Specifications Part 1 - Water Supply, Sewerage and Tube Well Works

14.8.4.8 The design of siphon should be made on the basis of pipe running full underpressure, It is therefore necessary to know the maximum available head. Alsothe losses of head due to friction, bends, etc. should be properly worked outand should be accordingly adjusted in the design of siphon.14.8.4.9 The inlet chamber should be provided with screens to remove silt, grit etc. fromsewage before it enters the siphon.14.8.4.10 It is advisable to provide a diversion for the siphon. Hence, when the siphon ischoked up due to any reason, the flow of sewage can be diverted.14.9 Construction14.9.1 To ensure self-cleaning velocities for the wide variations in flows, generally, twoor more pipes not less than 200mm dia are provided in parallel so that up to theaverage flows, first pipe is used and when the flow exceeds the average, thesecond and subsequent pipes take the balance flow.14.9.2 Siphons may need cleaning other than gravity sewers and hence should nothave any sharp bends either horizontal or vertical. Only smooth curves ofadequate radius should be used. The design criteria for inverted syphons aregiven in IS: 411 part -III. It is necessary to have a self-cleaning velocity of 1.0mps for the minimum flow to avoid deposition in the line.14.9.3 Provision should be made for isolating the individual pipes as well as thesiphon to facilitate cleaning.14.9.4 It is desirable to provide a course screen to prevent the entry of rags etc, intothe siphon.14.9.5 Inlet and outlet chambers: -14.9.6 In the multiple pipe siphon, the inlet should be such that the pipes coming toaction successively as the flow increases. This may be achieved by providinglateral with heights kept in accordance with the depth of flow at which one ormore siphon pipes functions. In the two-pipe siphon, the first should take 1.25to 1.5 times the average flow and second should take the balance of the flow.14.9.7 A manhole at each end of the siphon should be provided with clearance forrodding. The design of inlet and outlet chambers should allow sufficient roomfor entry for cleaning and maintenance of siphons. The outlet chambers shouldbe so designed as to prevent the flow of sewage into pipes, which are notbeing used at the time of minimum flow.14.10 Hatch box:Hatch boxes of adequate size in manholes shall be provided on the pipes so asto give access into the pipes for rodding.Urban Administration & Development Department Page 167