Water hammer is a phenomenon that originates from the principles of incompressibility. It occurs when there is a sudden change in the direction of a large volume of fluid flowing through a pipeline. Due to the inertia of the fluid, which is determined by its mass, it comes to a sudden stop, leading to a rapid increase in pressure. As a result of the non-elastic nature of the fluid, the excess energy cannot dissipate or be absorbed, resulting in the generation of pressure waves that travel through the pipeline until they find a way to dissipate the energy.
The peak pressure associated with water hammer is proportional to the volume, similar to what happens when a non-compliant object collides with a wall at high speed. For instance, a pipeline that is 24 inches in diameter and 50 kilometers long, filled with water, has a mass of approximately 16,000 tons. Any disruption to the momentum of such a substance requires a significant amount of energy.
Therefore, in large pipelines, this phenomenon can lead to catastrophic damage, ranging from vibrations and noise to the complete collapse of the pipeline.
There are various methods to prevent or mitigate water hammer, all depending on avoiding sudden changes in fluid movement or safely dissipating wave energy without harming the system. Valves play a crucial role in this phenomenon as their design allows for changes in fluid direction. Ensuring the proper functioning of valves in the system is fundamental to designing an effective pipeline system.
How to Operate Valves to Reduce or Prevent Water Hammer?
1. Extend Closure Time
An effective method is to extend the time when fluid motion undergoes a change. In pipelines, this is often achieved by prolonging the closure time of valves, allowing energy dissipation due to pressure loss. When a valve is closed, its flow capacity decreases, determined by the relationship between the pressure difference at the ends of the valve and the flow through the valve. At a given flow rate, the higher the closure rate, the lower the flow, and the greater the pressure difference for a given flow. The pressure peak generated by closing the valve is related to the time it takes to close the valve. Instantaneous valve closure produces the maximum pressure peak. While extremely slow valve closure may seem to eliminate water hammer entirely, it is not a feasible solution in all situations or for certain applications.
In many isolation valve or shut-off valve applications, valves must operate as quickly as possible, especially when dealing with emergency shut-off valves. This is not only to stop the fluid promptly but also because open/closed valves should not be kept partially open for extended periods, as it may lead to damage to closing membranes, valve seats, and other components due to corrosion. This challenge can be addressed by using a two-speed or variable-speed system to operate valves. Such systems allow operators to close the valve at high speed, for example, initially closing 80% of the valve and then gradually closing the remaining 20%, to dissipate energy to the maximum extent in the shortest time. This approach can significantly reduce the flow of the valve in a short time, achieving effective flow blocking while considering necessary pressure loss to mitigate or prevent water hammer.
The operation control device of the actuator relies on valve position feedback and the energy supply system to manage the system. For simple hydraulic actuator systems or systems using non-compressible fluids, synchronization of the target position setpoint is typically achieved using limit switches or position sensors. Once the valve reaches the desired position, the system reduces the flow to slow down the actuator’s stroke time. Similar operation configurations can be designed for pneumatic systems or systems using compressible fluids. Calculating the time required to change the fluid in the pipeline is determined through a transmission analysis of the pipeline system.
2. Reduce Flow Velocity
Another common technique to mitigate the potentially destructive effects of water hammer is to intentionally reduce the speed at which the fluid flows inside the pipeline. In turn, this speed is related to the nominal diameter of the pipeline. The key to the issue lies in understanding this relationship: as the pipeline diameter increases, it essentially provides a broader cross-sectional area through which the fluid can flow. Therefore, to maintain a constant flow rate, the fluid must flow at a lower speed in a larger diameter pipeline. Conversely, when using a smaller diameter pipeline, the fluid must flow at a higher speed to maintain the same volume of flow. The faster the fluid moves, the greater its kinetic energy, and thus, the more pronounced the pressure fluctuations caused by sudden changes.
Given this, the choice of pipeline diameter becomes a critical design consideration. While opting for smaller pipelines to meet theoretical minimum flow rate requirements may be tempting, this approach often leads to an increase in flow velocity in practice. These increased velocities, in turn, exacerbate the vulnerability to water hammer during valve operation or other flow interruptions.
Therefore, a prudent and proactive strategy includes selecting a pipeline diameter larger than the minimum diameter specified by flow calculations. By choosing larger pipelines, the flow velocity naturally attenuates at the same flow rate. This reduction in velocity acts as a buffer, aiding in more gradual adjustments to flow and pressure. This, in turn, lowers the likelihood of pressure peaks and associated potential risks of damage to the system.
Conclusion:
Properly designing pipeline valve drive systems is crucial to preventing or mitigating the impact of water hammer in pipeline systems. Valves play a role in changing the flow direction, affecting the momentum of the fluid within the pipeline. Depending on the application, valves may need to achieve these changes rapidly, but swift changes can lead to increased pressure peaks and pressure wave amplification. Introducing a variable-speed system for valve operation in the pipeline system design, coupled with detailed fluid transmission analysis and consideration of future flow increments, can provide a viable solution to water hammer issues in pipeline valve operations.