Book - School of Science and Technology

252 Piping design for indirect heating systemsTable 9.4 Cross-sectional areas of steel pipes (BS 1387: medium)Pipe size(mm)Area(m 2 )Pipe size(mm)Area(m 2 )Pipe size(mm)Area(m 2 )15 0.000 175 40 0.001 272 100 0.008 3820 0.000 326 50 0.002 070 125 0.013 0525 0.000 518 65 0.003 530 150 0.018 7032 0.000 927 80 0.004 905Table 9.5 Conventional velocities for saturated steamPositionVelocity(m/s)PositionVelocity(m/s)Boiler outlet connections (LP) 5±10 Trunk steam mains 30±50Boiler outlet connections (HP) 15±20 Unit heater connections 25±30Boiler headers (HP and LP) 20±30 Calorifier connections 25±30Selection by steam velocityMain headers in boiler houses and other plant rooms, off-takes from them and actualconnections to principal items of steam consuming plant are most conveniently sized onthe basis of velocity. Volume flow may be calculated from the energy load using the datalisted in Table 6.6 and, from that, velocity using the internal cross-sectional areas of pipes,in m 2 , given in Table 9.4. Velocities which are generally recommended for use in variousapplications are quoted in Table 9.5.Selection by pressure dropElsewhere in the piping system, selection of size is usually made on the basis of overallpressure drop. If no particular criteria were imposed and depending upon the extent ofthe distribution system, it is normal to allow for a pressure drop of between 5 and 10%of the initial pressure, subject to a limiting maximum velocity of 50 m/s. Since steam iscompressible, both the specific mass and the viscosity change with pressure: as a result,presentation of data relating pressure drop to flow is more complex than is the case withwater. Bearing in mind the many extraneous influences mentioned earlier, an approximateapproach to the relationship is required and in the context of the subject matter of thisbook, one which will provide adequate accuracy for velocities between 5 and 50 m/s andpressures between about 100 and 1000 kPa.A method which was developed originally by Rietschel* in 1922, and used in earlyeditions of this book, fulfilled this specification but became slightly dated and, in anyevent referred to a different quality of pipe than that available in the British Isles. For the1965 edition of the Guide, the basis for the expression was re-examined and a slightlymodified version was used to produce the data included both there and in later editionsincluding the current Section C4. This approach defines the relationship between steamflow and pressure loss as:Z 1 Z 2 ˆ 3:648 (M 1:889 L)/(10 6 d 5:027 )* Rietschel, H. and Brabbee, K., Leitfaden der Heiz-und LuÈftungstechnik, Julius Springer, 1922, 2, 28.