nfsa sloped ceiling analysis

7/30/2019 NFSA Sloped Ceiling Analysis

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ANALYSIS OF SLOPED CEILING AND SPRINKLER

ORIENTATION IMPACT ON DELIVERED DENSITY

Prepared for the National Fire Sprinkler Association

February 2013


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AbstractThe problem of how to best protect commodities from fire under sloped ceilings is an

unsolved fire sprinkler challenge. Because of a lack of understanding of this problem, there

is minimal engineering guidance on how to design sprinkler systems in this configuration.

Fire protection solutions in these situations are often developed through ad hoc design

concepts which are validated with large-scale tests. This empirical sprinkler system design

approach, requiring case-specific large-scale testing, is expensive leaving little room for

optimization and little opportunity for generalization to other protection challenges with

even the slightest differences.

In this study, we take a revolutionary new quantitative approach to address the sloped

ceiling commodity protection challenge. A commercially available k-14 sprinkler head was

characterized using the University of Maryland invented Spatially-resolved Spray Scanning

System (4S). The complete spatio-stochastic spray description obtained from the 4S was

used as input into the Custom Spray Solutions (CSS) SprayVIZ software tool along with the

geometric details of the protected space. The SprayVIZ software provides quantitative

visualization of the impact of ceiling slope on the delivered density to the protected

commodities based on the sprinkler activation scenario of interest.

The SprayVIZ software analysis reveals that when the sprinkler deflector is parallel to the

floor, the sloped ceiling acts as an obstruction. Further, the SprayVIZ analysis reveals that

as the slope increases the percentage of water delivered to the protected commodity

decreases when the sprinkler deflector is parallel to the floor.


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1. Introduction

The effect of sloped ceilings on sprinkler spray distribution performance is a subject thatcontinues to challenge the fire protection industry. There is minimal guidance that exists

for the practitioner and any design direction is typically a result of a large-scale test, the

results of which may not readily be transferred to other scenarios (e.g. different sprinkler

heads, pressure and flow characteristics, rack configurations and clearances, ceiling

heights, or ceiling slopes). This inability to provide concrete guidance stems from

fundamental knowledge gaps in sprinkler spray characteristics. In this study we bridge

these gaps through innovations that 1) provide detailed characteristics of the sprinkler

head spray properties and 2) predict delivered density to protected surfaces using these

head characteristics. We demonstrate how this new technology can be deployed to attack

real-world fire sprinkler design challenges through an evaluation of the impact of ceiling

slope on delivered density.

2. Current Guidance on Sloped CeilingsThere is a lack of clear guidance on how to design fire sprinkler systems with ceilings

having a rise of more than 2:12. Two of the leading authorities in fire protection, NFPA and

FM Global, provide some limited guidance; however, in several key areas they diverge from

each other. For example, NFPA requires that sprinkler heads be installed parallel to the

ceiling (due to spray density performance considerations) while FM Global requires that

they are installed parallel to the floor (due to spray momentum performance

considerations). Needless to say it is physically impossible to comply with bothrequirements. While each organization has salient points supporting their design guidance,

there is simply insufficient analysis to determine the performance benefits of one position

versus the other.

NFPA 13 provides only two solutions for slopes greater than 2:12, namely putting in place a

drop-ceiling (effectively eliminating the slope) or conducting a performance based analysis

to determine the requirements based on the specific configuration and sprinkler head

selected. It has generally been indicated that the NFPA standard guidance on coverage area

and flow does not apply in sloped ceiling situations. This leaves the practitioner in a

challenging predicament with little guidance other than experience.

3. Basic Scenarios EvaluatedA standard 4x4x4 pallet load double rack configuration was evaluated with a storage

height of 19 and a starting ceiling height of 25. Sprinkler drops were defined as 1 such

that the sprinklers are located 5 from the top of the load. This configuration is typical of

many storage facilities and can serve as a baseline for comparison with each of the sloped

ceiling configurations.


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3.1 Rack ConfigurationThe storage space was created in the model to simulate a double rack configuration. The

loads are 4 cubes spaced with 6 transverse and longitudinal flue spaces and a 4 aisle. A

plan view for the storage space of interest detailing the initial sprinkler locations relative tothe commodity can be seen in Figure 1.

