Modeling a Fire Hydrant

Modeling a Fire Hydrant

A typical fire hydrant installation is shown in Figure 1 and consists of a lateral segment of pipe connecting the water main to the fire hydrant. A gate valve is typically installed as an isolation valve in this lateral segment and will have a stem extension protected by a valve box.

Figure 1.  Typical fire hydrant installation.

Figure 2. Fire hydrant cutaway courtesy of Mueller Co.

The fire hydrant itself consists of a shoe that contains the main valve seat and plug, a lower barrel, and the upper barrel that contains one or more ports to connect with various sizes of fire hose. Figure 2 shows a Mueller Super Centurion 250 Fire Hydrant which has three ports (or nozzles): one 4-1/2" pumper nozzle used to supply fire water to a pumper truck, and two 2-1/2" hose nozzles when the hydrant is used directly. A single hose nozzle can be used or both could be used at the same time if necessary.

The hydraulic performance of the fire hydrant depends on how many and which ports are used. Figure 3 shows the hydraulic performance graph of the Mueller fire hydrant as the head loss (or pressure drop) as a function of the flow rate through the hydrant. The steepest curve is the performance when just one 2-1/2" hose nozzle is used (dash-dot line). If both 2-1/2" hose nozzles are used, the curve becomes less steep (dotted line). When the 4 -1/2" pumper nozzle is used the hydrant has even less resistance, as indicated by the shallower solid line. Some models can be configured with a 4-1/2" pumper nozzle, in which case the performance would be indicated by the dashed line.

Figure 3. Hydraulic performance graph for Mueller Super Centurion Fire Hydrant (courtesy of Mueller Co.).

To model a fire hydrant in PIPE-FLO®, first draw the main underground lateral from the water main, which can be modeled as an input flow demand or pressure boundary, as shown in Figure 4. A gate valve can be installed in this line to represent the fully open isolation gate valve.  The lower barrel can be modeled as a pipeline with the length and the inner diameter of the lower barrel obtained from the manufacturer's drawing. Although the hydrant's valve seat and plug are located in the lower barrel, the PIPE-FLO® model will have to be different because the hydrant can be used in different modes (pumper, 1-hose, or 2-hose operation).

Figure 4. Fire hydrant modeled in PIPE-FLO® Professional.

At the top of the lower barrel, connect three pipelines to branch out to the hydrant nozzles. Make one of these a 4 ½" pipeline and the other two 2 ½" pipelines. A component is added to each branch to represent the hydraulic performance of the hydrant when operated with one 2 ½" port being used, the 4-1/2" port being used, or both 2 ½" ports being used.  Lineups can be created to change which port is used and which ones are isolated based on the application being modeled.

A short length pipe is attached to the outlet of the component and a spray set to zero psig for a boundary pressure is added to show the operation if the hydrant nozzles are opened for cleaning, for example. Pipelines could be added instead of the spray boundary if the user wants to model the flow through the fire hoses and spray nozzles. 

The curve for each component can be generated from the hydrant's resistance curve obtained from the manufacture.  For the Mueller Super Centurion 250tm - Model 423 shown in Figure 3, a single data point was taken off the curve for each mode of operation, one from the 2 ½ inch single line, 2 ½ inch double line, and 4-1/2 inch single line.

These points are then used to generate the 2nd order curve for the components representing the port configuration being modeled.  The resistance curve data points used for this article are the following.

LINE                                FLOW                          PRESSURE LOSS
2 ½ inch single line         400 gpm                              1.25 psi
2 ½ inch double line       700 gpm                               2.30 psi
4-1/2 inch single line      1000 gpm                             2.48 psi

PIPE-FLO can generate the individual component resistance curves from these points. These are shown in Figure 5 below.

Figure 5. Component performance curves for the 4-1/2 inch port operation (left), a single 2-1/2 inch port in

operation (center), and both 2-1/2 inch port operated at the same time (right).

With the barrel length and diameters specified and each port configuration resistance curves in place, PIPE-FLO® can be used to model a fire hydrant.  It should be noted that the ports not being used are turned off in PIPE-FLO® by the on / off pipe command 

Lastly, a fire hose and nozzle could be added to each line to model the overall hydraulic system, as shown in Figure 6, 7 and 8.

Figure 6.  Fire hydrant model including two 100 feet of connected fire hoses and fire nozzles.  This is not a wye.

Figure 7.  Fire Hydrant model including one 4 ½ inch connected fire hose going to the Fire Truck.

Figure 8.  Fire Hydrant model including one 2 ½ in connected fire hose and fire nozzle.