Overview of Hydraulic Transients
A transient is a temporary flow and pressure condition that occurs in a hydraulic system between an initial steady-state condition and a final steady-state condition. When velocity changes rapidly in response to the operation of a flow-control device (for instance, a valve closure or pump start), the compressibility of the liquid and the elasticity of the pipeline cause a transient pressure wave to propagate throughout the system. If the magnitude of this transient pressure wave and the resulting transient flow variation is great enough and adequate transient-control measures are not in place, a transient can cause system hydraulic components to fail (for instance, a pipe burst).
In general, transients resulting from relatively slow changes in flow rate are referred to as surges, and those resulting from more rapid changes in flow rate are referred to as water hammer events. Surges in pressurized systems are different than tidal or storm surges, flood waves, or dam breaks, which can occur in open-water bodies. A water hammer wave travels much faster in a pressurized system and it can burst even the strongest pipes. In general engineering practice, the terms surge, transient, hammer, and water hammer are synonymous.
Transients can occur in pressurized systems conveying any fluid, including the following:
- Water (raw or treated) systems—transmission lines including booster stations, low-head pumps and piping in water treatment plants, or high-lift pump stations and connected networks or distribution systems with branching and looping pipes.
- Wastewater (sewage) systems—pressurized sewage forcemains, surcharged sewers flowing by gravity, and sewers that are partially pressurized and partially open channel.
- Combined sewers and tunnels—combined sewers under surcharge with deep-well pump stations, time-varying inflows from surface sewer systems to drop shafts, and large storage chambers or deep tunnel conveyance or storage systems.
- Hydro power—penstocks, turbines, and tailraces, including spherical valves.
- Slurry or oil pumping—mining slurries and tailings reclaim lines, oil transmission pipelines, airport refueling systems, and liquefied natural gas (LNG) pumping.
- Industrial fluid systems—closed loops, heaters, coolers, boilers, steam, and other water-conveyance or cogeneration systems. This requires a special version of Bentley HAMMER CONNECT to track the heat of the fluid. A transient analysis is critical for operator safety.
Bentley HAMMER CONNECT has been used extensively to analyze and design water and wastewater systems, as well as slurry and oil systems. GENIVAR has analyzed steam, industrial, and cogeneration systems with custom versions and has calculated transient forces on above-ground anchors.