M&E Engineering Flashcards
Hydraulic transients or water hammering
Hydraulic transients, often referred to as water hammering, occur when there is a sudden change in the flow velocity of a fluid within a pipe. This change causes pressure waves to propagate through the system, potentially leading to significant pressure surges, noise, vibrations, and even structural damage.
Causes of Hydraulic Transients
1. Valve Operations:
• Sudden closure or opening of valves can create rapid deceleration or acceleration of the fluid.
2. Pump Start or Shutdown:
• Abrupt changes in pump operation can disrupt the steady-state flow and trigger pressure waves.
3. Pipe Breaks or Sudden Obstructions:
• A sudden change in flow path due to damage or blockages can induce hydraulic transients.
Mechanism of Water Hammer
1. When fluid flow is abruptly stopped, the kinetic energy of the moving fluid converts into potential energy, creating a high-pressure wave.
2. This wave travels through the pipeline at the speed of sound in the fluid, reflecting back and forth until dissipated by friction and other resistances.
Key Parameters
1. Wave Speed (): Depends on the compressibility of the fluid and the elasticity of the pipe material.
• Typical wave speeds for water in steel pipes: .
2. Pressure Rise (): Determined using the Joukowsky equation:

Where:
• : Fluid density
• : Wave speed
• : Change in velocity
Effects
• Structural Damage: High pressures can burst pipes or damage fittings.
• Noise and Vibrations: Audible hammering sounds and pipe rattling.
• System Instability: Can lead to cavitation, flow separation, and pump damage.
Mitigation Strategies
1. Gradual Valve Operations:
• Use slow-closing valves to minimize abrupt flow changes.
2. Surge Tanks or Vessels:
• Install surge tanks to absorb pressure waves.
3. Air Chambers:
• Provide cushioning for pressure surges.
4. Pressure Relief Valves:
• Release excess pressure to prevent damage.
5. Pipe Design:
• Use materials and configurations that minimize wave propagation and reflection.
Applications and Relevance
Hydraulic transients are critical considerations in the design of water supply systems, industrial pipelines, and hydroelectric power plants, where the sudden movement of fluids can lead to catastrophic failures. Proper analysis and control mechanisms ensure system safety and longevity.
