NAC445A.6708. Storage structures: Design and construction.

     1. Storage tanks must:

     (a) Be designed by an engineer, structurally competent and constructed of materials that are acceptable to the Division or the appropriate district board of health.

     (b) Employ a foundation that is appropriate for the type of tank and complies with the American Water Works Association Standards, as adopted by reference in NAC 445A.6663.

     2. A supplier of water shall provide means for the drainage of storage structures. Storage structures that provide pressure directly to the distribution system must be designed in such a manner that they can be isolated from the distribution system and drained for cleaning or maintenance without any loss of pressure in the distribution system.

     3. Storage structures must have a device for overflow that is brought down to an elevation of not less than 12 inches nor more than 24 inches above the surface of the ground, is sloped for complete drainage and discharges over a drainage inlet, plunge pool or splash plate without causing erosion. The outlet of the drain must be protected with an angled flapper valve and located in such a manner that any discharge is visible. The device must be sufficiently large to dispose of overflow at a rate that equals the maximum rate for filling the structure. The device must have an air gap and must not discharge directly into a sanitary sewer or a storm sewer. Discharge from the device must be controlled in a manner that does not present a hazard to or cause a nuisance for any existing or contemplated development of property.

     4. Storage tanks must contain vents that:

     (a) Prevent external pressures from causing the tank to buckle; and

     (b) Are designed in such a manner as to:

          (1) Prevent the entrance of rain and surface water; and

          (2) Exclude dust, birds, insects and other animals as much as possible.

Ê For the purposes of this subsection, “vent” does not include a device for overflow.

     5. Vents, devices for overflow, drain outlets and other openings in a storage tank must be constructed and located in such a manner as to protect the stored water from contamination. Top and side vents must be screened and turned downward, except that mushroom vents in the center of the roof are acceptable. Screens used for venting air must be constructed of a stainless steel that is not susceptible to damage by corrosion and must have not less than 22 nor more than 24 mesh per inch. Drain outlets must have an air gap. Vents in buried structures must be not less than 24 inches nor more than 36 inches above the finished grade.

     6. The discharge pipes from all storage structures must be located in a manner that will prevent the flow of sediment into the distribution system. A removable silt stop, of not less than 4 inches nor more than 6 inches, must be installed on the floor of a storage structure over the discharge pipe.

     7. Storage structures must have a device for indicating the level of water in the structure. Automatic controls and set points must be provided which are adequate to maintain the level of water. Alarms to indicate respectively that the level of the water is too high or too low must be installed in or transmitted by telemetry to a prominent location. The design and operation of such a structure must provide for an adequate turnover of stored water. If a public water system has two or more storage structures located at different hydraulic elevations, the Division or the appropriate district board of health may require the public water system to install altitude control valves or similar controls.

     8. Steel storage tanks must have:

     (a) Two manholes, each with a diameter of 30 inches, in the side of the tank that allow entry into the interior of the tank for cleaning and maintenance.

     (b) One manhole on the roof of the tank. The manhole must have a curbing or frame around its opening that:

          (1) Extends at least 4 inches above the surface of the roof;

          (2) Is gasketed;

          (3) Is hinged on one side; and

          (4) Is equipped with a cover that:

               (I) Is watertight;

               (II) Can be locked; and

               (III) Overlaps the curbing by at least 2 inches.

     9. Storage structures and their appurtenances, including vents, riser pipes and devices for overflow, must be designed in such a manner as to prevent any freezing that would interfere with the proper functioning of the structures and their appurtenances.

     10. Each catwalk located over finished water stored by a public water system must have a solid floor with raised edges and be constructed in such a manner that shoe scrapings and other dirt will not fall into the finished water.

     11. Sampling taps must be provided in a vault at a storage tank which are appropriate for facilitating the collection of samples of stored water for chemical analyses and for ascertaining the concentration of coliform bacteria.

     12. If necessary to allow for any differential movement of a storage tank caused by settling or seismic activity, the inlet and discharge piping of the tank must be provided with flexible coupling.

     (Added to NAC by Bd. of Health, eff. 2-20-97; A by Environmental Comm’n by R194-08, 10-27-2009)