Fire sprinkler systems are needed to protect people, contents and property from the harmful effect of fire. Specified and installed to meet regulatory, insurers', business or property risk management requirements, they can be automatically operated. Most are wet pipe sprinkler systems but in certain circumstances they can be dry pipe sprinkler systems. Several regulatory factors affect their design and specification that sprinkler engineers are familiar with. We explain this in more detail here.
Automatic fire sprinkler systems
Historically, automatic fire sprinkler systems have been used in the US since the 19th Century, originally in factories, where at the turn of the century, fires were often disastrous in terms of economic and physical loss. Today an automatic sprinkler system should be designed and specified according to a variety of factors, including its location, ambient temperature and maximum time of water delivery.
There are several types of sprinkler systems available to fight fires purposes in Chicago including wet pipe, dry pipe, deluge, pre-action and combined preaction-dry pipe systems. The factors influencing the selection of the second most common type - dry pipe sprinkler systems - in this city, along with information on combined preaction-dry pipe systems, and details of their benefits and limitations, are also described here.
Dry pipe sprinkler systems
Dry pipe sprinkler systems are most often used in unheated buildings, parking garages or within external canopies attached to heated buildings (in which a wet pipe system could be provided), or in refrigerated coolers.
These sprinkler systems are the second most common sprinkler system and are used where wet or alternate systems cannot be. Pipes are filled with constantly pressurized air or nitrogen, below the water supply pressure level, and water is held back by a dry pipe control valve, a specialized type of check valve, outside of the protected area. If a sprinkler head opens during a fire, the air pressure drop, or differential, opens the valve and water flows into pipework and out through sprinkler head(s) to the point(s) of heat from the fire. In this way the water flow from sprinklers, needed to control the fire, is slightly delayed until the air is vented from the sprinklers.
Combined Pre-Action - Dry Pipe sprinkler systems can also be installed in Chicago. These employ automatic sprinklers attached to a piping system containing pressurized air with a detection system installed in the same areas as the sprinklers; operation of the detection system (which also serves as an automatic fire alarm system) actuates tripping devices which open “dry pipe valves” simultaneously and without loss of air pressure in the system. detection system operation also opens air exhaust valves at the end of the feed mains feed facilitating the filling of the system with water which usually precedes the opening of sprinklers.
Regulatory backdrop of Dry Pipe Sprinkler Systems in Chicago
Prior to full design and specification of dry pipe sprinkler systems, recourse should always be made to prevailing fire protection codes, liaison with relevant fire officers and authorities carried out as necessary and ideally sprinkler engineers consulted. The core regulatory factors influencing the design and specification of dry pipe sprinkler systems are described further here, for guidance purposes only.
Existing buildings or structures in Chicago, or parts thereof, are to be equipped with automatic sprinkler systems (which can include dry pipe systems) complying with the requirements of Chicago Municipal Code Chapters 15/16. Automatic sprinkler systems are also to be installed in all buildings having a floor area exceeding the maximum areas defined in Chapter 13 of this Code. There also special requirements and hazardous use unit sprinkler requirements under this Code.
Currently, the Illinois Office of the State Fire Marshall enforces NFPA 101 - Life Safety Code. Contrary to popular belief, this does not contain a blanket requirement to provide or retrofit buildings with automatic sprinklers. The sprinkler requirement for existing residential high-rise buildings is however likely to reappear in Chicago as the NFPA and others call for greater use of sprinklers in such locations.
According to the Chicago Municipal Code, a dry pipe system is permitted only where the ambient temperature is not high enough to prevent water freezing in all or sections of a system. In all other instances a wet automatic system is required.
Maximum time of water delivery:
Where National Fire Protection Association (NFPA) 13 applies, the time it takes water to reach the hydraulically remote sprinkler from the time that sprinkler is activated, or maximum time of water delivery, is limited to a maximum of 1 min. This time may need to be reduced, depending on the hazard classification of the area protected by the dry pipe sprinkler system.
Benefit of dry pipe fire sprinkler systems
1. Use where freezing is possible.
2. Dry pipe sprinkler systems provide automatic sprinkler protection where freezing is possible. These situations include unheated warehouses and attics, externally exposed loading docks and within commercial refrigerators.
Limitations of dry pipe fire sprinkler systems
1. Increased maintenance complexity: Additional supply components increases system maintenance complexity.
2. Lower design size flexibility: Regulatory requirements limit the maximum permitted size of individual dry pipe systems to 750 gallons. This is unless additional components are designed and provided to limit the time from sprinkler activation to water discharge to under 1 min, increase the number of sprinkler zones served from a riser in a building and reduce the possibility of system additions.
3. Increased response time: As piping is empty when a sprinkler operates, there is a time delay in delivering water to operational sprinklers as water travels from riser to sprinkler, partially filling pipework in the process. A maximum of 1min. is normally allowed by regulatory requirements from the time a single sprinkler opens until water is discharged onto a fire. However this fire suppression delay results in a larger fire prior to control, increasing property damage.
4. Increased corrosion potential: Following operation or testing, a dry pipe sprinkler system piping is drained, but residual water collects in piping low spots and moisture is retained within the piping. This coupled with the oxygen available in the compressed air in the piping, increases internal pipe corrosion, eventually leading to pin-hole leaks or other piping failures.
5. Higher installation and maintenance costs: Added complexity impacts overall dry pipe sprinkler system installation and maintenance cost. To combat corrosion pipe made of copper or stainless steel can be specified and dry nitrogen gas, instead of air, can be used to pressurize the system. Nitrogen generators can also be used as a permanent source of nitrogen gas.
6. System sharing impractical: If sprinklers share the same water supply system as the standpipe system supplying fire hoses, then water supply to the fire hoses would be severely reduced or even stopped.
Dry pipe sprinkler systems are the second most common automatic sprinkler system, however the limitations of their application outweigh their benefits for use where ambient temperatures are 40°F or more. Their configuration and maintenance requires consideration of multiple regulatory factors and their design and specification is, as in the case of the application of wet pipe sprinkler systems, best detailed by experts in the field of sprinkler engineering.