8.3 Hydraulic Structures and Machinery

Hydraulic structures and machinery are crucial components in water management systems. Dams, weirs, and spillways control water flow, while pumps and turbines harness its power. These elements work together to provide water supply, flood control, and hydroelectric energy.

Understanding the forces acting on hydraulic structures is key to their design and stability. Engineers must consider hydrostatic pressure, hydrodynamic forces, and uplift pressure when creating safe and efficient water management systems. Proper selection of pumps and turbines ensures optimal performance in various applications.

Hydraulic Structure Design

Dam, Weir, and Spillway Functions

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• Dams impound water creating reservoirs for water supply, flood control, and hydropower generation
• Weirs control water levels and measure flow in open channels and rivers
• Spillways safely release excess water during floods preventing dam overtopping and failure
• Hydraulic structure design considers hydrological data, site geology, structural integrity, and environmental impacts

Design Principles for Hydraulic Structures

• Dam design involves selecting appropriate type (gravity, arch, earthfill) based on site conditions
• Foundation treatment and seepage control measures crucial for dam stability and safety
• Weir design focuses on crest shape, approach conditions, and tailwater effects for accurate flow measurement
• Spillway design incorporates energy dissipation structures (stilling basins, flip buckets) to prevent downstream erosion
• Various spillway types used based on site conditions and required discharge capacity (ogee, chute, side channel)

Forces on Hydraulic Structures

Types of Forces

• Hydrostatic pressure acts perpendicular to structure surface, increases linearly with depth (Pascal's law)
• Hydrodynamic forces result from flowing water include drag, lift, and impact forces
  • Significant during flood events or high-velocity flows
• Uplift pressure caused by seepage under structure reduces effective weight and stability
  • Mitigated through drainage systems and impervious barriers (cutoff walls, grout curtains)
• Self-weight of structure contributes to stability against overturning and sliding

Stability Analysis

• Assess safety against failure modes overturning, sliding, and bearing capacity
• Calculate factor of safety for each mode comparing resisting forces to driving forces
  • Minimum acceptable values specified by design codes (USACE, USBR)
• Seismic forces considered in earthquake-prone regions requiring dynamic analysis
  • Additional stability measures may include shear keys or post-tensioning
• Stability analysis often performed using numerical methods (finite element analysis)

Hydraulic Machinery Principles

Pump Operating Principles

• Pumps convert mechanical energy to hydraulic energy increasing fluid pressure and/or velocity
• Centrifugal pumps use rotating impellers to impart kinetic energy to fluid
  • Kinetic energy converted to pressure energy in volute or diffuser
  • Most common pump type for water supply and irrigation systems
• Positive displacement pumps directly pressurize fluid by changing chamber volume
  • Reciprocating pumps use pistons or plungers (well pumps)
  • Rotary pumps use gears, lobes, or screws (oil pumps)

Turbine Operating Principles

• Turbines convert hydraulic energy to mechanical energy typically for electricity generation
• Reaction turbines operate fully submerged utilizing both kinetic and pressure energy
  • Francis turbines for medium head applications (hydroelectric dams)
  • Kaplan turbines for low head, high flow applications (run-of-river plants)
• Impulse turbines operate in air converting kinetic energy of high-velocity water jets
  • Pelton wheels for high head, low flow applications (mountainous regions)
• Performance characterized by flow rate, head, power, and efficiency
  • Represented in characteristic curves (head-discharge, efficiency-discharge)

Selecting Hydraulic Machinery

Performance Parameters

• Flow rate (Q) and head (H) primary parameters for pump and turbine selection
  • Different machinery types suited for specific Q-H ranges
• Specific speed (Ns) dimensionless parameter classifying pumps and turbines
  • $N_s = \frac{N\sqrt{Q}}{H^{3/4}}$ where N rotational speed, Q flow rate, H head
  • Guides selection based on optimal operating conditions
• Efficiency considerations include hydraulic, volumetric, and mechanical efficiencies
  • Overall efficiency critical factor in equipment selection and energy costs

Selection Process

• System curve analysis essential for pump selection
  • Considers pipe friction losses, static head, and system characteristics
  • Pump operating point determined by intersection of pump and system curves
• Turbine selection for hydropower depends on available head, flow variations, and operational flexibility
  • Francis turbines for 30-500m head range
  • Kaplan turbines for 10-70m head range
  • Pelton wheels for 300-1800m head range
• Life cycle cost analysis crucial for long-term operation
  • Includes initial cost, energy consumption, and maintenance requirements
  • Net Present Value (NPV) or Internal Rate of Return (IRR) used for comparison