Hydraulic Calculation Tutorial #1

7/27/2019 Hydraulic Calculation Tutorial #1

1/9


www.layouttech.com/calculation/calc1/calc1.htm

Calculation Menu

It's important to recognize this site is not intended to teach everything there is to know about fire sprinkler layout but to give a basic

introduction to hydraulically calculating fire sprinklers by hand.

Unlike the movies all the fire sprinklers don't go off at one time the system is designed to deliver a specified amount of water over a

hydraulically most demanding area.

The most common systems are designed using the area/density method according to the occupancy of the building and when I say

occupancy I don't mean how many people are in the building, or even what the building is constructed of, what occupancy means is what is

the building used for.

We will briefly discuss six different occupancies each followed by three examples which are are:

5.2* Light Hazard Occupancies.

Light hazard occupancies shall be defined as occupancies or portions of other occupancies where the quantity and/or combustibility of

contents is low and fires with relatively low rates of heat release are expected.

Examples are:

Educational (schools)

Hospitals

Offices, including data processing

5.3.1* Ordinary Hazard (Group 1). Ordinary hazard (Group 1) occupancies shall be defined as occupancies or portions of other

occupancies where combustibility is low, quantity of combustibles is moderate, stockpiles of combustibles do not exceed 8 ft (2.4 m), and

fires with moderate rates of heat release are expected.

Examples are:

Automobile parking and showrooms

Bakeries

Electronic plants

5.3.2* Ordinary Hazard (Group 2). Ordinary hazard (Group 2) occupancies shall be defined as occupancies or portions of other

occupancies where the quantity and combustibility of contents are moderate to high, stockpiles do not exceed 12 ft (3.7 m), and fires withmoderate to high rates of heat release are expected.

Machine shops

Metal working

Mercantile

Textile manufacturing

Mercantile is a cute name for stores but these do not include "big box" stores such as Wal*Mart and Home Depot. "Big box" stores

typically have commodity stored higher than 12'-0" either solid piled or stored on racks.

5.4.1* Extra Hazard (Group 1). Extra hazard (Group 1) occupancies shall be defined as occupancies or portions of other occupancies

where the quantity and combustibi lity of contents are very high and dust, lint, or other materials are present, introducing the probability of

rapidly developing fires with high rates of heat release but with little or no combustible or flammable liquids.

Upholstering with plastic foams

Saw Mills

Combustible hydraulic fluid use areas

5.4.2* Extra Hazard (Group 2). Extra hazard (Group 2) occupancies shall be defined as occupancies or portions of other occupancies with

moderate to substantial amounts of flammable or combustible liquids or occupancies where shielding of combustibles is extensive.

Asphalt saturating

Flammable liquids spraying

Manufactured home or modular building assemblies (where finished enclosure is present and has combustible interiors)

With a few exceptions you do not get into storage until whatever is stored exceeds 12'-0" in height or you have highly combustible storage

such as plastics, rubber tires, wood pallets or plastic pallets.
http://layouttech.com/calculation/calculation.htm


2/9



We'll concentrate on the Light and two Ordinary Hazard occupancies.

Figure 11,.2.3.1.5 the "Density/Area Curves".

Consider the local Foodway Grocery Store.

Foodway is a mercantile and since height of storage does not exceed 12'-0" we can be pretty safe in saying it is an Ordinary Hazard

Group II Occupancy.

As the layout technician it is your choice to pick any design criteria on the dark line labeled "Ordinary 2". It is up to you.

For a density area criteria the layout technician can select any point along the Ordinary 2 curve.

For example the Green line is at the lowest area... a density of .20 gpm over the hydraulically most remote 1,500 sq. ft..

The Purple line notes a density of .15 gpm over the hydraulically most remote 4,000 sq ft. while the Red line marks a density of .17 gpm

over the hydraulically most remote 3,000 sq ft.

For an Ordinary Hazard Group II Occupancy any one of the three points from the density/area curves are correct.

In actual practice 999 times (or more) out of 1,000 the least expensive system will happen when you select the lowest area which will give

you the highest density. With near 100% certainty I can assure you a system designed to a .20 over 1,500 sq. ft. will cost less than onedesigned to .17 over 3,000 sq. ft. and the most expensive design would be the .15 gpm over 4,000 sq. ft..

Ordinary sprinkler heads are listed to be spaced a maximum of 15'-0" apart but with few exceptions they can not be closer than 6'-0" apar

unless equipped with baffles.

