By GERARD J. NAYLIS
A fire department arrives at a well-involved two-story garden apartment fire that is burning throughout a second-floor dwelling unit; fire coming from the windows has spread into the combustible attic space. The buildings are situated l,000 to l,200 feet off the main road. The complex’s hydrant is fed by a dead-end six-inch water main. The first-arriving pumper [1,500 gallons per minute (gpm) with a 500-gallon tank] stops at the hydrant at the complex’s entrance and drops a five-inch supply line.
The fire’s volume requires heavy-caliber streams. The incident commander (IC) orders a deck gun to hit the main body of fire using the prepiped 1¾-inch smooth bore master stream. He orders a 2½-inch hoseline with a 11⁄8-inch smooth bore tip to the adjoining second-floor apartment where the fire is now spreading. Engine crews stretch a second 2½-inch hoseline with the same nozzle to combat the fire in the attic, which is now rapidly involving the entire building’s attic space.
|(1) A collapsing supply line is a clear indication that the water demand is outstripping the supply. Demand has to be reduced, or the pump will cavitate. (Photo by author.)|
As the pump operator charges the handlines, he realizes the supply line is collapsing. The IC calls for a second supply line, and crews stretch an additional five-inch supply line from the dead-end hydrant using two 100-foot lengths of hose. Although this improves the situation slightly, there is still a problem supplying adequate water to the single deck gun and two large hand-lines. (Note: These actual incidents are only being used to illustrate hydraulics tactics and are not meant as an endorsement of the overall fireground operations.)
The immediate assumption is that a problem exists with the hydrant and the public water supply. The initial static pressure was 70 pounds per square inch (psi), but it quickly bottomed out. So the problem must be the hydrant, right? A closer look at the tactics used reveals the answer.
Large-diameter hose (LDH) is a great tool to move large volumes of water. However, we must still adhere to basic hydraulic principles for LDH to work to our benefit. The friction loss in five-inch hose flowing 500 gpm is approximately two psi per 100 feet. This is the reason many believe there is little or no friction loss in LDH. Regardless of hose diameter, when you double the flow, the friction loss quadruples. So if we flow 1,000 gpm, our friction loss increases from two psi to eight psi per 100 feet.
On the surface, this does not appear to be a major factor. But in the case of this fire, the pumper laid 10 100-foot lengths of hose. This means that as soon as the first 2½-inch handline was opened, the pump attempted to flow 1,000-plus gpm. The friction loss would have been at least 80 psi (eight psi per 100 feet × 10 lengths = 80 psi). The problem is the initial static pressure was only 70 psi; opening the second 2½-inch handline further exacerbated the problem. The pump operator tried flowing approximately 1,300 gpm through a supply line that was barely capable of supplying the master stream alone. The second dead-end hydrant supply line helped improve the flow slightly, but by the time it was connected, the fire had burned the roof off and the building was heavily damaged.
In analyzing this case, flow tests from this incident demonstrated that the initial hydrant was connected to a 12-inch water main capable of supplying approximately 1,950 gpm at a 20-psi residual pressure. To solve this, have a second hydrant pumper with a 1,500-gpm-rated pump pumping to the attack pumper through the five-inch supply hose. To maximize supply, feed the hydrant from the 4½-inch steamer connection and both 2½-inch hydrant discharge butts. The supply line from the hydrant pumper to the attack pumper would pump at 150 psi at whatever the rated revolutions per minute (rpm) are to deliver the pump’s full capacity. Friction loss for 1,300 gpm flowing through a five-inch hose is approximately 13 psi per 100 feet. In this case, there were 10 100-foot lengths laid; the total hose friction loss would be 130 psi, leaving the attack pumper with a 20-psi residual pressure.
The next scenario also involves a two-story multiple-family garden apartment dwelling in which two wings are set at 90° angles from the center point. Each wing has eight dwelling units, four units on each of the two floors.
A fire starts in the second-floor unit of the right-side building closest to the intersecting center point. The resident’s attempt to extinguish the fire fails. This delayed notification allows the fire to grow and involve the entire dwelling unit. A passing neighbor spots the fire, but by now it is consuming the entire apartment and spreading to the attic space. The responding fire department must now confront a fire involving the entire building.
The IC orders the first-due engine to lay in from the hydrant approximately 200 feet due east of the fire building and begin a blitz attack with the deck gun. The deck gun has a variable tip fog nozzle that can deliver up to 1,000 gpm. As the hydrant is charged, the static pressure is noted at 70 psi. Once charged, the deck gun’s incoming pressure immediately drops to approximately 30 psi. The pump operator tries increasing the discharge pressure to achieve 100 psi for the variable fog master stream but quickly realizes that the supply line is collapsing and the pump is cavitating. The initial concern is a hydrant problem.
