Document RaxZX1rELOvkJjOMQewGZmG0z

386 CHAPTER 27 1959 Guide outlet. Proper foundations should be provided to prevent settlement and to insure proper pitch of piping at all tunes. Manholes may be required at intervals to provide access to valves, traps, and expansion joints. They should be drained. Some safe means of disposing of condensate from piping should be provided. There are many types of conduits. They fall into six fol lowing general classifications: Box-type, Solid-pour, Pre fabricated, Tile, Monolithic, Granular Fused, and Water proof. Cross-sections of a few of the typical types are shown in Fig. 1. Box-type conduits are typical of those built with common materials and have many variations. Conduit A, Fig. 1, shows a commonly used type in which a corrugated metal form is placed as an inverted U over the insulated pipe line in stalled on a concrete slab. The concrete may be reinforced or not, depending upon the load on the conduit. Drainage is provided by the drainage pockets filled with crushed stone or gravel and may be installed on either or both sides of the conduit. They conduct water from the top of the conduit to the drain below which is connected to the sewer or some other disposal facility. Variations of the concrete-box type have side walls constructed of hollow vitrified tile, concrete blocks, or other suitable materials, and covered with a re inforced concrete slab. The top and sides are sometimes covered with a waterproofing membrane of asphalt or hot pitch and roofing paper. The solid-pour construction, Conduit B, fig. 1, is also usually made with common materials and consists of poured structural concrete which is vibrated and tamped around a conventionally insulated steam line. If the insulation has a high compressive strength it can support the pipe, other wise, the pipe should be supported independently. Reinforc ing is sometimes used to prevent settlement. An eccentric space may be left around the pipe by using insulation larger the outside diameter of the pipe to permit upward movement of the pipe during the warm-up periods. During construction, pipe is suspended by wires. Precautions should be taken to prevent the pipe from floating. The top of the conduit should be coated with asphalt emulsion or tar to help 'seal shrinkage cracks. Anchor stresses are higher with this type of construction. Variations of this type include en closure of the pipe and insulation in tile, concrete, steel, or asbestos pipe and then pouring of concrete around outer pipe. This method provides space for pipe supports, if needed. External drainage can be provided by -methods similar to the concrete box type construction. Conduits constructed of common materials are often pre ferred by the district beating industry because of the condi tions 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 in sulated 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 insulated pipe is placed concentrically within the corrosion-resistant metal jacket and the inter vening space between jacket and insulation poured full of high melting-point asphalt or left as an air space. In these conduits, the pipes are supported by precast rings or guides BROKEN STONE LONGITUDINAL SECTION 1 CROSS SECTION District Heating 387 to TnR-wrtRM* alignment and to prevent pressure on insulation. Fittings are available for joints, bends, etc. Some of these conduits arc 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 the main 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 in sulating concrete, serves also as a conduit and is poured di rectly 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 con crete to obtain its insulating qualities. The poured structure is often protected by a water-proofing membrane and strengthened by a concrete duct. 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 insula tion, 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 perma nency and thoroughly tight joints are required. Some of the prefabricated conduits, C and D, Fig. 1, can be supplied with water-tight cast-iron carings and fittings. The casing shown in I, Fig. 1, consists of cast-iron pipe with lead-caulked joints enclosing the insulated steam mam It is water tight and intended for special applications, where, for instance the duct may be submerged, subject to flooding, or laid in ex tremely wet soil. All of the preceding conduits have their application and the choice depends upon local conditions. Any underground conduit is only as good as, among other things its founda tions to prevent settlement, its external and internal drain age, its ability to stand up under the corrosive action of cer tain ground conditions (see Buried Pipe Lines, Chapter 55), its strength to carry the load imposed on it, its field in stallation, and its insulating properties. Manholes When conduits are used for underground piping, manholwa may be required to house valves and other accessory equip ment. They are built of a variety of materials such as rein forced 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. Pro vision should be made for lifting necessary equipment from the pipe line, for drainage, for ventilation, and for auto matic water-removal equipment if a sewer is not accessible. Details of typical manhole construction housing a cingla expansion joint are shown in the NDHA District Heating Handbook, Third Edition, p. 164. If more equipment is in stalled, the size should be increased accordingly. Where water removal equipment is required, sumps are installed under the manholes. Water be removed periodi cally with portable piumps, 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 compartment which is ventilated to protect it from heat and vapors. Precautions should be taken against corro sion 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 pro vide underground passage between buildings. Tunnels are usually built of reinforced concrete, brick, con crete 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 precautions should be taken to reduce temperatures and protect equipment. Heat loss from pipes and supports 'can be reduced by insulating return mains and nring more than normal insulation on steam main?! A metal jacket over the pipe covering can permit insulation to carry the weight of the pipe* The precautions on sizing, design, installation, and pro tection against corrosion, for accessory equipment, as dis cussed in the sections Piping and Accessory Equipment, and Conduits for Piping, should be taken for tunnels. Special precautions should be taken to prevent rapid deterioration of enclosed electrical conduit. All electrical boxes, junctions and outlets should be vapor proof. Ventilation of the tunnel can be obtained by installation of suitable fans or by natu ral means. Piping and Accessory Equipment A district heating distribution system is usually buried in the ground where it is not readily accessible for repairs or replacement. This mn-lrua it imperative that the system be carefully designed and installed. The American Standard Code for Pressure Piping ASA B-31.1--1955* has sections, covering district heating piping systems and fabrication de tails, from which general specifications and minimum re quirements for piping and accessory equipment can be ob tained. Suggestions for selection and installation of materials based on long operating experience of the district heating industry are given in following paragraphs. More detailed informa tion is available in the District Heating Handbook, Third Edition.** Pipe. Either steel -or wrought-iron pipe is satisfactory. Welded construction is most desirable. Valves. Flanged cast-iron valves for pressures under 125 psig are satisfactory. For pressures from 125 to 300 psig flanged-steel and welded-end valves are used. Outside screw and yoke valves are preferred. Fittings. Forged or rolled steel is used for pressures in excess of 125 psig. Where pipe-line apparatus is to be re movable, flanged fittings are preferred. Expansion Joints. Pipe bends to absorb expansion are preferable wherever feasible in order to eliminate the in spections, maintenance, and housing required by slip joints. Slip joints should be guided either externally or internally, or both. Corrugated-type joints should have stainless steel