Hydraulics at the nozzle starts with "What is he capacity of the water system?" Knowing the capacity of the water system is valuable information to a fire officer or the pump operator. Knowing the available capacity of the water system is essential when making fire plans.
The results of a flow test can be used to advantage by both the fire and water departments of a community. Since test results indicate weak points in a water distribution system, they can be used by the water personnel when improvements in an existing system are planned and when extensions to newly developed areas are designed. Tests that are repeated at the same locations year after year may reveal a loss in carrying capacity of water mains and they may emphasize the need for strengthening certain arterial mains.
It is important for every fire officer to know the ultimate capacity of the water system. Fire fighting defenses cannot be intelligently planned without all of the known facts of the situation, and capacity of the water system and its flows in a given area are certainly important factors.
A hydrant flow test for an area is a means by which certain facts can be established. Flow tests are the only positive means readily available for fire fighting. The size of the water mains and static pressure in the mains are not in themselves sufficient to assure factual conclusions.
The following terms must be understood in order to conduct flow and pressure tests:
1. Static pressure. The word static means at rest or without motion. Pressure may be produced by an elevated water supply (a stand pipe), by atmospheric pressure, or by a force pump A water flow definition of static pressure is: stored potential energy that is available to force water through pipe and fittings, fire hose, and adapters. Normal static pressure is 35 psi.
2. Normal operating pressure. A water flow definition of normal operating pressure is: that pressure which is normally found on a water distribution system during normal consumption demands. Normal operating pressure is generally considered to be static pressure for fire flow purposes.
3. Residual pressure. A water flow definition of residual pressure is: that part of the total pressure that is not used to overcome friction or gravity while forcing water through pipe, fittings, fire hose, and adapters. The term "residual pressure" is also used to represent the pressure remaining in the water system while water is flowing for either test purposes or fire fighting. The word residual means a remainder or that which is left.
4. Flow pressure (PSI). A water flow definition of flow pressure or psi is: the forward velocity pressure at a discharge opening that is recorded by a pitot tube and gauge while water is flowing. the PSI or flow pressure can be measured by using a Pitot Tube Gauge. The flow pressure from a 2-1/2" hydrant outlet will most likely be the same as a residual pressure reading on an opposite 2-1/2" outlet because they are close together.
Useful Links:
Lenntech Water & Air Treatment Systems
GLI International, Inc.
