Editor’s Note: This is the last of a three-part series on fire apparatus water delivery system design. The first part covered the water’s route from the fire pump volute outlet through the pump discharge manifold, apparatus plumbing, discharge valve and fire hose to the nozzle or other fire appliance. The second part covered the suction inlet side of the system, and the third looks at the pump drive system.
The fire pump drive system must be matched to the flow and pressure required and the water supply available. The process guarantees the performance capability of the apparatus at the end of the hose, where the wet stuff meets the red stuff.
There are several different pump drive systems. Each has several major components to consider, and two have drive system limits, beyond available engine power, that need to be taken into consideration.
The most common setup is the split driveline pump drive. This design cuts the main drive shaft between the chassis transmission and the rear axle and adds inline a shiftable gearbox, which allows either driving down the road or operating the pump. This system requires a chassis transmission that can be held in the correct gear for pumping and that doesn’t have a first gear ratio high enough to multiply the engine’s torque to the point where it exceeds the pump’s lower drive torque rating. The truck transmission should also not have any oil temperature problems while pumping.
Full Power Reliably
The good news is this setup is very good at delivering full engine power reliably for more hours than most departments could ever use. The engine selected for this setup, as well as the others, needs the power to meet the National Fire Protection Association 1901 apparatus standard rating, but it also needs the power to meet the designed pumping discharge and intake requirements.
The second most common setup is the transmission power takeoff pump drive. This system requires a chassis transmission that has a PTO opening and driving gear that can be used to drive the fire pump. The PTO, which is bolted to the chassis transmission, must clear the cab, chassis frame and front suspension. Additionally the PTO must have a continuous duty torque rating suitable for both the NFPA 1901 ratings and the designed pumping requirements. The PTO also needs a design life that matches the department’s yearly average usage at the designed pumping performance. This drive setup has the limitation of the PTO’s design torque rating and designed life. The transmission should also not have any oil temperature problems while pumping.
Front Engine Pump Drive
The third and least common setup is front engine pump drive. This design drives the pump off the front of the engine crankshaft. This requires a cooling system designed for a front PTO drive. The other big issue is the crankshaft’s ability to transmit power. This rating is usually lower than the available engine power and could be a limiting factor. Front PTO drives are not available on many chassis. This system requires only the chassis transmission to be in neutral.
No matter which drive is used, the design pumping requirements could be very different than the basic NFPA 1901 ratings. The goal is to meet NFPA while performing the designed performance at the lowest practical engine rpm. For most applications where the designed operating pressure is 120 to 140 psi, the simultaneous total flows are less than 80 percent of the pump rating and the engine is at least 320 hp and over 860 foot-pounds of torque, it is common to be pumping close to the rpm that coincides with the maximum torque rating speed of the engine. The drive should have the component life suitable for the department’s usage history and projected future usage. With a split driveline setup, this is normally not an issue, but it can be on some PTO drive applications with big pumps and high usage.
What does all of this really mean? Spec the performance you want at each discharge, match the discharge requirements to the suction intake layout, design the pump drive to match the performance requirements and maximize the performance of your next pumper.
Editor’s Note: Gary Handwerk is global pump product manager for Hale Products. He has been involved with the fire service industry for more than 36 years, working for various fire apparatus and pump manufacturers. He has been a member of the National Fire Protection Association (NFPA) Fire Apparatus Standards Committee for more than 15 years.