FACTORS DETERMINING THE DESIGN OF ESTATE SEWER DRAINAGE SYSTEM

Transcription

1 FACTORS ETERMINING THE ESIGN OF ESTATE SEWER RAINAGE SYSTEM COVER STORY ER. SAHIL KANSAL Introdction The general nderstanding while designing estate services i.e. the site services for Sewage rainage System and Storm Water rainage System is that higher the pipe size, less slope is needed for the pipe and hence lower will be the invert level. As a reslt of a lower invert level, the cost of civil work redces. This kind of practice is not recommended as the design of these systems involves a mathematical expression which is very sefl while designing them. Please note that this method is to be sed only for systems which are nder gravity flow in circlar condits. Selection of Material While designing the sewerage system it is very important to note that there are many factors which inflence the correct design of the system i.e. the selection of correct material of the pipe in relation to the flow characteristics in different pipes; ease of handling and installation; water tightness; simplicity of assembly; physical strength; resistance to acids, alkalis, gases, solvents etc.; resistance to scoring; drability and cost inclding cost of handling and installation. No single material will meet all the conditions that may be encontered in sewer design. Selection shold be made for the particlar application and different materials may be selected for varios parts of a single project. The determination of the sitability of the pipes in all respects, for any work, is to be decided by the concerned engineer. The different materials of sewers which may be sed are as nder: Brick Brickwork is sed for constrction of sewers particlarly in larger diameters. Many old brick sewers are still in se and the failres are mainly de to the disintegration of the bricks or the mortar joints. Becase of the comparatively higher cost, larger space reqirement, slower progress of work and other factors, brick is now sed for sewer constrction only in special cases. The advantage of brick sewers is that these cold be constrcted to any reqired shape and size. Precast Concrete Egg Shaped Brick Sewers Plain cement concrete pipes are sed in sewer systems on only a limited scale and generally, reinforced concrete pipes are sed. Non-pressre pipes are sed for gravity flow and pressre pipes are sed for force mains, sbmerged otfalls, 4 / Jly 2017 / Indian Plmbing Today

2 inverted siphons and for gravity sewers where absolte water-tight joints are reqired. Nonpressre pipes sed for constrction of sewers and clverts shall confirm to IS 458. Certain heavy-dty pipes that are not specified in IS 458 shold conform to other approved standards. Stoneware or Vitrified Clay These pipes are normally available in lengths of 90 cm and the joints need calking with yarn soaked in cement mortar and packing in the spigot and socket joints which reqires skilled labor. The resistance of vitrified clay pipes to corrosion from most acids and to erosion de to grit and high velocities gives it an advantage over other pipe materials in handling acid concentrations. The strength of vitrified clay pipes often necessitates special bedding or concrete cradling to improve field spportive strength. Asbestos Cement Stoneware Pipes They are sbject to corrosion by acids, highly septic sewage and by highly acidic or high slphate soils. Protective measres as otlined in corrosion protection in sewers shall be provided in sch cases. While sing AC pipes, strict enforcement of approved bedding practices will redce possibility of flexible failre. Cast Iron The advantage of cast iron pipes are long laying lengths with tight joints, ability (when properly designed) to withstand relatively high internal pressre and external loads and corrosion resistance in most natral soils. They are however sbject to corrosion by acids or highly septic sewage and acidic soils. Inside coating shall be by cement mortar and oter coating shall be coal tar - both carried ot at the manfactrer s works and conforming to the relevant BIS standards/codes of practice. Steel Pressre sewer mains, nder water river crossings, bridge crossings and necessary connections for pmping stations, self-spporting spans, railway crossing and penstocks are some of the sitations where steel pipes are preferred. Steel pipes can withstand internal pressre, impact load and vibrations mch better than CI pipes. They are more dctile and withstand water hammer better. ctile Iron Pipes ctile iron pipes are normally prepared sing the centrifgal cast process. These pipes are sally provided with cement mortar lining at the factory by centrifgal process to ensre a niform thickness throghot its length. They have excellent properties of machinability, impact resistance, high wear and tear resistance, high tensile strength and dctility and corrosion resistance. I pipes, having same composition of CI pipe, will have same expected life as that of CI pipes. They are strong, both inner and oter srfaces are smooth, free from lmps, cracks, blisters and scars. One of the disadvantages of dctile iron pipes is that they are not cost effective, neither are they easy to install de to their weight and jointing method. PVC Pipe The chief advantages of PVC pipe are resistance to corrosion, light weight for transportation, toghness, rigidity, economical installation, easy jointing and maintenance. To prevent boyancy the pipes can be tied to poles driven into the grond. IS deals with non-pressre nplasticized polyvinylchloride (PVC) for se in ndergrond sewerage system. IS 9271 deals with the PVC single wall corrgated pipes for drainage. PVC Pipes Indian Plmbing Today / Jly 2017 / 5

