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I I I I I I I I I I I I I I I I I I I 4-4 and the pressure differential across the valves (system water pressure less atmospheric pressure). The valves must be protected against freezing and they must be located above ground or in an underground vault or manhole. Isolation valves. The pipeline should be fitted with isolation valves at both ends of the line. Valves will also be required to permit diversion of flow through the raw water storage reservoir or directly to the treatment plant. Head Dissipation. A head dissipation structure or valve will be required at the downstream end of the pipeline to destroy the kinetic energy in the water before it is introduced to the terminal reservoir or to the water treatment plant. At present, a gate valve is used for this purpose. This valve vibrates significantly and consequently creates some noise. Use of valves in series is an appropriate way to destroy head, but the choice of the type of valve for this purpose could be improved. Butterfly valves or globe valves are appropriate, with the butterfly valve probably being less costly. In the early years of operation of the line, an orifice plate could be inserted into the line to dissipate energy. Selection of the most appropriate device should be made during detailed design, after options are costed. Construction sequence The construction sequencing of the new pipeline installation is critical in four locations. These are (1) the Dolores Turnout connection, (2) the connection to both sides of the existing pipe under the State Highway 145, (3) the branch connection into the terminal reservoir, and (4) the connection into the treatme'nt plant supply line near the new valve vault. While the new pipeline is being constructed, the existing pipeline must be kept in service to maintain water supply to the plant. It is likely that the existing line may be isolated for up to 24 hours at a time to permit cross connections. During detai led design, a program for interconnecting the new pipeline should be developed. The time of the year during which these connections are to be made will largely dictate the measures which are to be taken. presently, the ne,l pipeline construction is scheduled for the last six months of 1986. Connections will be made during the winter time while demands are low, and the storage capacity in the treated water system will permit a longer period of time without raw water supply. The new line, above the state highway crossing, could be placed in service at a different time than the new line below this point. Partial use of old and new lines may be required to develop a practical conversion program. BROWN AND CALDWELL C CONSULTING ENGINEERS