By Elliott R. Berrin, P.E.
Copyright 1989 by Elliott R. Berrin, P.E.
The single most effective fire fighting tool available is the automatic sprinkler system. The advantages of sprinkler systems include:
Sprinkler heads create a heavy mist of fine water droplets that fall, by gravity, directly onto burning combustibles. The means of sprinkler actuation will be discussed later in this paper. These fine water droplets evaporate into steam when heated by the fire. Because the heat of vaporization of water is very high (980 BTU per pound of water), the heat of the fire is removed readily, as the water droplets easily evaporate into steam.
These water droplets have the effect of actually cooling the burning materials below their ignition temperature. When this occurs, unburned material immediately adjacent to burning material cannot be heated to its ignition temperature, and, thus, the fire extinguishes. While there are various types of automatic sprinkler systems, they all rely on this same principle of the "cooling effect."
In most sprinkler systems, a single sprinkler head, directly overhead, actuates, at the time of a fire. In many cases, this one head controls or extinguishes a fire; if it cannot, more heat is generated by the fire, and a second head operates. The combined effect of the two heads is to provide approximately twice the volume of water droplets and, therefore, approximately twice the amount of "cooling effect." This process can continue until the quantity of heat removed (in BTU's per unit time) by the droplets exceeds the rate of heat generation of the fire. When this happens, the burning material is cooled below its ignition temperature, and the fire is extinguished.
In the United States, there is no accurate statistical information on the effectiveness of automatic sprinkler systems. This is so because many small fires go unreported: a single sprinkler head can extinguish a fire in its early stages, a small amount of debris and water are removed, and that is the end of it. No Fire Department is ever called and no insurance claim (e.g., under the deductible) is ever filed.
Larger fires, of course, are in some manner, recorde d; these are the available statistics. Even these statistics are good; however, the National Fire Protection Association no longer reports them because of the known understating of the effectiveness of sprinklers.
In a worldwide search for more accurate statistics, Russell P. Fleming, P.E.,**1. Russell P. Fleming, P.E., Why Sprinkler Trade-Offs Work, September, 1980.* learned that in Australia and New Zealand, every fire is reported, by law. A study of this data yielded the information that 99.76% of fires in sprinklered properties are controlled or extinguished by the sprinklers. His actual figures are expressed in the following table.
| Number of Sprinklers Operating | Percent Extinguished or Controlled | Cumulative Percent Controlled | ||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| | | | | | | | | | | |
Fires Extinguished or Controlled = 99.76%
Unsatisfactory Performance = 00.24%
TOTAL 100.00%
As can be seen, most fires in sprinklered properties are extinguished before catastrophic proportions are reached. These are the statistics which this author [Berrin] relies upon when giving expert testimony in court.
One "side effect" of automatic sprinklers that is of great assistance to the fire investigator, is the fact that when sprinklers do operate properly, they have the effect of "freezing" the action early in the progress of the fire. That is, the evidence of cause and origin is not destroyed in the later stages of a fire, simply because the later stages never occur. One classic example of this occurred several years ago in an art gallery in mid-town Manhattan. The gallery was newly opened in a renovated brownstone. Automatic sprinklers were installed and new electrical wiring was installed behind new sheet rock interior partitions. Somewhere along the way, the electrician had erred during renovation, and, shortly after the grand opening of the gallery, arcing occurred between wires within a newly installed partition. The arcing ignited wood studs and the fire progressed inside the wall. Because there are no sprinkler heads within walls, the fire progressed for a short period of time. In short order, the heat resulting from the arcing and the fire caused the sheet rock partition to be breached.
Once the fire broke out into a room of the gallery, heat was able to reach a sprinkler head, which operated properly. As a result, the fire was extinguished with one head. Of more significance, however, is the fact that a large copper bead, on a wire in the wall, caused by the initial arcing, remained intact. This was near the bottom of a classic Vee pattern. This evidence of cause and origin was preserved solely because of the action of the sprinkler system.
Because of this sprinkler system, the gallery and much valuable art was saved, actual damage was small (in terms of both the physical extent of the fire and the dollar loss), and the evidence of cause and origin was preserved. This writer has observed the preservation of evidence by the automatic sprinkler system in other fires in different occupancies; take advantage of this by-product of the automatic sprinkler system.
Because of unique, or special, circumstances, there will be times when the common "wet pipe" sprinkler system found in the majority of properties is not a good choice for automatic fire protection.
Such unique circumstances include unheated buildings, refrigerated areas, freezer areas, occupancies with high combustible loading, and contents that are unusually susceptible to water damage. Thus, there are four basic types of sprinkler systems that can be installed individually, or in combination with each other.
A. THE WET PIPE SYSTEM.
This is the most prevalent system, with water in the piping at all times that the system is in service (ready and waiting for a fire). When a fire occurs, hot air created by the fire rises to the ceiling and heats a fusible link holding the head in the closed position. When the link reaches a pre- determined temperature, solder holding it together melts, and several components of the linkage, including a cap over the orifice (or nozzle), are forcibly ejected by water pressure. Water then flows out of the orifice, toward a deflector, which converts the solid stream of water into a fine spray which then falls, by gravity, onto the burning combustibles below.
