PRESSURIZED SYSTEMS
BY WILLIAM F. ECKMAN
DESIGNING THE WATER SUPPLY
Fire departments tend to rely on one of the three methods described above and ignore the others, but many progressive suburban, rural, and even some larger city departments make use of all three, depending on the situation. Regardless of the method chosen to establish the water supply, safety of the attack crews is the most important consideration. All water supply operations must be designed to ensure that the flow to attack lines being used inside a structure is not interrupted.
Regardless of the method chosen to transport it, a sustained water supply requires a good source. A fire hydrant, located on an approved public water system, generally is the most reliable water source for fire protection. Some private systems also have hydrants, but they may not have enough flow for the structures being protected. The quality of the system is determined by the amount of water in storage, the maximum production rate, the distribution system, and the installation and maintenance of fire hydrants.
The most common pressurized water system is the public water system, which may be operated by public agencies, quasi-public agencies, or even private companies. the water company`s primary objective as a public utility is to deliver potable water to its customers. Maintenance and operation of fire hydrants may assume a low priority since fire hy-drants do not pay water bills. Even where the local government pays a fire hydrant rental fee to the water company, some of the people in charge of day-to-day operations have very little knowledge of the fire service`s needs and frequently may not place very much importance on maintaining fire hydrants.
If a public water system is expected to provide a good level of fire protection, fire department officials, and in particular the water supply officer, have to develop and maintain a close relationship and spirit of cooperation with the people running the system. A good hydrant system can produce and store enough water to meet the total water supply needs of the area it serves. It also needs to have a distribution system that will sustain an adequate delivery rate throughout the area to be protected.
AVAILABLE FLOW
The amount of water available for fire protection from a public water system is limited by the normal usage in the system, storage, and production capabilities. In determining how much water is available for fire protection, the average daily minimum storage is a measure of the maximum reserve capacity. When the system`s peak usage by customers exceeds the maximum production rate, the average minimum storage will be somewhat less than the total capacity, since the level in the storage tanks will be lowered by the de-mand on the system at certain times.
For fire protection purposes, it is advisable to store water in elevated tanks so that the water will be readily available at all times without requiring mechanical or electrical equipment to access it. Reservoirs or other surface storage can be substituted for elevated tanks where the terrain provides a difference in elevation great enough to generate the natural head pressure needed to maintain a good working pressure in the system as the level in the storage facility drops with usage of water.
When standpipes or low-level storage is used where there isn`t sufficient ground-level elevation differential, the residual pressure in the system may drop to the point at which 50 percent or less of the water in storage is available for fire protection. When hydro-pneumatic storage tanks are used, as little as 20 percent of the tank`s total capacity may be available for fire protection purposes; and the tank must be sized proportionately.
When ground-level storage is used and pumps are required to offset lack of elevation, the fire hydrant`s flow rate will be limited by the system`s pump`s ability to deliver it at the normal working pressure. When estimating the total capacity, the minimum daily average storage, supply pump capacity, emergency supplies, suction sources, and any supplemental supply the fire department can maintain should be included. The Insurance Services Office (ISO) specifies that the capacity of the system should be sufficient to maintain needed fire flows up to 2,500 gpm for two hours, up to 3,500 gpm for three hours, and greater flows based on the degree of risk.
DISTRIBUTION SYSTEM
The fire department is inclined to assume that the water needed for fire protection will be readily available where hydrants have been provided. This is not necessarily true. Many public water systems are very old and will not supply the amount of water modern fire apparatus require for effective fire suppression. Some metropolitan areas have outgrown the capabilities of their water system. Even if the system were adequate at the time it was constructed, it may have deteriorated with age and have significantly less capacity now.
Two types of pressure losses occur in a water distribution system. Static losses are a function of the difference in elevation between the water in storage and various points in the system. Dynamic losses are caused by the friction that occurs when water moves through the system.
