"Pump Size Matters" in the December 2005 issue of In Service Online presented several reasons why departments might want to consider specifying big pumps on their next apparatus purchases. Although those are all good reasons, big pumps have some limitations as well.

To understand the limitations, you need to understand that each pump can have several different ratings depending on the input horsepower and rpm. Within that range of ratings, a pump will operate most efficiently at a single flow and pressure called the best efficiency point, or BEP. For example, the Darley LDM pump is sold with ratings ranging from 1,000gpm at 150psi up to 1,750gpm at 150psi. The BEP of this model is about 1,600gpm at 150psi.

Most single-stage centrifugal pumps can operate at flowrates down to about 50% of their BEP without any problems. Pumps designed for the fire service can go even lower. As flowrates drop to about 10–20% of the BEP, however, two potentially serious situations can occur. The first is overheating of the water. The second is recirculation cavitation. Both can cause poor performance and result in cumulative damage to the pump.

In general, larger pumps have higher BEPs than smaller pumps and are therefore more likely to have problems at low flowrates. To minimize these problems, Mike Ruthy, chief engineer of Darley, recommends that departments operating big pumps at low flowrates should recirculate water back to the tank, keep the engine and pump rpms low and draft from the lowest lifts possible.

Departments that routinely have to use their structure pumpers to engage in low-flowrate operations over extended periods, such as fighting grass and brush fires, may want to use small engine-driven auxiliary pumps rather than risk damaging their main pumps. This type of pumper configuration is commonly used in the West for wildland/urban interface operations.