A fire pump is a specialized water pump that provides constant, high pressure for a water system used to extinguish fires. The fire pump intake can be connected to an underground water pipe or a static source, such as a water tank or reservoir. Fire pumps may be powered by an electric or diesel motor. The fire pump starts up when the water system dips below a certain pressure threshold, which can happen as sprinkler system heads, fire hoses and other firefighting connections are opened. There are two main types of fire pumps: positive displacement and centrifugal.
A positive displacement fire pump moves water by trapping a fixed amount of it, then forcing that water out the discharge pipe. Positive displacement pumps can be further divided into two categories: reciprocating pumps like piston and diaphragm pumps, and rotary pumps like screw and gear pumps. Of these types, rotary pumps are most commonly used in firefighting systems as a primer pump to fill hoses, lines and the main pump.
Centrifugal pumps are commonly used as main pumps in firefighting systems. A centrifugal pump uses a revolving device called an impeller to accelerate water radially outward from the center of the pump housing, imparting it with velocity. This velocity, when coupled with the resistance of the pump housing and discharge line, creates pressure, which propels water out of the pump. Additionally, a vacuum is created at the impeller input opening that continuously draws in more water.
The main difference between the two types of pumps is that a positive displacement pump has the ability to pump air, whereas the centrifugal pump cannot. This makes the positive displacement pump a necessary choice for filling empty water lines, which is why it is commonly used as a primer pump. The centrifugal pump is used as a main fire pump because of higher flow rate and efficiency.
- pump image by Einar Bog from Fotolia.com