Air Pockets and Pressure Surges: The Invisible Threat

What Are the Consequences of a Pressure Surge?

If caught early, minor pressure surges will have little effect on a facility’s fire protection system. Gauges can indicate that a pressure surge is occurring and the appropriate measures can be taken to rectify the origin of the problem before it causes damage to the system. However, if a surge goes unnoticed and the issue escalates, the consequences can be destructive to the system and its effectiveness compromised.

When a pressure surge occurs within a water supply system, pressure levels can reach upwards of 300 pounds per square inch (PSI) and become trapped on the system side of a sprinkler system valve. The standard equipment associated with sprinkler systems in power generating and chemical processing plants typically have a rating of 175, in accordance with NFPA standards. When sprinkler equipment is subjected to pressures that exceed their rating, equipment can fail and components such as valves and pumps are susceptible to damage. Even if pressure surges of a lower magnitude occur repeatedly, the repeated stress cycles can weaken the system components.

Fire protection systems that suffer damage as a result of pressure surges are rendered ineffective while repairs are being made, putting the facility at risk until the system is functional again. Alternatively, when damage is not immediately recognized, facilities are subjected to an even greater threat because it is possible that the faulty equipment will not be realized until proven ineffective in the event of a fire.

Perhaps the most prevalent consequence related to pressure surges in sprinkler systems is the occurrence of false tripping of dry valves caused by surges. False discharge that is triggered by a pressure surge is burdensome for plant staff and also takes a critical component of the fire protection system out of service. Once discharge has occurred, plant staff must shut the system down and drain it, exposing the facility to risk while the system is being serviced. Furthermore, if an accidental discharge occurs in freezing conditions and it is not immediately recognized, there is a possibility that pipes could freeze, causing long term impairments to the system.

Beyond the issues related to damaged equipment and false trips, the overall performance of a dry sprinkler system can be impeded by the presence of air. If the differential values of air pressure within the system pipes are skewed to compensate for surges, it can take longer for water to discharge in the event of a fire. Excessive air pressure and restrictions caused by pockets of air can result in slower delivery of water to the open head, allowing fires to grow in intensity, making them more difficult to control.

Safeguarding Fire Protection Systems from Pressure Surges

Taking into consideration all of the potential problems that can stem from pressure surges caused by trapped air in a water supply system, the best solution is preventing pressure surges from occurring altogether. There are different methods for preventing air from becoming trapped within a water supply system and the most effective solution involves careful planning that begins in the design phase of a project.

Because air inherently migrates to the high points of a system, underground designs that include many changes in elevation are more problematic than level designs. 
One of the most critical measures that must be taken to prevent air from becoming trapped within the system takes place after the system has been installed and is being filled with water. Lines should be filled slowly, allowing air ample opportunity to escape from the piping. Additionally, hydrants and lead ins should be flushed in a systematic manner, beginning with the back of the system and moving towards the front, ensuring that every section of the system is thoroughly flushed before being put into use. Air should be bled from all of the high points of the system to expel any remaining air that exists. And, after the system is operational, these actions should be performed on a regular basis to prevent any accumulation of air within the sprinkler pipes.

Diligent Maintenance Mitigates Risk

The negative effects of neglecting trapped air within fire protection systems can be dire when the performance of the system is inhibited as a result. Damaged equipment can not only be a costly and time-consuming problem, but the safety of a plant is diminished when vital components of a fire protection system are not operating at peak performance. Less obvious impediments, such as a line restriction, can also have harmful effects if not adequately addressed. Diligent execution of simple, yet effective practices, such as venting pockets of air from the high points in the system can help ensure that fire protection systems are functioning at their optimal capability in the event of a fire.