Using FLOW-3D to Model Valves and Water Hammer Effect
System design (water tank at left;
butterfly check valve at right)
Valve opening by flow
Results of simulation, showing
Water hammer refers to pressure fluctuations caused by a sudden increase or decrease in flow velocity. These pressure fluctuations can be severe enough to rupture a water main. Potential water hammer problems have to be considered when pipe and valve design is evaluated, to avoid costly malfunctions in a water distribution system.
Recent simulations with FLOW-3D have shown its ability to model the water hammer effect. This can lead to greater understanding and the ability to design systems that better control the adverse effects of water hammer.
What is Water Hammer?
Water hammer (or hydraulic shock) is the momentary increase in pressure that occurs in a water system when there is a sudden change of direction or velocity of water in the system. When a rapidly-closed valve suddenly stops water flowing in a pipe, pressure energy is transferred to the valve and pipe wall. Shock waves are set up within the system. Pressure waves travel backward until encountering the nearest solid obstacle, then forward and back again until dissipated by friction losses. The pressure wave’s velocity is equal to the speed of the sound; it “bangs” as it travels back and forth. Anyone who has lived in an old house has heard the “bang” that resounds through the pipes when a faucet is suddenly closed. This is an effect of water hammer.
The operating conditions of a water transport system are seldom at a steady state. Pressures and flow rates change continually as pumps start and stop, demand fluctuates and tank levels change. In addition to these normal events, unforeseen events such as power outages and equipment malfunctions can sharply change the operating conditions of a system. Any change in flow rate, regardless of the magnitude of change, requires that the fluid be accelerated or decelerated from its initial flow velocity. Rapid changes in flow rate require large forces that are seen as large pressures, which cause water hammer.
Valves as Control Mechanism
Water hammer often damages pumps when electrical power fails. In this situation, the best form of prevention is to have automatically-controlled valves, which close slowly. These valves do the job without electricity or batteries. The direction of the flow controls them. Closing the valve slowly can moderate the rise in the pressure when a change in flow rate is detected. However, the valve design must be carefully chosen so as to enable it perform the task for which it is intended without producing adverse side effects.
Shown in this article are some results from recent simulations run with FLOW-3D for a prospective customer involved in valve design and manufacturing. In this particular case, the customer was interested in modeling the operation of a butterfly valve used as a check valve, and in detecting water hammer and its effect on the check valve, when the main valve is shut down.
The first step was to model the opening of the butterfly valve as caused by the flow of water in the pipe, using FLOW-3D's new General Moving Objects model. The animation above shows the results of this simulation.
Next, to simulate the water hammer effect, a restart simulation was initiated from the end of the valve opening simulation. In the restart, the water flow was shut down instantaneously, causing the propagation of water hammer, as seen in the animation above.
The description of water hammer and its effects was drawn from a Tech Brief by Z. Michael Lahlou, Ph.D., Technical Assistance Consultant for the National Drinking Water Clearinghouse at West Virginia University.