3 High ensity Polyethylene (HPE) Pipes The advantages of these pipes are smooth interior srfaces offering relatively highest resistance to corrosion and their availability with a solid wall. When laid in straight gradient, withot hmps or depressions, they can easily offer longer life cycle. Methods of jointing are sally fsion welded or flange joint depending on straight rns or fittings. Standard specifications have been framed by the BIS in IS for sewerage application. Strctred Wall Piping These pipes can be manfactred in PVC-U, PP and PE as per EN / IS The walls of these pipes are either doble walled or ribbed wall. The BIS for pipes and fittings with PVC-U material having smooth external srface Type A is IS (Part-1) and for pipes and fittings with PE and PP material having non-smooth external srface Type B is IS (Part-2). Type B pipes are generally known as oble Walled Corrgated (WC) pipes. In India, WC pipes are prodced in sizes 75 mm I to 1,000 mm I with a standard length of 6 m for easy transportation and handling and to redce the nmber of joints reqired. In the case of hilly locations, the minimm diameter of 150 mm shall be adopted. The hose sewer connection pipe to pblic sewer shall be (a) minimm 100 mm or higher based on the nmber of hoses/flats connected and (b) sbject to the receiving pblic sewer being of higher diameter. Flow in Circlar Sewers If the velocity and depth of flow is the same for the length of a condit, it is termed steady flow otherwise it is non-steady flow. The hydralic analysis of sewers is simplified by assming steady flow conditions thogh the actal flow conditions are different dring morning peak flows and varying flows dring other periods of a 24 hor cycle. In the design of sanitary sewers, an attempt shall be made to obtain adeqate scoring velocities at the average or at least at the maximm flow at the beginning of the design period. The flow velocity in the sewers shall be sch that the sspended materials in sewage do not silt p; i.e., the velocity shall be sch as to case atomatic self-cleansing effect. The generation of sch a minimm selfcleansing velocity in the sewer, at least once a day is important, becase if deposition takes place and is not removed, it will obstrct free flow, casing frther deposition and finally leading to complete blocking of the sewer. WC HPE Pipes Apart from the above Glass Fibre Reinforced Plastic Pipes (GRP), Fibre Glass Reinforced Plastic Pipes (FRP), Pitch Fibre Pipes are also available. Minimm Size of Circlar Sewers As per the Central Pblic Health & Environmental Engineering Organization s (CPHEEO) Manal on Sewerage & Sewage Treatment Systems Part A Engineering (MSSTS) pblished in November 2013, the minimm diameter may be adopted as 200mm for cities having present/base year poplation of over 1 lakh. However, depending on growth potential in certain areas even 150 mm diameter can also be considered. However, in towns having present/ base year poplation of less than 1 lakh, the minimm diameter of 150 mm shall be adopted. The smooth interior srface of a sewer pipe gets scored de to continos abrasion cased by the sspended solids present in sewage. It is, therefore, necessary to limit the maximm velocity in the sewer pipe. This limiting or nonscoring velocity will mainly depend pon the material of the sewer. Ths the sewers are designed on the assmption that althogh silting might occr at minimm flow, it wold be flshed ot dring peak flows. Erosion of sewers is cased by sand and other gritty material in the sewer and by excessive velocity. Minimm Velocity for Preventing Sedimentation As per the CPHEEO s MSSTS, the minimm design velocities which needs to be ensred in gravity sewers to prevent sedimentation are as nder No Criteria Vale 1 Minimm velocity at initial peak flow 0.6 m/s 2 Minimm velocity at ltimate peak flow 0.8 m/s 3 Maximm velocity 3 m/s 6 / Jly 2017 / Indian Plmbing Today

