What Is Transient Flow?

Paul Scott

Transient flow is a fluid dynamics condition where the velocity and pressure of a fluid flow change over time due to changes in system status. These changes may be caused by the starting or stopping of a pump, opening or closing of valves, or fluctuations in supply pressure from reservoirs or tanks. It is usually characterized by a powerful pressure wave or waves that may persist for a significant time after the precipitating event has ceased. Transient flow events can be extremely destructive if the magnitude and velocity of the pressure wave exceeds the capacity of the system in which it takes place. Solving recurring transient flow problems is not easy, and is generally only achievable with sophisticated software simulations.

Man with a drill
Man with a drill

When fluids such as water are driven unimpeded and at a constant rate through any system of pipes and valves, the velocity of the fluid and the pressure that it exerts on the system remain steady and predictable. When some part of that balance is upset, the starting of a booster pump or the closing of a valve for example, then a condition known as transient flow is initiated. This condition is characterized by a sudden peak in pressure and fluid flow velocity, the magnitude of which depends on the type of initiating event and how rapidly it occurred. As water is a resonant material, this wave is transferred along the entire column of fluid, the results of which are usually experienced throughout the system.

This condition may be characterized by a single or series of waves that can, depending on ambient conditions, remain active in the fluid column for a long period after the initiating event has ceased. These waves frequently cause a loud banging sound in the piping, giving rise to the phenomenon's common name of water hammer. In cases where the pressure wave and resultant increase in fluid flow velocity are of sufficient magnitude, the transient flow can cause damage or even complete failure of pipes and valves along the system. Not only can these events cause considerable damage, but they are also highly disruptive to pressure and flow-rate-sensitive manufacturing processes.

Accurately defining the root cause of transient flow events with a view to eliminating them is difficult. In most cases, a thorough knowledge of the exact nature of the fluids involved, as well as pipe, valve, and pipe support details are required as a starting point. Complex and sophisticated computer software is employed thereafter to simulate conditions in the system that can, in most cases, identify the source and potential solutions for transient flow events.

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