Establishing Pipe Sizes and Water Supply Acceptability
In the past, pipe schedules were the accepted method of determining the adequacy of system pipe sizes; however, the current standards no longer recognize the pipeschedule method for new construction. The accepted method for the determination of pipe sizes and water supply adequacy is the performance of hydraulic calculations as outlined in NFPA 13.
The installing contractor may perform hydraulic calculations if qualified, but there is significant advantage to the engineer performing the hydraulic calculations and establishing pipe sizes and water supply acceptability before the bidding process begins. In performing hydraulic calculations, the sprinkler piping layout, nature of the hazard protected and water supply information must be known. Based on this information the area of sprinkler operation, appropriate design density and minimum sprinkler pressures can be used to perform the necessary hydraulic calculations.
While it is beyond our scope to further describe the hydraulic calculation procedure an excellent resource is Fire Protection Hydraulics and Water Supply Analysis, by Pat Brock, published by Fire Protection Publications of Oklahoma State University.
There are numerous types of sprinkler piping currently accepted for the installation of sprinkler systems. In all cases the piping specified for a sprinkler system must be installed and used within the parameters of its U.L. Listing.
Most sprinkler piping specified today is black or galvanized, welded and seamless steel pipe. Normally smaller pipe sizes are specified as Schedule 40 and larger sizes as Schedule 10 black steel pipe. The joining methods for sprinkler pipe include the use of flanged fittings, prefabricated welded outlets, cast or malleable iron fittings or mechanical grooved fittings. In all cases, sprinkler pipe and fittings must be capable of withstanding pressures of 175 psi. For further details NFPA 13 and pipe manufacturer catalogs should be consulted.
Areas Protected by Sprinklers. To provide a fully sprinklered building, which is the intent of most building codes and insurance industry sprinkler requirements, all areas of a structure, with few exceptions, must be provided sprinkler protection.
Generally, all spaces that are accessible, combustible or intended for storage or occupancy require sprinkler protection. The guidelines for permissible sprinkler omissions in fully sprinkled buildings are contained in NFPA 13. Except for void spaces in walls and noncombustible concealed spaces there are few exceptions and where exceptions do exist they are extremely specific with respect to accessibility, construction and dimensions.
With respect to individual sprinkler spacing requirements, the maximum area protected by one sprinkler should not exceed the area specified for the specific sprinkler as indicated in the manufacturer’s specification sheets. Standard spray sprinklers are listed for light hazard, ordinary hazard and extra hazard occupancies for 225-, 130- and 100-2 protection areas, respectively.
In most installations, the area or coverage of each sprinkler is usually less than the maximum areas listed.
In areas where construction is unobstructed, sprinkler deflectors should be parallel to and within 12 inches of the ceiling. Where construction is obstructed sprinklers must be within 22 inches of the roof deck above. In all cases, when locating sprinklers the maximum expected ambient temperature of the area being protected must be considered such that unwanted sprinkler activation does not occur.
Undesirable sprinkler water spray obstructions must also be considered when locating sprinklers. Where obstructions such as ducts are greater than 4 feet wide and are located below sprinklers, additional sprinklers should be added to spray below the obstruction. Furnishings such as tables are not considered obstructions unless they are within 18-inches, measured vertically of a sprinkler. When locating all sprinklers NFPA 13 and the sprinkler manufacturer’s guidelines must be followed to prevent unwanted water spray obstructions.
Sprinkler System Layout. A sprinkler system is generally laid out as a ‘‘Tree’’, ‘‘Loop’’ or ‘‘Grid’’ type system. Whatever the case, sprinklers are attached directly to pipes called branch lines. Branch lines, normally the smallest of sprinkler pipes, are supplied water from cross mains or feed mains which are directly connected to the system riser.
The riser, configured to control the water supply and monitor water flow and valve position, may support a single sprinkler system or if manifolded, many systems (Fig. 14.19). In any case, the maximum area per floor to be protected by a single riser is 52,000-^2 for light and ordinary hazard areas and 40,000-2 for extra hazard areas.
In high-rise buildings where standpipes and sprinklers are required a combined standpipe/ sprinkler system is normally used. In these situations there may be no true system riser; rather, each floor is provided with a floor control valve (Fig. 14.20) consisting of a control valve, drain, test connection and flow switch. In this configuration the individual floor control valves accomplish the function of the system riser. An inherent advantage of using floor control valves is that individual floors can be isolated so sprinkler system repairs on one floor do not reduce the level of protection on another floor.
As a practical matter, when lying sprinkler piping out it is advantageous to consider the pipe hanging arrangement. Where construction consists of joist construction, mains should be run parallel to the joist channels. This accommodates pipe hanging since the branch lines, which out number the mains, can be hung directly off the joists. Where construction is concrete pipe hanging is an easier task but one should give consideration to the arrangement of beams and bays such that unnecessary fittings and pipe lengths can be avoided.
Drainage of Sprinkler Systems. Provisions must be made for draining all parts of a sprinkler system. For that purpose, valve-controlled drains must be provided at low points in the system. The primary drain for most sprinkler systems is the main drain, normally a 2 inch drain located at the system riser. All drains should discharge directly to outdoors or to a sump capable of handling full flow drain capacity.
Special consideration must be given to the drainage of dry-pipe systems and portions of preaction systems subject to freezing. The branch lines of these systems should be pitched 1⁄2 in per 10 feet and mains 1⁄4 inch per 10 feet of length towards a suitable drain connection to accommodate total system drainage. Where trapped piping in dry-pipe systems exceeds 5 gallons capacity, a means must be provided to drain the trapped area without accidentally tripping the dry-pipe valve. This is usually accomplished with use of a drum drip assemble, an assembly which permits isolating trapped water and draining it without loosing air pressure.
Inspector’s Test Connections. All sprinkler systems should be tested periodically to ensure their proper function. A test connection for wet- and dry-pipe systems consists of a connection at least 1 inch in diameter with a test valve terminating in a smooth-bore, corrosion-resistant orifice. This orifice connection should be sized to provide a test flow equivalent to the smallest orifice size sprinkler installed in the system. For most systems the test valve connection can be located anywhere down stream of the alarm valve or flow switch, whichever is provided; however, on dry-pipe and double-interlock preaction systems the alarm test valve must be located at the hydraulically remote point of the system. This is to ensure that water will reach the remote end of the sprinkler system without undue delay, usually 60 seconds for large systems.
Approvals of Sprinkler-System Design. In all cases, before a sprinkler system is installed or modified, the applicable drawings and hydraulic calculations should be submitted to the authority having jurisdiction and the insurance underwriter as necessary. Since beneficial reductions in insurance rates may be obtained by suitable installation of sprinkler systems, it is important that the underwriter have sufficient time for a full review of the plans before construction begins. Similarly, municipal approval of the sprinkler-system plans is necessary before the structure can be occupied. In actual construction, the installing contractor generally secures the necessary municipal approval. The existence of this approval should always be confirmed before construction starts.