Type Of Fluid Flow In Pipes

Fluid Flow in Pipes

Fig. 1-9. Streamline flow

STREAMLINE, or laminar, flow is the ideal type of fluid flow in hydraulic power system because all of the particles of a fluid move in parallel lines as shown in Fig. 1-9(A). During the flow, the layer of fluid next to the surface of the pipe moves the slowest because of friction between the fluid and pipe. Each inner layer of fluid slides along on the next outer layer of fluid with less and less friction until the fluid layers near the center of the flow passage move the fastest. Figure 1-9(B) shows that the velocity of flow near the center of the pipe is greatest.

Fig. 1-10. Turbulent Flow

TURBULENT flow condition usually occurs because the fluid passage is too small for the required flow velocity or because the viscosity of the hydraulic fluid is too low. Also, rough or irregularly formed fluid passages, sudden enlargement or reductions in the diameter of the fluid passages, and sudden changes in the direction of flow as shown in Fig. 1-10 contribute to turbulence and should be avoided.

Turbulent flow heats up the hydraulic fluid more than laminar flow does, wastes power by requiring more fluid pressure, and tends to wear out hydraulic equipment rapidly. In addition, turbulent flow can release the air that suspended in the hydraulic oil, thus forming large bubbles or pockets in the lines and component. This is called CAVITATION. Cavitation is undesirable because air bubbles make the hydraulic system sluggish and less responsive. Large air pockets in the hydraulic system can also render the system completely inoperative.

Fig. 1-11 Good Piping Constriction

When fluid must pass through a passage of reduced size, the restriction should be as gradual as possible, as shown in Fig. 1-11. As the amount of fluid passes through the constriction, the flow increase in velocity. 


Bernoulli's Principle
Hydraulic fluid in a system possesses two types of energy-KINETIC and POTENTIAL. Kinetic energy is present when the fluid is in motion. Potential energy is a result of the fluid pressure. The total energy of the fluid is the sum of the kinetic and potential energy. Bernoulli's principle state that, “the total energy of the fluid always remains constant," Therefore, when the flow in a system increases, the pressure must decrease. You may note that when fluid starts to flow through a hydraulic system the pressure drops slightly. When the flow stops, the pressure rises. The pressure gauge shown in Fig. 1-11 indicates this more clearly.