When buying a water storage tank, the configuration and selection process often is something that needs to be done in the early stages of project design. To apply for and secure funding that may be available, timing of construction and overall project costs will play a factor in the determination. Because many municipalities have a single source of water storage, the tank design plays a crucial role in meeting the current and future needs of a community.
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Some of the key details essential to the ultimate selection of the tank configuration process are an assessment of community demands for current and anticipated water supply, site conditions, pressure requirements, long-term maintenance, ease of access and overall costs.
There are three types of liquid storage tanks available that are considered for municipal water storage applications: glass-coated bolted steel, welded painted steel and concrete.
1. Design & Configurations
Standpipes, reservoirs and composite elevated tank (CET) designs are the different types of configurations used when selecting glass-coated bolted steel tanks. Standpipes are tanks where the water is elevated in a tall column to achieve gravity-fed pressure, which is required to properly feed the system. The tank height is greater than the tank diameter. The elevation of the water is accomplished by storing the required “water on top of water.” Standpipe height usually does not exceed 140 ft.
The most common configuration used for water storage is the reservoir. Reservoirs have a greater diameter than height and can be used with a pumping system or can be gravity fed. The width of these tanks can reach 250 ft with capacities up to 6 million gal. Similar to standpipes, the CET design is used in applications where height is used to achieve the head pressure needed to properly operate the system. The CET column is constructed in a hollow concrete pedestal on which the tank is then built. Structural rebar and steel embedded in the concrete reinforced walls that can exceed 10 in. thick and a top cap of concrete 4 ft thick.
There is plenty of space offered in the interior of the concrete pedestal for municipal maintenance equipment, pump stations, office space and other uses. The CET design does not have height restrictions and capacity can be as great as 1.5 million gal.
Depending on the diameter, snow loads and other factors, the roof of glass-bolted tanks can vary. They can be a free-span aluminum geodesic dome, consisting of panels mounted on a rigid structural frame, or the same glass-fused-to-steel material.
The tank floors usually are constructed with reinforced concrete or they can be glass-coated panels depending on site and design conditions.
2. Manufacturing Process
The technology and manufacturing process of this equipment sets these tanks apart from painted steel or concrete structures. When using the factory manufacturing process, the uncontrolled variables are eliminated, unlike field manufactured products such as painted steel or concrete tanks. Worker experience and extreme climatic environmental conditions that are proven to have a significant effect on in-field manufactured products have minimal effect on the glassing process. In addition, the tanks can be erected year-round as the manufacturing is completed in the factory and only the assembly of the components is required in the field.
3. Coating
All storage tanks have a coating. The coatings available today consist of either paint, concrete or glass. The impermeability and features of glass offer advantages.
The glass coating process begins with a glass frit that is mixed with other minerals and water to create a liquid slurry. This glass slurry is then robotically sprayed at precise amounts and thicknesses onto previously cut and rolled, punched, grit-blasted and cleaned steel sheet panels. Companies like CST run panels through a furnace at ° F. This heat melts the silica glass slip into the surface of the grit- blasted steel. This completes the mechanical bond, as well as the chemical bond between the steel and the silica glass.
Different coatings that are available for other tanks rely on a mechanical bond of the coating to the underlying material. The chemical bond strength is many times the holding strength of the conventional mechanical bond and prevents any undercutting of the coating, which can allow spreading of corrosion on the primary steel material. This benefit can best be explained by imagining a scratch on an automobile. Because that coating only has a mechanical bond, if the steel is exposed, corrosion will occur. Left untreated this corrosion will expand and creep beneath the surrounding painted surface and compromise the remaining coating.
This often is witnessed with raised bubbles, spreading rust and weakened substrate. The chemical bond of the glass-fused-to-steel coating prevents this spreading of corrosion in the event the coating were compromised.
With the goal of making the storage tanks as maintenance free as possible, companies like CST manufacture rounded sheet edges to exact radii to ensure adherence of the glass for complete encapsulation on all four sides of the sheet.
4. Tank Construction
A jacking system is used when erecting a glass-coated bolted steel storage tank. Once the starter sheet (bottom ring) is either embedded into the concrete foundation or constructed utilizing a glass-fused-to-steel floor design, the top ring of the tank is constructed on the jacks. The roof of the tank then is erected and the ring and roof are jacked up. Each additional ring is then assembled below the top ring by bolting the sheets together and applying a urethane sealant between the seams.
Tanks are assembled from the top down allowing for a safer and faster construction environment. The erection process normally is completed within a week or two, which saves costs to the owner if prevailing wages for onsite labor are being used. Additionally, the manufacturer requires that all building crews be factory-trained and certified in the erection process, ensuring quality control in the field.
5. Maintenance/Life Time Value
Glass-coated bolted steel tanks have a long lifetime. Glass coating never needs painting because it is permanent. Glass-coated tanks often are placed in areas, where long-term pleasing visual appearance is sought. The budgeted dollars that may be used to repaint a painted tank or repair aged concrete can be saved and funneled to other projects in a municipality.
