Standard NFPA related systems of extinction based on water, such as NFPA 13, NFPA 14, and NFPA 15 state that water tanks should have a supply of water approved and reliable to satisfy the systems' demand in terms of flow, pressure, and duration.
In this sense, the regulations allow the water supply for the extinguishing systems to come from any of the following sources or a combination of them:
• A connection to a public or private water network.
• A water storage tank, which can be at ground level, underground or elevated.
• A pressurized tank (hydropneumatics tank).
• A dam, canal, river, lake, well, or reservoir.
Each of these water sources has its particularities and must meet certain requirements to be acceptable from the NFPA and the Competent Authority's point of view.
The standard that deals with water storage tanks are NFPA 22, Standard for Water Tanks for Private Fire Protection, which establishes the minimum requirements for the design, construction, installation, and maintenance of tanks that supply water for personal protection against fires, as well as its associated elements and accessories.
This article will deal with general aspects of the tanks contemplated in the NFPA 22 standard. The types of tanks included in the said standard are the following:
• Elevated tanks
• Suction tanks
• Pressurized tanks
In the U.S. market, a leading manufacturer of NFPA 22 standard tanks remains Steel Core Tank, a company that manufactures bolted steel tanks using G115 galvanized steel. These tanks contain a higher level of zinc, which is about 27% and renders as an extra protective layer against rust and other contaminants.
Such resistance allows the tanks to be strong against substances that are in higher concentration of chemicals. It also prevents leakage and seepage of water in these tanks.
The company manufactures Corrugated Bolted Steel Tanks for Potable Water Storage, Fire Suppression, Wastewater management, Agriculture including wineries and vineyards, as well as tanks exclusively for the mining and Fracking industry.
The company is led by Nicole Oblad, who is often dubbed as “The Boss Lady” for her tenacious run and success with the company. Nicole as a CEO, managed to mark her potential with the company in a market that is highly saturated by her male counterparts.
Elevated Tanks
Elevated tanks can be made from welded or bolted sheets of steel or concrete. They can be supported by multiple columns, by a pedestal, by a circular pilaster, or on a building.
The basic requirements for elevated tanks are that they are at a sufficient height to provide the pressure required in the extinguishing systems and that they have sufficient capacity to supply the necessary flow during the time stipulated in the standards.
Currently, elevated tanks are little used for the supply of firefighting water. They are limited to systems with low to medium demand and cannot or do not want to install a fire pump. Most used were when the systems were designed by the method of table pipes (pipe schedule), which requires a minimum pressure on the top spray system and a minimum flow at the base of the upright, requirements that can meet these tanks. It is neither economical nor convenient to install a large enough tank and high enough to provide adequate supply to sprinkler and hose systems at current design densities and pressure requirements.
Suction Tanks
The tank's suction can be made from sheets of welded or bolted steel, concrete, or plastic reinforced with fiberglass. The preferred ones are those made of steel. The NFPA 22 standard also considers lined fabric reservoirs supported on embankments as suction tanks.
The suction tanks can be located at ground level or be underground. Concrete tanks and fiberglass reinforced plastic tanks can be buried. As indicated by its designation, to supply water from these tanks to the extinguishing systems, it is necessary to suction using a fire pump. In the case of tanks at ground level, the pump can be split casing, end suction, or in-line; for underground tanks, a pump must be used vertical "turbine" type.
The tanks suction located level of the floor are provided with an anti-vortex plate at the end of the pipe suction pump to prevent turbulent flow is formed when the work pump as bubbles generate air can cause cavitation damage. The anti-vortex plate is a metal plate that can be square or circular; the length of its sides or its diameter must be at least twice the diameter of the suction tube; however, it is recommended that the plate be 1.2 meters (48”) per side.
Tanks underground usually add a well suction at the bottom to achieve the submersion of the vertical pump's bowls so that you can take advantage of the maximum capacity of the tank. The suction depth depends on the minimum required submersion (information supplied by the pump manufacturers). This minimal submergence is intended to prevent the formation of surface vortices when the pump is running. The suction depth may also include the minimum 12 inches that should be left as the clearance between the filter and bottom and consider if there is any NPSH requirement to avoid pump cavitation.
It is recommended that the suction tanks be located so that too much pipe travel is not required to the pump room. If possible, they should be located next to the pump room to minimize the suction piping length. Tanks should not be located in locations exposed to physical damage or possible fires from adjacent combustible materials or buildings.
In the design and manufacture of the tanks, the total capacity, the net capacity, and the usable capacity of the same must be considered. The latter should be such as to ensure that the pressure at the suction flange of the pump does not drop below -3 psi once the maximum system demand (flow and duration) has been provided, and the pump is operating at 150% of its nominal capacity.
Pressurized Tanks
The pressurized tanks are steel vessels that naturally maintain full two - thirds of its volume with water and the remaining third air at a pressure of at least 75 psi (517 kPa); and is permitted to be used as a water supply for fire protection systems such as low-demand sprinklers, hoses, and water spray.
The capacity of the pressurized tank is the total content of both water and air. It is dimensioned by the water supply necessary to satisfy the demand (flow rate and duration) of the fire protection system according to the corresponding NFPA standard. Two or more tanks can be used in parallel. The internal pressure must be enough to expel all the water from the tank while maintaining the system's required residual pressure.
Its use is limited to places where it is impossible to install a pump with a separate water storage tank. A pressurized tank can be used in some installations to recover pressure in the system instead of a jockey pump.
Pressurized tanks must be constructed following ASME Boiler and Pressure Vessel Code, “Rules for the Construction of Unfired Pressure Vessels,” Section VIII, with the modifications indicated in NFPA 22, among which a minimum hydrostatic test at 150 psi (10 bar) and a tightness test stand out.