No matter what pump you choose or what type of apparatus you choose to have built, the intake and discharge manifolds, piping, and valves will affect operational performance.
National Fire Protection Association (NFPA) 1901, Standard for Automotive Fire Apparatus, and NFPA 1906, Standard for Wildland Fire Apparatus, only go so far and do not always provide the maximum performance possible from the pump and drive system selected.
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| 1 This is a 5,250-gpm NFPA 1901 rated pump installed on a 600-hp custom pumper being tested using four sets of eight-inch suction hose. (Photos by author.) |
Obtaining the maximum available performance is critical on high-flow applications such as industrial or other large property fires but is also important at the extreme opposite end of the market on slip-style wildland/grass apparatus where the engine driving the pump is very small, without the extra power to overcome the losses in the plumbing.
The current NFPA standards reflect historical conditions, situations, and technologies and at best reflect the commonly used state-of the-art technologies. This safely covers most apparatus being built but may not always provide the optimum performance opportunities available from the pump and engine selected. This is the first of two articles that look at intake and discharge systems on most types of fire apparatus. They will look at the current standards, current practices, and what would work better for many fire departments to optimize performance.
Performance Limitations
Intake performance when drafting is limited to the atmospheric pressure available to push the water into the pump. Losses in the suction hose and intake manifold system consume some of that atmospheric pressure. The lower the losses, the more performance can be derived from the pump. When you consider that atmospheric pressure can be no more than 14.7 pounds per square inch (psi), even small losses make a big difference. Even when pumping from a pressurized source, losses in the intake hose and manifolds are robbing potential performance.
The suction intake systems, as we know them, are based on using the smallest possible suction hose to get the minimum performance. Why? Because we used hard black rubber, very heavy suction hose for decades, and 4½-inch hose was much lighter than a five-inch let alone six-inch suction hose. Today we have lightweight hose and couplings that are easy to handle and deploy. The hose also bends better for easier setup. So, the size hoses we are used to for a given size pump could be looked at in a different light. Even eight-inch modern hose is easier to handle than the old six-inch, black, hard suction we used for decades.
As a pump designer, I look at the velocity of the water traveling in the pipe, waterway, or hose as a reference point in any evaluation of this type. The water speed is commonly measured in feet per second. To calculate this, I use the following formula: Velocity in feet/second = [0.32 x gallons per minute (gpm)]/the area of the waterway in square inches.
Further study of pump and system design books and various industrial standards reveals that 12 feet per second is hydraulically the ideal maximum design speed in an intake waterway/hose system. So, let’s look at what is commonly pur