TAKING THE PRESSURE OFF
HYDRAULICS
Over the past 10 years the fire service has experienced radical changes in water delivery methods. Large-diameter supply hoselines, l 3/4-inch and 2-inch attack lines, and advances in nozzle design have helped fire departments deliver more water to a fire more efficiently than ever before.
Unfortunately, these advances have coincided with a general trend toward an overall reduction in available fire scene suppression personnel. Although pumpers carry ing more than 1,000 feet of large-diameter hose and capable of pumping more than 1,500 gpm are becoming more common in the nation’s fire departments, more often than not these units are arriving on the fire scene staffed with only two or three firefighters.
Recent surveys show that most of the offensive interior firefighting in this country is accomplished w ith two or three firefighters attacking a fire with a medium-diameter hoseline flowing 95 to 125 gpm. While this flow’ range generally has proven adequate in the past, many departments report that interior structure fires are becoming more intense, generating more heat than 125 gpm can suppress. Researchers believe there are three major causes of this alarming trend: increased loading of flammable contents within structures, especially dwellings; extensive use of plastic and other synthetic materials as structural building materials, furniture, plumbing, and interior finishes; and changes in building construction methods, especially the widespread use of lightweight roof and floor trusses, which expose more surface area of the material to combustion than do older construction methods.
Many departments have experimented with new tactics to combat this overall increase in fire volume. Most of these new tactics call for the application of greater amounts of water. However, the traditional method of simply increasing the number of lines to increase overall flow at a fire scene is not proving practical because of the trend toward manpower reduction in both paid and volunteer departments.
A department attempting to maximize attack potential with higher flow through larger hose and nozzles must take into consideration the fire-flow stress imposed on the firefighters operating the line in hostile environments. Not only must the crew contend with excessive heat and reduced visibility, they also must exert themselves physically while advancing the charged hoseline and then expend more effort counteracting nozzle reaction. It all comes down to safety and effectiveness. Chief Neil Svetanics of the St. Louis Fire Department puts it succinctly when he says, “It’s not easy to advance a line while nozzle pressure has you pinned against a wall in the stairwell.”
(All photos by author.)
LOW-PRESSURE FOG NOZZLES
EXPERIMENTS WITH LOW-PRESSURE FOG NOZZLES
While officials grapple with the personnel replacement problem, the Los Angeles City, Chicago, and San Francisco fire departments, among others, have been trying to solve flow/ personnel problems via improved waterflow hardware. Captain Gil Moreno, head of San Francisco’s Equipment Bureau, says his department’s testing of new hose and nozzles had a single goal: “We wanted a maximum of flow using the least amount of people.”
The Los Angeles City Fire Department began an extensive testing program in the early 1980s that addressed the same concerns. The evaluation group, under the direction of Battalion Chief Claude Creasey, attempted to determine the maximum effective flow that could be safely controlled by two firefighters using a 2’/2-inch hoseline. Tests were devised to simulate offensive firefighting as closely as possible.
In one of the tests firefighters were told to move in different directions while attempting to keep the stream within a 15-inch target opening on a large panel. Firefighters of different physical sizes and varying firefighting experience were rotated on the nozzle and the results were videotaped for review. Other tests included measuring the amount of nozzle reaction generated by various nozzles and pressures using a device created by the department.
Chief Creasey and his evaluation group found that the most effective streams that two firefighters could handle were produced by lowering the nozzle pressure on a standard combination fog nozzle. He knew that by lowering the nozzle pressure, flow was reduced; therefore, in 1984, after working with nozzle manufacturers, the department purchased nozzles that would flow their rated 250 gpm capacity at 75 psi rather than the traditional 100 psi. Success of the 2½inch program led to further testing utilizing the low-pressure concept on smaller attack lines.
At about the same time these tests were taking place, the Chicago Fire Department evaluated a number of different nozzle styles to facilitate the adoption of a new attack program they called “quick water.” Through testing they found that by changing to D/4-inch hoseline and high-flow combination nozzles, they could put more water on the fire than with the 1½inch hose and conventional nozzles they were then using without increasing the physical effort expended by firefighters to stretch the line.
