All pipeline systems are susceptible to surge and water hammer, two powerful forces that can cripple critical infrastructure. Surge and water hammer can lead to short- and long-term damage to piped systems by causing conditions such as vibration, rupture, leaks, and displacement. As a result, pipelines may collapse, become contaminated, or fail prematurely.
To avoid loss of key services and expensive repairs, engineers designing piped infrastructure must consider the potential for surge and water hammer and determine the most efficient, cost-effective mitigation strategy.
Hydraulic transients occur when a water column's steady-state conditions are altered; this leads to surge or water hammer. Typical causes of surge andwater hammer include pump start-up and shut down, power failure, sudden open or closure of line valves, and column separation.
Surge is most commonly observed in a rigid, or incompressible, system. The phenomenon results from relatively slow changes in pipeline flow and is oftenassociated with the formation of vapor cavities (water column separation). The rate of change for a surge event is moderate compared with the near instantaneous change observed in a water hammer event.
Water hammer occurs in systems with some element of compressibility of the liquid and elasticity of the pipeline. This elastic process yields a pressurewave due to a rapid change in fluid velocity.
Surge and water hammer are controlled by a number of components, each with advantages and disadvantages.
Pump control valves offer automatic control but may prove ineffective in power failures unless provided with backup stored energy sources to close thevalve.
Surge relief valves, whether spring or pilot operated, can be set to open at 10% to 15% above maximum pump pressure, but they require drain piping to adischarge point and are generally not effective on down surges (low-pressure events such as those caused by pump power failure).
Surge anticipator valves are pilot operated and programmable. They open immediately on power failure and await the pressure wave arrival. Valve closingis controlled to limit pressure upsurges (high pressure that can be caused by valve closure). However, these valves are not effective in controllingnegative pressures, or they must be combined with other valves to cover all possibilities.
Surge tanks act as combination shock absorbers and water accumulators. The tanks, typically located near the pump station, are effective on both up anddown surges, but they are large, expensive, and require varying levels of maintenance, including freeze protection.
Flywheels control surge and water hammer by slowly reducing motor/pump speed. However, flywheels are expensive and can increase motor starting torque.