By Chad Lueders, P.E.
The vast majority of fire protection systems utilized in buildings today rely partially or completely on the municipal water system to provide the water necessary to control or suppress the fire. While seemingly only a small part of the overall fire protection system, an understanding of how municipal water systems are operated, and what their limitations are, is vital to ensuring a reliable and functional fire protection system.
At the most basic level, water departments typically maintain their system pressure or hydraulic grade in one of two ways.
The first, and probably most common, is with an elevated storage tank(s), commonly referred to as a water tower. With this arrangement the water system “floats” on the level of the water in the tank. The tank elevation and the water level in the tank set the system pressure. Pumps either at a water treatment plant, or a booster station are operated based on tank level. When the level drops to a certain point, pumps are brought on line to refill the tank. Larger systems with multiple pumps may bring on pumps in a staggered arrangement, with the first pump coming on at a certain water level and additional pumps brought on if the level in the tank continues to drop. On some systems, the pumps deliver water directly to the tank; however it is more common that the pumps discharge into the distribution system which, when pumping capacity exceeds system demand, will fill the tank indirectly. This method is, appropriately, called an indirect pumping arrangement.
With this arrangement water system operators must strike a balance between maintaining adequate water in their tank (for emergencies such as a fire), and providing adequate water turn-over in the tank to meet water quality standards. As water sits in a tank, the quality begins to degrade. Emptying and refilling the tank with fresh water limits this problem, and must be done on a regular basis for potable water systems. With some exceptions, most water departments will operate their tanks between 95% and 60% full. On larger systems some pumps may be operated continuously even during periods of low demands. On smaller systems pumps may be completely shut down during periods of low demand (ranging from a few hours to all night) to allow the tank to drain enough to provide adequate turn-over.
Elevated storage tanks range in height and capacity. Common heights are approximately 100 feet plus the bowl depth of the tank, which is typically 30 to 50 feet. Shorter tanks can be used, particularly to match the pressure of an existing system. Taller tanks are also available. A typical tank 100 feet high with a bowl depth of 40 feet will provide a pressure at the base of the tank of approximately 60 psig (when full). If placed in the highest part of the system being served, such pressures are generally considered adequate for most municipalities. Tank capacity can vary from less than 100,000 gallons to over 2,000,000 gallons.
Ground storage tanks can also be operated as elevated storage tanks provided there is adequate elevation difference between the tank and the area being served. This is common in mountainous areas where tanks can be built on the edges of a municipality at a higher elevation.
Systems fed from elevated storage tanks have several advantages when used as a water source for fire protection systems. First the tank represents a large store of water that is available even with an impairment to the pumping stations. Secondly, acting as a fixed pressure source, systems with elevated tanks generally have less pressure drop when flowing the large amounts of water required for fire protection systems. Finally, a system fed from a tank will generally have a consistent operating pressure over the long term. Compared to pumping station or pressure reducing valve stations, significantly changing the pressure on a water system fed from a tank is very difficult and expensive, as such a fire protection system can be designed to the system pressure with less fear that it will change significantly over time.
One disadvantage to systems fed from elevated storage is they generally have less pumping capacity for a given population than direct pumping systems. This is due to the fact that the elevated storage can provide the difference during times when system demand exceeds pumping capacity. While not a concern in larger systems, small towns may not have adequate pumping capacity to meet the fire flow requirements, resulting in a large tank level drop, or in extreme examples, a complete draining of the tank and a loss of water pressure in a major fire event. These factors must be taken into account when evaluating systems like this.
At the other end of the spectrum are systems that rely solely on pumps to maintain system pressure. Pumps are turned on and off to either maintain a certain discharge pressure, or a certain system pressure (utilizing a remote pressure transducer). Using this control scheme additional pumps are brought on line as system pressure drops. Even more precise control can be accomplished utilizing variable speed pumps which can often maintain desired discharge pressures to within 1 or 2 psi over a wide range of flow rates.
Often called direct pumping systems, these types of systems have several advantages for fire protection. First, without elevated storage, the system must have adequate pumping capacity to meet the maximum system demand, so for a given population, a direct pumping system will usually have more total pumping capacity. Secondly, well designed pumping stations, particularly those driven by variable frequency drives, can maintain very precise system pressures. While an elevated tank may see a 5 to 10 psi variation throughout the day as the water level rises and falls, a variable speed pumping station may see little if any pressure variation, and therefore would require less safety factor in the design of the fire protection system.
There are some disadvantages to direct pumping systems. First, if there is a power failure at the pumping station, the water system would lose pressure in a matter of minutes, while a system equipped with an elevated storage tank may have several hours or more of reserve capacity in the tank. Therefore pumping stations in direct pumping systems must have adequate redundancy both in pumps that may fail as well as the energy source which drives the pumps to ensure water is always available. Another disadvantage, at least from a fire protection standpoint, is that a pumping station’s set pressure can easily be changed throughout the course of the day, week, or year. It is common for water departments to change the pressure throughout the day, raising or lowering it to match system demand or to maintain adequate pressure at a remote location. Also common are seasonal variations, particularly in dry areas where water departments may lower system pressure to reduce water usage and/or waste. Using this type of system to provide water for fire protection requires a careful investigation to ensure adequate safety factors are utilized.
The two methods described above represent the most common methods of operating a water system at the simplest level. However there are several other types of systems and the majority of water utilities often operate different parts of their systems in different ways. In the next installment we will look at some of these, and discuss the importance of a proper understanding of a water system prior to using it for a source of water for a fire protection system.
Chad Lueders is an engineer at Alternatives In Engineering. He has a bachelor’s degree in Mechanical Engineering and holds a Professional Engineer license in the state of Missouri. Mr. Lueders has traveled throughout the United States performing flow tests and analyzing water supplies for numerous commercial and industrial projects.