The department experimented with automatic and fixed-gallonage nozzles and rated each on firefighting effectiveness, maintenance considerations, cost, and training. It determined that given the average volume of dwellings and other hazards found within the city, 150 gpm was an ideal target flow for a single interior attack hoseline. The department also found that the nozzle reaction caused by the 150 gpm flow’ was the most a single firefighter could handle safely and effectively in a fire situation. At the suggestion of a manufacturer, they purchased low-pressure fog nozzles that were designed to flow’ the 150 gpm rate at 75 psi nozzle pressure, which reduced nozzle reaction even more.
The net effect was that the new hardware could deliver more water on the fire than the previous system, while the reduction in back pressure had the desirable effect of reducing firefighter stress. The reduction in nozzle operating pressure also had the desirable effect of providing stream qualities such as greater reach in highheat situations, which proved ideal for their methods of direct interior attack.
Additionally, both Los Angeles City and Chicago were concerned with initial purchase price and maintenance considerations. Because fixedgallonage nozzles have fewer internal moving parts than automatic or adjustable-gallonage designs, they predicted that the simpler constructions would prove to have a lower repair rate. The Los Angeles City nozzle is constructed of only four parts, an important consideration during anticipated maintenance evaluations.
As a result of their extensive evaluation and testing, both Chicago and Los Angeles City departments were among the first to utilize low-pressure fog nozzles on all medium-diameter attack handlines.
READVOCATING THE SMOOTH-BORE
In another attempt to reduce firefighter stress and increase firefighting effectiveness, a number of departments are readvocating the use of smooth-bore tips in place of combination nozzles. For years relegated to the back of dusty storerooms, smoothbore nozzles are being placed back in service in increasing numbers. Now made of machined aluminum, smooth-bore nozzles are less expensivc, have no moving parts, and are almost impossible to clog. The smooth-bore exhibits less nozzle reaction compared with combination nozzles at the same flows. Its stream can penetrate deep into high-heat areas and, unlike fog streams, tends to disrupt the fire’s thermal balance and create unwanted steam less. It has developed an enthusiastic following among progressive departments.
Departments such as New York City, which uses the smooth-bore as its standard attack line nozzle, report great success with the nozzle, citing its high knockdown potential because it is able to penetrate and cool intense fires. Lieutenant Steve Krupa of FDNT’s Bureau of Research and Development says that smooth-bore nozzles used on l 3/4-inch attack hoselines work well for his department’s style of aggressive firefighting and that even though company officers have a choice of fog or smooth-bore, the smooth-bore is the most popular. However, combination tips are immediately available for situations such as flammable liquid fires in which spray streams may be better suited for the task.
HURDLING THE PARADOX
The arguments advocating the use of certain styles of nozzles for interior fire attack are far from settled; each type has a distinct, positive advantage. One of the biggest drawbacks in using the standard combination fog nozzle is that the high nozzle pressure it requires for proper operation causes high nozzle reaction at high flows. On the other hand, many departments do not feel comfortable using straightbore nozzles on attack lines. Although they appreciate that the lower pump pressures required for smooth-bore nozzles cause less nozzle reaction and therefore reduce operator stress, they do not want to give up the versatility offered by variable stream patterns of fog nozzles. For these departments, the testing conducted by Los Angeles, Chicago, San Francisco, and other fire departments could have an impact on future firefighting tactics. The St. Louis Fire Department, for one, recently completely revised its tactics for aggressive interior attacks whereby 1 ⅛-inch hoselines are being used at lower engine pressure and attacks made with a combination of spray and smooth-bore nozzles.
Meanwhile, attempts to find answers to the paradox of flowing more water using less personnel go on. Perhaps not so surprisingly, a number of widely separated departments have begun reaching similar conclusions almost simultaneously and have worked in cooperation with manufacturers to develop a low-pressure fog nozzle. Major nozzle manufacturers now market fog nozzles that operate at pressures rivaling those used for straight-bore nozzles. A low-pressure fog nozzle is simply one that flows its rated capacity at 75 psi rather than 100 psi.* Low-pressure nozzles are usually of a fixed-gallonage style, having flows of 150 to 200 gpm for 1 3/4inch lines and 250 gpm for 2 1/2-inch lines. Some manufacturers of automatic nozzles now deliver their products with the automatic flow control set to activate at 75 psi rather than 100 psi if requested. All low-pressure models presently on the market have constant flow for all stream positions and all but the Los Angeles nozzle have a flush feature.
