A. Pressure Requirement. Water systems shall be designed to maintain a minimum residual pressure of 30 psi at the point of delivery under maximum demand flow conditions, excluding fire demand. For water systems requiring fire flow capability, the design shall be adequate to maintain, under fire flow conditions, positive pressure throughout the system and a 20 psi residual pressure in mains supplying fire hydrants in use (Chapter 246-290 WAC).

B. Pipe Sizing. Water mains shall be sized using the DOH "Water System Design Manual." Minimum main size may be established by a licensed engineer using recognized hydraulic analysis techniques. Water main size shall be adequate to deliver fire flow, if required, and to maintain the pressure requirement defined above.

C. Isolation Valving. Valving shall be installed in a configuration which permits isolation of distribution system lines. A valve is not generally required for short block lines of less than 100 feet. In addition, unvalved lengths of pipe serving commercial, multi-family or school areas, should not exceed 400 feet. Greater spacing between isolation valves is allowed on transmission lines, to be determined on a case-by-case basis by the purveyor.

D. Air and Air-Vacuum Relief Valves. To minimize problems associated with air entrainment, the purveyor shall provide for installation of air or combined air-vacuum relief valves at appropriate points of high elevation in the system.

E. Blow-off Valves. A blow-off assembly shall be installed on all dead end runs and at designated points of low elevation within the distribution system. The blow-off assembly shall be installed in the utility right-of-way except where an access and construction easement is provided for in writing by the water purveyor. In no case shall the location be such that there is a possibility of back-siphonage into the distribution system.

F. Storage. Storage requirements are based upon three components;

G. General Facility Placement. Below-ground facilities shall be located in accordance with applicable municipal or County ordinance. Where no ordinance applies, water mains shall be installed at a location which is compatible with the existing water system, the terrain, and the location of other utilities.

In addition, all piping, pumping, source, storage, and other facilities shall be located on purveyor-owned property and/or public rights-of-way or dedicated utility easements. Utility easements must be a minimum of 15 feet in width, and piping shall be installed no closer than 5 feet from the easement's edge. Exceptions to this minimum easement may be approved by the operating water purveyor. Purveyor access shall be provided to all public water system lines and their appurtenances and public fire hydrants that are maintained by public agencies or purveyors.

H. Pipe Cover. The depth of trenching, installation of pipes, and backfill shall be such as to give a minimum cover of 36 inches over the top of the pipe. This standard shall apply to transmission, distribution, and service piping. Backfilling up to 12 inches over the top of the pipe shall be evenly and carefully placed. Remaining depth of trench to be filled in accordance with applicable construction standards identified in General Provision. Materials capable of damaging the pipe or its coating shall be removed from the backfill material.

I. Separation Distances. Whenever possible, transmission and distribution water piping shall be separated at least 10 feet horizontally from on-site waste disposal piping, drainfields, and/or wastewater gravity or force mains. At a minimum, the bottom of the water main shall be 18 inches above the top of the sewer. Where local conditions prevent such horizontal and/or vertical separation, closer spacing may be permissible where the separation is mitigated in the design and construction, and meets requirements set forth in DOE's "Criteria for Sewage Works Design," and WSDOT/APWA "Standard Specifications for Road, Bridge, and Municipal Construction."