Automatic Sprinkler System Calculations

1 SECTION FOUR. CHAPTER 3. Automatic Sprinkler System Calculations Russell P. Fleming Introduction Types of Sprinkler Systems Automatic Sprinkler systems are considered to be Applications Where Water Is Appropriate the most effective and economical way to apply water to Water is the most commonly used fire extinguishing suppress a fire. There are four basic types of Sprinkler agent, mainly due to the fact that it is widely available systems: and inexpensive. It also has very desirable fire extin- 1. A wet pipe System is by far the most common type of guishing characteristics such as a high specific heat and Sprinkler System . It consists of a network of piping high latent heat of vaporization. A single gallon of water containing water under pressure. Automatic sprinklers can absorb 9280 Btus ( kJ) of heat as it increases are connected to the piping such that each Sprinkler from a 70 F (21 C) room temperature to become steam at protects an assigned building area. The application of 212 F (100 C).

2 Heat to any Sprinkler will cause that single Sprinkler to Water is not the perfect extinguishing agent, how- operate, permitting water to discharge over its area of ever, and is considered inappropriate for the protection of protection. certain water reactive materials. In some cases, the use of 2. A dry pipe System is similar to a wet System , except that water can produce heat, flammable or toxic gases, or ex- water is held back from the piping network by a spe- plosions. The quantities of such products must be con- cial dry pipe valve. The valve is kept closed by air or sidered, however, because application of sufficient water nitrogen pressure maintained in the piping. The oper- can overcome the reaction of minor amounts of these ation of one or more sprinklers will allow the air pres- materials. sure to escape, causing operation of the dry valve, Another drawback of water is that it is more dense which then permits water to flow into the piping to than most hydrocarbon fuels, and immiscible as well.

3 Suppress the fire. Dry systems are used where the wa- Therefore, water will not provide an effective cover for ter in the piping would be subject to freezing. burning hydrocarbons, or mix with them and dilute them 3. A deluge System is one that does not use Automatic to the point of not sustaining combustion. Instead, the hy- sprinklers, but rather open sprinklers. A special deluge drocarbons will float on top of the water, continuing to valve holds back the water from the piping, and is ac- burn and possibly spread. To combat such fires, foam so- tivated by a separate fire detection System . When acti- lutions can be introduced into the water to provide an ef- vated, the deluge valve admits water to the piping fective cover and smother the fire. Applying water in a network, and water flows simultaneously from all of fine mist has also been successful. the open sprinklers. Deluge systems are used for pro- However, even when water from sprinklers will not tection against rapidly spreading, high hazard fires.

4 Suppress the fire, its cooling ability can protect structural 4. A preaction System is similar to a deluge System except elements of a building by containing the fire until it can be that Automatic sprinklers are used, and a small air extinguished by other means. pressure is usually maintained in the piping network to ensure that the System is air tight. As with a deluge System , a separate detection System is used to activate Russell P. Fleming, , is vice president of engineering, National Fire Sprinkler Association, Patterson, New York. Mr. Fleming has a deluge valve, admitting water to the piping. How- served as a member of more than a dozen NFPA technical commit- ever, because Automatic sprinklers are used, the water tees, including the Committee on Automatic Sprinklers. He cur- is usually stopped from flowing unless heat from the rently serves on the Board of Directors of NFPA. fire has also activated one or more sprinklers. Some 4 72. Automatic Sprinkler System Calculations 4 73.

5 Special arrangements of preaction systems permit vari- very rudimentary calculation methods available with re- ations on detection System interaction with Sprinkler gard to the most fundamental aspect of Sprinkler systems, operation. Preaction systems are generally used where , the ability of water spray to suppress fires. there is special concern for accidental discharge of wa- The reason that calculation methods are not used is ter, as in valuable computer areas. simply the complexity of the mechanisms by which water suppresses fires. Water-based fire suppression has to this These four basic types of systems differ in terms of point not been thoroughly characterized to permit appli- the most fundamental aspect of how the water is put into cation of mathematical modeling techniques. As a result, the area of the fire. There are many other types of sprin- the fire suppression aspects of Sprinkler System design kler systems, classified according to the hazard they pro- are empirical at best.

