Wet systems are continuously filled with water under pressure that flows out immediately when a sprinkler head operates. These systems are generally less prone to corrosion in comparison to dry systems because of the limited oxygen present. An exception is when they are emptied and refilled frequently, as for example, in changing office or store environments. About 96% of the oxygen that is the source of corrosion results from air trapped in the piping when it is filled with water. This can be significantly reduced by a simple but precise and effective method of purging the piping with nitrogen gas prior to re-filling with water.


A wet pipe fire sprinkler system is a number of connected long and narrow vessels mostly filled with stagnant water and periodically drained and re-filled at which time air that ends up trapped under pressure in the higher locations. The result of draining and filling activity is that the oxygen in the trapped air results in corrosion. This then produces insoluble corrosion products and corrosion activity can be focused and produce localized pitting under the debris.

There are three main areas/stages of corrosion activity:

  • Piping has been drained and exposed to air: Wet metal surfaces have direct access to dissolved oxygen from the 21% that is contained in air, resulting in rapid corrosion.
  • Incompletely filled sections of pipe: At the air/water interface wherever trapped air remains in the system after filling – corrosion will be more severe at this interface.
  • Under deposits of corrosion products: Localized severe corrosion can occur leading to pitting.



Holtec is on the frontline of utilizing nitrogen gas in wet sprinkler systems to control corrosion. Nitrogen gas from cylinders, or in large facilities with multiple zones, from an installed generator, is the effective and environmentally acceptable means of treating this costly problem. The approach to excluding oxygen is quite simple and is used frequently, especially in the gas & oil industry, to purge internal oxygen from complex equipment and piping to prevent possible fire or explosion. Holtec was the first to recognize that this technology could be transferred for use in complex fire sprinkler piping to eliminate or minimize oxygen in trapped gas within the piping before being filled with water.

The method uses a series of three or four successive steps of pressurizing the piping with nitrogen gas followed by manual venting back to atmospheric pressure.

The technique is described further below:

Inerting a new wet sprinkler system with nitrogen:


  1. Install fittings for nitrogen injection on the wet zone rise (obtained from Holtec).
  2. Hook up a suitable supply of nitrogen with pressure controlled output that is pressure regulated to 30 psig.
  3. With no drains or vents open to the system, start nitrogen flowing into the sprinkler piping until the pressure reaches 30 psig
  4. Vent the pressure down to atmospheric pressure (DO NOT VENT IT INTO A SMALL UNVENTILATED ROOM, SEE NOTICE BELOW)
  5. Repeat steps #2 through #4 two or three more times and the oxygen concentration of the gas remaining in the piping will be under 2% compared to 21% at the start.
  6. Re-fill the system with water up to normal pressure.

Sources of nitrogen gas can be from an existing in-house source if available, high pressure cylinders, a dedicated Holtec nitrogen generator or a Holtec mobile nitrogen generator HCMNG. Contact Holtec for more information.

Venting of trapped gas is not essential once nitrogen gas has replaced air in the piping.


SAFETY NOTICE: Always exercise extreme caution when venting nitrogen into occupied areas to ensure oxygen concentration does not fall to a level that could endanger life. Always vent the nitrogen to the outside atmosphere if there is any doubt. We have created computer calculation tool to determine if a space is sufficiently large to assimilate the gas being vented without creating a hazard. The tool being made available to all, so please contact us for your copy.