Because water is already in the piping, under pressure, the instant that the fusible link operates, water spray is applied to the fire. The system is fast, simple and highly reliable.
At the beginning of the wet pipe system, where water initially enters from the source of supply, a device known as an alarm check valve is installed. It is the "heart" of the system, and as its name implies, serves two functions: it provides alarms and it is a check valve. It has fittings that permit the connection of both local and remote location (e.g., central station) alarms; and, it acts as a check valve, permitting water to flow in one direction only (toward the sprinkler heads).
B. THE DRY PIPE SYSTEM.
It is common knowledge that water freezes at 32 degrees fahrenheit . Thus, if a sprinkler system is to be located in a building where temperatures can go below the freezing point of water, a wet pipe sprinkler system would be inappropriate. The water would freeze, expanding in volume at the same time, into ice. The expansion would create tremendous pressures, and sprinkler piping would crack or split apart. When thawing occurred, the now liquid water would pour forth, creating damage to everything beneath the piping.
To provide safe, reliable, effective automatic sprinkler protection in areas subject to freezing, a dry pipe system should be installed. For our purpose, the dry pipe system is identical to the wet system, except that the water is replaced by compressed air, and the alarm check valve is replaced by a dry pipe valve.
When a fire occurs, the same sprinkler head as in the wet system operates in the same way, but compressed air is released from the pipes, instead of water. The function of the compressed air is to hold the dry pipe valve shut, preventing water on the other side from entering the system.
When sufficient compressed air is released, the pressure in the system drops to a point where it can no longer hold the dry pipe valve closed against the water pressure on the other side; it is then that water enters the system, flows through the piping, and is converted to spray by the sprinkler head deflector.
These systems are used only in areas subject to freezing because there are certain disadvantages to them:
C. THE DELUGE SYSTEM.
There are times when the combustible loading within a building becomes too high for effective extinguishment by a wet pipe sprinkler system. In other words, the contents within a building, when burning, can generate heat at a faster rate than the spray of one, two, or even ten heads can cool. This situation is described as the fire "outrunning the sprinkler system." Typical occupancies where this can occur are aircraft hangars (jet fuel in the wings) and chemical warehouses (drums of flammable liquids). Such an occupancy can be protected by a sprinkler system which will discharge water spray from every head on the system simultaneously. This is done by the deluge system. This system is markedly different from the wet and dry systems, in that the sprinkler heads are all open (no fusible links and no caps to hold back water), the content of the piping is ordinary air at atmospheric pressure, and the entire system is turned on when a "deluge" valve at the beginning of the system (water sup ply side) automatically opens. This valve is actuated by detectors (usually heat detectors) mounted at ceiling or roof level, above the sprinkler heads.
When a fire occurs, hot air rises and "trips" a detector; a signal is sent to the deluge valve, which opens automatically, admitting water to the system. Now water is emitted from every head in the system because they are permanently open heads. Such a system is intended to prevent the fire from outrunning the actuation of sealed sprinkler heads.
The main disadvantages of this system are:
D. THE PRE-ACTION SYSTEM.
Certain occupancies are sensitive to water damage from even the small amounts of spray from a wet system. Examples include libraries and museums with irreplaceable documents and objects. In this situation, it is desirable to permit sprinkler system operation only when there is no question that a fire is really occurring. This is a condition where an accidental operation of a sprinkler system in the absence of a fire is intolerable; yet, to not provide sprinkler protection would result in a total loss when a fire really occurred.
One way to achieve a high degree of reliability (i.e., extremely small probability of accidental spray discharge, is to require that two events occur prior to operation of a sprinkler system. If we were to design the system so that a fusible link on a sprinkler head and a heat detector had to operate before water is applied, then we have achieved a high degree of reliability.
This is accomplished by installing, believe it or not, the above described deluge system, but with sealed heads. Now two conditions must exist before water is released:
If only one of the above events occurs, no water spray will be applied to the contents of the building.
The disadvantage: cost. This is an expensive system, but necessary in specific circumstances.
VI. PIPE SCHEDULE VERSUS HYDRAULICALLY BALANCED SPRINKLER SYSTEMS
Regardless of whether a sprinkler system is of the wet, dry, deluge, or pre- action type, the sizes of piping used are important if an acceptable flow rate (gallons per minute) and pressure (pounds per square inch) are to exist at each sprinkler head. If pipes are too small in diameter, or too long in length, friction losses will occur to such an extent that insufficient water will come out of each sprinkler head. Historically, there are two methods of designing sprinkler piping to obtain a sufficient flow of water at each head.
A. The Pipe Schedule is the older of the two methods, and consists of looking up the permitted pipe sizes in NFPA13, Installation of Sprinkler Systems.**2.National Fire Protection Association, Quincy , MA, Standard for the Installation of Sprinkler Systems.* This publication has, since its inception many years ago, provided for minimum permissible pipe sizes for "light, " "ordinary," and "extra" hazard occupancies. These schedules state how many sprinkler heads are permitted on a given size of pipe, and how close together the sprinkler heads must be placed.
