May 1, 2006
By James Dockrill, J & S Sprinkler Design & Consulting and Lee Norton, P.Eng., The Mitchell Partnership
In the last decade many new issues with respect to fire protection standards and codes have arisen, as well as growth in the number of equipment options. The designers of fire protection systems, and...
In the last decade many new issues with respect to fire protection standards and codes have arisen, as well as growth in the number of equipment options. The designers of fire protection systems, and the people installing, testing and approving them, have to be more knowledgeable than ever.
This article looks at some typical issues facing sprinkler system designers and the newest equipment available to them.
Perhaps the first and most important question is which standard to use. For example, both the National Building Code of Canada and the Ontario Building Code reference Standard 13 of NFPA-1999 issued by the National Fire Protection Agency in the United States. Specifically the Canadian codes reference NFPA 13, Standard for the Installation of Sprinkler systems; NFPA 13R, Standard for the Installation of Sprinklers in Residential Occupancies up to Four Stories; and NFPA 13D, Sprinkler Installation in One and Two Family Dwellings.
From the information we have, the new edition of the Ontario Building Code due this year will still reference NFPA-1999. However, what the designer may not always realize is that he or she may use the more recent NFPA-2002 Standard 13 if it will be of help. It’s the designer’s choice.
Each new edition of a standard refines and closes “loopholes” as well as giving a greater degree of protection compared to the previous version. A newer edition of a standard may also give the designer opportunity for savings. For example, the 1999 edition of NFPA 13 allowed the use of quick response sprinklers, which in turn allowed for a reduction in the design areas for “Light and Ordinary Hazards,” as in a typical commercial office.
The sprinkler “design area,” which is used to prove the validity of the whole system, is hydraulically the most demanding area of the system. A fire protection classification requires a specific flow over a prescribed area, such as 0.10 gallons per minute (gpm) per sq.ft. over 1,500 sq. ft. = 150 gpm. If the area to be covered can be reduced to, say 0.10 gpm/sq.ft. over 900 sq.ft. = 90 gpm, then the required total flow can be reduced, which in turn reduces the pipe sizing and associated costs.
Numerous architectural decisions will affect the designer of fire protection systems.
For example, attics of non-combustible steel and concrete construction do not require sprinkler protection whereas combustible wood framed buildings do. When steel prices rose sharply last year, developers chose the more affordable wood frame construction, but then found that they had to add the cost of a sprinkler system.
The slope of a roof also has a major impact. The effective water throw of the sprinkler is reduced when the head is placed at an angle to the floor. Normally, any roof slope less than 2-in-12 is considered flat and no special requirements are indicated.
In the 1999 NFPA Standard 13, pitched roofs with slopes above 2-in-12 are lumped into one category. However, the 2002 Standard 13 divides pitched roofs into two categories: above 2-in-12 up to 4-in-12, and 4-in-12 and above. Any pitch less than 2-in-12 is still considered flat. Those responsible for the 2002 standard found that the previous requirements of 1999 did not adequately protect the building on steeper slopes.
In Ontario, therefore, the designer has to decide whether he should provide the minimum coverage as required by the Ontario Building Code, or the enhanced coverage required by NFPA-13 2002, knowing that the updating of the requirements was made due to the inadequacies of the previous version.
It is important to consider how flexible the sprinkler system would be if there are future expansions to the building or a change in tenants. For example, a designer may use quick response sprinkler heads for ceilings up to 20 ft. high. However, if the system is designed for a 20-ft. ceiling and not designed for the 25-ft. deck height, and the next tenant wants to remove the ceiling for an open concept, substantial costs will be involved to change the sprinkler system. If the original designer had anticipated this problem, the new tenant’s needs could be met by simply raising the sprinkler heads with “candlesticks” to within 12 inches of the deck.
Selecting the sprinkler
Engineering drawings usually give the NFPA standard under which the project is to be designed along with the category of the required design parameters for each area.
However the cost of complying with these specifications may vary substantially according to which sprinkler head is chosen. While the standard sprinkler heads may comply with the specifications, specifically approved sprinkler heads can cover more area at a lower flow rate. They require fewer heads and smaller piping, so despite their higher cost, they make the job less expensive to install.
In the NFPA-13 standards there is a statement that “Nothing in this standard shall be intended to restrict new technologies or alternate arrangements, provided the level of safety provided by the standard is not lowered.” The statement allows the designer to use the most current issue of the standard or code as well as opening the door for manufacturers to develop new products.
Of course, care must always be exercised to install the sprinklers in accordance with the particular standard and individual sprinkler head data sheet.
Sprinkler categories include:
* Standard Coverage Sprinklers
* Quick Response Sprinklers
* Dry Sprinklers
* Extended Coverage Standard Response
* Extended Coverage Quick Response
* Residential Sprinklers
* Large Drop Sprinklers
* Specific Application Sprinklers
* ESFR (Early Suppression Fast Response)
Note, also, that there are numerous non-standard heads available on the market. Specialized sprinklers designed for a particular purpose can supersede the NFPA standard in their specific applications.
Since January 1, 2001, each sprinkler head is designated an SIN number. It indicates the manufacturer, the orifice size, the deflection characteristics, and the thermal sensitivity.
Following are descriptions of four of the main sprinkler head types mentioned above.
* Extended Coverage Sprinkler Heads may save significant money when there is an open area to be protected. The added coverage per head can be used to reduce the total number of heads required to meet the design criteria, as shown in the table above.
They can be listed as a quick response type or as a standard response type but how a particular head is listed depends on the hazard it is protecting. For example, a K=11.2 extended coverage sprinkler is listed as a light hazard quick response sprinkler to cover an area of 400 sq.ft., but when used in the higher ordinary hazard classification it is listed only as a standard response sprinkler.
