Storage Facilities
High-Bay and Automated Storage

Warehouse-type facilities with rack storage
present a complex fire protection problem.
As indicated in the fire test data included
in NFPA 13, Standard for Installation of
Sprinkler Systems, and NFPA 30, Flammable
and Combustible Liquids Code,
large-scale fire tests have demonstrated
that many of these fires can become very
large and difficult to control within two
minutes or less.

Determining the correct
automatic sprinkler system solution can be
a challenge. The increasing popularity of
large storage warehouses that have rack
storage heights from 40 ft to over 100 ft is
increasing the level of complexity for the
automatic sprinkler system design.

Currently, very little full-scale fire testing
with automatic sprinklers has been done
with rack storage in excess of 40 ft, which
limits the automatic sprinkler choices and
protection schemes that are available for
these applications.

The complexity of protecting storage
occupancies with sprinkler systems seems
to originate with the large number of
sprinklers and categories available. For
those designing this type of sprinkler
system, it is important to understand that
the science and math involved is universal
and obvious to those trained in the field.

Sprinkler hydraulics, sprinkler spacing,
response times, fire control and fire
suppression are subfields of sprinkler
technology that use the same mathematics
and science that have been applied for over
100 years. There are six significant
characteristics in the manufacture of
automatic sprinklers that can be changed
in the manufacturing process to achieve
different performance. These are:
1. Thermal sensitivity
2. Temperature rating
3. Orifice size
4. Installation orientation
5. Water distribution characteristics
6. Special service conditions

The science of thermal sensitivity is well
documented. Transfer of heat governs all
aspects of fire, from ignition through
final extinguishment. Heat is transferred
by one or more of three mechanisms:
conduction, convection, or radiation.

Convective heat transfer (heated air
from a fire rising to the ceiling) is the
primary means by which a sprinkler is
activated. The heated air rises in a
plume to the ceiling. When the plume
hits the ceiling, it produces a ceiling gas
jet. The heat responsive elements of the
sprinklers within the jet are then heated
by conduction of the heat from the air.

When the heat responsive element
reaches its operating temperature, the
sprinkler will activate.
It is necessary to measure this process in
order to determine the thermal sensitivity
of the sprinkler. Thermal sensitivity is the
measure of how fast a heat responsive
element operates when installed in a
sprinkler. Factory Mutual developed a
method to measure thermal sensitivity that
is used today which utilizes the concept of
a response time index (RTI). The device
used to conduct this measurement is
commonly referred to as a “plunge oven”;
the test is referred to as an “oven heat test”
in Underwriters Laboratories Standard UL
199.

Within the plunge oven an air stream is
moving through a metal duct at a constant
velocity and temperature. The sprinkler,
which is at room temperature, is plunged
into the heated air stream. A measurement
is made of the time, in seconds, required
to raise the temperature of the heat
responsive element, approximately 63
percent of the temperature of the heated
air stream. This measurement is called a
“tau factor.”

Multiplying this factor by
the square root of the velocity provides the
number which is the RTI of the sprinkler.
RTI = TU1/2
T = tau factor
U = air velocity

NFPA 13 section 3.6.1(a)(1) defines the
response time for fast response sprinklers
as having a thermal element with an RTI
of 50 (meter-seconds)1/2 or less, while
section 3.6.1(a)(2) defines standard
response as having a thermal element with
an RTI of 80 (meter-seconds)1/2 or more.

The standard does not have a separate
defined RTI for quick response sprinklers.
Section 3.6.2.9 defines quick response as a
type of spray sprinkler that meets the
criteria of 3.6.1(a)(1), making it a type of
fast response sprinkler.

The sprinkler’s temperature rating can
have a dramatic effect on the sprinkler’s
performance in a storage occupancy fire. A
good example of this is illustrated in the
density/area curves for rack storage of class
I through IV commodities less than 25 ft,
and rack storage of class I through IV
commodities above 25 ft. Full-scale fire
testing in rack storage up to and including
25 ft indicated a more effective performance
for high temperature-rated
sprinklers when compared to ordinary
temperature-rated sprinklers. Ordinary
temperature-rated sprinklers typically
operated outside the actual fire area, which
will essentially rob water from the
sprinklers in the fire area.

High temperature-rated sprinklers did not operate in the
same manner, which resulted in greater
discharge in the fire area. As a result, the
density/area curves for rack storage of class
I through IV commodities up to and
including 25 ft allow a reduction in
density when using high temperature-rated
sprinklers.

