Document By3yOagGooD53JbQQJQpZMOm4

726 CHAPTER 28 1957 Guide from which general specifications and minimum requirements for piping and accessory equipment can be obtained. Suggestions for selection and installation of materials based on long operating experience of the district heating industry are given in follow ing paragraphs. More detailed information is available in the District Heat ing Handbook, Third Edition.10 Pipe. Either steel or wrought-iron pipe are satisfactory. Welded con struction is most desirable. Valves. Flanged cast-iron valves for pressures under 125 psig are satis- faetoiy. 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 are used for pressures in excess of 125 psig. Where pipe-line apparatus is to be removable, flanged fittings are preferred. Expansion Joints. Pipe bends to absorb expansion are preferable wher ever feasible in order to eliminate the inspections, 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 or copper corrugations depending upon operating conditions. Spare ex pansion capacity should be allowed for contingencies such as higher pressures, superheat, and installation tolerances. Anchors. Anchors can be of structural steel, special fittings, or U-shaped steel straps which partially encircle the pipes. Anchors should be firmly bolted or welded to a short length of cast or stainless steel set in concrete. Anchors are often fabricated by welding structural steel to pipe and embed ding the ends in manhole walls or concrete poured in connection with con duit. Anchors are subject to humid conditions in conduits and manholes, and can deteriorate rapidly. They should be designed for adequate strength, intended life, and the steam pressure to which they can be sub jected, with liberal factors of safety and protection from corrosion. An chors should be attached to both sides of the pipe. Lugs should be welded to pipe on both sides of loop-type anchors to prevent slippage. The canti lever type of anchor should be avoided. Cast steel is preferred to cast iron for anchor material. Alignment Guides. To assure pipe alignment and proper operation of expansion joints, pipes should be adequately guided. Special precautions should be taken if the pipe deviates from a straight line. Pressure Reducing Valves. Suitable single-seated valves are preferable. The valves should be selected from the manufacturer's rating regardless of pipe size. Pressure gages are necessary at valve and control points. Valves require some auxiliary source of power for valve positioning, if they are to be controlled from some remote point to regulate terminal pressure. Traps. To assure adequate drainage of piping, extra trap capacity and large drip pockets should be provided. Extra heavy or corrosion-resistant, trap discharge piping is desirable. On steep grades or in high velocity steam mains, special deflector scoops or baffles should be placed in the line to direct condensate into traps.11 Supports. Due to humid conditions in conduit and manholes, it is de sirable to use stainless steel supports. Instruments. The instruments required will be the same as for other steam systems as described in Chapter 38, Automatic Controls, or Chapter 52, Instruments andMeasurements, with modifications depending on the District Heating 727 records desired with regard to pressures and temperatures maintained or the records of response, of the system to varying load demand.12 Branch Connections. Branch connections should be made at or near anchor points; if not, ample clearance must be provided around the branch to take care of pipe movement. Insulation Insulating materials constitute a primary element of district steam dis tribution structures and must be suitable for underground service. In sulation should be noncombustible, vermin-proof, moisture-resistant, durable, non-corrosive to steel pipe especially when wet, and able to retain its position in relation to the pipe, line. It should be of a type which, if flooded, will dry out and regain its original physical, chemical and insulat ing properties. It should be relatively unaffected by such chemicals as are usually found in ground or other contaminated waters. It should have the lowest practical heat conductivity consistent with the given requirements. Insulating properties vary with soil conditions, weather variations, types of conduits used, and, if in tunnels, the amount of ventilation. The selec tion of insulation to provide for the various conditions is covered in Dis trict Heating Handbook--Third Edition. Where underground insulation is applied directly to the pipe, copper wire is normally used to band the covering. Insulation may be poured as a monolithic structure in a suitable duct. Where insulation in an existing conduit has deteriorated the line may be reinsulated with a minimum amount of excavating by use of some of the materials used in monolithic conduits.13 Insulation is discussed further in Chapter 27, and studies of heat loss from bare and insulated buried pipe are given in District Healing Handbook14 and in Piping Handbook.li RETURN OF CONDENSATE If the condensate can be returned to the plant to be used for boiler feed water at an overall cost less than that required to process raw water, the installation of return lines parallel to the steam supply lines should be given consideration. A typical analysis of comparative costs of returning con densate and processing feed water is given in District Heating Handbook." Return of condensate by gravity is desirable, but usually because Of terrain conditions or user's return facilities, the condensate is pumped back under pressure. Pipes carrying condensate are more subject to corrosion than other parts of the system. Care must be taken to give proper pitch to the pipes and to provide proper venting of non-condensable gases. (See Chapter 42, Corrosion.) BUILDING PIPING AND EQUIPMENT There are certain fundamental principles that should be followed in the resign of a building.heating system which is to be supplied from district steam mains. Some of these are unique to district heating and some, al though they may apply to any building have been demonstrated to be parrcularly important when district steam is used. District heating com panies have certain regulations regarding the consumer's installation to "teguard the mutual interest of supplier and consumer and to insure satisfactory and economical service. When an installation is designed, local egulations and code requirements13 should be observed. Typical service connections are shown in Fig. 2 for low-pressure service