Blog of Aaron J.: system design

Important to fire sprinkler effectiveness is sprinkler discharge pattern development. A fire sprinkler system should be designed in a manner that provides full water coverage from the fire sprinkler system. Inherent building construction or design elements can pose potential obstructions to the full coverage or discharge pattern of the sprinkler system. However, these buildings and elements must still be protected. NFPA 13, Standard for the Installation of Sprinkler Systems, provides sprinkler coverage guidance for these obstructed areas.

To determine the proper application it must first be determined if the construction is obstructed or unobstructed. Then it must be determined if the obstruction is continuous or noncontinuous.

Is this obstructed or unobstructed construction?

To be considered obstructed, the construction elements must be composed of “beams, trusses, or other members” that will impede heat flow, water distribution, or otherwise affect the ability of the sprinkler to control or suppress the fire. [13:3.7.1]

Construction types that may create obstructions, include:

Beam and girder
Concrete Tee
Composite wood joist
Panel construction
Bar joist with fireproofing
Steel purlin

Unobstructed construction includes any “beams, trusses, or other members” that do not impede heat flow, water distribution or the ability of the fire sprinkler to control or suppress fire. This generally consists of construction members that are not solid.[13:3.7.2]

Unobstructed construction elements may include:

Bar joist
Open-grid ceilings
Smooth ceilings
Standard mill (heavy timber)

Is the obstruction continuous or noncontinuous?

If the obstruction affects the sprinkler discharge pattern of two or more adjacent sprinklers then the obstruction is considered to be continuous. However, if only one sprinkler discharge pattern is affected then the obstruction is noncontinuous.

Different sprinkler head types and orientations have different requirements for designing to protect obstructed areas. These are addressed in the following sections of NFPA 13.

General guidance: 13:8.5.5
Pendant and upright sprinklers: 13:8.6.5
Sidewall sprinklers: 13:8.7.5
ESFR sprinklers: 13:8.12.5

A great reference tool for quickly determining the design criteria and requirements for obstructed areas is this ceiling obstructions reference tool/calculator provided by Meyer Fire.

NFPA 409, Standard on Aircraft Hangars defines hangar group classifications, construction features, and fire protection requirements for aircraft hangars. Group I and II hangars require foam and foam-water type systems. The design the criteria for these is referenced in NFPA 409, Chapter 6. Determining the correct system design is essential to proper functioning of these systems. In, Design of Special Hazard and Fire Alarm Systems, Robert Gagnon outlines a 12 step design method for aircraft hangar protection.

Step 1. Determine aircraft hangar group and select protection system type.

Each hangar group permits only specific types of fire protection systems designs. These options can include a foam-water deluge system, with underwing supplementary protection, automatic sprinkler with low-level foam, low-level high expansion foam, or a closed-head foam water system.

See also, Guide to Hangar Classifications.

Step 2. Determine foam application time.

These times can vary based on the hangar group classification and foam systems utilized.

Low-expansion foam - 10 minute application time
High-expansion foam - 12 minute application time
Foam-water hand hose stations - 20 minute application time

Step 3. Determine system design density.

This will be based on the system coverage area, sprinkler spacing, type of foam used, and design density as outlined in the various component sections of NFPA 409:6.2.

Step 4. Estimate protection discharge rate.

Use the formula:

D = (A) x (R)

D = foam solutions discharge rate, gpm

A= hangar floor area, square feet

R= application rate (from Step 3), gpm per square foot

Step 5. Estimate concentrate quantity for protection.

Use the formula:

Q = (A) x (R) x (T) x (%)

Q= foam concentrate quantity, gallons

T= foam discharge time

% = concentrate percentage, decimal

Step 6. Determine aircraft wing area.

Hangars that house aircraft having a wing area in excess of 3,000 sq.ft. are required to have supplementary under-wing protection. Without this under-wing protection the low-expansion foam system may be blocked from accessing the fire. The most common and effective supplementary under-wing protection is the use of oscillating monitors.

Step 7. Determine under-wing oscillating monitor location.

These should be located perpendicular to the fuselage to provide unobstructed protection beneath the wings.

Step 8. Determine oscillating monitor coverage area.

Monitors by different manufacturers will throw water in a certain radius and distance. When the radius is obtained the area of monitor coverage must be determined. To determine coverage area use the following formula:

Monitor area = [(3.1416) x (r2)] x (area of coverage/360)

Step 9. Apply oscillating monitor discharge time and application rate.

Discharge time is 10 minutes. Application rate is 0.10 gpm per square foot.

Step 10. Determine oscillating monitor discharge rate and concentrate quantity.

Use the formula:

D = (A) x (R) x (N)

Q = (A) x (R) x (N) x (T) x (%)

N = number of monitors installed

Step 11. Determine supplementary hose discharge requirements.

A minimum of (2) hose lines at 60 gpm each for 20 minutes is required.

Step 12. Determine hose discharge rate and concentrate requirement.

Use the formula: