Document BR84pKQNdQEw239jmm7JEYoao

620 CHAPTER 28 1952 Guide " steam requirements such as laundry, cooking, or process, should be indi vidually calculated. The steam requirements for water heating should be taken into account, but in most types of buildings this load will be relatively small compared with the heating load, and will seldom occur at the time of the heating peak. Unusual features, such as large heaters for swimming pools, should not be overlooked. _The pressure at which the steam is to be distributed will depend upon (1) boiler pressure, (2) whether exhaust or live steam, and (3) pressure requirements of apparatus to be served. If steam has been passed through electrical generating units, the pressure will be considerably lower, than if live steam, direct from the boilers, is used. The advantages of low pressure distribution (2 to 30 psig) are: (1) smaller heat loss per square foot of pipe surface; (2) less trouble with traps and valves; (3) simpler problems in pressure reduction at the buildings; and (4) general reduction in maintenance costs. With distribution pressures hot exceeding 40 psig, there is little danger even if the full distribution pres sure should build up in the radiators through.the faulty operation of a reducing valve; but with pressures higher than 50 psig,a second reducing valve or some form of emergency relief is usually desirable to prevent ex cessive pressures in the radiators. The advantages of high pressure distribution are: (1) smaller pipe sizes; (2) greater adaptability of the steam to various operations other than building heating; and (3) wider flexibility in allowance for maximum pres sure drop and ability to serve equipment requiring higher pressures. . Frequently the different kinds of apparatus which must be served require various minimum pressures. Kitchen equipment requires from 5 to 15 psig, the higher pressures being necessary for apparatus in which water is boiled, such as stock kettles and coffee urns. An increased amount of heating surface, which is easily obtained in some kinds of apparatus, re sults in'quicker and more satisfactory operation at low pressures. For laundry equipment, particularly the mangle, a pressure of 75 psig is usually demanded, although 30 psi is sufficient if the flat work ironer is equipped with a large number of rolls, and if a slower rate of operation is permissible. Pressing machines and hospital sterilizers require about 50 psig. Where pressures are not as high as desired, higher pressures can be obtained by a steam compressor. Important points in laying out underground conduits are: 1. The depth of the buried conduit should be kept at a minimum. Excavation costs are a large factor in the total cost. 2. An expansion joint, offset, or bend should be placed between each two anchors. Advantage should be taken of the flexibility of piping to absorb expansion wherever possible. Information on provisions for expansion will be found in Chapter 26. 3. A proper hydrostatic test should be made on the assembled line before the insulation and the top of the conduit are applied. The hydrostatic test pressure should be one and one-half times the maximum service pressure, and it should be held for a period of at least two hours without evidence of leakage. Since it is difficult to make a concrete or masonry conduit absolutely water-tight, provision should be made for some seepage. The pipe should be protected by a waterproof jacket over the insulation, and the seepage drained from the inside of the conduit. Underdrainage of the conduit is generally provided for by a tile drain laid in crushed stone or gravel under- District Heating 621 neath the conduit. The tile underdrain should be carried to the sewer or some other drainage point. Manholes are required at intervals for access to valves, traps, and some types of expansion joints. Where steam and return piping are installed in the same conduit, the return piping usually follows the same grade as the steam piping. In general, the condensation is pumped back under pressure. Where it is possible to use basement or sub-sidewalk space for the dis tribution piping, the cost of installation and maintenance is greatly reduced. "''ono Layer Offer Ptper * Covered WWi Hot Thr And Pitch Standard Ihhknest Pipe Covering wilh Vtertrproof Jacket vfVit. Sewer Crock Bell & Spigot Joint Fiq. 1. Constbuction Details of Conduits Commonly Used Pipe Sizes. The lengths of pipe, steam quantities, and initial and terminal pressures having been chosen, the pipe sizes can readily be cal culated by means of Babcock's pressure drop formula given in Table 2 of Chapter 20. CONDUITS FOR PIPING Conduits for steam pipes buried underground should be reasonably waterproof, able to withstand earth loads and to take care of the expansion and contraction of the piping without strain or stress on the couplings, or without affecting the insulation or conduit. Expansion of the piping must be carefully controlled by means of anchors and expansion joints or bends so that the pipes can never come in contact with the conduit. Anchors can be anchor fittings or U-shaped steel straps which partially encircle the pipes, and are firmly bolted to a short length of structural or cast steel set in concrete. In general, cast steel is preferable to structural steel.