What is the principle of energy grade lines and hydraulic grade lines?

Short Answer:

The principle of energy grade lines (EGL) and hydraulic grade lines (HGL) helps visualize the energy levels in a flowing fluid. The energy grade line shows the total energy (pressure + velocity + elevation), while the hydraulic grade line shows only pressure and elevation energy (excluding velocity).

These lines are used to understand how energy varies in pipes or channels, and help civil engineers design systems to prevent overflow, backflow, and ensure smooth flow. Knowing the position of EGL and HGL helps determine pipe size, slope, and pressure requirements.

Detailed Explanation:

Principle of energy grade lines 

In civil engineering, especially when designing water pipelines, sewer lines, and open channels, it is very important to know how energy is distributed in the flowing fluid. This is where the energy grade line (EGL) and the hydraulic grade line (HGL) come into use. These are graphical tools that help engineers see how pressure, velocity, and elevation vary along the flow path.

Understanding these lines ensures that systems are designed to avoid issues such as pipe bursting, negative pressure, overflow, and inefficient flow conditions.

Energy Grade Line (EGL)

The energy grade line represents the height to which the total mechanical energy of the fluid can raise water. This includes:

  • Pressure head (P/ρg)
  • Velocity head (V²/2g)
  • Elevation head (z)

In mathematical form:
EGL = z + P/ρg + V²/2g

The EGL lies above the actual pipe or channel profile. It gives the total energy per unit weight of the fluid. At any point, the fluid cannot rise above this line unless extra energy is added (like from a pump). It shows the highest possible point the fluid can reach.

Hydraulic Grade Line (HGL)

The hydraulic grade line shows the pressure and elevation energy only—it does not include the velocity head. It represents the height to which fluid would rise in a piezometer tube inserted into the pipe or channel.

In mathematical form:
HGL = z + P/ρg

This line is always below or equal to the EGL. If the velocity is zero, then the HGL and EGL coincide. In open channel flow, the HGL is equal to the free water surface.

Principle and Use in Engineering

The difference between EGL and HGL is the velocity head. So, if a pipe has high velocity, the gap between EGL and HGL becomes larger. Understanding this helps in designing safe systems:

  • In pipelines, HGL helps check if pressure stays above atmospheric to avoid air suction or pipe collapse.
  • In pump systems, EGL is used to determine the required pump head to move fluid between elevations.
  • In open channels, the HGL is the actual water surface level, and the EGL lies slightly above due to velocity.
  • For safety, pipes should be placed above HGL to avoid backflow or flooding in gravity systems.

Real Examples

  1. Pumps and Valves: Engineers use EGL to select pump size and valve strength to handle energy changes.
  2. Surge Tanks and Reservoirs: HGL ensures fluid does not overflow or fall below critical levels.
  3. Water Distribution Systems: HGL helps avoid negative pressure zones that can suck contaminants into the pipeline.
Conclusion:

The principle of energy grade lines and hydraulic grade lines is essential for visualizing and managing energy in fluid systems. The EGL represents total energy, while the HGL shows only pressure and elevation. These lines guide civil engineers in designing safe, efficient, and well-functioning pipelines, channels, and hydraulic structures by indicating energy levels throughout the system.