6. Flexibility
The bolted design and erection of this product yields flexibility. Because manufacturing is completed in a factory, large staging areas needed when a product is manufactured onsite are eliminated. The construction of the tank typically can be completed with a cleared area of roughly 6 to 10 ft around the tank diameter. This small footprint can save thousands of dollars on the overall project beyond the price of the tank itself. The panels themselves can be hand-carried and easily assembled without cranes or special equipment, allowing this tank to be installed in many locations that would be impossible for other tank types.
7. Expandability
The glass-coated bolted tank design allows the tanks to be vertically expanded. If a community or industry experiences growth and additional capacity is necessary, the tanks’ jacking process allows the end-user to gain capacity quickly and cost-effectively. The factory-trained professional building crew unbolts the bottom ring from the original starter sheet, jacks the tank up and adds the number of rings necessary to achieve the new capacity. When these tanks are expanded, there is no difference in appearance between the original panels and the new panels.
Conclusion
Initial construction costs, anticipated life and long-term maintenance costs are significant factors relative to the various tank designs and materials available today. The long-term maintenance costs and life cycle during a tank evaluation all must be considered when selecting the appropriate product for a specific project.
Because project financing can vary depending on several factors, a complete analysis of initial costs and, lower maintenance should help a community decide which type of product is best suited for its needs.
In North America, municipal water systems are often robust, with distribution piping sized large enough to provide adequate water supplies to satisfy the demand for many water-based fire protection systems, including sprinkler and standpipe systems. When a municipal water supply is not sufficient, it is typically due to a lack of required pressure, in which case a listed fire pump can be installed to increase the pressure from the municipal water supply.
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By Shawn Mahoney
In cases in which the municipal water supply is not available, reliable, or it cannot provide adequate flow for the fire pump, a fire pump and water storage tank needs to be used. This is because a fire pump alone can only increase the water pressure; it cannot increase the flow available. Water storage tanks and pumps are usually seen in facilities with systems that have a significant water supply demand, such as large warehouses. Water storage tanks can be made of various materials such as wood, steel, concrete, fiberglass-reinforced plastic (FRP), and rubberized fabric.
NFPA 22, Standard for Water Tanks for Private Fire Protection, provides the minimum requirements for the design, construction, installation, and maintenance of tanks and accessory equipment that supply water for private fire protection, including:
This blog will explore some of the most common types of water tanks as well as introduce some of the common water tank equipment and accessories used.
There are several different types of water storage tanks. Depending on the needs of the system that the tank is supporting and the geographical and topographical location of building, the type of tank that is used will vary. Each type of water storage tank can have vastly different system components.
Gravity Tanks
Water pressure increases by 0.433 psi for every foot of elevation gain (0. bar per meter). Gravity tanks, otherwise known as elevated tanks, take advantage of this by increasing the height of the water to increase the pressure created by gravity. These types of tanks are typically not used for private fire supplies and instead are part of municipal water supplies.
Suction Tanks
Suction water tanks are located on the ground. Because of this, they do not create pressure. Instead, they are used to provide water to a fire pump, which creates the needed pressure for the fire protection system. These are the most common type of tank used because they tend to be more cost effective, but they take up much more space than the next option.
Underground Tanks
Underground tanks are like suction tanks in that they do not create their own pressure but rather they supply a fire pump with water, and the fire pump creates the needed pressure. These tanks require a vertical turbine pump or the pump be located below the level of the tank.
NFPA 22 does not provided requirements for water tank capacity. Instead, it directs you to the standard for the fire protection system that is being supplied. The required capacity is calculated by multiplying the system demand flow rate by the required duration. For example, NFPA 13, Standard for the Installation of Sprinkler Systems, requires specific minimum durations based on the hazard classification and sprinkler system supervision. If a light hazard sprinkler system had a system demand of 100 gpm (380 L/min) for a duration of 30 minutes, then the minimum required capacity would be 3,000 gallons (11,400 liters).
Table 19.2.3.1.2, NFPA 13
There are many accessories and equipment that are needed for a water tank to ensure that it is a reliable water supply.
Tank Heater
Water tanks and the associated piping needs to be maintained at or above 42°F (5.6°C) to protect against freezing. Calculations are needed to determine if this protection can be afforded with either insulation alone or if a tank heater is needed. If the lowest one-day mean temperature is greater than or equal to 5°F (-15°C), heating systems are only required for elevated tanks. If the lowest one-day mean temperature is less than 5°F (-15°C), then calculations are needed to determine if a heater is needed and to determine the capacity of the heating system.
Water Level Gauge
A water level gauge is needed on all water tanks to be able to see the level of the water in the tank. A listed high and low water alarm can be used in lieu of a water level gauge.
Anti-Vortex Plate
The discharge outlet of every suction tank is required to be equipped with an anti-vortex plate. This anti-vortex plate is installed to eliminate the possibility of a vortex being created, similar to the vortex seen when draining a sink or bathtub. This vortex would allow air to enter the suction pipe, travel to the fire pump, and lead to cavitation.
Figure B.1(o), Suction Nozzle with Anti-Vortex Plate for Welded Suction Tanks, NFPA 22
There must be a permanently installed inlet pipe to fill the water tank, and it must be sized to fill the tank in 8 hours or less. Additionally, the water level needs to be maintained within 4 in. (100 mm) of the required fire protection level (determined by the required capacity). Typically, this is done with an automatic tank fill, but can be done manually if approved by the authority having jurisdiction (AHJ).
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