7.10.2.2 Cst Systems

CST systems are the preferred buried system. These systems consist of concrete, at grade tunnels, that allow access along the entire route. These systems can be used for all the listed media. The CST design should include the following:

• Detailed piping layouts showing all support locations

• Clearances inside the trench system, insulation, and jacketing selection

• Concrete trench wall and floor design (cast-in-place)

• Concrete top design (precast or cast-in-place) complete with lifting devices

• All road crossings

• Grading to keep groundwater from ponding over the trench system

• Sealant types and locations

• System slope 1 in./20 ft [25 mm/6096 mm] minimum, to ensure the trench floor will drain to the valve manholes)

• Vent locations

All drains and traps must be located in the valve manholes. Vents may be located in the trench system only if access is provided to them with manhole lids poured in the trench top.

The use of manholes in these systems to provide housing for drains and traps must be evaluated concerning confined space entry provisions.

7.10.2.3 Buried Conduit (preapproved type)

Due to many premature failures, buried conduit (preapproved type) is the last choice in buried distribution systems. These systems consist of insulated steel carrier pipe enclosed in a drainable and dryable steel conduit. These systems are not preferred except in an unusual situation that precludes the use of any other system (e.g., flood plain areas).

Manufacturer's Responsibility

Buried conduit systems are transported to the site in factory assembled sections. The manufacturer is responsible for the design of pipe supports, expansion compensation devices, end seals, insulation types, conduit design, and universal protection of the con­duit. The manufacturer must submit expansion stress calculations for the designer to review compliance with project specifications.

Designer's Responsibility

The designer is responsible for all the general design considerations listed previously and should also include the design of the buried conduit system's penetration into the con­crete valve manhole and the detailed routing of the system on the site.

The use of manholes in these systems to provide housing for drains and traps must be evaluated concerning confined space entry provisions.

7.10.2.4 Buried Conduit (not preapproved type)

ВС systems (not preapproved type) consist of an insulated metallic or nonmetallic car­rier pipe covered by a nonmetallic conduit. Due to the lower pressures and temperatures of these media, these systems have proven effective.

ВС systems (not preapproved type) are similar to the preapproved buried conduit in that these systems are delivered to the site in factory assembled sections. However, the designer

has less control with the not preapproved system. The designer chooses the items listed for general design considerations, and, in addition, provides detailed piping layouts, insulation type and thickness, conduit selection, carrier pipe selection, and valve manhole entrances.

The use of manholes in these systems to provide housing for drains and traps must be evaluated concerning confined space entry provisions.

7.10.3 Design of Heat Distribution Systems

The design of heat distribution systems includes, but is not limited to the following:

• Mechanical—expansion compensation, piping system design (fittings, valves, insulation), equipment selection, equipment sizing, and pipe sizing and routing

• Structural—reinforced concrete design, pipe supports, valve manhole design, and other miscellaneous structural designs

• Electrical—electrical service to equipment and controls, and universal protection (if required)

• Civil—excavation and backfill, grading, road crossings for buried systems, area drainage design, system plans and profiles, and site coordination to ensure sys­ tem integrity (especially for CST) fits into the site properly

7.10.4 Existing System Capacity

The designer must determine if the system has adequate capacity to tie into the exist­ing heat distribution system. The designer must also determine if the connecting points for the existing lines have adequate hydraulic capacity (are large enough) to satisfactorily han­dle the new loadings under variable operational scenarios.

Each installation should have hydraulic analysis data to indicate what the new loading impact is on the existing system. This information must be provided by the designer. The designer must update the hydraulic analysis, while considering possible future expansion impacts, as part of any new system design.

7.10.5 General Design Considerations

The following general design considerations should always be considered:

• Survey—A survey in the location of the distribution system must be done com­ plete with soil borings and information on groundwater, soil types, and soil resis­ tivity. The survey data should be noted.

• Utilities—A utility investigation must identify all existing utilities within a mini­ mum utility corridor of 25 ft (7.6 m) of the new distribution system (including information on type, piping material, size, and depth). This investigation includes the engineering determination of where to connect the new distribution system to the existing system. All new connections must be at or near existing system anchor points to avoid damage to the existing utility system.

• Pipe sizing—All new pipes must be sized in accordance with prescribed engi­ neering design procedures. Minimum line sizes for any system should be 1.5 in. (38 mm) (nominal). The use of better performing pipe materials for specific trench soils should be a consideration.

• Expansion—Expansion compensation calculations are necessary to ensure the new lines are properly designed under the engineering allowable values for stresses, forces, and moments. A computer finite element analysis program can be used to determine these values. Only loops and bends are to be used for expansion com­ pensation. No expansion joints should be permitted in the design and installation.

• Valve manholes—Concrete valve manholes must be completely designed includ­ ing structural grated or concrete covers, internals (including valves, traps and drip legs), clearances, and reinforced concrete design.

• Drainage—All valve manholes must either be gravity drained to an existing storm drain line with backflow protection or to a remote sump basin complete with duplex sump pumps, which discharge to an existing storm drain line or to grade.

• Grading—Regardless of the system, grading must be designed to prevent groundwater from entering the valve manholes.

• Plan/profile—Plans and profiles should be drawn for all systems showing, at a minimum:

—System routing and piping slope elevations

—System stationing

—All existing utility and other major interferences (depths if known)

—All adjacent roads and buildings clearly labeled

—Current types of surface conditions along the new utility corridor (asphalt,

grass)

—Both new and existing grade contour lines (plan) —Exact support locations for the new utility system —Dimensioning (consistent English or metric units) to ensure accurate utility

routing

7.10.6 Identification

Provide a brass name tag for each valve and temperature control device installed in all mechanical systems.

All exposed or concealed piping in accessible spaces should be identified with color-coded bands and titles in accordance with American National Standards Institute (ANSI) Standard A13.1, Scheme for Identification of Piping Systems.

• Pipes in buildings are categorized as pipes related to —Fire protection systems

—Critical piping in essential and hazardous facilities —All other piping

• All water pipes for fire protection systems in seismic zones 1, 2, 3, and 4 will be designed under the provisions of the current issue of the Standard for the Installation of Sprinkler Systems of the National Fire Protection Association (NFPA No. 30). To avoid conflict with these NFPA recommendations, the criteria in the following subsection are not applicable to piping expressly designed for fire protection.

• Ductwork in buildings is categorized as

—Critical ductwork in essential and hazardous facilities —All other ductwork

Consistent system identification provides a basis for future communication to mainte­nance and operations personnel, users of the system, and emergency providers.