Static losses. The static pressure at a fire hydrant may not be a good measure of its capabilities. Static pressure is only an indication of the difference in elevation between the pressurized water source and the point of measurement. Where pressure is developed by elevated storage tanks, static pressure in the system probably will average 50 to 70 psi. In mountainous terrain, static pressure in the system can go as high as 200 psi when reservoirs or ground-level storage tanks are located at a much higher elevation. Other systems depend on ground-level storage within the system or surface treatment plants, and the static pressure depends on the location and type of pumps being used to supply the distribution network.
Dynamic losses. Distribution systems may be required to supply a large volume of water for fire protection. As the flow increases, pressure losses occur as a result of the friction caused by water moving at high speed through pipes in the system. In general, the higher the flow, the greater the pressure loss and the larger the pipe diameter, the lower the friction loss.
SYSTEM DESIGN
The size of the pipes in the system is the most important single factor in designing a good water system, but the configuration of the main distribution lines contributes significantly to the end result. By designing a “gridded system” in which the water moves through the system through a number of different paths, the flow through any one pipe is lower and the friction loss is reduced. The worst situation occurs when a fire hydrant is connected to a “dead-end” main where all of the water comes through a single line and the maximum flow is limited by the size and condition of the pipe. Not only do dead-end mains tend to be installed with smaller pipe, but the flow through them is generally minimal; and they may have their capacity reduced even further by a buildup of sediment and corrosion inside the line.
EMERGENCY OPERATION
When a major fire occurs, additional flow can be expected when all pumps are running and all wells or treatment facilities are operating at maximum capacity. It is a good practice to arrange with the public utility that operates the water system to have all pumps turned on manually when an emergency occurs and water demand is high.
HYDRANT OPERATION
To prevent freezing, all dry-barrel hydrants listed for installation by Underwriters Laboratories are equip-ped with a valve to drain the barrel of the hydrant each time it is closed. The operating shaft of a fire hydrant is designed so that the drain valve on the hydrant is open when the main valve to the system is closed and closed when the main valve is open. This happens only when the hydrant is fully opened or fully closed. If the hydrant is not opened fully when it is used, the drain can be open at the same time the main valve from the system is open. If this happens, water will escape from the drain under pressure and may cause the ground from the base of the hydrant to wash away. Over a period of time, the hydrant could break because of the lack of support and become unusable. On the other hand, if the hydrant is not fully closed, the drain valve may not open and allow the water to completely drain from the hydrant barrel. In warm weather, this doesn`t cause a problem; but if the hydrant is not dry when the weather turns cold, the water in the barrel can freeze and make the hydrant unusable when it is needed.
A big concern when operating from a hydrant is the potential for a “water hammer” that may damage the system if the flow is cut off suddenly. To keep this from happening, fire hydrants are designed to require many turns on the operating shaft to open or close the main valve. This ensures that changes in flow cannot occur rapidly enough to damage the system. Anytime a valve that can be opened or closed rapidly is connected to a hydrant, the system can be damaged; extreme caution should be used when opening or closing it.
HYDRANT MAINTENANCE AND OPERATION
A thorough knowledge of the design and capabilities of the water system is essential for prefire planning and maintaining adequate fire streams during major fires. The only way to be sure of the fire flow available throughout the system is to conduct regular flow tests on each of the hydrants.
After the system has been tested, it would be helpful to color-code each hydrant to indicate its capacity to the fire department. NFPA 291, Testing and Marking of Hy-drants, specifies the classification and marking standard listed in the box above.
Standard operating procedures should include a rule that prohibits more than one pumper from hooking to the system at a time in an area where fire hydrants are color-coded orange or red.
FIRE DEPARTMENT RESPONSIBILITIES
The fire department should make sure that the water department is testing all fire hydrants regularly and making repairs promptly. It also should verify that detailed records for each hydrant in the water system are readily available. Such records are essential to ensure that a reliable water supply will be available when needed and qualify for maximum credit during an ISO evaluation. n
WILLIAM F. ECKMAN spent 25 years in the volunteer fire service and served as a chief officer in Pennsylvania and Maryland. He is a fire protection consultant specializing in department evaluation and master planning with an emphasis on water supply. He has conducted numerous seminars and specialized training programs involving fire pumps and fire department water supply and authored Fire Department Water Supply Handbook (Fire Engineering Books, 1994).