Figure 1 Plan view of modeled space

The loads are stacked 4 high with 1 spacing between the loads, thus the top of the load is at

a height of 19, while the ceiling is located at 25 and the sprinklers are on a 1 drop placing

them at a height of 24 and 5 above the top of the load. This three dimensional orientation

can be seen in Figure 2 which shows the configuration in the SprayVIZ software.

4

4 4

10

10

10

0.5


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Figure 2 3D view of wet commodity from SprayVIZ software.

3.2 Ceiling Slope & Sprinkler LayoutIn each case the modeled sprinkler flowed at a discharge rate of 60 gpm, for the k-14

sprinkler head tested this corresponds to 18 psi.

In the flat ceiling configuration the sprinklers are spaced 10 on center, for the sloped

ceilings the sprinklers are spaced 10 on center aligned with the slope of the ceiling (i.e. the

greater the slope the shorter the horizontal separation between the heads). Spacing was

configured such that Sprinkler 03 was always located directly above the center of the

commodity as depicted in Figure 1.

Three separate ceiling slopes were evaluated with 2:12, 4:12, and 6:12 rises being

compared with the flat ceiling.

3.3 Nozzle Parallel with Ceiling vs. FloorThe orientation of the sprinkler with respect to the ceiling has been a debated issue for

some time. NFPA and FM offer differing guidance on what orientation is correct for sloped

ceilings however no quantitative evidence has been provided to date to indicate which


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orientation offers better protection and what impact, if any, the change in orientation has

on the delivered density.

4. ResultsFor each of the results below a k-factor 14 sprinkler head operating at 18 psi was utilized.

4.1 Flat CeilingThe control scenario was the flat ceiling, Figure 3; each of the other scenarios was

compared to the results from this scenario. As discussed above the sprinklers were spaced

10 on center with sprinkler 03 located above the middle of the box.

(a) (b)

Figure 3 Commodity View Reference room, flat ceiling. (a) Isometric view; (b) Plan view delivered density to

commodities

4.1.1 Sloped CeilingsA total of 6 different sloped ceiling configurations were examined and modeled. Each slope

(2:12, 4:12, 6:12) was modeled with the sprinkler parallel to the floor and also parallel to

the ceiling. The main difference for each configuration, other than the slope itself, is thatthe spacing of the sprinklers is measured along the slope, thus effectively making the

sprinklers closer together horizontally in relation to the floor with increasing slope.

The results from each of the sloped configurations are somewhat similar when compared

to flat ceiling reference geometry as shown in Figure 3 which served as a baseline for much

of the analysis conducted. However, the 6:12 slope differs most from the other

configurations in that there is a significant quantity of spray that hits the roof for both the


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parallel to the floor and parallel to the ceiling sprinkler orientations. In this situation it was

observed that the parallel to the floor configuration results in the highest quantity of water

spray impacting the roof with the majority of it being directed at the lower portion of the

slope.

Representative figures for the 6:12 configuration are provided below which show the detail

of the spray for isometric, plan views, and ceiling views. These figures, Figures 4 & 5, show

the detailed spray patterns in the full geometry and water distributions to the ceiling and

commodity for the two sprinkler orientations.

Figure 4 Isometric view of 6:12 slope with sprinklers parallel to the floor, density distribution in mm/min


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5. Analysis5.1 Impact of Sloped CeilingFigure 6 below shows the difference in the delivered density to the top of the commodity for each slope with the sprinkler

parallel to the ceiling. The difference is calculated by subtracting the delivered density of water of the sloped configuration

from the reference room (flat ceiling) density. The legend has been provided to show areas that have a deviation greater than

20% from that of the reference average flux. As can be observed there are significant areas where the delivered flux from the

sprinkler on a sloped ceiling is within 20% of the reference flux (green shade) and also significant areas where the delivered

flux from the sloped ceiling sprinkler is more than 20% less than the reference ceiling (blue shade). The flux from the sloped

ceiling sprinklers exceeds that of the reference flux by more than 20% over a much smaller surface area (red).

(a) (b) (c)

Figure 6 Difference in delivered density (flat ceiling-sloped ceiling) to the surface of the commodity with sprinkler parallel to the ceiling. (a) 2:12 slope, (b)

4:12 slope, (c) 6:12 slope


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Figure 7 below is similar to Figure 6 with the exception that the sprinkler is parallel to the floor in these simulations.

