standards / manuals / guidelines for small hydro development - AHEC

At surface power stations with open channels following measures of desilting are taken:i) Silt extruders have been provided near under sluice gates of the barrage.ii) About half a kilometer downstream of the head regulator a silt ejector has beenconstructed in the bed of the power channeliii) In some projects desilting basin is constructed.iv) Forebay is also used as desilting basing with flushing conduits2. Cavitation & Cracking of RunnerThe problem of cracking of runner and Pelton buckets in few power stations has beenreported. This can be due to following reasons:i) Faulty designii) Poor metallurgyiii) Metal fatigueThe cavitation phenomenon occurs due to the vaporization of flowing fluid in a zone ofexcessive low pressure. Cavitation is inherent even in the best designed turbines andcavitation damage occur under unfavourable operating conditions, limits of metal removalhave been specified beyond which cavitation becomes harmful and requires repair. Tominimize cavitation following steps are necessary:i) Periodically (annually) inspect the runner and other turbine parts and take remedialmeasures.ii) Operate the machines as per guide lines given by manufacturers. The unit should notbe run below certain load to avoid cavitation pron zones.iii) At design stage itself, ensuring proper submergence use of cavitation resistantmaterial and adoption of optimize runner profile based on mode tests, cavitation caneffectively controlled.iv)As a result of draft tube pulsation and surges at no load or part gate opening excessivenose, vibrations and cavitation is experienced. To minimize pulsation of draft tubefollowing measures must be taken:-Air admission through air admission / vacuum braking valve installed at top cover.-Provision of fins or flow splitter in draft tube to break the vortex flow.-Provision of a byepass arrangement for releasing the pressure built up below the topcover.Normally the discharge side surface of buckets or blades, areas on the crown on thethroat ring and the tip of the blades and the upper portion of the draft tube liner are affectedby the action of cavitation. In rare cases, there may be pitting on the pressure faces of thebuckets or blades due to a unusual amount of over hung of the guide vanes improper designor unusual operating conditions.The power House authorities have over the years satisfactory expertise for repair andwelding of the runner at site, but it becomes a regular maintenance problem if the crackingoccurs too often.3. Precaution in welding of runnersi) Surface should be prepared by chipping or grinding.ii) To locate crack etc. die penetration test must be carried.AHEC/MNRE/SHP Standards/ E&M Works – Guidelines for Operation and Maintenance of Small Hydropower Station 23