Taking a look at rack storage of class I
through IV commodities over 25 ft, we see
the exact opposite, as NFPA 13 recommends
the use of ordinary temperature-rated
sprinklers over high temperature-rated
sprinklers. Again, this is the result of
full-scale fire testing. In these tests, in-rack
sprinklers (which are now required) show
their effect on the ceiling sprinklers.

Essentially the sprinklers operating at the
lower levels reduce the heat release rate of
the fire dramatically. Therefore, the
sprinkler operating area did not increase
with ordinary rated sprinklers as it did
with less than 25-ft storage without
in-rack sprinklers.

Sprinkler orifice size and K-factor determine
the amount of water that can be
discharged from a sprinkler at a given
pressure. The fundamental formula for
water discharge from an orifice can be
traced back to Torricelli’s Flow Theorem,
Q=av, discovered in 1644.8 This theorem
has been used by hydraulics engineers,
physicists, and all who have ever hydraulically
calculated a sprinkler system as
documented in Harold Wass’ 1999
second-edition book Sprinkler Hydraulics
and What It’s All About. Pages 45 and 46
describe how this formula is mathematically
converted to the standard formula
used in the sprinkler systems (shown
below):
Q=29.84cd2√p
This can be simplified further to:
Q= k√p

All sprinkler K-factors, whether or not
they are currently used in the sprinkler
industry and/or storage occupancies,
are developed from the same mathematical
formula.

Installation orientation and water
distribution are also critical to a sprinkler’s
performance. Installation orientation
can have an impact on water
distribution and the ability of the
sprinkler to control or suppress the fire,
when in close proximity to building
structural members. The concepts of
suppression and control are well-known
and are linked to the actual delivered
density of water (ADD) versus the
required delivered density (RDD). The
required delivered density will increase
as the heat release rate of the expected
fire increases. In other words, a more
intense fire will require more water for
control or suppression.

Current CMDA and In-Rack
Sprinkler Design for Rack Storage
of Class I through IV Commodities
Above 25 Ft

The requirements for
rack storage protection are contained in
chapter 16 for Class I-IV commodities
and chapter 17 for plastics and rubber
commodities. These chapters are separated
into sections for rack storage below
25 ft and storage above 25 ft. When
required to use control mode density/area
sprinklers, understanding the requirements
of NFPA 13 can be simpler when
rack storage is over 25 ft than when it is
less than 25 ft. For rack storage over 25
ft, the size of the design area and the
required density are established and very
few adjustments are made; whereas, they
are not for rack storage less than 25 ft.

However, additional difficulty comes in
with the requirements for in-rack
sprinklers.

When control mode density/area sprinklers
have been selected for installations
involving rack storage over 25 ft, in-rack
sprinklers are required. While the hydraulic
calculations are not overly difficult, the
figures describing the in-rack locations
within NFPA 13 will require the designer
to dedicate some time to review the charts
for a complete understanding of proper
in-rack sprinkler locations.

Chapters 16 and 17 also allow for the use
of control mode specific application
(CMSA) and early suppression fast
response (ESFR) sprinklers in specific
situations. Using these types of sprinklers
can provide several benefits, including
elimination of in-rack sprinklers and, in
some cases, a less costly design. The
primary limitation to CMSA and ESFR
sprinklers is the limit in storage and ceiling
height. If the rack storage and building
ceiling height exceed 40-ft storage and
45-ft high ceilings as defined in Chapters
16 and 17 of NFPA 13, they cannot be
considered for use. The designer is then
limited to the selection of CMDA
sprinklers with in-rack sprinklers. The
rules for installation will be found in
several different locations of the document,
including Chapters 8, 12, 16, 17
and 22.

NFPA 13 Chapter 12 allows the following
CMDA sprinklers to be used in
storage applications. Standard response K
5.6 sprinklers are allowed where the
required density is .20 gpm/ft2 or less.
Standard response K 8.0 sprinklers are
allowed where the required density is .34
gpm/ft2 or less. Where the required
density exceeds .34 gpm/ft2, standard
response K 11.2 sprinklers that are listed
for storage must be used. Quick response
sprinklers can be used if they have been
listed for storage applications.