ONE COMPANY’S TESTS
My own fire company recently evaluated a group of low-pressure fog nozzles to determine if they could add to our firefighting efficiency. Because of limited personnel availability, especially during the day, we sometimes experienced delays in quick, effective application of high-volume interior attack streams.
The company responds to more than 600 fire runs a year in a district that includes factory buildings of mill construction, threeand four-story baloon-frame apartments, Victorianstyle singleand multiple-family dwellings, a railroad yard, and newer types of single-story mercantile and high-rise construction. We were using automatic nozzles on our preconnected lines with acceptable results so we were able to approach the tests without having to prove a concept or advance a cause.
Using a portable flow meter and special line gauges, we evaluated three fixed-gallonage nozzles set by the manufacturers to flow 150 gpm at 75 psi. After calibration, all three were flowed together off a three-way wy e. The shapes and actions of the streams did differ between each nozzle; however, we found the reach of the lowpressure streams to be about the same and the spray pattern differences of little consequence in practical firefighting.
To provide a side-by-side comparison, a manufacturer supplied a matched set of 150-gpm nozzles, with one calibrated to flow its rating at 100 psi and the other set at 75 psi. Testing at identical flows found the low-pressure nozzle to exhibit less nozzle reaction, making it much easier to handle. We rotated company members on each nozzle to verify this result.
We were surprised to find that the droplet size exiting from the lowpressure nozzle was much larger than the size from the high-pressure nozzle. The larger droplet size allowed more water to travel farther; the stream more efficiently penetrated into high-heat areas and was not degraded by thermal currents as much as higher-pressure streams. When comparing streams from smooth-bore nozzles and low-pressure nozzles side by side, we were impressed by their similarity in reach and compactness of the water column.
While the evaluations were in no way comprehensive, they did prove the following points to company members:
* No one seems to know why 100 psi was chosen as a standard operating pressure for spray nozzles. All nozzle manufacturers even admitted that they had no idea why that pressure was chosen. Some speculated that early nozzle designs had rather small internal passageways and the high pressure was needed to overcome friction loss within the nozzle body. The National Fire Protection Association Standard 1964 calls for rating spray nozzle flows at 100 psi operating pressure, but this standard is only advisory, allowing departments to determine their own operating pressures.
- The low-pressure fog nozzles had considerably less reaction force and are much less stressful to operate at the same flows than automatic, fixedgallonage, or adjustable-gallonage fog nozzles designed to operate at 100 psi.
- Because the nozzles operated at less nozzle pressure, the engine delivery pressure was reduced, causing the hose to be less stiff, making it much easier to maneuver in tight hallways and small rooms. We also found that the reduction in operating pressure did not increase the tendency for the hose to kink.
- If the nozzle was overpressurized, the flow and reaction would increase, but only up to about 100 psi —no worse than the reaction from fog nozzles designed to operate at a higher pressure.
- An important discovery was the low-pressure nozzles’ larger droplet size. When compared side by side with nozzles operating at 100 psi, the larger droplets were not affected as much by wind and heat and allowed more water to land on the test target, especially in the narrow’ fog positions. Later, in actual fire situations, we found that more water was being delivered directly to the flame/fliel interface, especially in high-heat situations.
- When compared side by side with smooth-bore tips operating at the same flows, the low-pressure fog nozzles in the straight-stream position exhibited similar characteristics of stream compactness and delivery of water on target. This observation was attributed in part to a noticeable decrease of the amount of air entrained with the exiting w ater, which is common in all fog nozzles. This fact was verified by several nozzle manufacturers.
Our company does not advocate the use of indirect attack w ithin a fire building. Our personnel are taught to apply the water directly on the burning material, not into the overhead. We do this because directing the stream at the burning material will extinguish a fire more quickly and will not subject personnel to high heat and thermal burns caused when water applied overhead rapidly expands into steam and then travels downward on top of the nozzle crew. There are times w hen the nozzle must be worked across the ceiling to distribute the water over a wider area. We found the low-pressure narrowfog and straight streams to be more effective for this tactic than highoperating-pressure nozzles, because the larger, heavier water droplets dropped to the burning material, with very little being converted into unwanted steam.
Our evaluation reinforced the results of tests in other departments and demonstrated the advantages of the low-pressure fog nozzle’s ability to deliver a maximum amount of water with a minimum amount of firefighter stress and in an extremely effective form.
