Pressure losses are pressure or energy drops due to the resistance encountered by moving fluids. These resistances are due to the viscosity of the fluids, the roughness of the pipe and all the obstacles encountered by the fluid in its flow.
They split into two parts:
 - linear pressure losses caused by friction along the straight lengths of pipes;
 - the particular pressure drops which occur at the level of special parts of a network such as tees, elbows, valves, reductions or other organs such as valves, valves, meters, etc.
The estimation of the individual pressure losses, in the case where the number and type of accessories is known with precision, is done by the use of the charts and tables of the manufacturers of these organs or by specific formulas. They are generally evaluated between 5% and 15% of the linear pressure losses on the discharge pipes.
For a suction pump,
Total Head = Geometric suction height + Pressure drop at suction + Geometric delivery height + Pressure drop at discharge + Residual pressure required at the return point.