A FAST-RESPONSE RESIDENTIAL SPRINKLER HEAD
FIRE PROTECTION
Tests show that this head with special alloy sensor and actuator can open in one-third the time of an ordinary head.
The need for fast-response fire sprinklers in residential applications has been well established in recent studies. The common commercial sprinklers, intended for industrial use, prove to be too slow to maintain a survivable environment and prevent fire spread in the residential setting. Due to relatively small residential room sizes, the concentrations of carbon monoxide and smoke, along with the temperature, soon rise to fatal levels if fire extinguishment is not begun promptly.
On the other hand, early sprinkler actuation leaves a smaller, less-involved fire to combat, reducing the probability of excessive fire damage due to flashover. The cooling effect of the spray minimizes the possibility of opening additional sprinklers, thus concentrating the water in the area of fire origin and preventing unnecessary water damage.
USFA contract sponsored research
As a part of the United States Fire Administration’s effort to curb residential fire losses, Battelle’s Columbus Laboratories was awarded a contract to develop a residential sprinkler head using nitinol as the sensor and actuator. A previous program at Battelle had identified nitinol as a promising new method of fast-response heat sensing for sprinklers.
Nitinol is the generic name given to a series of alloys that are compounds composed of approximately 55 weight percent nickel and 45 weight percent titanium. Nitinol’s unique property is that it can be given a so-called memory shape by annealing it at about 900 degrees F. If it is then deformed, it can be made to return to the memory shape by the application of heat. As nitinol returns to its memory shape it can exert considerable force and do very significant mechanical work. This recovery of the memory shape is the sensing and actuating phenomenon used in the nitinol sprinkler where the hot gases of combustion created by the fire serve as the heat source.
There are several benefits to using nitinol as a means of achieving fast response. It serves as combined sensor and actuator since it actually converts thermal energy to mechanical energy. Its strength properties are much better than solders, and its energy output greater than bimetal elements or wax motors of equally low response time. The available energy per unit volume is also superior to other thermal-tomechanical energy conversion devices. In addition, it is estimated that less than $1 worth of wire will go into each sprinkler head.
How it works
The basic configuration of the nitinolactuated sprinkler is derived from that of the existing commercial model (Grinnell Duraspeed) that uses a pair of soldered copper cups as the heat sensor, with a lever and column arrangement to amplify the retaining force of the soldered joint.
The nitinol element is in the form of a 0.058-inch diameter wire, anchored to the deflector plate in a vertical arrangement with its upper tip serving to lock the lever in place. The column and lever are made from spring phosphor bronze and both have holes in them to minimize blockage of hot air flow to the nitinol. The vertical orientation of the wire puts its long dimension always perpendicular to hot air flow in the ceiling area.
When the nitinol is heated, it contracts and releases the lever which, along with the column and orifice seal, falls away and allows water to flow. Actuation temperature has been verified at 134 to 135 degrees in water-bath tests.
To determine sensitivity, “plunge tests” were conducted in which the prototype heads were placed into a 275 degree air stream moving at 245 feet per minute, and the time to actuate was measured. Where the standard commercial 135 degree sprinklers will require a minute or more to respond, the nitinol actuator will trigger in one-third that time. Room-fire tests at a recognized testing laboratory confirmed the fast-response characteristic. Eye-level temperatures beneath the sprinkler nearest the fire never exceeded 115 degrees, and ceiling temperatures directly above the fire peaked at 230 degrees.
Can turn on or off
Along with its ability to change shape in response to a temperature increase, nitinol can be conditioned to return to the original shape when the temperature drops. The amount of resulting motion (and energy output) over a relatively small temperature change is quite significant. This characteristic of nitinol enables it to be used as a sensor-actuator in an on-off sprinkler. Such an on-off design is shown. Like the on-only design, it incorporates a lever and column to provide the primary sealing function. A butterfly valve, also activated by the nitinol, acts to shut off the sprinkler when the temperature drops. It will then reopen if the fire flares up again and the nitinol contracts.
The butterfly does not provide highpressure sealing like the orifice seal, but reduces the flow to a slow leak, cycling on and off as needed, until the water is manually shut off. This will serve to reduce water damage and usage, and help to maintain system pressure at those sprinklers still requiring it. The slow leak in the off position will not add significantly to water damage.
Though this application of nitinol is fairly specialized, its capability as a heat sensor and actuator is quite diverse. The actuation temperature is closely controllable over a very wide range by varying the composition and processing of the wire. Response time varies with wire diameter, and can be reduced to much less than that of the sprinkler, as long as the load on the contracting wire does not exceed 20,000 pounds per square inch of wire cross-section. The distance which the wire contracts is about five or six percent of its original length.
