If the pressure heads at the different sections of the pipe are plotted to scale as vertical ordinates above the axis of the pipe and all these points are joined by a sloping straight line, which is known as
- AHydraulic gradient lineCorrect
- BTotal energy line
- CTotal head line
- DEnergy grade line
Solution
Concept
The line joining the pressure head (piezometric head) points along a pipe is the hydraulic gradient line.
Formula
HGL = p/(ρg) + z
Step-by-step solution
1. Plot the pressure head p/(ρg) at each section above the pipe axis. 2. Join these points with a line. 3. This line shows the piezometric head, so it is called the Hydraulic Gradient Line (HGL).
Explanation
The pressure heads plotted above the pipe axis and joined together give the hydraulic gradient line. It shows the level water would rise to in a piezometer tube. The energy line is above the HGL by the velocity head V2/(2g).
Why the other options are wrong
B: Total energy line also includes the velocity head V2/(2g). C: Total head line is another name for the energy line, not the pressure-head line. D: Energy grade line is the same as the total energy line, so it lies above the HGL.
Shortcut
Pressure head only = HGL. Pressure head + velocity head = TEL.
Common mistake
Students confuse HGL with TEL. TEL is always above HGL by V2/(2g).
Correct answer: A) Hydraulic gradient line