Focus on Fire Protection
By Matthew Roy
Designing Combined Standpipe and
Sprinkler Systems
In the past, automatic sprinkler systems were installed
only 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 fire. Due to the relatively high demands required by
extra and special hazard occupancies, the sprinkler piping
and the standpipe piping were kept separate.
Today, however, virtually every multi-occupant residential
and commercial building is fitted with an automatic sprinkler system in addition to the traditional standpipe system
as a result of such tragedies as the Seton Hall University fire
in January of 2000, which claimed three lives and resulted in
the hospitalization of dozens of building occupants. Boland
Hall, the freshman dormitory in which the fire occurred, was
built in 1952 and was not equipped with a sprinkler system.
Due to the lower fire flow demands of light or ordinary
hazard occupancies, both NFPA 13: Standard for the Installation of Sprinkler Systems and NFPA 14: Standard for the
Installation of Standpipe and Hose Systems allow the standpipe and sprinkler piping to be shared. The shared piping,
known as a combined system, dramatically reduces installation costs, making automatic sprinkler systems economically
feasible for more new and existing buildings. The shared distribution piping does not compromise the reliability of the
system, and it reduces system maintenance.
With past versions of NFPA 13 and NFPA 14, applying both
standards to an installation resulted in system designs with
conflicts and inconsistencies. For the most part, these conflicts and inconsistencies have been resolved. In particular,
the standards work together quite well for residential and
commercial buildings classified as light or ordinary hazard.
The most recent NFPA 13 (2002) and NFPA 14 (2003)
define requirements for combined sprinkler/standpipe systems. Which standard does one use as the driving force for a
particular design? The answer to this question is somewhat
muddy, but it becomes much clearer if you evaluate a building using the two design approaches independently.
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. It is important to note that sprinkler and standpipe requirements vary
by jurisdiction, so you should make sure you know which
applicable international and local codes apply for a particular project. This article assumes use of NFPA 101: Life
Safety Code. Many jurisdictions that have not adopted NFPA
101 use the International Building Code (IBC) as a driver
for overall fire protection design. Specialized installations
such as health care facilities may be subject to other code
requirements such as Joint Commission on Accreditation
18 Plumbing Systems & Design JANUARY/FEBRUARY 2006
of Healthcare Organizations (JACHO) standards for health
care facilities. Finally, this article assumes that hydraulic calculations are not required to be provided on a project and
that the pipe schedule method of design can be applied in a
particular jurisdiction.
The NFPA 13 Approach
Chapter 11 of NFPA 13 defines the design approaches for
sprinkler systems. It is noteworthy that Section 11.1.1 allows
three design approaches for any sprinkler system.
1. Occupancy Hazard Approach. For light and ordinary
hazard occupancies, hydraulic calculations are not
required. In these applications, the hazard approach can
be used for design.
2. Storage Design Approach. Storage facilities must be
hydraulically calculated and must follow the requirements of Chapter 12.
3. Special Design Approach. 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 the occupancy hazard approach.
Using this method, NFPA 13 Table 11.2.2.1 requires light
hazard applications to be designed for capacities of 500750 gallons per minute for durations of 30-60 minutes (see
Table 1). Ordinary hazard applications should be designed
for 850-1,500-gpm capacities for durations of 60-90 minutes.
Table 1 Water Supply Requirements for Pipe Schedule Sprinkler Systems
Acceptable Flow at Base
Minimum Residual of Riser, Including Hose
Occupancy
Pressure Required
Stream Allowance
Duration
Classification
(psi)
(gpm)
(minutes)
Light hazard
15
500 - 750
30 - 60
Ordinary hazard
20
850 - 1,500
60 - 90
Note: For SI units, 1 gpm = 3,785 L/min; 1 psi = 0.0689 bar
Source: NFPA 13 (2002) Table 11.2.2.1
The capacities defined in this table cover both sprinkler and
hose stream demands.
The NFPA 14 Approach
NFPA 14 requires a system to be designed based on the
number of standpipes and the type of hose valves (standpipe
class). NFPA 101 no longer allows Class II standpipe systems
(those with 1½-inch hose valves for most buildings), so I’ll
limit the discussion to combined sprinkler/standpipe systems of the Class I and Class III types.
For Class I and Class III systems (any system with 2½-inch
hose valves for fire department use), NFPA 14 Section 7.10.1.1
requires 500 gpm for the first standpipe plus 250 gpm for
each additional standpipe to a maximum of 1,000 gpm for
fully sprinklered occupancies (1,250 gpm for partially sprinklered occupancies). An important consideration in Section 7.10.1.1.2 is that 750 gpm should be used for any single
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FOCUS on FIRE PROTECTION
standpipe system in which individual branches off the standpipe
riser service three or more hose cabinets or fire department hose
valves. This section ensures that at least three hose streams are
available to fight fires in large buildings regardless of whether one
or more standpipes are used in the design. In terms of pressure,
100-pounds-per-square-inch residual pressure is required at the
most hydraulically remote standpipe outlet. It is important to note
that some jurisdictions, primarily those where the fire department
uses straight-bore nozzles, still allow 65-psi residual pressure for
standpipe systems.
