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Dry Pipe Nitrogen Inerting (DPNI) – FAQs

Where does oxygen corrosion occur in dry (preaction) fire sprinkler systems?

Oxygen corrosion typically occurs under pools of trapped water within the mains of the dry (preaction) fire sprinkler systems.

What exactly is Dry Pipe Nitrogen Inerting (DPNI)?

Simply stated, DPNI is a process of sequential dilution of the pressure maintenance gas within a dry (preaction) fire sprinkler system with nitrogen gas. Our nitrogen dry pipe systems uses sequential dilution which is performed over a period of several day using a “fill and purge” breathing process wherein nitrogen is added incrementally and gas is vented from the piping to remove oxygen.   After the atmosphere within the dry (preaction) fire sprinkler piping has achieved 98%+ nitrogen concentration nitrogen gas is used as the pressure maintenance gas for the fire sprinkler system.  

What equipment is required to perform DPNI on a dry or preaction fire sprinkler system?

There are three (3) essential components required to perform Dry Pipe Nitrogen Inerting (DPNI) on a dry (preaction) fire sprinkler system:
  • A continuous source of dry nitrogen gas of 98%+ purity (nitrogen generator)
  • An integral venting device attached to the dry (preaction) piping to facilitate removal of oxygen gas from the system
  • A breathing system to perform the pneumatic “fill and purge” breathing process in conjunction with the nitrogen generator and the integral venting device

What level of nitrogen gas purity for the inerting gas is sufficient to reduce the corrosive attack of the mild steel or galvanized steel fire sprinkler system piping?

It is the cumulative amount of oxygen gas that is added to the piping network over time that determines the corrosion rate for the fire sprinkler system piping.  Under a persistently moist, 21% oxygen atmosphere many dry pipe fire sprinkler system will experience leaks within 10 years from initial construction.  It is not uncommon for dry pipe fire sprinkler systems constructed using galvanized schedule 10 piping to experience leaks within 2 years after initial construction.  This is because the pressure maintenance gas from a typical fire sprinkler compressor delivers compressed air containing 21% oxygen every time it turns on.  Each time fresh oxygen is added to the piping the corrosion reaction within the pipe is reignited.

Using nitrogen gas to inert the fire sprinkler system piping network can reduce the average corrosion rate from 20+ mils per year of penetration to less than 1.0 mil per year.  This rate of corrosion control can be achieved in three steps:
1. Convert the pipeline gas atmosphere to 98%+ nitrogen by “fill and purge” breathing
2. Close the vent to stop the gas exchange venting process
3. Use 98%+ nitrogen gas as the pressure maintenance gas for the system  

Are there benefits to continuously venting the fire sprinkler piping with dry nitrogen gas to “dry out” the piping by removing the trapped moisture?

Even using 99.5% nitrogen gas with 0.5% oxygen will cause significant corrosion damage over a long period of time if the system is vented continuously.  Every molecule of oxygen that is added to the piping network will react with the pipe.  The benefits of using a continuous venting process to remove trapped water are outweighed by the risk of adding a continuous small amount of oxygen.  We recommend that once the atmosphere in the fire sprinkler pipe achieves 98% nitrogen (2% oxygen) the venting should be stopped.  At that point, that level of residual oxygen will be consumed in a few days and the corrosion reaction will stop as the system achieves equilibrium.

Is a nitrogen gas storage tank necessary in the application of nitrogen generators for fire sprinkler systems?

The nitrogen generator requires a continuous supply of compressed air (100 psig minimum) to facilitate efficient separation of nitrogen gas from the inlet air.  Using membrane separation, the process is approximately 50% efficient.  That means for a continuous 20 scfm compressed air supply, the membrane separator will continuously produce 10 scfm of 98%+ nitrogen gas.  The production of a continuous stream of nitrogen gas requires a continuous stream of compressed air.  Any gas storage tank simply puts a buffer in the system with a fixed amount of gas in storage.  The placement of the storage tank upstream of the nitrogen generator as a compressed air receiver tank is the recommended approach.  It is not necessary to store compressed nitrogen gas.  

Can I use my existing dry (preaction) compressor in conjunction with the nitrogen generator?

There are two specific requirements for the compressed air supply for use with membrane type nitrogen separators:
  • Minimum output pressure of 100 psig
  • Sufficient air output volume to meet the DPNI requirements of the dry (preaction) zone(s) that is being inerted

Many of the compressors typically used in dry (preaction) fire sprinkler system applications cannot meet these two criteria.  It is also important to have durable, reliable compressors that are rated for continuous duty service.  

How can redundancy be built into the nitrogen generator design for dry (preaction) fire sprinkler systems?

The membrane type nitrogen generator requires a dedicated power supply.  Air compressors will also require dedicated power supply in various power configurations.  The possible sources of failure for the nitrogen generation system are as follows:
  • Complete loss of power to the nitrogen generator, SMART vents or air compressor
  • Loss of compressed air supply resulting in the inability to maintain system pressure
  • Poisoning of the nitrogen separation membrane resulting in the deterioration of the nitrogen output gas quality

Providing for a secondary supply of compressed air, e.g. a second compressor, a duplex compressor or a design with multiple nitrogen generator/compressor sources can provide the necessary redundancy.

Is continuous monitoring of the pipeline gas composition a reliable method for determining the corrosivity within the pipeline system?

By measuring the instantaneous oxygen concentration in the fire sprinkler pipeline gas, it is possible to track the corrosion potential of the system.  Monitoring of the pipeline gas composition can provide a warning about the deterioration in the output nitrogen gas quality.  Sampling can be done continuously or intermittently.  

All oxygen monitoring devices require a fresh sample of gas to determine the oxygen concentration in the gas.  The current continuous monitoring design uses a small continuous stream (less than 1.0 scfh) of pipeline gas to provide in-situ monitoring of the oxygen concentration.  This provides an indirect measure of the corrosivity of the system.

ECS In-Line Corrosion Monitors are designed to match (black steel and galvanized steel) the fire sprinkler system piping can be used to provide an “early warning” signal that elevated corrosion activity is occurring within the system.  If the monitor indicates that a pin hole leak has occurred, the in-line monitor can be removed from the system to determine the pit shape, pit depth and metal loss characteristics.  The in-line monitor is the most reliable method for monitoring the corrosion rate.

What are the maintenance requirements associated with the ECS Nitrogen Generator Systems?

The preconditioning filters within the nitrogen generator cabinet require annual replacement.  A complete kit is available.  The compressor that provides the compressed air supply to the nitrogen generator also requires routine maintenance and upkeep as per the manufacturer’s recommendations.  The automatic air vents include a small in-line filter which requires bi-annual replacement.  Finally, the gas separation membrane within the generator has a minimum expected life of 20 years.  The entire maintenance protocol for the system is included with the O&M Manual.