TANDEM PUMPING
When operating hoselines from standpipe systems having either plain hose valves, overriding-type pressure-restricting valves, or removable pressure-restricting devices, it usually is an easy matter for a single pumper to supply the volumes and pressures needed by suppression crews operating on the upper floors; these control devices are installed on standpipe systems having a maximum operating pressure of 175 psi.
High-pressure standpipe systems that have riser pressures of up to 600 psi must have pressure-regulating valves (PRVs) installed to safely reduce high-riser pressures to around 100 psi at the hose outlet. It is important that the fire department inspect each standpipe system installed in its jurisdiction and make note of the pressure at which each operates. In the event of building fire pump failure, the department must supplement the system through the standpipe Siamese. Unless die designed operating pressure for the building’s system is maintained, the PRVs will not function properly and will not supply the necessary water.
Fire department pumpers are tested to deliver their maximum capacity at 150 psi. They also are tested to deliver 50 percent of their capacity at 250 psi, the maximum pressure available from draft. Figuring head loss at 5 psi per floor and a desired pressure at the floor hose valve of at least 75 psi, 250 psi can deliver water at usable pressures to about the 25th floor through a conventional standpipe system. If a system is designed to operate at pressures more than 250 psi or if the building is taller than 25 stories, a fire department has to pump into the Siamese at pressures higher than those that can be supplied by a single pumper.
Centrifugal fire pumps have the advantage of being able to use intake pressure to boost their output pressure. For example, if a hydrant system is supplying 75 psi residual pressure on the intake of a pumper, then that pressure will boost the highest output pressure of the pumper from 250 psi to 325 psi. If hydrant pressure alone is not enough to boost a pumper’s output pressure to that needed to supply a standpipe system, then another pumper can be used in an operation called in line or tandem pumping.
For a tandem pumping operation, a pumper takes suction at a hydrant or drafting source, and a line or lines are laid from its discharges to the intake of a second pumper. Then lines are laid from the second pumper’s discharges into a building’s Siamese connection. To limit friction loss, the pumpers should be located no farther than 200 feet apart. Depending on the pressures needed, the hydrant pumper charges its lines to about 200 psi. The second pumper, with 200 psi on its intake, boosts pressure to the level required to supply the system.
Because the second pumper may be supplying pressures close to the design limits of the discharge hose, all couplings must be secured with rope tools to prevent their thrashing about should a connection slip from the hose. Worn attack line hose should not be used for this operation, and it is a good practice to use screwthread couplings on discharge lines from the second pumper. At pressures more than 200 psi, the gasket may blow out of ⅛-turn Storz couplings, which could cause the couplings to disconnect.
While not an every day operation, tandem pumping may be the onlyway to provide suppression companies operating on upper floors with hose streams having enough pressure and flow to successfully extinguish a fire.