Adding the sprinkler system to the picture, Section 7.10.1.3
defines what additional flows are required. For fully sprinklered
buildings, no additional flow needs to be added for the sprinkler
demand. For partially sprinklered buildings, 150 gpm and 500
gpm 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 NFPA 14 design approaches
for combined systems, you can see that the two codes very closely
agree regarding calculations. Note that NFPA 14 Section 7.6.2
further requires the normal 4-inch minimum standpipe sizing to
be increased to 6 inches for combined systems. In answering the
question of which approach to use, the answer is clearly both.
The following procedure is a simple approach to designing a
combined system.
1. Using NFPA 101, determine the location and number of risers
for the system. The risers and all supply piping should be sized
6 inches minimum.
2. Use NFPA 14 to locate hose connections and valves, being
careful to ensure each pressure-regulating device has a 2½inch test connection to a 3-inch drain riser.
3. Use NFPA 14 to determine both capacity and residual pressure requirements for the standpipe system. Pressure drops
through the distribution system should be calculated using
the Hazen-Williams Formula (also defined in NFPA 14).
4. Add the sprinkler allowance for partially sprinklered buildings. Though buildings should be fully sprinklered, this step
applies to jurisdictions where complete sprinkler coverage is
not required.
5. Locate sprinklers and size distribution piping per NFPA 13.
6. If a water supply tank is required to meet the capacity requirements of the system, use NFPA 13 Table 11.2.2.1 to size the
tank for the appropriate duration. Refer to NFPA 22: Standard
for Water Tanks for Private Fire Protection for installation
requirements.
7. If the water supply is a city main, use NFPA 24: Standard for
the Installation of Private Fire Service Mains and Their Appurtenances to design the water supply piping. Determine the
available static and residual supply pressures at the main from
a hydrant test (defined by NFPA 291: Recommended Practice
for Fire Flow Testing and Marking of Hydrants).
8. Size the fire pump to meet the pressure and flow requirements
and design the pump room to comply with NFPA 20: Standard
for the Installation of Stationary Pumps for Fire Protection.
As a final note on combined systems, keep in mind that NFPA
14 Section 6.2.5.1 requires “each connection from a standpipe that
is part of a combined system to a sprinkler system shall have an
20 Plumbing Systems & Design JANUARY/FEBRUARY 2006
individual control valve and check valve of the same size as the
connection.”
Example
Consider a 10-floor multi-usage residential and commercial
building with 50 dwelling units. The commercial space is offices
and has no rack storage or combustibles stacked higher than 8 feet.
The building height is 110 feet from the municipal water supply
entrance to the topmost fire department hose valve outlet. The
floor plan is 250 feet by 60 feet and is effectively rectangular. Stairwells are located at opposite ends of the building, and the building
is completely protected by an automatic sprinkler system.
In this arrangement, NFPA 101 would require one standpipe
in each exit stairwell with 2½-inch valves located on the landings between floors. NFPA 14 would require a design flow for the
standpipe system of 750 gpm (500 gpm for the first standpipe plus
250 gpm for the second standpipe). Being a combined system, the
risers would have to be sized a minimum of 6 inches. No additional
flow would be required for the sprinkler system because the entire
building is sprinklered. Note that NFPA 13 Table 11.2.2.1 requires
500-750 gpm for this system including sprinkler system demand
and hose stream allowance. Thus, the two standards agree.
If a water supply tank is required for this building, it would need
to be sized for 750 gpm for a duration of 30 minutes at a minimum.
The tank would need a minimum volume of 22,500 gallons and
would be required to be installed in accordance with NFPA 22.
Regarding pressure, you would be required to flow 500 gpm at
the two topmost outlets of the standpipe while maintaining a 100psi residual pressure at the acceptance test. Using the Hazen-Williams Formula for riser pipe losses, 500 gpm would induce a 7-psi
loss in the 6-inch riser. Elevation would account for another 48 psi,
resulting in a pressure requirement at the base of the riser of 155
psi.
For losses entering the building, you would need to count the
valves and fittings and calculate pressure losses based on equivalent lengths of pipe. As a final note, ensure that the appropriate
losses for the reduced-pressure principle backflow preventer are
considered in the calculations.
Upon further study of the combined system, it becomes clear
that NFPA 13 and NFPA 14 offer a relatively simple and elegant
approach to design. Though occasional loopholes or inconsistencies occur between standards, the NFPA requirements for the
entire water-based fire protection system mesh completely and
beautifully for light and ordinary hazard applications.
Matthew Roy is Marketing Manager for the Fire
& Plumbing Product Groups for Armstrong Pumps.
For more information or to comment on this article,
e-mail articles@psdmagazine.org.
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