Aquifers, Hydrostatic Pressure, and Hydraulic Systems

What is an Aquifer and How Does it Relate to Hydrostatic Pressure?

An aquifer is a geological formation capable of storing and transmitting water through its pores and cracks, making it hydraulically usable. Aquifers are classified based on their hydrostatic pressure:

• Free Aquifer: The free surface of the water is below the ceiling of the aquifer, so it is at atmospheric pressure.
• Confined Aquifer: The water is above atmospheric pressure, filling all the pores. When the roof of the aquifer is pierced, the water rises to reach a hydrostatic equilibrium.
• Leaky Aquifer: A type of confined aquifer where the wall and/or ceiling are semipermeable, acting as an aquitard.

An aquitard is a geological formation that contains water but transmits it very slowly. Aquitards are not suitable for locating underground deposits but can allow vertical recharge of other aquifers under specific conditions.

Devices Connected to a Hydraulic Drive

The following devices are typically connected to a hydraulic drive:

• Foot-valve with strainer: A check valve with a filter that prevents large solids from reaching the pump.
• Crusher: Placed before the drive to reduce the size of solids, preventing obstruction of filters or grates.
• Priming: Ensures water supply to the pump.
• Valves (downstream and upstream): Allow isolation for maintenance.
• Check valve: Placed behind each hydraulic pump to prevent short circuits.
• Vents and traps: Prevent vacuum problems, air obstructions, and starting difficulties.
• Waste disposal: Empties the conduit in case of failure.

Understanding Vacuum and its Role in Hydraulic Drives

Vacuum pads, also known as air vents, allow air to enter during a negative pressure wave, preventing cavitation. They can perform three functions:

• Air admission
• Air expulsion
• Purging of occluded air pressure

Air vents should be placed at:

• Elevation changes
• Significant reductions in slope in the direction of airflow, every 600-700m in long runs with a gentle slope, with check valves
• Branches downstream of traps, with a length greater than 600m

Types of Air Vents

• Suction trap: Air is expelled through a small hole during pipe filling. When fluid fills the cup body, a float rises, closing the outlet with a cam system.
• Float-ball vent: Air passes through the body of the cup and out through a large hole. When fluid enters, the float rises, blocking the exit.

Relief Valves and Rupture Discs in Hydraulic Systems

Relief valves and rupture discs are set above the working pressure. When pressure rises significantly due to water hammer, the valve opens or the disc ruptures, providing an atmospheric contact point that quickly eliminates the water hammer. Their primary function is to eliminate water hammer.

A specific case is a check valve with a by-pass differential, used in large-diameter pipes with reversible lifting groups. The main drawback is their failure to address depressions that may cause cavitation. Their main advantage is their low cost, both in terms of space and money.

Determining the Hydraulic Profile of a Channel

Here's the procedure for determining the hydraulic profile of a channel:

1. Consider the system (river, torrential, or critical) in each section of interest.
2. Depending on the regime, adopt a known height and the direction in which it will be measured.
3. Choose a different section than the one adopted for the sections prior to the known section.
4. With the known draft and course, define sections, hydraulic radius, velocity, Bernoulli's principle, and losses.
5. Using the derived Bernoulli's formula, determine the distance 'x' to the section.
6. Repeat the process from the draft and position of the assumed section.