The lowest density for this application
described in Table 16.3.1.1 is .25 gpm/ft2
limiting the sprinkler selection to standard
response K 8.0 sprinklers, no special listing
required, or K 11.2 and larger that have
received a listing for storage. The majority
of the required densities are in the range
where K 11.2 or larger would be the most
hydraulically efficient. Chapter 8, Table
8.6.2.2.1 limits the spacing of standard
pendent and upright sprinklers to 100 ft2
where the required density is greater than
or equal to .25 gpm/ft2. Given that most
of the required densities for this type of
storage application are .3 gpm/ft2 or
greater, the K 11.2 becomes a likely
selection for the ceiling sprinklers.

100 x .3 = 30 gpm
(30 gpm / 11.2)2 = 7.17 psi end-head
pressure

Section 8.13 provides the basic installation
rules, including the maximum protection
area for any single system of sprinklers in
racks which is 40,000 ft2 The in-rack
sprinklers can be either standard or quick
response and the allowed K-factors are 5.6,
8.0 and 11.2.

This section of Chapter 8 also clarifies that
the installation rules for in-rack sprinklers
do not follow the installation rules of ceiling
sprinklers. For example, in-rack sprinklers
are allowed to be placed less than 6 ft on
center and do not have to meet the
clearance and obstruction criteria of section
8.5. Chapters 12 through 20 will define the
positioning of these sprinklers.

Another commonly overlooked section is
8.16.1.6, which requires separate control
valves and drains for the sprinklers
installed in racks. The only exception to
this requirement is for systems with 20 or
fewer in-rack sprinklers. The intent of this
requirement is to allow for the isolation of
the in-rack sprinklers from the ceiling
sprinklers. Because in-rack sprinklers are
susceptible to damage due to their
location, it is essential to allow for the
ceiling system to remain in service while
repairs are being made.

When CMDA sprinklers are used with
in-rack sprinklers the ceiling sprinklers are
intended to protect the upper level of
storage, whereas the in-racks are intended
to control the growth and spread of fire
within the rack structure at the lower
levels.10 Chapter 22, section 22.8 requires
the pipe supplying the in-racks to be
hydraulically calculated and the water
demand for the in-racks to be added to the
ceiling sprinkler water demand over the
same protected area at the point of
connection. The demand shall be balanced
to the higher pressure.

Balancing can be done with the formula
listed below:
Qadj = (Ql) (√Ph/Pl)
Where:
Qadj = the adjusted flow into the lower
pressure line.
Ql = the calculated flow in the lower
pressure line.
Ph = the higher of the two pressures
Pl = the lower of the two pressures
Example: At the point of connection the
sprinkler system demands 984 gpm at
62.4 psi. The in-rack sprinklers have been
calculated to require 322 gpm at 57 psi.

To balance the pressure and determine the
adjusted required flow:
Qadj= (322) (√62.4/57)
Qadj = 336 gpm
336 gpm will be added to 984 at the point
of connect for a total water demand of
1,320 gpm.




High-Bay and Automated Storage

Concepts for the Future Special service conditions for sprinklers used in racks will define new technology in the future. Unique applications such as automated storage will likely require special sprinklers. Automated storage systems utilize computer-controlled equipment to move and stack com¬modity, typically on racks with narrow aisle widths, or on racks that move horizontally, which are referred to as movable racks. These buildings can be designed for storage heights over 100 feet. The racks are typically connected to the walls and roof to provide support for the structure. NFPA 13 will require a substantial amount of in-rack sprinklers to be installed in applications such as this. Manufactur¬ers will surely seek alternative methods of protection in the future.

