Fluid Mechanics: Topic 7.3.1 - Energy grade line (EGL) & Hydraulic grade line (HGL)

Summary

This lesson explains the concepts of Energy Grade Line (EGL) and Hydraulic Grade Line (HGL) in fluid mechanics. It covers how these lines represent the total mechanical and potential energy in a fluid flow, respectively. Using a pipe system example, the lesson illustrates how the EGL and HGL change through various devices such as reservoirs, pipes, valves, pumps, nozzles, and turbines. The flow behavior and energy changes are visualized with the help of these lines, offering insight into how mechanical energy, pressure, and velocity evolve through a fluid system.

Takeaways

• 😀 The Energy Grade Line (EGL) represents the total mechanical energy in a fluid flow, including pressure head, velocity head, and elevation head.
• 😀 The Hydraulic Grade Line (HGL) represents the total potential energy in a flow, consisting only of pressure head and elevation head.
• 😀 The difference between EGL and HGL gives the velocity head, which represents the kinetic energy in the flow.
• 😀 Energy loss occurs at various devices in the flow system, such as sharp-edged entrances and valves, leading to a rapid decline in EGL and HGL.
• 😀 In a pipe system, friction gradually reduces the pressure and energy, causing both EGL and HGL curves to drop gradually.
• 😀 Pumps increase the mechanical energy in the system, causing both EGL and HGL curves to rise sharply.
• 😀 Nozzles accelerate the flow and cause a reduction in pressure, causing a gradual decline in EGL and a sharp decline in HGL due to increased velocity head.
• 😀 Turbines extract mechanical energy from the flow, resulting in a sharp drop in both EGL and HGL curves.
• 😀 The HGL curve can be observed by measuring the height of liquid in pressure taps along the system.
• 😀 The EGL curve can be observed using pitot tubes, with the height of liquid in the tubes corresponding to the energy grade line.
• 😀 While the curves may appear sharp in the example, in real-life applications, the transitions are generally smoother, reflecting continuous changes in flow conditions.

Q & A

What are the Energy Grade Line (EGL) and Hydraulic Grade Line (HGL)?

The Energy Grade Line (EGL) represents the total mechanical energy in a flow, combining pressure head, velocity head, and elevation head. The Hydraulic Grade Line (HGL), on the other hand, represents the total potential energy in a flow, combining pressure head and elevation head only.

How do EGL and HGL help visualize energy changes in a fluid system?

EGL and HGL help visualize how mechanical energy and potential energy in a fluid change as it flows through various devices such as pipes, valves, pumps, and nozzles. The difference between the EGL and HGL curves represents the velocity head, indicating kinetic energy in the flow.

What is the significance of the difference between EGL and HGL curves?

The difference between the EGL and HGL curves represents the velocity head, which corresponds to the kinetic energy of the fluid. This difference indicates how much energy is being converted into velocity (kinetic energy) as the fluid moves.

Why do the EGL and HGL curves decline steeply at sharp-edged entrances and valves?

At sharp-edged entrances and valves, the fluid experiences frictional losses and flow separation, which causes a rapid loss in mechanical energy. This is reflected in the steep decline of both the EGL and HGL curves.

What happens to the EGL and HGL curves when a pump is introduced into the system?

When a pump is introduced, it boosts the mechanical energy of the fluid, primarily by increasing pressure. As a result, both the EGL and HGL curves rise sharply at the pump, indicating an increase in the total energy and potential energy of the fluid.

How do the EGL and HGL curves behave in a nozzle?

In a nozzle, the fluid accelerates, which increases the velocity head (kinetic energy), causing a gradual decline in the EGL curve. The HGL curve drops more rapidly at the nozzle due to the conversion of pressure head into velocity head.

What is the role of pipe friction in the behavior of EGL and HGL curves?

Pipe friction causes a gradual pressure drop in the system. This is reflected in a gradual decline in both the EGL and HGL curves as the fluid moves through the pipe, with the friction losses reducing the pressure head.

How does the elevation head affect the EGL and HGL curves?

The elevation head, represented by the height 'z', contributes to both the EGL and HGL curves. The distance from the fluid to the HGL curve represents the pressure head, while the distance between the EGL and HGL curves represents the velocity head. The elevation head impacts both curves by adding to the potential energy of the fluid.

How is the EGL measured in a fluid system?

The EGL can be measured using a series of L-shaped tubes called 'pitot tubes'. These tubes are inserted into the flow, and the height of the liquid in the tubes indicates the energy grade line at those points.

What are some key differences between the behavior of EGL and HGL curves?

While both the EGL and HGL curves generally decline in the presence of friction or flow losses, the EGL includes the velocity head (kinetic energy), causing it to decrease more gradually in certain areas like nozzles and straight pipes. In contrast, the HGL curve focuses on pressure and potential energy, showing more significant drops in cases of rapid flow changes like valves and turbines.