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What is waterhammer mitigation?

What is waterhammer mitigation?

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Waterhammer mitigation is any action taken to prevent waterhammer 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 to mitigate the damage they can cause.

 

How can waterhammer be mitigated?

 

There are two main categories of waterhammer mitigation: operation-based mitigation and equipment-based mitigation. As the names imply, the first approach sees waterhammer mitigated by changing the way the system is operated, while the second approach involves adding pieces of surge-suppression equipment to dampen out waterhammer effects.

 

Operation-Based Mitigation

 

Mitigating waterhammer via system operation involves changing process actions to avoid causing waterhammer events. If a valve closure occurs too quickly, closing it more slowly can lower the maximum surge 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 waterhammer 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 maximum pressures seen in the system.

 

One approach to designing a valve closure is to follow the 80/20 rule. For waterhammer and valve closures, 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. Operating a valve following the 80/20 rule helps lengthen the time where the valve is effective at slowing the fluid while reducing the wasted time where the valve does not control the system. The valve closes quickly without abruptly bringing the fluid velocity to zero. The engineers over at AFT have a good discussion about the 80/20 rule in a parallel valve system.

 

Another common cause of waterhammer events is a rapid, unplanned pump shutdown. Allowing a pump (or pumping station) to more gradually slow down can reduce 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 extending a valve closure. If the pump is a constant speed pump, its shut-down is more difficult to control, 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, staggering them out will gradually reduce flowrate. Instead of allowing an Emergency Shut Down (ESD) valve to close without tripping an upstream pump, making sure the pump trips at the same time avoids forcing more fluid into a pipe with no exits.

 

In many cases, adjusting system operation can be the most cost-effective approach to waterhammer mitigation. However, there are still situations where waterhammer effects persist, warranting equipment-based mitigation.

 

Equipment-Based Mitigation

 

Mitigating waterhammer via equipment involves adding surge suppression devices or modifying the existing components of the physical system. This approach is also the most common approach taken when systems experience waterhammer. Engineers will typically add equipment including relief valves, surge tanks, gas accumulators and more to their systems.

 

The primary objective with this approach is to address problematic locations in a system. If system measurements reveal excessively high pressures during waterhammer 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 waterhammer issues. Accumulators that have not been sized correctly can amplify pressure waves in a system, exacerbating the event. Air valves that release air from the system too quickly can cause high-pressure transients after mitigating low-pressure transients.

 

A properly sized and located piece of surge suppression equipment rarely fails to mitigate waterhammer. However, adding equipment can be expensive and more complicated than other mitigation approaches. The prudent engineer will always look first at addressing the causes of waterhammer before mitigating its effects.

 

When can waterhammer be mitigated?

 

Design Stage

 

Ideally, waterhammer 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.

 

At this point in the process, the engineer can also help determine operational procedures to avoid waterhammer events before the system is even built out. If a proposed operation will cause dramatic effects in the system, they can simulate alternative operational steps before they are put into action.

 

Operation Stage

 

If the system has already been designed and put into operation, making changes to combat waterhammer 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 complication to the system.

 

Instead, if the system is already operation, it’s best to first consider changing system operation practices. Again, a 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 don’t require modifications to the system and often provide minimal disruption to existing operational practices.

 

 

Waterhammer mitigation is an important part of the design and maintenance stages in a system’s lifecycle. Mitigation should be considered even in systems without obvious signs of extreme waterhammer events. Small waterhammer events can cause cyclic fatigue, weakening the system over time until failure occurs. However, proper operational and equipment-based mitigation strategies can protect a system over its entire lifecycle.

 

 

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