Crews lay a second supply line in from the second-due hydrant, located approximately 600 feet north of the fire building; it is opened and the static pressure is noted at approximately 75 psi. As the line begins flowing, the pressure drops below 30 psi and quickly falls as the pump throttles up. The department abandons the master streams and deploys several 1¾-inch handlines in a defensive posture to effect exposure protection. The fire results in the total loss of two apartments, significant water damage to the two units below the fire apartments, and varying degrees of smoke and water damage to the other 14 apartments. Fire walls erected during construction limit the fire spread and most likely prevent the complete destruction of the entire building.
Flow tests are conducted following the fire to determine the available water flow and if there are any water distribution system infrastructure issues.
The fire building was one of numerous multiple-family garden apartment-type buildings in a segregated complex. A site plan examination for the complex revealed an elevation difference of approximately 85 feet between the building’s finished floor at the complex entrance and the fire building’s finished floor. The water main depth beneath the surface was not available but was believed to be consistent based on the depth of the curb box valve shutoffs for the hydrants. Calculating elevation loss at 0.433 psi per foot results in a 37-psi elevation pressure loss at the outset. As the master stream pressure was increased, the residual pressure quickly decreased. The second hydrant had a determined elevation of approximately 10 feet lower than the primary hydrant, resulting in the five psi difference in pressure.
Additionally, when the department flowed the hydrant, the results were drastically lower than the primary hydrant flow, indicating an obstruction between the water main and the flow hydrant. Although a partially closed curb box valve was suspected, another potential cause to consider was an obstruction in the lateral piping.
In both cases, although the circumstances were slightly different, the need to test water supplies and preplan the site was overlooked and proved critical. In the first case, the lack of preplanning was compounded by the lack of fundamental hydraulic principles—trying to move too great a volume of water without sufficient pressure. Preplanning in the second case would have identified the elevation difference and the need for on-site auxiliary pumps for the water supply or an on-site water supply at the higher elevation such as a water tower (elevated tank) or belowground cistern for fire protection purposes. Additionally, water supply testing would have discovered the problem with the second hydrant, and corrective action could have been taken before the fire occurred.
The third scenario involves a three-inch supply line at a fire involving a large tree in the park. The officer wants to use a new large-caliber nozzle recently acquired for a “blitz attack.” The officer tells the engine to lay in a single three-inch supply line. The engine company drops a 600-foot-long supply line and lays out the 200-foot three-inch blitz attack hoseline with a large-caliber nozzle with the intent to flow 500 gpm.
Members open the hydrant (the static pressure is noted at 65 psi) and charge the large-caliber nozzle line, but as they increase the throttle, the supply line collapses and the pump begins to cavitate. The minimum flow for this large-caliber nozzle is 400 gpm, but the pump could not supply even that volume.
Static pressure and the flow yield the answer to this problem. A three-inch supply line can easily flow 500 gpm, but the friction loss is approximately 20 psi per 100 feet of three-inch hose; reducing the flow to 400 gpm drops the friction loss to approximately 13 psi per 100 feet of three-inch hose. The problem here is the original static pressure of 65 psi. This means 120 psi in friction loss if we flow 500 gpm over the 600-foot hoselay. Reducing the flow to 400 gpm drops the total friction loss in the supply hose to 78 psi. Unfortunately, both of these far exceed the starting static pressure of 65 psi.
For the third scenario, a smaller handline would have been wiser from a hydraulic point of view. A single 2½-inch hose with a 11⁄8-inch tip and 50-psi nozzle pressure would flow approximately 265 gpm. The friction loss in the supply would have been less than 30 psi total. If the officer wants to use the large-caliber nozzle fed by a single three-inch hose, crews should position the apparatus at the hydrant and pump through the hoseline to the large-caliber nozzle. In this alternative, the pump would generate sufficient pressure to overcome the friction loss of 20 psi per 100 foot of three-inch hose and deliver the necessary 100 psi at the nozzle. However, the overall stretch would be limited to 750 feet of three-inch hose (7.5 100-foot sections of hose × 20 psi friction loss per 100 feet + 100 psi nozzle pressure = 250 psi pump discharge pressure). Two hundred and fifty psi is the last point on the pump curve as seen on the design information on the pump’s nameplate. This is especially true of newer nozzles and appliances that many manufacturers recommend operating at or below 250 psi.
All three scenarios reinforce the need to understand and apply basic hydraulic principles relating to friction loss and stress the need for pump operators to have a working knowledge of the water supply system’s capabilities. Additionally, these cases underscore the need for all firefighters, and especially pump operators, to preplan their local districts’ locations that contain hidden firefighting challenges. Once you have identified these challenges, conduct drills and training exercises to adequately prepare pump operators to meet the challenges that will confront them.
GERARD J. NAYLIS is a 36-year fire service veteran who has worked as a firefighter and fire officer in career and volunteer fire departments. He has a bachelor’s degree in fire science from Jersey City State College and a master’s degree in administrative science from Fairleigh Dickinson University. He is a certified fire instructor who has taught throughout the United States, Canada, and the United Kingdom. He is also a fire service author and the fire series book acquisitions editor for Fire Engineering Books and Videos.