Mar 2009

Automatic Sprinkler Protection for Refrigerated Warehouses

When considering fire protection for situations where temperatures are below freezing, automatic fire sprinklers provide the best fire protection advantages; but sprinkler systems use water, and water presents a freezing problem. This is the dilemma you face and the following approaches that can be used to accommodate these seemingly opposed issues.

For small storage situations such as a freezer room within a larger sprinkler protected space, the simplest approach is the use of dry sprinklers. These are sprinklers secured in an extension nipple that has a seal at the inlet end to prevent water from entering the nipple until the sprinkler operates. They can be attached to water filled piping above or near the freezer space and essentially only the sprinklers are in the cold space. When using this type of protection, make sure there is at least one foot between the freezer and the water filled pipe to prevent possible freezing. Also, make sure the hole where the pipe penetrates the freezer is well sealed to prevent sweating at the cold to warm interface, which could lead to corrosion over time. This relatively simple approach offers the advantage of easy maintainability. However, it is limited to relatively small spaces where less than 20 sprinklers would be needed. The maintenance for the system involves periodic checks of the penetration seals, and a test every 10 years to validate sprinkler viability for continued service or replacement.

For more complex situations, another relatively simple approach is an antifreeze sprinkler system. In this arrangement a somewhat conventional sprinkler system is used and usually filled with a glycerin or propylene glycol based water solution in the cold space. The cold space sprinkler piping is supplied by sprinkler piping in a warm space. Before entering the cold space a fairly standard piping arrangement is used and typically includes a drain valve, fill valve and shut off control valve for the antifreeze system. There is no restriction on the hydraulic design of an antifreeze system, so the system can be treated as if it were a wet sprinkler system for providing an adequate design for protection of the storage. The limiting factor will be the water flow and pressure that can be supplied to the point of connection to existing water filled piping in the area. Because of the antifreeze, care must be taken with regard to backflow contamination of potable public water supplies and requirements can vary by locality, so be sure to check with local authorities before installing such a system. The maintenance requirements would be as required for any wet pipe sprinkler system, plus checking the freezing point of the antifreeze annually and adjusting the solution if necessary.

Beyond the above types of systems are those with no liquids in the piping of the system protecting the cold space – these are known as Dry pipe and Preaction sprinkler systems.

A Dry pipe system uses water up to the location of a dry pipe valve, where air or nitrogen under pressure in the downstream system of pipe and sprinklers precludes water from proceeding until an automatic sprinkler opens from heat of a fire and releases the air or nitrogen pressure. Once this happens, the system acts like a wet pipe sprinkler system. Since there is water on one side of the dry pipe valve and air (nitrogen) on the other, the dry pipe valve must be in a heated area or enclosed in a heated room. Because of the detrimental effect of time delay, the size of these systems is proscriptively limited to 750 gallons to reduce the time until water is applied to the fire. For situations where system size becomes an issue, there are various options to permit larger volume systems to be installed, such as test or calculation or special quick opening devices. The air pressure in the system just be maintained and so a compressor is necessary. The air (nitrogen) supply to the dry pipe system must be relatively dry to reduce complications of water condensation and freezing inside, so a dehydrator or other regenerative air dryer should be used. It is worth noting that gridded piping is not permitted for dry pipe systems, which limits the hydraulic efficiency of the system. Likewise, Early Suppression Fast Response (ESFR) sprinklers are not permitted. These two factors tend to make the use of dry pipe systems less attractive for high challenge occupancies such as high rack storage of more combustible products. The air pressure must be maintained sufficiently high or the system will “trip,” permitting water to enter the cold space whether a sprinkler has operated or not. Once this occurs you have quite a difficult time disassembling piping to check for and eliminate ice plugs that may form. Therefore it is in your best interests to keep on top of maintenance and inspections and provide alarm indications for abnormal conditions. System maintenance should include a required trip test of the dry pipe system annually, and piping arrangements to facilitate this may be needed.

Preaction sprinkler systems provide the advantages of dry pipe systems, but offer the benefit of several additional options. These systems can be arranged as non interlocked, single or double interlocked, and a fire detection system is needed in addition to the sprinkler system in the cold space. A non interlock system allows water to enter the cold space if detection systems or sprinklers operate. The single interlock system requires only actuation of the detection system. For this arrangement the air pressurization is somewhat superfluous for system operation but is still required. The double interlock requires both detection and sprinklers to operate. This latter arrangement provides the highest degree of “fail safe” design to preclude inadvertent introduction of water into the cold space. The decision of which to use depends on your desire for quick action vs. concerns about inadvertent water release. One type of system is bound to suit your needs. Like a dry system, an air supply is required and the same requirements should be observed. The double interlock system is not permitted to use “gridded” piping, similar again to the dry pipe system. The volume of the system is also limited to that of a dry pipe system, due to the same concerns about delay of water application through sprinklers that operate. There is a limit of 1,000 sprinklers for a preaction system and for high rack storage, ESFR type sprinklers are not permitted with these systems.

For storage situations where there is high piled storage of ordinary combustibles (considered Class III Commodities), there are a number of factors that work against you. Some have been mentioned already, but to recap and expand:

  • No ESFR sprinklers.
  • Limited use of sprinklers most appropriate for high challenge storage – Specific Application Control Mode (SACM) allowed only on wet pipe systems.
  • Design penalty for use of dry pipe systems (including preaction systems) in density/area designs.

The following are basic recommendations that apply to gas fired boilers:

Due to the above, the protection of ordinary combustibles in cold areas is limited to 20 ft. to 25 ft. high or less, depending on the storage configuration. When rack storage is used, any higher storage requires in-rack sprinklers. Most warehouse operators would prefer to avoid this requirement. Obviously there was a need for some more palatable options and this has been happening, little by little.

Based on existing code “equivalency” sections and performance based designs using alternate method and material approaches, there is now a code acceptable alternative system package (UL Listed, FM Global Approved) that has coupled a SACM 286°F rated sprinkler with a double interlock preaction system that can be used to protect refrigerated warehousing up to 40 ft. high in a space of maximum 48 ft. high ceilings. It has also been established as capable of protecting higher hazard commodities (cartoned Group A Plastics) to 30 ft. high storage in a 35 ft. high building. This should find good application in new high bay cold storage warehouses.

Risk Logic, Inc. can help in a layout and design that will meet all applicable codes and insurance company requirements. Please contact our office and we can work with you or directly with your insurance carriers, contactors, or an in-house design department.