For Light Hazard Occupancies a sprinkler can not cover more than 225 sq. ft. and for both Ordinary Hazard Group I and II Occupancies the

maximum coverage area is reduced to 130 sq. ft..

Consider the typical layout for Ordinary Hazard on the right. Sprinkler lines are spaced 12'-0" apart with sprinklers on lines spaced 10'-0"

for a sprinkler head spacing of 120 sq. ft. per head.

Assume a design density of .20 gpm over the most remote 1,500 sq. ft. is used what would be the minimum amount of water that would be

needed to be discharged from sprinkler #1?


3/9



Obvious Sprinkler #1 covers 120 sq. ft.. With

density of .20 amount of water required is

120*0.20=24.0 gpm.

If using a 1/2" sprinkler with a k-factor of 5.6 what

pressure is required at the sprinkler to discharge

24.0 gpm?

Answer: 18.4 psi

Review and practice:

Consider the layout below:


4/9



Sometimes it is not possible to symmetrically lay

out a sprinkler system and often enough we end

up with a system as shown below where distance

between sprinklers and wall is not one half the

distance between sprinklers.

What is the square footage area covered by

sprinkler #1?

From NFPA #13 - 2002 Edition

8.5.2 Protection Areas per Sprinkler.8.5.2.1 Determination of the Protection Area of

Coverage.

8.5.2.1.1 The protection area of coverage per

sprinkler (As) shall be determined as follows:

(1) Along branch l ines as follows:

(a) Determine distance between sprinklers (or to

wall or obstruction in the case of the end sprinkle

on the branch line) upstream and downstream.

(b) Choose the larger of either twice the

distance to the wall or the distance to the nex

sprinkler.

(c) This dimension will be defined as S.

(2) Between branch lines as follows:

(a) Determine perpendicular distance to the sprinkler on the adjacent branch line (or to a wall or obstruction in the case of the last

branch line) on each side of the branch line on which the subject sprinkler is positioned.

(b) Choose the larger of either twice the distance to the wall or obstruction or the distance to the next sprinkler.

(c) This dimension will be defined as L.

8.5.2.1.2The protection area of coverage of the sprinkler shall be established by multiplying the S dimension by the L

dimension, as follows: As = S L

Sprinkler #1 covers 130 sq. ft..

If the required density is .20 gpm what would be the minimum required discharge from Sprinkler #1?

Answer: 130*.20=26.0 gpm

If using a 1/2" sprinkler having a k-factor of 5.6 what would be the required pressure at the sprinkler head to discharge 26.0 gpm?

Answer: (26.0/5.6)2=21.6 psi

What if we used a 17/32" "large orifice" sprinkler having a k-factor of 8.0 what would be the required pressure at the sprinkler head to

discharge 26.0 gpm?

Answer: (26.0/8.0)2=10.6 psi

Consider the drawing below.


5/9



What is the area covered by Sprinkler #1? Sprinkler #2? Sprinkler #3? Sprinkler #5? Sprinkler #6?

Referring to NFPA #13 Section 8.5.2.1.1 the area covered by sprinkler #1 is not 130 sq. ft. but 140 sq. ft.. Spacing of 140 sq. ft. is not

allowed as standard spray sprinklers have a maximum coverage area of 130 sq. ft..

In addition to Sprinkler #1 Sprinkler #2 and #3 each cover 140 sq. ft. which is not allowed when using standard spray sprinklers. Sprinkler

#5 and #6 each have a coverage area of 120 sq. ft..

What about the 1,500 sq. ft. area of calculation, how is that defined?

From NFPA #13 - 2002 Edi tion

14.4.4.1.1 Densi ty-Area Method.

14.4.4.1.1.1 Where the design is based on the density-area method, the design area shall be a rectangular area having a

dimension parallel to the branch lines at least 1.2 times the square root of the area of sprinkler operation (A) used, which shall

permit the inclusion of sprinklers on both sides of the cross main.14.4.4.1.1.2 Any fractional sprinkler shall be carried to the next higher whole sprinkler.

14.4.4.1.1.3 In systems having branch lines with an insufficient number of sprinklers to fulfill the 1.2 requirement, the design area shall

be extended to include sprinklers on adjacent branch lines suppl ied by the same cross main.

On the drawing below the green "Bounding Box" indicates the minimum area while the magenta colored "Bounding Box" indicates the

actual area to be calculated.


