What is Water Hammer Mitigation?
Water hammer mitigation is any action taken to prevent water hammer effects from damaging a system or interrupting a process.
Each series of articles are written by pipe flow analysis engineers from Applied Flow Technology. As industry leaders in water hammer and surge analysis, AFT has collected models and data from projects around the world to use as reference materials for published technical papers, case studies, and blogs. Visit www.aft.com for more information on analysis tools.
What is Industrial Water Hammer Mitigation?
Water hammer mitigation is any action taken to prevent water hammer effects from damaging a system or interrupting a process. Undesirable events include excessive pressure events, low-pressure events and cavitation, slam events caused by sudden changes in fluid velocity, and many more.
In extreme cases, these events can damage the physical equipment in a system or even lead to a spillage of the process fluid. Thus, it is desirable to prevent these events from happening and mitigate the damage they can cause.
How can water hammer be mitigated?
There are two main categories of water hammer mitigation: operation-based mitigation and equipment-based mitigation. As the names imply, the first approach sees water hammer being mitigated by changing the way the system is operated, while the second approach involves adding pieces of surge-suppression equipment to dampen out water hammer effects.
Mitigating water hammer via system operation involves changing process actions to avoid causing water hammer events. If a valve is closing too quickly, closing it more slowly can lower the maximum pressure to an acceptable level. If a pump trip is causing cavitation, ramping the speed down more slowly or closing a downstream valve at the same time can reduce the extent of cavitation seen in the system.
The most common cause of water hammer events is an abrupt valve closure. Ideally, a valve should be closed such that the flow velocity is reduced linearly over time. In practice, most of the fluid velocity change is seen in the last 10% of the closure due to how a valve’s installed characteristics affect its effective closure time. Changing how a valve is closed can substantially reduce the magnitude of those events. The most intuitive way to adjust a valve closure is to increase the closure time. Closing the valve over 5 seconds instead of 1 or 2 seconds may dramatically reduce the maximum pressures seen in the system.
A good approach to designing a valve closure is to follow the 80/20 rule. For water hammer and valve closures, taking this approach means closing the valve 80% of the way in the first 20% of the available closure time, then closing the last 20% over the remaining 80% of the time. This approach allows the valve to still close quickly without abruptly bringing the fluid velocity to zero. The engineers over at AFT have a good discussion of what this looks like in a parallel valve system.
Another common cause of water hammer events is a rapid, unplanned pump shutdown. Allowing a pump (or pumping station) to more gradually slow down can the low-pressure extremes or cavitation events in a system. If a pump is installed with a VFD, the shutdown profile can be lengthened or manipulated similar to a valve closure. If the pump is a constant speed pump, it becomes difficult to control the shut-down, but adding a flywheel can increase the pump’s inertia and the time required for it to come to a complete stop.
The ordering of valve closures and pump trips can also be considered. Instead of tripping a set of parallel pumps at the same time, stagger them out to gradually reduce flow rate. Instead of allowing an Emergency Shut Down (ESD) valve to close without tripping an upstream pump, make sure the pump trips at the same time to avoid forcing more fluid into a pipe with no exits.
In many cases, adjusting system operation can be the most cost-effective approach to water hammer mitigation. However, there are still situations where water hammer effects persist, necessitation equipment-based mitigation.
Mitigating water hammer using pieces of equipment involves adding surge suppression equipment or modifying the existing components of the physical system. This approach is also the most common approach taken when systems experience water hammer. Engineers will commonly add equipment including relief valves, surge tanks, gas accumulators and more to their systems.
The logic behind this mitigation approach is addressing specific problematic locations in a system. If system measurements reveal excessively high pressures during water hammer events, adding a relief valve or a gas accumulator at the high-pressure location can lower the maximum pressure. If system measurements reveal excessively low pressures, adding an air valve or a surge tank can keep vacuums from forming.
It is important to note that adding a piece of equipment to a problematic system is not a guaranteed solution to water hammer issues. Accumulators that have not been sized correctly can amplify pressure waves in a system. Air valves that release air out of the system too quickly can cause high-pressure transients after mitigating low-pressure transients.
A properly sized and placed piece of surge suppression equipment rarely fails to mitigate water hammer. However, adding equipment is typically expensive and more complicated than other mitigation approaches. The prudent engineer will always look first at addressing the causes of water hammer before mitigating specific situations.
When can water hammer be mitigated?
Ideally, water hammer is simulated and mitigated during the design stage. With hydraulic transient simulation software such as AFT Impulse, the engineer can model their system and various operational events to see how it will respond. At this stage, the engineer can more easily change aspects of the design including pipe diameter and pipe material. They can also change the type or size of pump selected along with which valves are installed where in the system.
At this point in the process, the engineer can also help determine operational processes to avoid water hammer events from the beginning. If a proposed operation will cause dramatic effects in the system, they can simulate alternative operational steps before they are put into action.
If the system has already been designed and put into operation, making changes to combat water hammer becomes more involved. The gut reaction of many engineers is to add in surge-suppression equipment. Adding a relief valve or an air valve or a surge tank (as space allows) can be the least disruptive to production, even if it is expensive. However, adding those pieces of equipment can be expensive and add additional complications to the system.
Instead, if the system is already operational, it’s best to first consider changing system operation practices. Again, hydraulic transient simulation software allows the engineer to test multiple options to see which works best. Changing the duration of a valve closure or the order in which pumps start up and trip doesn’t require modifications to the system and often provides minimal disruption to existing operational practices.