Extended coverage sprinklers have certain limitations under NFPA 13(2002) Section 8.4.3. The most notable is that they cannot be installed within bar joists having web members exceeding a 1-in. width or where the trusses are spaced less than 7 1/2 ft. on-centres. This provision excludes the types of trusses typically used in big box stores for example. However, there are extended coverage sprinklers such as the Tyko K=25.0 that are specifically listed for extended coverage, extra hazard or storage occupancy in the above type of construction.
* Early Suppression Fast Response (ESFR) heads discharge a high volume of water in order to penetrate the fire plume. On average, one sprinkler head will discharge approximately 100 gallons per minute. The systems have very strict installation requirements in order to ensure they will operate as intended.
High pile storage
warehouses often use ESFR heads, which are installed under the roof deck, rather than using in-rack fire protection. In-rack systems can be expensive to install and susceptible to damage by tow motors. If damaged, the heads could discharge and cause water damage to the goods being stored.
A designer using ESFR heads designs the system according to the storage height and building height, so these systems are good for Class 1 to Class 4 storage.
New ESFR heads are constantly being developed with higher “K” factors. The higher the K, the lower the water pressure that is required for the head to function. A K=14.0 head in a 25-ft. high storage area in a 30-ft. high building needs 50 psig of water pressure to provide the required flow, but a K=25.0 in the same situation needs only 15 psig. Here the designer has to weigh the benefits. The K=25.0 costs much more (e.g. $25 vs. $8) but may save the owner the cost of providing a fire pump.
* Quick Response Sprinklers have heads designed to react in less time and thus contain the fire more effectively. With these sprinkler heads, the design area can be reduced without revising the required water density flow. They are most cost effective when used in large open areas.
* Large Drop Sprinklers produce characteristic large water droplets. Because the larger droplet will penetrate down to the flames rather than first evaporating, these sprinklers are often used in very high storage facilities up to a maximum 35 ft. building with up to 25 ft. storage, depending on the commodity or as listed by the sprinkler head manufacturer.
Besides the many new sprinkler heads, all types of materials for fire protection are changing, from pipe hangers to valves and devices. For example, the designer can choose between Schedule 40, 10, 7 or Schedule 5 in steel pipe. (A pipe “schedule” refers to the wall thickness of the pipe.) Schedule 40 pipe is usually referred to as standard pipe (although not in all sizes), but NFPA allows piping down to Schedule 5 to be used.
The lower schedule pipe has thinner walls, which give less resistance and better flow characteristics but can lead to problems when cutting grooves or threads in them. It is up to the designer whether to accept this pipe or go with his own experience or conservative instincts and specify thicker walled pipe such as schedule 10 or schedule 40.
Copper and plastic pipe is also an option for sprinkler systems in certain circumstances, such as residential installations (NFPA-13D). Here it is possible to design the copper domestic water distribution system to be used for fire protection. Plastic pipe is allowed as long as it is covered by walls.
Dry valves, often used in cold storage areas, are now available that can operate with less than 20 psig, considered the old minimum value. The previous dry pipe valves worked on a pressure differential of approximately 6:1 (1 psig air will hold back 6 psig of water). Developments have led to a new type of dry valve that no longer requires this differential. Therefore, lower air pressures may be used. The valve’s required trip time is also reduced and larger dry systems can be installed and still maintain the 60-second time interval for water to get to the farthest sprinkler head.
Groove type couplings connecting lengths of pipe with modified triple seals are also now available, which is beneficial to dry systems.
In conclusion, sprinkler systems are being used more frequently in buildings and we must ensure that the expected level of life safety and property protection is provided. We predict the future will bring increased mandatory sprinkler coverage of all types of buildings along with further increases of options. Indeed, the discipline of sprinkler design is becoming so complex it is no longer the domain of a designer who practises it only as a part of his work but often requires a dedicated specialist.
James N. Dockrill, C.F.P.S., NICET Level IV, is the principal of J&S Fire Sprinkler Design & Consulting, St. Catharines, Ont. E-mail,email@example.com.
Lee Norton, P.Eng., is with the Mitchell Partnership and principal of TMP Niagara, St. Catharines.
Maximum coverage of standard & extended coverage heads
|Light Hazard Max Area (sq.ft.)||Ordinary Hazard Max Area (sq.ft.)||Extra Hazard Max Area (sq.ft.)|
|Extended Coverage||400 (20' x 20')||400||196|
Note: The maximum coverage noted above has been dictated by NFPA-13 (i.e. a sprinkler head manufacturer cannot provide a head that exceeds this coverage).
DIFFERENT HAZARD CLASSIFICATIONS
|Light Hazard||Auditoriums, office areas|
|Ordinary Hazard Group 1||Moderate combustibles, mechanical rooms|
|Ordinary Hazard Group 2||Moderate/high combustibles, repair garages,machine shops, stores|
|Extra Hazard Group 1||Arena multi-purpose, RV Shows, sawmills|
|Extra Hazard Group 2||Paints, spray booths, plastic processing|
|Storage less than 12′||Misc. storage goes under OH or EH|
|Storage greater than 12′||High pile storage per classifications (below)|
|Storage Class 1||No combustibles on wood palettes, e.g. bottles and cans|
|Storage Class 2||Class 1 stored in combustible packaging; bottles &cans filled with products|
|Storage Class 3||Products with a percentage of plastic, candies, chocolates|
|Storage Class 4||Products with an appreciable percentage of plastic; waxcoated paper, furniture, disposable diapers, matches, rubber|
|Plastics Group A||Most demanding group — polystyrene|
|Plastics Group B||Silicones, natural rubber|
|Plastics Group C||PVC|