6 Tect (such as residential, in-rack, or exposure protection);. Some, but not all, of the current Sprinkler System de- additives to the System (such as antifreeze or foam); or sign criteria are based on full-scale testing, including the special connections to the System (such as multipurpose criteria originally developed for NFPA 231C and 13D, and piping). However, all Sprinkler systems can still be cate- parts of NFPA 13, such as the material on the use of large gorized as one of the basic four types. drop and ESFR (early suppression fast response) sprin- klers. Most of the NFPA 13 protection criteria, however, Applicable Standards are the result of evolution and application of experienced NFPA 13, Standard for the Installation of Sprinkler Sys- judgment. In the 1970s, the capabilities of pipe schedule tems (hereafter referred to as NFPA 13), is a design and in- systems, which had demonstrated a hundred years of sat- stallation standard for Automatic Sprinkler systems, isfactory performance, were codified into a System of referenced by most building codes in the United States area/density curves.

7 This permitted the introduction of and This standard, in turn, references other hydraulic Calculations to what had become a cookbook- NFPA standards for details as to water supply compo- type method of designing Sprinkler systems. It allowed nents, including NFPA 14, Standard for the Installation of System designers to take advantage of strong water sup- Standpipe and Hose Systems; NFPA 20, Standard for the In- plies to produce more economical systems. It also permit- stallation of Centrifugal Fire Pumps (hereafter referred to as ted the determination of specific flows and pressures NFPA 20); NFPA 22, Standard for Water Tanks for Private available at various points of the System , opening the Fire Protection (hereafter referred to as NFPA 22); and door to the use of special sprinklers. Special sprinklers NFPA 24, Standard for the Installation of Private Fire Service are approved for use on the basis of their ability to ac- Mains and Their Appurtenances. complish specific protection goals, but are not inter- For protection of warehouse storage, NFPA 13 tradi- changeable since there is no standardization of minimum tionally referenced special storage standards that con- flows and pressures.

8 Tained Sprinkler System design criteria, including NFPA Because of this history, the calculation methods avail- 231, Standard for General Storage Materials (hereafter re- able to the fire protection engineer in standard Sprinkler ferred to as NFPA 231); NFPA 231C, Standard for Rack Stor- System design are only ancillary to the true function of a age of Materials (hereafter referred to as NFPA 231C); Sprinkler System . The sections that follow in this chapter NFPA 231D, Standard for Rubber Tire Storage; and NFPA address hydraulic Calculations of flow through piping, 231F, Standard for Roll Paper Storage. However, beginning simple Calculations commonly performed in determin- with the 1999 edition of NFPA 13 these standards were all ing water supply requirements, and optional Calculations merged into NFPA 13 to produce a consolidated Sprinkler that may be performed with regard to hanging and brac- System design and installation standard. ing of System piping. The final section of this chapter Other NFPA standards contain design criteria for spe- deals with the performance of a System relative to a fire, cial types of occupancies or systems, including NFPA 13D, and the material contained therein is totally outside the Standard for the Installation of Sprinkler Systems in One- and realm of standard practice.

9 This material is not suffi- Two-Family Dwellings and Manufactured Homes (hereafter re- ciently complete to permit a full design approach, but ferred to as NFPA 13D); NFPA 15, Standard for Water Spray only isolated bits of total System performance. Fixed Systems for Fire Protection; NFPA 16, Standard for the Installation of Foam-Water Sprinkler and Foam-Water Spray Systems; NFPA 30, Flammable and Combustible Liquids Code; Hydraulic Calculations NFPA 30B, Code for the Manufacture and Storage of Aerosol Products; and NFPA 409, Standard on Aircraft Hangars. Density-Based Sprinkler Demand Occupancy hazard classification is the most critical Limits of Calculation aspect of the Sprinkler System design process. If the haz- in an Empirical Design Process ard is underestimated, it is possible for fire to overpower the sprinklers, conceivably resulting in a large loss of Engineering Calculations are best performed in areas property or life. Hazard classification is not an area in where an understanding exists as to relationships be- which calculation methods are presently in use, however.

10 Tween parameters. This is not the case with the technol- The proper classification of hazard requires experienced ogy of Automatic Sprinkler systems. Calculation methods judgment and familiarity with relevant NFPA standards. are widely used with regard to only one aspect of sprin- Once the hazard or commodity classification is deter- kler systems: water flow through piping. There are only mined and a Sprinkler spacing and piping layout has 4 74 Design Calculations been proposed in conformance with the requirements of Water is provided only for the number of sprinklers the standard, the System designer can begin a series of in the design area, since no water is needed for the sprin- Calculations to demonstrate that the delivery of a pre- klers that are not expected to open. The actual number of scribed rate of water application will be accomplished for sprinklers in the design area depends, of course, on the the maximum number of sprinklers that might be rea- spacing of the sprinklers.