What is the difference between the two?
Single Stage Fire Pump
Single stage pumps use a single impeller to produce water pressure. There are no Volume/Pressure (Series/Parallel) Transfer valves to manipulate. Single stage fire pumps usually have larger impellers than Two-Stage Pumps to generate the desired volume and pressure combinations. Impellers of pumps are classified based on the number of points that the liquid can enter the impeller and the amount of webbing between the impeller blades. Impellers can be either single or doublesuction. A single suction impeller allows liquid to enter the center of the blades from only one direction. A double-suction impeller allows liquid to enter the center of the impeller blades from both sides simultaneously and thus generate higher water flows.
Two Stage Fire Pumps
Two stage pumps were the standard in the Fire Service for many years and some departments still use them. The primary difference between the two stages is the volume of water pumped at a range of pressures. Pumps are transitioned between Pressure and Volume Modes using pump panel mounted Transfer Valve Controls.
Pressure Mode – When the pump is in Pressure mode, a Transfer Valve located on the Engine Pump Panel closes clapper valves on one side of the intake piping, and one side of the discharge piping. These Clapper valves direct water into a “series” of two sequential volutes, hence this mode may also be referred to as Series. Water enters the first volute and gains pressure from the spinning impeller. This pressurized water exits the first impeller and enters the second volute where a second impeller further boosts the pressure before the water is routed to the discharge manifold. This mode is used where higher discharge pressures are needed due to friction loss, elevation loss, or other factors that mandate pressure over volume. Higher pump pressures do not equate to higher volumes of water. Pressure mode is not appropriate for pumping more than one-half of a pump’s rated capacity, i.e. Pressure Mode is not appropriate if operations require more than 625 GPM from a 1250 GPM rated pump. Pressure mode provides higher pressures at a cost of lower capacities.
Volume Mode – When the pump is in Volume mode, all of the clapper and change-over valves are open and intake water is distributed evenly to both impellers at the same time. The two impellers are operating in parallel to each other to feed water to the discharge manifold, hence this mode may also be referred to as Parallel. Volume Mode delivers the maximum flow capacity available from a pump. This volume is provided at a cost of lower operating pressure. Volume Mode should be used when pumping more than one-half the rated capacity of a pump. It should be used during defensive operations (i.e. Master Streams) and when drafting.