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TEAM
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Hydro -pneumatic Tanks
Water in the USA is something we Americans usually take for granted. We go to the faucet, turn on the spigot and out it comes. Sometimes with less pressure than we would like, but there it is. Producing that water and getting it to the faucet is not considered. Starting with the source, whether it is a river, a lake or a well, it must then be retrieved, treated, stored and then distributed to that faucet. Vessels or tanks for storing water are critical to the efficient operation of any potable water distribution system. Storage tanks serve two major purposes. One is to provide storage volume and the other is to provide pressure to the distribution system. A particular tank can serve one or both purposes depending on its location within the system and its type of configuration. Hydro-pneumatic Tanks One of the most common types of vessel is the hydro-pneumatic tank. These are used in almost every home in America that is not served by a public water system. Hydro-pneumatic tanks are also used to provide pressure to very small public water systems such as resorts, mobile home parks and very small communities. They are not a good storage vessel for fire protection purposes due to the small volume of water within the vessel. This article will concentrate on hydro-pneumatic tanks used for small public water systems. Volume Sizing A consulting professional engineer best performs the design and sizing of a water storage tank. The process is quite complex and involves many considerations. The operators need to have a basic knowledge of the recommendations for storage volume within a system. The current recommendations are that the storage volume should be equal to the average daily demand for the system, not including fire protection requirements. It is also recommended that two tanks be provided for each system. When one tank is out of service for cleaning, inspection and maintenance a backup tank will assure uninterrupted service. Design Since hydro-pneumatic tanks are pressure vessels they must be constructed to meet the codes and standards for pressure vessels. All fittings and accessories must be properly designed for the design pressures. The design code used for hydro-pneumatic tank design is ASME Section VIII. Foundation design is an important aspect of the design. Water weighs 8.33 lb/gallon. A 30,0000 gallon tank will weigh more than 250,000 lb. If the soil bearing is 2500-psf, the foundation foot will need to be at least 100 square feet. Record Keeping Keeping records of the tank design and construction information is important. If a tank is code stamped and the registration number is available, construction records can be obtained from the National Board of Boiler and Pressure Vessel Inspectors. If no construction records are available, it may be required to downgrade the working pressure. All repairs and upgrades should be recorded and qualifications of the welders who worked on the tank kept on file. Operation Normally a hydro-pneumatic tank will contain approximately two-thirds water and one-third air at full capacity. An air compressor is required to maintain a proper volume of air within the tank at the necessary pressure. At low operating level the tank will contain about one-third water and two-thirds air. The air is pressurized to provide a system head and operates at about a 20-pound-per-square-inch pressure difference between high and low water levels. A system using a hydro-pneumatic tank with a need for an average operating pressure of 40-psi would then have a 50-psi pressure at high levels and a 30-psi pressure at low levels. The proper operation of a water storage tank is critical to both the overall system operation and the life of the system components. Improper operation can result in large repair and maintenance costs in addition to shortening the system's components useful life. The operating principles of all the pressure-creating tank types are the same. Routine Maintenance A systematic maintenance program should be established that address the following components:
Assessment Inspection A hydro-pneumatic tank must be periodically removed from service and inspected by trained qualified inspectors who are thoroughly knowledgeable in this process. This inspection should review metal loss due to corrosion, interior and exterior coating condition, component assessment and recommendations for repairs and upgrades. The Florida Department of Environmental Protection (FDEP), 62.555 requires every finished water storage tank to be cleaned and inspected at least once every five (5) years if you furnish potable water to the public. Tank Engineering And Management Consultants, Inc. can help with this clean out and tank inspection, including a complete assessment of your water tank system. |
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