We also found during our evaluation that differences in flow’ and nozzle pressure were detected among various fixed-gallonage and automatic nozzles. Nozzles with identical manufacturers’ flow ratings performed differently at the same engine pressures and hose lengths, indicating the need for individual departments to perform their own evaluations to ensure that their particular needs are met by the equipment. No progressive department would put a pumper in service without testing its waterflow capabilities; likewise, no department should put into service such an important piece of equipment as a nozzle without testing its waterflow capabilities. We found a portable flow-metering device to be invaluable in our evaluations.
CALCULATING REACTION FORCE
Newton’s Third Law of Physics states that for every action there is an equal reaction in the opposite direction. As a stream flows from a shaping device, it creates a force in the opposite direction known as nozzle reaction. The amount of reaction depends on two things: the weight, or amount of water flowing, and the amount of pressure forcing it through the device.
These reaction forces can be mathematically calculated by using accepted formulas—one for smooth-bore nozzles and one for combination nozzles.
The smooth-bore reaction force fomula is expressed as RF = 1.57 X (BD X BD) x NP, where RF = nozzle reaction in force/ pounds; BD = bore diameter in inches; and NP = nozzle pressure.
The combination nozzle reaction force formula is expressed as RF = GPM X NP x .0505, where RF = nozzle reaction in force/pounds; GPM = gallons per minute flowing; and NP = nozzle pressure.
Using these formulas, Akron Brass Company calculated the following chart showing the reaction force of various gallonages and pressures. Tests have shown that an average 175-pound Firefighter can handle about 60 pounds of nozzle reaction force during offensive operations. How long the firefighter can handle the reaction depends on additional body weight and physical condition. If a department thinks it is flowing 200 gpm on a handline yet the line is easily handled by one firefighter, the flow rate is probably much less than was computed. The easiest and most effective method to check actual flow is with a flow meter.
Manufacturers state that many fixed-flow, constant-gallonage fog nozzles now in service can be converted easily to high-flow/low operating pressure use simply by changing the tip baffle, adding or removing a baffle spacer, or returning the unit to the factory for recalibration. In some cases, changing the nozzle configuration can be done by the department, but flow test equipment should be available to help calibrate the modifications. All low-pressure fog nozzles, whether purchased new or modified from existing nozzles, should be permanently marked with their flow rating and operating pressure. This will help prevent identification problems in the future in case they are mixed with nozzles designed to operate at higher pressures.
It’s interesting to note that the development and introduction of low-pressure nozzles were not a result of the nozzle manufacturers’ marketing departments. The low-pressure nozzle was a result of a number of fire departments requesting a device to meet the challenge of allowing fewer personnel to safely handle higher fire flows.
In fact, while all manufacturers offer versions of low-pressure nozzles, their sales forces have tended to treat them as the dowdy, ugly stepsisters— keeping them hidden from view unless they have a chance for a quick, arranged marriage. Since many distributors may not be familiar with the existence of low-pressure nozzles, equipment officers may have to expend a little extra effort to get ahold of the devices for research, evaluation, and purchasing.
If combination fog nozzles are preferred for interior attack, lowpressure nozzles at 75 psi operating pressure offer the following advantages over combination nozzles operating at 100 psi:
- They have less nozzle reaction force at the same flows than nozzles rated at 100 psi operating pressure.
- There is less air mixed with the water stream, which tends to increase the stream’s penetration into hot fire areas.
- The spray pattern’s water droplets are larger and tend not to vaporize as easily in undesired, overhead areas.
- Because of the lower operating pressure, the hose is not as stiff, allowing it to be more easily maneuvered inside buildings.
- Fixed gallonage versions are relatively inexpensive, they’re more durable, and firefighters are more easily trained in their use than with automatic or adjustablegallonage nozzles.
- In actual use, operators will flow the rated gallonage rather than make attempts to reduce the reaction force by reducing pump pressure or by partially closing the shutoff. Both actions will reduce nozzle flow rate considerably.
A battering ram can easily poke an inspection hole in a ceiling, but that does not mean it is the best tool for the job. Low-pressure fog nozzles may not be ideal for all situations; however, as proven by many departments, they offer some great advantages to help make a fire suppression crew more efficient in performing knockdown while helping reduce firefighter stress.