B. The newer, Hydraulically Balanced System is the result of a different approach to sprinkler system design. In this method, the specified parameters are how much spray can be applied per unit of floor area, and over what total floor area it must be applied. A typical example would be that a restaurant kitchen, with its grease deposits and garbage, would require 0.2 gallons per minute per square foot of floor area over "any, including the most remote," 1500 square feet.
This statement says that the sprinkler system must be capable of providing 2/10's of a gallon per minute over each square foot of floor area, and that the system must do this simultaneously over any 1500 square foot area.
This approach specifies an amount of water needed to extinguish a fire in a particular occupancy. At the same time, the engineer is given considerable latitude in matters of head spacing, pipe diameters, and pipe lengths. The correct design is the one that delivers the required water spray, at the least cost.
Now that we have a basic understanding of how automatic sprinkler systems work, and how highly reliable they really are, lets examine the means by which they can fail, thus permitting a building to burn to the ground.
A. A shut water supply valve is the most obvious way that an automatic sprinkler system can fail; if there is no water to the system, the fire cannot be extinguished. When this happens, it is usually because the valve was shut during maintenance or repair procedures and forgotten.
B. A broken water main. If city water is the sole source of supply, and the authorities are in the midst of repairs, water is unavailable to sprinklers.
C. Gravity, suction, or pressure tank empty, resulting from a failure of the responsible parties to periodically check the tank.
Again, if there is no water, we don't really have a sprinkler system.
D. Failure of a fire pump to start automatically. If it is electric motor driven, a power failure would incapacitate the pump ; if it is diesel engine driven, poor maintenance, dead batteries or lack of fuel would prevent operation. All privately owned fire pumps should be tested weekly by turning them on and running them for 15 to 30 minutes.
E. Pump shut down for maintenance or repairs.
F. Obstruction of public or private water mains. Some unusual items have been found in piping after a catastrophe: stones, shoes, and a workers lunch pail are a few examples. Mains must be flushed clean when installed, repaired, or replaced. G. Obstruction of sprinkler branch lines and cross mains due to corrosion and tuberculation. As pipe ages, deposits, sediment, and corrosion build up on the interior walls of piping. The effect is to reduce the diameter of the piping available for water flow. Periodic flushing of pipes as recommended in NFPA 13A, "Maintenance of Sprinkler Systems,**3.National Fire Protection Association, Quincy, MA, Recommended Practice for the Inspection, Testing and Maintenance of Sprinkler Systems.* should be performed. After a catastrophe, branch piping should be examined internally for such buildup.
H. Old style sprinkler heads (not the modern, spray sprinkler head ) which are still found in service today, are not capable of providing the fine spray required for an effective system. These heads should have been replaced before the fire.
I. Corroded or painted sprinkler heads cannot respond efficiently or quickly (or may not operate at all) because corrosion or paint acts as a heat insulator preventing the fusible link from separating. Corrosion is a problem in corrosive atmospheres, such as acid bath rooms.
J. A defective dry pipe valve caused by corrosion and accumulation of sediment internally that results in the valve being stuck in the "set" or closed position. Dry pipe valves must be cleaned periodically in accordance with NFPA 13A.
K. Freezing of water left anywhere in a dry pipe system. The resulting ice is 100% effective as an obstruction to the flow of water.
L. Partial sprinkler systems are foolhardy. Since we cannot predict where a fire will start, every portion of an occupancy must be protected. The most common areas left unprotected are under wide cutting tables in a garment worker, under wide conveyor belts in a factory or warehouse, small offices erected in the middle of a factory, and under mezzanines. Wherever there are combustibles in occupancy and construction, there should also be sprinklers. A fire originating in an unsprinklered area will continue to enlarge in that unprotected area. Surrounding sprinklers will eventually operate ineffectively, because the spray cannot reach the burning materials, while causing water damage to non-burning contents.
M. Overloading of combustibles. For example, a sprinkler system hydraulically balanced for a metal worker cannot be expected to protect flammable liquid storage. When a change in occupancy occurs, the sprinkler system must be analyzed for adequacy, and modified as necessary. Underwriters who charge sprinklered rates in this situation are inviting financial disaster.
N. A water supply that is simply not capable of supplying the necessary flow rate (in gallons per minute) or pressure (in pounds per square inch) for an otherwise well designed sprinkler system.
Installing a correctly designed hydraulically balanced system is to no avail, if the water is not there in sufficient quantity to flow through the pipes.
Although a separate topic altogether, alarm systems deserve some comment now: even if something is wrong with a sprinkler system, that could lead to a catastrophe, detecting the condition before, or in the early stages of a fire, just might enable human beings to correct the situation before it is too late.
The following conditions of sprinkler systems and their water supplies should be supervised at a location that is constantly attended:
As stated at the beginning, sprinkler systems are extremely reliable: they rarely fail. However, when they do fail, one or more of a multitude of reasons for failure do exist. It is important to investigators, adjusters, attorneys, and underwriters to determine why they have failed in a specific fire. A thorough engineering examination of the sprinkler system and water supplies is required to determine why a sprinklered property burned to the ground.