There will always be some departments that insist on buying the biggest pump available that can utilize every single horsepower provided by today’s largest diesel engines.
Some obvious direct penalties for larger pumps will include more weight, a larger pump compartment and higher cost.
The indirect associated penalties can make more of an impact and may include: a larger and more expensive engine, transmission, drivelines, axles, brake system, cooling system, and less available compartment space for a given truck size.
For departments that want the most appropriate pump for their trucks, as opposed to the largest available, the real question should be, do you usually draft.
If your department usually drafts and wants to flow big water, the large body pumps are a no-brainer. The best arguments made by Gary Handwerk in support of large pumps all boil down to the fact that large body pumps draft more water under all conditions.
This is true because the pump’s suction passages, double-suction impeller inlets and impeller diameters are all physically larger in size. Less friction loss when entering the pump and impeller, combined with lower impeller speed, will always improve flow from draft.
For those departments that usually do not draft, which is probably an overwhelming majority, the 1,250 gpm and smaller pumps can provide almost equivalent performance while leaving more money available for additional tools like a foam system or a compressed air foam system (CAFS).
Flowing More Than Rating
Any fire pump meeting the National Fire Protection Association (NFPA) 1901 performance rating can flow at least 1.5 times its rating when operating from a suitable pressurized hydrant, provided the engine has sufficient power.
Your pump manufacturer can tell you exactly how much engine horsepower would be required based on your hydrant pressure and desired discharge pressure.
Unfortunately, this information is not usually published because the NFPA 1901 performance rating points are biased toward operating from draft. These smaller pumps certainly can draft when necessary, but only at flows up to their rating under the normal 10-foot lift test. A new pump performance standard for most pumps that usually do not draft is long overdue.
Double Suction Impellers
If you are considering a 1,250 gpm or smaller pump rating, there are a few more points to consider. Small body midship pumps, with double-suction impellers, will always draft better than those with single-suction impellers.
The single-suction impeller pump’s drafting performance is optimized when it is configured as a rear-mount pump with the suction straight in from the back of the truck, any suction manifold added will decrease drafting performance.
Editors note: Rich Teske has been involved in the fire service industry for 24 years. He is currently a vice president at Trident Emergency Products, Hatboro, Pa. Previously, he spent 8 years as manager of research and development, followed by 9 years as vice president of engineering at Hale Products. Prior to that he was a research and development engineer for ITT Domestic Pump Division.