In order to develop new products that can benefit the end-user and provide the necessary fire protection for this challenging environment, manufac¬turers will look to expand upon the known math and science that is well-established within the sprinkler industry. Rack storage schemes using horizontal barriers have a history of success and have been well-docu¬mented within NFPA 13. This type of concept separates the rack storage into more manageable areas where fire control or suppression is more easily achieved. Viking worked with a European furniture manufacturer in 2005 to conduct testing of rack storage using vertical barriers as an alternative to existing fire protection schemes defined within NFPA 13. While the tests were successful, Viking sought improvement using the barrier concept. Seeking improved results leads to the concept of using horizontal barriers within the racks with improved in-rack sprinklers. The horizontal barriers would be placed at different elevations above the picking level of the forklift operators. This configuration will eliminate the flue space to the ceiling, helping toprevent the fire from spreading vertically, which will assist in con¬taining the fire growth. It also reduces the volume of commodity being protected and exposed to activated sprinklers.
The European furniture manufacturer test used K11.2 extended coverage sprinklers as the in-rack sprinklers. The flatter spray pattern helped to negate the obstructions presented by the rack structure and helped to reduce the overall amount of in-rack sprinklers required. The tests were conducted with the sprinkler flows at 60 GPM and 80 GPM. The sprinklers were positioned with the deflectors 1¼ inches below a simulated rack beam, and the framed arms were positioned both parallel and perpendicular to the simulated rack frame.
The test resulted in water spread within simulated rack structure. The discharge spray was able to discharge in rack structure and apply water to face of rack. Some dry spots were visible at 60 GPM discharge. Relative¬ly no dry spots at 80 GPM discharge. The results were relatively equal with the frame arms parallel or perpendicu¬lar to the simulated rack frame.
The results of this testing will likely result in an extended coverage in-rack sprinkler designed to provide down¬ward thrust in the flue space and spread water to the face. The use of extended coverage sprinkler technol¬ogy will continue to expand in use within the storage market. This type of technology used in storage can expand upon the same concepts and rules established in the 1980s for ceiling protection, such as the square coverage areas for pendent and upright sprin¬klers. As described in the NFPA 13 handbook, using the square coverage area will promote inter¬changeability and allow positioning on the branch lines without regard to the sprinkler frame arms. Those familiar with sprinkler design will see the natural benefit to such a concept.

Developing a concept for the sprin¬kler system installations in warehous¬es will likely require different ap¬proaches and the use of new or existing technology to achieve improved performance. It is highly likely that these new protection schemes will use horizontal or vertical barriers for compartmentation. Ideas that are being evaluated include the use of interstitial space sprinkler tech¬nology for in-rack sprinklers. These sprinklers are designed to be installed in small combustible concealed spaces where there is limited space for water distribution to develop. The flat deflector technology combined with larger K factors and fast response thermal elements may address the installation issues encountered with in-rack sprinkler.
Another concept is to combine fast response thermal elements, K factors of 11.2 or larger, varying temperature ratings with the deflector technology used for suppression and specific application technology for in-rack sprinklers. This type of technology may result in im-proved fire control or suppression with a limited amount of sprinklers. Used in conjunction with horizontal barriers, this may result in lower overall water require¬ments and localize the number of sprinklers operating to a very small area of commodity. Other sprinkler technol¬ogy being considered included the use of directional type sprinklers, such as sidewalls and flat spray nozzle within rack storage. Each one of the existing technol¬ogies has the potential to be modified based on the six characteristics we discussed earlier in order to be applied in a new application.
Based upon the full-scale fire testing for rack storage covered earlier, the benefit to larger volumes of water with fire control and suppression are obvious and docu¬mented by numerous research agencies. This, combined with fire modeling, paints a clearer picture of how dry sprinkler systems operate in fire condi¬tions. While the concept of fire modeling may sound new to some, using models to predict sprinkler performance on dry systems can be traced back to the early 1970s as discussed in an article “The Number of Sprinklers Opening and Fire Growth” by R. Baldwin and M.A. North, published in NFPA periodical Fire Technology. The models referenced in this paper predicted the number of sprinklers that would operate on a dry system as it relates to fire growth. The science used in these early models will continue to be used in research for dry systems in storage applications.
All current design criteria described within NFPA 13; actual full-scale fire testing to date; current fire modeling as indicted in the report released by the NFPA Fire Protection Research Founda¬tion in 2007, titled “Review of NFPA 13 Dry System Water Delivery Provi¬sions;” and fire modeling as described above indicate that more sprinklers will operate on dry sprinkler systems than wet sprinkler system during a fire due to the inherent water delay. With this knowledge and understanding of the science the use of concepts that address the large number of open sprinklers and larger volume of water needed for control and suppression will certainly be used in high warehouse storage applica¬tions in the future.
Conclusion As the construction of warehouses with rack storage of com¬modities over 40 feet continues to grow, the fire protection industry must develop a variety of new sprinkler system solu¬tions. We are fortunate that much of the way has been paved for us by the efforts of many professionals who have done the research. The math and the science will be consistent. What is old will be new again as manufacturers seek to apply fundamental principles discussed earlier to develop new products.

Selecting the most appropriate sprinkler to be used in storage applications can be a challenging activity. Designers should not allow themselves to become distract¬ed by the terminology and language of the installation standards. The designer should be confident that the science and math are universal and that many of the principles described above remain the same for all types of sprinklers.