What is friction in a pump system (cont.)
Another cause of friction is all the fittings (elbows, tees, y’s, etc) required to get the fluid from point A to B. Each one has a particular effect on the fluid streamlines. For example in the case of the elbow, the fluid particles that are closest to the tight inner radius of the elbow lift off from the pipe surface forming small vortices that consume energy. This energy loss is small for one elbow but if you have several elbows and other fittings the total can become significant. Generally speaking they rarely represent more then 30% of the total friction due to the overall pipe length.
Frictional Head Losses
The total frictional head losses in a system are comprised of the frictional losses in the suction piping system and the frictional losses in the discharge piping system. Frictional head losses = frictional losses in suction piping system + frictional losses in discharge piping system The frictional losses in the suction and discharge piping systems are the sum of the frictional losses due to the liquid flowing through the pipes, fittings and equipment. The frictional head losses are usually calculated from the Darcy-Weisbach equation using friction factors and fittings factors to calculate the pressure loss in pipes and fittings. Darcy-Weisbach equation:
In order to calculate the frictional head losses you therefore need to know the lengths and diameters of the piping in the system and the number and type of fittings such as bends, valves and other equipment.
Even in a horizontal pipe, friction inevitably occur due to the viscosity of the fluid. In this case, the associated head loss can indeed be shown very clearly. One can imagine small vertical tubes attached to the pipe. Due to the static pressure in the flowing liquid, the fluid in the vertical tubes is pressed upwards by a certain amount. Due to the friction losses in the pipe, however, the static pressure downstream decreases (assuming a constant pipe cross-section). The fluid in a downstream tube only reaches a lower height. The difference in the liquid levels corresponds to the head loss of the horizontal pipe.