Sprinkler Ice Plugs
Automatic fire sprinkler systems that are used in cold storage freezer installations present unique problems that require specialized design considerations regarding corrosion and potential ice plug obstructions. Historically, fire sprinkler designs for freezers have involved the removal of water content from the pressure maintenance air to prevent ice plugs at the point of penetration of the dry pipe main into the freezer. The chemistry and physics of the corrosion reaction at freezing temperatures are very different from installations at room temperature and above.
The removal of water from the compressed air stream used for pressure maintenance in dry and preaction fire sprinkler systems for freezer service has historically been accomplished by using a regenerative desiccant type air dryer in conjunction with the air compressor. Using this approach, the air entering the freezer would have a lower dew point than the temperature within the freezer which should prevent condensation and freezing of moisture in the pipeline. Ice plugs in sprinkler mains have the potential to greatly restrict or completely obstruct water flow into the freezer.
The FM Global Property Loss Prevention Data Sheet 8-29 entitled Refrigerated Storage suggests that the dew point target of the pressure maintenance air for the dry or preaction fire sprinkler system should be 20°F below the temperature inside of the freezer that is being protected. This reduced dew point would achieve a relative humidity of 30% for the air within the piping at a temperature within the freezer.
Using Nitrogen to Eliminate a Sprinkler Ice Plug
A nitrogen generator provides several advantages over traditional air compressor/dryer configurations:
- The 98% nitrogen gas produced using a nitrogen generator has a much lower dewpoint of -70°F to -90°F than the dry air produced by a dryer at about -40°F.
- Nitrogen generators eliminate oxygen corrosion (less than 2% oxygen) that occurs in supply mains outside of the cold storage area compared to the 21% oxygen supplied by dryers
- Nitrogen generators require less maintenance and cost less to operate because they run less frequently than a regenerative desiccant dryer which must constantly use dry air or heat to regenerate the desiccant bed
- Nitrogen generators can supply a significant volume of supervisory gas to multiple dry pipe sprinkler systems, minimizing the equipment needed at large facilities
- Nitrogen generators cost approximately the same as regenerative desiccant dryers for equivalent size dry and preaction system installations
Case Study: Nitrogen Generator vs Dry Air Pac in Cold Storage Facility
- Original installation of dry air packs in four (4) riser rooms.
- Two (2) dry air packs had reached the end of their life cycle and required replacement.
- In May 2016 one nitrogen generator replaced two dry air packs.
- An internal investigation performed six (6) months later revealed the following:
- Ice plugs found in dry pipe systems supplied by existing dry air packs.
- No ice plugs found in systems supplied with the ECS nitrogen generator.