BUILDING SOLUTIONS
CODES AND STANDARDS
By Vincent T. Favale, PE, CPD, CFPS, NICET Level II, WSP USA, New York City
Understanding fire pumps, their applications and sizing Learn about fire pump types, drivers, sizing and components
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ire pumps are an essential part of most fire protection systems, as they allow for taller buildings, smaller piping and higher pressure and flow rates in systems. Incorrectly sized or selected pumps can result in not enough pressure or flow being created, which can cause several problems: • Systems function incorrectly. • Spray patterns from sprinklers don’t develop properly. • Not give enough pressure to hose valves at the top of a high-rise building. • Too much pressure, causing components to burst and break open during use.
Learning
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To size and select the correct pump, it is important to have an understanding of the different oper• Know the different types of fire ating principles, types, drivers and pumps, including operating applications for fire pumps. principles, types and driver There are two main principles of options and their applications. operation of pumping water, centrifu• Learn how to size a pump based gal and positive displacement. on the most demanding factor. Centrifugal pumps use the prin• Understand the other ciple of a spinning action to genercomponents needed to design a ate centrifugal forces — water starts fire pump system. off in the center of an impeller and as the impeller rotates it is thrown to the outer parts of the impeller. This is similar to a spinning ride at a carnival; as the ride speeds up the people inside are pushed against the wall. The faster the spin, the more force is applied to the people inside. Positive displacement uses the principle of capturing a select amount of liquid in a single revolution and using a mechanical process to displace
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OBJECTIVES
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November 2020
consulting-specifying engineer
that liquid. Think of an air compressor, with an outside energy source and pistons to compress the air. A defined volume is allowed into the chamber when the piston at the bottom and the intake are open. As the outside motor spins, the shaft the intake is closed and the piston is forced up. In the case of water and water-based fluids, they can’t be compressed, so they build up pressure as the piston moves up. Once the outlet valve is opened, that water can flow out of the chamber. Each one of the operating principles have its advantages and disadvantages. Centrifugal pumps are better at higher flow rates when compared to positive displacement, which provide constant flow at a range of pressures and typically have low flow rates and higher pressures. Popular applications of centrifugal pumps are low- and high-rise buildings and utility/campus central pumping stations. Positive displacement pumps are used to pump fluids that are not always water, such as foam concentrate or in systems that require high water pressures, such as water mist systems.
Fire pump types
Positive displacement pumps come in two main types, reciprocating and rotary. Reciprocating is described above and consists of plunger-style pumps. Rotary-style pumps use a spinning internal mechanism, such as lobe pumps, to capture water and move it through the chamber of the pump. This is similar to rotary engines in some automobiles — inside a specially designed chamber, a center lobe or rotor with vanes rotates and forces water through the pump body. A select volume of water is captured in each rotation by the lobes and the vanes and pressure is added to the system. Centrifugal pumps come in several types: horizontal split-case, vertical in-line, vertical line shaft and end suction pumps. Horizontal split case is one www.csemag.com
