Engineers find it useful to employ the "energy grade line"
(EGL) and the "hydraulic grade line" (HGL) in working with the pipe
systems. These imaginary lines help the engineers find the trouble spots in the
system (usually points of low pressure).
Energy Grade Line
- Energy Line shows total head for certain cross section in system.
- Total head = P/γ + V2/2g + z
- P/γ = Pressure head. It represent the height of fluid column that produces the static pressure,P.
- V2/2g = Kinetic Energy Head. It represent the elevation needed for a fluid to reach the velocity,V during frictionless free fall.
- z = Elevation Head. It represent the potential energy of the fluid.
Hydraulic Grade Line
- Hydraulic Grade Line shows piezometric head for a certain cross section in system.
- Piezometric head is measurement of pressure or compressibility by using piezometer.
- Piezometric head = P/γ + z
Example Energy Grade Line & Hydraulic Grade Line.
Energy Grade Line & Hydraulic Grade Line [Conclusion]
- Energy Grade Line always higher or same with hydraulic grade line.
- The difference between 2 value is kinetic energy head,V2/2g
As the velocity goes to zero, the HGL and the EGL approach each
other. Thus, in a reservoir, they
are identical and lie on the surface.
The EGL and
HGL slope downward in the direction of the flow due to the head loss in the
pipe. The greater the loss per unit length, the greater the slope. As the
average velocity in the pipe increases, the loss per unit length increases.
A sudden change occurs in the HGL and the EGL whenever a loss occurs due to a sudden geometry change as represented by a valve or a sudden enlargement, or reduction.
At points where the HGL passes through the centerline of the pipe, the pressure is zero. If the pipe lies above the HGL, there is a vacuum in the pipe, a condition that is often avoided, if possible, in the design of piping systems; an exception would be in the design of a siphon.
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