Book - School of Science and Technology

Provision for thermal expansion 255Table 9.7 Approximate data for condensate flow in steel (BS 1387: heavy) and copper (BS 2871: Table X)Flow of condensate (g/s) for listed conditions and pressure loss of4 mm/m run (40 Pa/m)Pipingnominal bore(mm)Liquid condensatewithoutair or vapourLiquid condensatewith someair and vapourMixture of liquidcondensate withair and vapourSteel Copper Steel Copper Steel Copper Steel Copper10 12 15 10 10 10 5 ±15 15 30 20 20 15 10 520 22 65 65 40 40 20 2025 28 120 135 75 85 35 4032 35 260 240 170 155 80 7540 42 400 410 260 265 125 12550 54 765 835 500 540 240 25565 67 1560 1500 1030 975 500 46080 76 2420 2130 1600 1380 780 655100 108 4920 5580 3260 3640 1590 1740. Gravity flow immediately following trap discharge where air and flash steam will bepresent in quantity. A drop in pressure ten times that due to water flow alone may beanticipated.For these three very different situations, Table 9.7 provides values for condensate flowin g/s based upon a pressure drop of 40 Pa per m run of condensate main. It will beappreciated that these values are not the result of a deeply theoretical analysis: the samecould be said, however, of the proven traditional approach which was to use condensatepipes one size smaller than the associated steam pipe! This somewhat intractable problemmay be summarised by advocating that a well designed system of condensate collectionwill provide for flow by gravity, using consistent grading, to an adequate number of pumpand receiver units. In no circumstances should a trap discharge be connected to the pumpdelivery pipe from such a unit.Provision for thermal expansionWhilst the coefficients of linear expansion for steel and copper are low in comparison withthose of other metals and only a tenth of those for pipeline plastics, this does not meanthat their effects may be ignored. Table 9.8 shows the extent of the growth in length whicharises, relative to temperature increase. When carrying out a design for thermal expansion,the temperature at which the piping is installed should always be taken intoconsideration, and this should be regarded as 0 C. Provision to accommodate thesechanges must be made in the physical layout of system and, as is obvious, the seriousnessof the matter increases for those which operate at higher temperatures or where piping isin straight runs and rigidly fixed between critical points.In all instances, short rigid connections between the various system components,boilers, pumps etc., should be avoided since situations may arise where a temperaturedifferential exists between piping and component, positive or negative as the case may be.Dependent upon the pipework configuration, the shape change of such a connection mayimpose a torsional movement upon materials not best suited to the resulting stress, even at