Causes of Transient Initiation
The cause of a hydraulic transient is any sudden change in the fluid itself or any sudden change at the pressurized system's boundaries, including:
- Changes in fluid properties—such as depressurization due to the sudden opening of a relief valve, a propagating pressure pulse, heating or cooling in cogeneration or industrial systems, mixing with solids or other liquids (may affect fluid density, specific gravity, and viscosity), formation and collapse of vapor bubbles (cavitation), and air entrainment or release from the system (at air vents and/or due to pressure waves).
- Changes at system boundaries—such as rapidly opening or closing a valve, pipe burst (due to high pressure) or pipe collapse (due to low pressure), pump start/shift/stop, air intake at a vacuum breaker, water intake at a valve, mass outflow at a pressure-relief valve or fire hose, breakage of a rupture disk, and hunting and/or resonance at a control valve.
Sudden changes such as these create a transient pressure pulse that rapidly propagates away from the disturbance, in every possible direction, and throughout the entire pressurized system. If no other transient event is triggered by the pressure wave fronts, unsteady-flow conditions continue until the transient energy is completely damped and dissipated by friction.
The majority of transients in water and wastewater systems are the result of changes at system boundaries, typically at the upstream and downstream ends of the system or at local high points. Consequently, you can reduce the risk of system damage or failure with proper analysis to determine the system's default dynamic response, design protection equipment to control transient energy, and specify operational procedures to avoid transients. Analysis, design, and operational procedures all benefit from computer simulations with Bentley HAMMER CONNECT.
The three most common causes of transient initiation, or source devices, are all moving system boundaries.
Figure 14-1: Common Causes of Hydraulic Transients
Pumps—A pump's motor exerts a torque on a shaft that delivers energy to the pump's impeller, forcing it to rotate and add energy to the fluid as it passes from the suction to the discharge side of the pump volute. Pumps convey fluid to the downstream end of a system whose profile can be either uphill or downhill, with irregularities such as local high or low points. When the pump starts, pressure can increase rapidly. Whenever power sags or fails, the pump slows or stops and a sudden drop in pressure propagates downstream (a rise in pressure also propagates upstream in the suction system).
Turbines—Hydropower turbines are located at the downstream end of a conduit, or penstock, to absorb the moving water's energy and convert it to electrical current. Conceptually, a turbine is the inverse of a pump, but very few pumps or turbines can operate in both directions without damage. If the electrical load generated by a turbine is rejected, a gate must rapidly stop flow, resulting in a large increase in pressure, which propagates upstream (in the penstock).
Valves—A valve can start, change, or stop flow very suddenly. Energy conversions increase or decrease in proportion to a valve's closing or opening rate and position, or stroke. Orifices can be used to throttle flow instead of a partially open valve. Valves can also allow air into a pipeline and/or expel it, typically at local high points. Suddenly closing a flow-control valve (with piping on both sides) generates transients on both sides of the valve, as follows:
- Water initially coming towards the valve suddenly has nowhere to go. As water packs into a finite space upstream of the valve, it generates a high-pressure pulse that propagates upstream, away from the valve.
- Water initially going away from the valve cannot suddenly stop, due to its inertia and, since no flow is coming through the valve to replace it, the area downstream of the valve may "pull a vacuum," causing a low-pressure pulse to propagate downstream.
The similarity of the transient conditions caused by different source devices provides the key to transient analysis in a wide range of different systems: understand the initial state of the system and the ways in which energy and mass are added or removed from it. This is best illustrated by an example for a typical pumping system (see Figure 14-2: Typical Locations where Transient Pulses Initiate):
- A pump (upstream source device) starts up from the static HGL and accelerates flow until its input energy reaches a dynamic equilibrium with friction at the steady HGL.
- A power failure occurs and the pump stops supplying hydraulic energy; therefore, the HGL drops rapidly at the pump and a low-pressure pulse propagates downstream towards the reservoir. Subatmospheric pressures can occur at the high point (minimum transient head), but the reservoir maintains downstream pressure at its liquid level by accepting or supplying liquid as required, often several times during the transient event. Note: As the HGL drops to the pipeline elevation, a vacuum breaker valve can be installed at the local high point to supply or expel air from the system in a manner analogous to the reservoir. This tends to maintain atmospheric pressure at the valve, minimizing subatmospheric pressures when air is admitted and often reducing high pressures when air is expelled.
- The pressure pulse is reflected toward the pump, but it encounters a closed check valve (designed to protect the pump against high pressures) that reflects the pulse as a high pressure toward the reservoir again (maximum transient head).
- Friction eventually attenuates the transient energy and the system reaches a final steady state: static HGL, in this case, since pumping has stopped and flow at the reservoir is zero.
The foregoing discussion illustrates the typical concepts to consider when analyzing hydraulic transients. Computer models are an ideal tool for tracking momentum, inertia, and friction as the transient evolves, and for correctly accounting for changes in mass and energy at boundaries. Note that transients propagate throughout the entire pressurized system.
Figure 14-2: Typical Locations where Transient Pulses Initiate