Document V3K7p3NJ2zGV72364QomnOoKN

724 CHAPTER 28 1957. Guide include enclosure of the pipe and insulation in tile, concrete, steel, or as bestos pipe and then pouring of concrete around outer pipe. This method provides space for pipe supports, if needed. External drainage can be pro vided by methods similar to the concrete box-type construction. Conduits constructed of common materials are often preferred by the district heating industry because of the conditions imposed in congested street beds, heavy traffic loads, and drainage conditions or because of the ease of opening and closing the conduit, for branch pipe connections. Prefabricated conduits, C and D, Fig. 1, are commonly constructed in standard lengths. In some, as in C, the insulated pipe or pipes are placed within a corrosion resistant helically-corrugated metal jacket which is protected with a heavy asphaltic coating. Similar casings are made of steel protected with an anti-corrosive compound. Both are of welded construction and can be tested under air pressure. In conduit D the in sulated pipe is placed concentrically within the corrosion resistant metal jacket and the intervening space between jacket and insulation poured full of high melting-point asphalt or left as an air space. In these con duits, the pipes are supported by precast rings or guides to maintain align ment and to prevent pressure on insulation. Fittings are: available for joints, bends, etc. Some of these conduits are also available with cast-iron casings... Tile conduit E, Fig. 1, is constructed of specially-shaped glazed vitrified bell-and-spigot tile. In the conduit shown, the base drain is made in separate units and is designed to support themain.conduit and pipe lines, and .to drain the conduit. All joints are cemented. There are several types of tile conduits. Monolithic conduits F and G, Fig. 1, are similar to the solid-pour type except that the insulation, referred to as insulating concrete, serves also as a conduit and is poured directly around the steam main. The insulation consists of expanded-mica, shredded-rubber or other cellular materials mixed with Portland cement or asphalt; or of a special foaming material mixed with Portland cement. . The latter process creates a multitude of minute air cells in the concrete to obtain its insulating qualities. The poured structure is often protected and strengthened by a concrete duct or water-proofing membrane. Granular-fused type conduits such as H, Fig. 1, consist , of a specially sized and selected granular bitumen which is poured in the trench around the pipes. The manufacturers advise that, the conduit is formed when heat passing through the pipes forms three distinct concentric zones, all resistant to water penetration. A plastic area surrounds the pipe, outside of which is a sintered zone providing thermal insulation, and finally, an outer layer of unconsolidated particles providing further insulation value. This unconsolidated zone is also the load-bearing portion of the structure. Waterproof conduits are intended for use where permanency and thoe oughly tight joints are required. Some of the prefabricated conduits, C and D, Fig.. 1, can be supplied with water-tight cast-iron casings and fittings. The casing shown in I, Fig. 1, consists of castdron pipe with lead-caulked joints enclosing the insulated steam main. It is water, tight and intended for special applications, where, for instance the duct may be submerged, subject to flooding, or laid in extremely wet soil. .All of the preceding conduits have their application and the choice de pends upon local conditions. Any underground' conduit is only as good as, among other things, its foundations to prevent settlement,- its external and internal drainage, its ability to stand up under the corrosive action ot District Heating 725 certain ground conditions (see Buried Pipe Lines, Chapter 42), its strength to carry the load imposed on it, its field installation, and its insulating properties. ... Manholes When conduits are used for underground piping, manholes may- be re quired to house valves and other accessory equipment. They are built of a variety of materials such as reinforced concrete block! They should have adequate volume to permit men to work on equipment. They should be as nearly waterproof as possible and precaution should be taken to prevent entrance of surface water around the covers.. Provision should be made for lifting, necessary equipment from the pipe line, for drainage, for ventilation, and for automatic waterrremoval equipment if a sewer is not accessible. , Details of typical manhole construction housing a single expansion joint are shown in the NDIIA District Heating Handbook, Third Edition, p.764. If more equipment is installed, the size should be increased accordingly. Where water removal equipment is required, sumps are installed under the manholes. Water can be removed periodically with portable pumps, or, where water conditions are bad or periodic inspection is not feasible, by installed sump pumps or water ejectors. The motors for electric pumps should be enclosed in a separate compart ment which is ventilated to protect it from heat and vapors. Precautions should be taken against corrosion as mentioned in the Section Piping and Accessory. Equipment.. Piping Tunnels Because of their relatively high first cost as compared with conduits walking tunnels are usually not provided for steam mains unless, as in some institutions, they are required to accommodate miscellaneous other utility services or to provide underground passage between buildings. Tunnels are usually built of reinforced concrete, brick, concrete blocks or other suitable structural materials, and are waterproofed on the outside. Provision for drainage and ventilation is essential. Since tunnels and conduits are apt to be hot and humid,' special pre cautions should be. taken to reduce temperatures, and protect equipment. Heat loss from pipes and'supports can be reduced by insulating return niams and using more than normal insulation on steam mains. A metal jacket over the pipe covering can permit insulation to carry the weight of the pipe. The precautions. on sizing, design, installation, and protection against corrosion, for accessory equipment, as discussed in the sections Piping and Accessory Equipment, and Conduits, should be taken for tunnels. Special precautions should be taken to prevent rapid deterioration of the electrical conduit. All electrical boxes, junctions and outlets should be vapor proof. entilation of the tunnel can be obtained by installation of suitable fans 01 by natural means. Piping and Accessory Equipment A district heating distribution system is usually buried in the ground - ere H is not readily accessible for repairs or replacement. This makes it Aiye that the system be carefully designed and installed! The. efican Standard Code for Pressure Piping ASA B-31.1--1955" has secns covering district heating piping systems and fabrication details,