length of travel of the fluid.

The resistance through various valves and fittings will also contribute to the overall head loss. A method to model the resistances for valves and fittings will be of minor significance to the overall head loss, many designers choose to ignore the head loss for valves and fittings at least in the initial stages of a design.

Frictional loss is that part of the total head loss that occurs as the fluid flows through straight pipes. The head loss for fluid flow is directly proportional to the length of pipe, the square of the fluid velocity, and a term accounting for fluid friction called the friction factor. The head loss is inversely proportional to the diameter of the pipe.

The friction factor has been determined to depend on the Reynolds number for the flow and the degree of roughness of the pipe’s inner surface.

THEORY

For an incompressible fluid flowing through a pipe the following equations apply:

Notation:   

Q  Volumetric flow rate (m3/s) 

V  Mean Velocity (m/s)

A  Cross sectional area (m3) 

Z  Height above datum (m)

P  Static pressure (N/m2)

hL  Head Loss (m)

ρ  Density (kg/m3) 

g   Acceleration due to gravity (9.81m/s2)

HEAD LOSS

The head loss in a pipe circuit falls into two categories: 

That due to viscous resistance extending throughout the total length of the circuit, and; 

That due to localized effects such as valves, sudden changes in area of flow, and bends. 

The overall head loss is a combination of both these categories. Because of mutual interference between neighboring components in a complex circuit the total head loss may differ from that estimated from the losses due to the individual components considered in isolation.

Head Loss in Straight Pipes 

The head loss along a length, L, of straight pipe of constant diameter, d, is given by the expression: 

Where f is a dimensionless constant, which is a function of the Reynolds number of the flow and the roughness of the internal surface of the pipe.