6/9



With each sprinkler covering 120 sq. ft. we need a minimum of 1,500/120=12.5 or13 sprinklers (always round up) located in our

calculated area.

What if we increase the area of application from 1,500 sq. ft. to 2,000 sq. ft.?

Figure 11.2.3.1.5 allows for our design density to decrease from .20 gpm to .19 gpm.

Below we find an example where the length of the calculated area is less than the stipulated area of 1.2 times the square root of the area.


7/9



For 1,500 sq. ft. the length should be 46.48 feet but the entire length of the branch line is only 43'-0" in which case we simply calculate all

sprinklers on the entire branch line.

If calculating to a density of .20 gpm what would be the minimum water required from sprinkler #1 (upper right)? Answer: 26.0 gpm.

While it is true the actual area covered is only 120.25 sq. ft. we are required to calculate twice the distance from a wall or the distance

between sprinklers which ever is greater. Twice 7'-6" (the distance from the wall) is 15'-0" so the area for calculations is

15'-0"*8'-8"=130.0 sq. ft.. 130*.20=26.0 gpm.

If calculating to a density of .20 gpm what would be the minimum water required from sprinkler #6 (upper left)? Answer: 26.0 gpm again.

While it is true the actual area covered is only 115.63 sq. ft. we are required to calculate twice the distance from a wall or the distancebetween sprinklers which ever is greater. Twice 7'-6" (the distance from the wall) is 15'-0" and with sprinklers spaced 8'-8" on the branc

line (use the 8'-8" between sprinklers on the line because it is more than twice the 4'-0" off the wall) so the area for calculations is

15'-0"*8'-8"=130.0 sq. ft.. 130*.20=26.0 gpm.

With a density of .20 gpm what would be the minimum discharge requirement from sprinkler #7? Answer: 12'-9*8'-8"=110.50 sq ft.

110.5*.20=22.1 gpm.


8/9



For light and ordinary hazards 1,500 sq. ft. is not always the minimum area of coverage. From NFPA #13 Section 11.2.3.2.3:

11.2.3.2.3 Quick-Response Sprinklers.

11.2.3.2.3.1 Where listed quick-response sprinklers, including extended coverage quickresponse sprinklers, are used

throughout a system or portion of a system having the same hydraulic design basis, the system area of operation shall be

permitted to be reduced without revising the density as indicated in Figure 11.2.3.2.3.1 when all of the following conditions are


9/9


wwwlayouttech com/calculation/calc1/calc1 htm

satisfied:

(1) Wet pipe system

(2) Light hazard or ordinary hazard occupancy

(3) 20 ft (6.1 m) maximum ceiling height

(4) There are no unprotected ceiling pockets as allowed by 8.6.7 and 8.8.7 exceeding 32 ft2 (3 m2)

The graph in the standard is hard to read so I prefer the formula where y is the percent of reduction to the design area and

x is the ceiling height.

For ceiling heights less than 10'-0", y=40

For ceiling heights greater than 20'-0", y=0

Exercize and practice:

An Ordinary Hazard Group II occupancy. Selecting a density of .20 gpm over 1,500 sq. ft. what can we reduce the area of applicaton to

with a ceiling height of 8'-6"? Answer: 900 sq. ft. Design would be to a density of .20 over a 900 sq. ft. area.

A Light Hazard occupancy. Selecting a density of .09 gpm over 2,000 sq. ft. what can we reduce the area of applicaton to with a ceiling

height of 12'-0"? Answer: 1,350 sq. ft. or a 32.5% reduction in area. Design would be to a density of .09 over a 1,350 sq. ft. area.

An Ordinary Hazard Group I occupancy. Selecting a density of .15 gpm over 1,500 sq. ft. what can we reduce the area of applicaton to wit

a ceiling height of 19'-9"? Answer: 1,119.38 sq. ft.. Design would be to a density of .15 over a 1,119.38 sq. ft. area.

An Ordinary Hazard Group II occupancy. Selecting a density of .20 gpm over 1,500 sq. ft. what can we reduce the area of applicaton to if

the eave height is 14'-0" and the building peak is 17'-6"? Answer: 1,068.75 sq. ft. because you always use the highest ceiling height in abuilding with a sloped ceiling. Design would be to a density of .20 over a 1,068.75 sq. ft. area.

We will get into steeply sloped ceiling heights later.

Calculation Menu
http://layouttech.com/calculation/calculation.htm

Hydraulic Calculation Tutorial #1

Documents

Transcript of Hydraulic Calculation Tutorial #1