(a) (b) (c)Figure 7 Difference in delivered density (flat ceiling-sloped ceiling) to the surface of the commodity with sprinkler parallel to the floor. (a) 2:12 slope, (b)

4:12 slope, (c) 6:12 slope

As can be observed in both figures the greater the slope the greater the percentage of surface area that deviates from the

reference configuration. As the slope increases the sprinklers higher on the slope provide more of their water to commodities

further away from the intended area of protection.


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When a single sprinkler head, Sprinkler 02, is isolated and the impact of the slope is observed, with the sprinkler parallel to

the floor, it is clear that the sprinkler spray is still centered over the protected commodity. It is also clear that the increased

distance from the head to the protected commodity increases the coverage area of the sprinkler but simultaneously reduces

the delivered density. Figure 8 shows the delivered density of a single sprinkler with the sprinkler parallel to the floor for each

configuration.

(a) (b) (c) (d)

Figure 8 single sprinkler distributions with sprinkler parallel to the floor. (a) Reference flux for flat ceiling; (b) 2:12 slope; (c) 4:12 slope; (d) 6:12 slope,

density in mm/min.


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5.2 Impact of Sprinkler OrientationThe orientation of the sprinkler head clearly has an impact on the delivered density to the surface and affects the quantity of

water that impacts the roof, as shown earlier. When the sprinkler orientation is parallel with the floor in a sloped

configuration there is a greater percentage of water that impacts the ceiling. While orienting the sprinkler parallel to the

ceiling results in less flux to the ceiling, as the slope increases the flux to the ceiling does as well. Figure 9 shows the

percentage of total water flowed that is delivered to the ceiling and to the top of the protected commodities as a function of

ceiling slope and sprinkler orientation. Figure 10 shows the impact of the orientation on the delivered density.

(a) (b)Figure 9 Percent of total flowed water delivered to the protected commodities and to the ceiling as a function of sprinkler orientation and ceiling slope. (a)

Percent of water delivered to ceiling (note in the 4:12 configuration approximately 0.1% of the total flowed water was delivered to the ceiling); (b) Percent

of total flowed water delivered to the top surface of the protected commodities.

0

1

2

3

4

5

6

7

0 2:12 (9.5) 4:12 (18.4) 6:12 (26.6)

Perc

entageofTotalWatertoCeiling[%]

Ceiling Rise (Angle [deg])

Parallel to Floor Parallel to Ceiling

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37

39

41

43

45

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0 2:12 (9.5) 4:12 (18.4) 6:12 (26.6)Perce

ntageofTotalWatertoCommodity[%]

Ceiling Rise (Angle [deg])

Parallel to Floor Parallel to Ceiling


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(a) (b) (c)

Figure 10 Impact of sprinkler orientation on delivered density. (a) Reference distribution; (b) 6:12 slope with sprinkler parallel to t he floor; (c) 6:12 slopewith sprinkler parallel to the ceiling


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6. ConclusionsThis analysis examined the impact that ceiling slope and sprinkler orientation has on thedelivered density to the protected commodity. For this specific configuration (sprinkler

type, commodity geometry, pressure, etc.) it was shown that both sprinkler orientation and

ceiling slope impact the spray distribution. With a single sprinkler activating it is clear that

the protected coverage area is shifted based on the orientation of the head as well as due to

the increased clearance between the sprinkler and the protected commodity due to the

slope.

Furthermore for the tested sprinkler head it appears that orienting the sprinkler head

parallel to the ceiling reduces the quantity of water that strikes the ceiling (the ceiling is

less of an obstruction) and that it also increases the quantity of water that is delivered tothe protected commodities. This effect increases as the slope increases.

7. Additional ApplicationsThe CSS methodology can also be used to address many of the other sprinkler protection

challenges facing the industry, namely:

The sprinkler droplet momentum interaction with fire and plume dynamics The high clearance sprinkler problem in industrial operations Impact of pressure on actual delivered density Differences in sprinkler performance based on model and manufacturer Cloud and other complex ceiling arrangements and designs Sloped, curved, and other ceiling configuration impact on spray distribution Deflector orientation effects on commodity protection High cost of large scale testing

In addition to the current design challenges above, the methodology can be applied in many

other situations where a non-standard solution is required or as a screening tool prior to

conducting large scale testing.

nfsa sloped ceiling analysis

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