Transient flow is a fluid dynamic condition where changes in the system cause pressure and velocity changes, resulting in powerful pressure waves that can damage pipes and valves. It is difficult to eliminate, but can be simulated with software to identify potential solutions.
Transient flow is a fluid dynamic condition in which the velocity and pressure of a fluid flow change over time due to changes in the state of the system. These changes can be caused by a pump starting or stopping, valves opening or closing, or supply pressure fluctuations from tanks or reservoirs. It is usually characterized by a powerful pressure wave or waves that can persist for a significant time after the precipitating event has ceased. Transient flow events can be extremely destructive if the amplitude and velocity of the pressure wave exceed the capability of the system in which it occurs. Solving recurring transient flow problems is not easy and is usually only achievable with sophisticated software simulations.
When fluids such as water are guided unimpeded and at a constant velocity through any system of pipes and valves, the velocity of the fluid and the pressure it exerts on the system remain constant and predictable. When some part of that balance is upset, such as the starting of a booster pump or the closing of a valve, a condition known as transient flow is set in motion. This condition is characterized by a sudden surge in pressure and fluid flow velocity, the magnitude of which depends on the type of triggering event and how quickly it occurred. Since water is a resonant material, this wave is transferred along the entire column of fluid, the results of which usually occur throughout the system.
This condition may be characterized by a single or a series of waves which may, depending on the environmental conditions, remain active in the fluid column for a long time after the inciting event has ceased. These waves often cause a loud banging noise in the pipes, giving rise to the common name of the water hammer phenomenon. In cases where the pressure surge and resulting increase in fluid flow velocity is large enough, the transient flow can cause damage or even complete failure of pipes and valves throughout the system. Not only can these events cause considerable damage, but they are also highly destructive to pressure- and flow-sensitive production processes.
It is difficult to pinpoint the root cause of transient flow events in order to eliminate them. In most cases, a thorough understanding of the exact nature of the fluids involved, as well as the details of the hose, valve and hose support is needed as a starting point. Subsequently complex and sophisticated software is employed to simulate conditions in the system which can, in most cases, identify the source and potential solutions for transient flow events.
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