What is Water Hammer?
Water hammer (or, more generally, fluid hammer) is a pressure surge or wave resulting when a fluid (usually a liquid but sometimes also a gas) in motion is forced to stop or change direction suddenly (momentum change). Water hammer commonly occurs when a valve is closed suddenly at an end of a pipeline system, and a pressure wave propagates in the pipe.
Design Challenges Resolved in Flownex:
- Prediction of the maximum pressure in the pipeline enables the designers to:
- Create a safe design.
- Predict pressure surges.
- Ensure pipe integrity.
- Determine accurate sizing.
- Save costs by not being overly conservative.
- Provide quantitative data to base cost decisions on when upgrading existing equipment/ systems
- Determine the maximum allowable closing time of valves. To prevent water hammer, the rate of change of the velocity in the pipe must be kept low, which means that valves should not be closed too quickly. The closing rate should be less than the specific pipe’s wave period.
- Inclusion of complex factors into the water hammer analysis. Factors such as the elastic behaviour of the fluid and the pipe wall, frictional losses and other flow losses make it possible to predict maximum wave pressures.
- Flexibility in calculation model. The ability to perform various sensitivity analyses and design optimisation techniques is very important. Being able to quickly adapt and optimise the model for different scenarios can save on time and cost.
- Performing a fully transient simulation of a pump start-up and shutdown procedure. Water hammer is an inherently transient event. All the momentary dynamic forces cannot be accommodated by a simple steady state calculation, therefore Dynamic analysis is essential.
Flownex enables engineers to design, test and optimise system using its dynamic analysis capabilities, listed below are some of the characteristics modelled in Flownex to reduce or eliminate water hammer:
Water Hammer Handling Measures Simulated in Flownex:
· Flownex enables the designer to push the envelopes of his design where high flow velocities are required. Instead of relying on rules of thumb, Flownex enables a designer to quantify exactly the operating boundaries of his system.
- Model variable valve closing speeds to reach acceptable max pressures in system.
- Provide quantitative data to ensure expensive high pipeline pressure ratings are not required.
- Test and optimise pipeline control (start-up and shut-down procedures).
- Determine the necessity and location of mitigating measures to systems. These include accumulators or expansion tanks, vacuum breakers/air admission valves and pressure relief valves.
Accumulator Component in Flownex
- Test and optimise component geometries, closing times and system layout to lower fluid velocities, this keeps water hammer low, pipe-sizing charts for some applications recommend flow velocity at or below 5 ft/s (1.5 m/s).
- Ensure correct design and maintenance of systems because the water hammer can cause water pipes to fail catastrophically.
- Flywheel on rotating machinery such as pumps and turbines.
- Test multiple scenarios, e.g. Pumping station bypass
- Air valves are often used to remediate low pressures at high points in the pipeline.
- Shorter branch pipe lengths.
- Shorter lengths of straight pipe, i.e. add elbows, expansion loops. Water hammer is related to the speed of sound in the fluid, and elbows reduce the influences of pressure waves.
- Arranging the larger piping in loops that supply shorter smaller run-out pipe branches. With looped piping, lower velocity flows from both sides of a loop can serve a branch.
Water Hammer Example:
The water hammer effect of a pump trip in a mine de-watering pipeline modelled in Flownex is shown below:
The water hammer effect of a pump trip in a mine