The NFPA 20 Report on Proposals Meeting was held in San Diego in March. During the meeting, 166 public proposals were reviewed and responded to. Particularly novel and controversial issues discussed included the installation of break tanks as backflow and pressure control devices, air-cooled engines, and text involving alarms and signals. The Report on Proposals (ROP) will be available in June of this year. To receive a copy, visit the NFPA web site at www.nfpa.org. Any comments with respect to the report must be submitted by September 2nd of this year to be reviewed for the 2006 publication of the code.
The “Focus on Fire Protection” section of PM Engineer Magazine’s March issue features a paper on high-rise standpipe system design in accordance with NFPA 14. If you are involved in installation and design of high-rise fire protection systems, this article is a “must read”. If you do not currently receive the magazine, you can view the feature on-line at http://www.pmengineer.com/CDA/ArticleInformation/features/Features_Index/1,2733,14-65809,00.html
or subscribe to the magazine at http://www.pmengineer.com.
COMBINED STANDPIPE & SPRINKLER SYSTEMS
In the past, automatic sprinkler systems were only installed to protect extra and special hazard occupancies. A separate set of piping, the standpipe system (a water distribution system within a property servicing hose cabinets and fire department hose valves or hydrants) was installed to provide adequate water flow for hose streams used to extinguish the fire. Due to the relatively high demands required by such occupancies, the sprinkler and standpipe piping were kept separate.
Today, virtually every multi-occupant residential and commercial building is fitted with an automatic sprinkler system and standpipe system. Due to the lower demands for these occupancies (light or ordinary hazard), both NFPA 13 and NFPA 14 allow the standpipe and sprinkler piping to be shared. The shared piping, known as a combined system, reduces the cost of the installation dramatically making sprinkler system installation economically feasible for more and more buildings. If done properly, the shared distribution piping does not compromise the reliability of the system, and reduces system maintenance.
This article will focus on some of the important considerations in calculating water supply requirements (water flow, pressure, and duration) for a combined fire protection system for a light or ordinary hazard application.
Both NFPA 13 and NFPA 14 define requirements for combined systems. Why is this, and more importantly, which code does one use as the driving force in the design? The answer to this question is somewhat muddy, but becomes much clearer if you evaluate a building using the two design approaches independently.
The NFPA 13 Approach
Chapter 11 of NFPA 13 defines the design approaches for sprinkler systems. It is noteworthy that Chapter 11 section 11.1.1 allows three design approaches for any sprinkler system:
- OCCUPANCY HAZARD APPROACH: For light and ordinary hazard occupancies, hydraulic calculations are not required. In these applications, the “pipe schedule” approach can be used for design.
- STORAGE DESIGN APPROACH: Storage facilities must be hydraulically calculated and must follow the requirements of Chapters 12.
- SPECIAL DESIGN APPROACH: Once again, special hazard applications are required to be hydraulically calculated, and must follow the requirements of Chapter 13.
Combined systems for light and ordinary hazard occupancies can be designed using method #1. Using the pipe schedule approach, Table 184.108.40.206 (illustrated below) requires that light hazard applications be designed for capacities of 500 to 750gpm for durations of 30 to 60 minutes. Ordinary hazard applications should be designed for 850 to 1500gpm capacities for durations of 60 to 90 minutes. The capacities defined in this table cover both sprinkler and hose stream demands.
The NFPA 14 Approach
NFPA 14 requires designs to be based on the number of standpipes and the type of hose valves in the system (standpipe class). For Class I and Class III systems (any system with fire department use 2˝” hose valves), section 220.127.116.11 requires 500gpm for the first standpipe plus 250gpm for each additional standpipe up to a maximum of 1250gpm. An important consideration in section 18.104.22.168.2 is that 750gpm should be used for any single standpipe system where individual branches off the standpipe riser service 3 or more hose cabinets or fire department hose valves. This section ensures that at least three hose streams are available to fight fires in larger buildings regardless of whether one or more standpipes are used.
Moving on to the combined system, section 22.214.171.124 defines what additional flows are required for the sprinkler system. For fully sprinklered buildings, no additional flow needs to be added for the sprinkler demand. For partially sprinklered buildings, 150gpm and 500gpm are to be added to the standpipe demand for light and ordinary hazard occupancies, respectively.
The Combined Approach
Having looked at the NFPA 13 and 14 design approaches for combined systems, it is not hard to see that the two codes are in close agreement for calculations. You will note that NFPA 14 further requires that the normal 4” minimum standpipe sizing be increased to 6” for combined systems (per section 7.6.2). In answering the question of which approach to use, the answer is clearly both.
To recommend a simple approach to designing a combined system:
- Open NFPA 14.
- Determine the location and number of risers for the system. The risers and all supply piping should be sized 6” as a minimum.
- Locate hose connections and valves (where permitted) ensuring any pressure regulating device has a 2˝” test connection to a minimum 3” drain riser.
- Use NFPA 14 to determine both capacity and residual pressure requirements for the standpipe system. Pressure drops through the distribution system should use the Hazen Williams method as defined in NFPA 14.
- Add the sprinkler allowance for partially sprinklered buildings. Though buildings should be fully sprinklered, this step applies for jurisdictions where complete sprinkler coverage is not required.
- Close NFPA 14 and open NFPA 13.
- Locate sprinklers and size distribution piping per NFPA 13.
- If a water supply tank is required to meet the capacity requirements of the system, use Table 126.96.36.199 to size the tank for the appropriate duration. Refer to NFPA 22 for installation requirements.
- If the water supply is a city main, design the water supply piping to NFPA 24. Determine the available static and residual supply pressures at the main from a fire flow test (per NFPA 291).
- Size a fire pump to meet the pressure and flow requirements and design pump room to comply with NFPA 20.
Upon further study of the combined system, it becomes clear that there is a relatively simple and elegant approach to the design. It is noteworthy that though occasionally loopholes or inconsistencies occur between code documents, the NFPA requirements for the entire water-based fire protection system mesh completely and beautifully for light and ordinary hazard applications.
In closing, Armstrong would like to thank Jonathan Ladd at the Virginia Polytechnic Institute and State University for initiating the discussion that spawned this article.
FIRE PUMP TIMPS
Question: Can a fire pump main relief valve be piped back to the pump suction?
Answer: Yes, section 5.18.7 of NFPA 20 allows this arrangement. A few important items should be considered when designing a relief valve into an application where the waste pipe returns to suction are as follows:
- The main relief valve can no longer be sized per Table 5.25. Because the suction pipe is not at atmospheric pressure, the backpressure in the waste line must be considered in the relief valve sizing. A relief valve piped back to suction may need to be sized larger than the minimum sizing requirements of Table 5.25.
- Main relief valves should not be installed as pressure regulating devices. They should be installed only on diesel fire pumps where required by section 188.8.131.52 and on systems with a variable speed pressure limiting control device as required by section 184.108.40.206.
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