A Sprinkler System is a Closed Vessel
In analyzing the fill and purge pressure cycling process it is appropriate to consider the piping network in a dry or preaction fire sprinkler system as an irregularly shaped closed vessel. Gas movement through an irregularly shaped vessel is much different from water movement through that same vessel. Water is not compressible and it will flow into an opening in its path. Water is also greatly affected by gravity and it will self-level as the vessel fills.
Gas is compressible and it will always move in the direction of the lowest pressure within the closed vessel. Gas does this much more quickly and easily than water. Two different gases will also mix much more efficiently than two liquids. With gases of similar densities like oxygen and nitrogen, the effect of gravity is very minimal.
Remote Nitrogen Purge Valve
When nitrogen gas is injected into a port in the riser room with a nitrogen purge valve on the end of a remote sprinkler pipe on a typical dry or preaction fire sprinkler system the nitrogen gas will move very efficiently toward the pressure drop at the nitrogen purge valve. If the nitrogen gas is injected at a low flow rate the nitrogen is more likely to move directly to the pressure drop with very limited dynamic mixing in route to the purge valve. This process can be appropriately characterized as “inject and vent”. Injecting and venting a dry or preaction fire sprinkler system with no pressure cycling is characterized by the following conditions:
- Nitrogen purge valve installed remotely on system piping.
- Short-circuiting of the nitrogen gas from the injection port directly to the purge valve.
- Very limited gas movement in the piping that is not in the line between the injection port and the nitrogen purge valve.
- No removal of the unreacted oxygen gas from branch lines that do not have a purge valve.
Empirical trials on dry pipe sprinkler systems have demonstrated that when a nitrogen purge valve is placed at the end of a supply main nitrogen injected at the riser predictably follows the path of the main until it reaches the purge valve. Branch lines that do not have purge valves receive very little flow of nitrogen gas.
Fill and Purge Vs Inject and Vent
Unlike the “inject and vent” process, fill and purge breathing allows the vent to be positioned anywhere within the system piping because during the purge portion of the cycle all of the gas within the system moves toward the pressure drop that occurs at the vent wherever it is located. Placing the vent in the riser room adjacent to the nitrogen injection port during the “fill” cycle is beneficial in several ways:
- It is easier to install the vent at the sprinkler riser.
- A vent in the riser room creates less leak risk than a vent installed at a remote and potentially critical building location.
- During fill and purge breathing the nitrogen gas moves deeply into the fire sprinkler system piping away from the riser during the “fill” phase and conversely during the “purge” phase the nitrogen enriched gas mixture moves in the opposite direction toward the riser; this promotes mixing and homogenization to efficiently remove oxygen.
Systems utilizing an "inject and vent" method will perform even more poorly when the nitrogen purge valve is located at the sprinkler riser near the nitrogen injection port. The lack of dynamic mixing described above means that the nitrogen gas will travel a minimal distance into the system during the nitrogen injection process before it is redirected back to the purge valve to follow the path of least resistance towards the pressure drop. With no remote purge valve in this configuration, gas movement is further limited within the system and oxygen remains in the system to react with metal piping. In these systems, it is more likely that the oxygen concentration is reduced due to the consumption of oxygen by the metal pipe (oxidation) than the dilution process of the "inject and vent" method.