4 Manning s Formla for Sewer nder Gravity Flow In case of SI Units V 1 n x R2/3 x S 1/2 Where, V Velocity in metre per second n Manning s Coefficient of roghness R Wetted perimeter in metre Area Circmference (π / 4 x 2 ) π x, where iameter of pipe in metre 4 S Slope of Hydralic Gradient Note: In case yo need to calclate the Velocity in Imperial Units a factor of K needs to be mltiplied to the reslt Hence, in case of imperial nits V K x 1 n x R2/3 x S 1/2 Where, V Velocity in metre per second n Manning s Coefficient of roghness R Wetted perimeter in metre Area Circmference (π / 4 x 2 ) π x, where iameter of pipe in metre 4 S Slope of Hydralic Gradient K (A constant mltiplied to convert V from SI Units to Imperial Units) Also, the discharge in the pipe can be expressed as, Q V x A Where, Q ischarge in the pipe V Velocity of the sewer A Cross sectional area of the pipe Ths, we can say that the slope of pipe, velocity of the sewage and discharge in the pipe are all inter related. Also, the discharge carrying capacity of the pipe is inversely proportional to the Manning s coefficient (n), i.e. less the vale of n more will be the discharge carrying capacity of pipe and less slope will be reqired. Ths, pipes having smooth inner srface need less slope for the same discharge. esign epth of Flow The sewers shall not rn fll as otherwise the pressre will rise above or fall below the atmospheric pressre and condition of open channel flow will cease to exist. Moreover, from consideration of ventilation, sewers shold not be designed to rn fll. In case of circlar sewers, the Manning s formla reveals that: The velocity at 0.8 depth of flow is 1.14 times the velocity at fll depth of flow. The discharge at 0.8 depth of flow is 0.98 times the discharge at fll depth of flow. Accordingly, the maximm depth of flow in design shall be limited to 0.8 of the diameter at ltimate peak flow, also it provides the best design and most cost effective, as the discharge carrying capacity at 0.8 depth of flow is almost same as at fll depth of flow, whereas the velocity is 1.14 times at fll depth of flow. Comparison in Stoneware and PVC pipes in terms of the slope It can be often seen, especially in government departments, that earlier the sewer was designed at 0.5 depth of flow which sed to reslt in higher pipe sizes and the depth at the last reach sed to increase. It is always recommended to design the sewer at 0.8 depth of flow ntil the system allows since it provides the most cost effective and high efficient system as compared to 0.5 depth of flow, which is now an old practice. It may be made clear with the example below. Assming the sewer is rnning at fll depth of flow, Actal Velocity / esired Velocity or V actal 0.6 m/s As we know that for 0.8 depth of flow the V 0.8 V Fll x V Fll m/s m/s 1.14 Therefore, if we will design the sewer with V Fll V m/s we will get 0.6 m/s which is desired. Indian Plmbing Today / Jly 2017 / 7

5 Type of Material Condition Manning's n Salt glazed stone ware (a) Good (b) fair Cement concrete pipes (with collar joints) (a) Good (b) fair Spn concrete pipes (RCC & PSC), with S/S Joints (esign Vale) (a) Neat cement plaster (b) Sand and cement plaster (c) Concrete. steel troweled Masonry (d) Concrete. wood troweled (e) Brick in good condition (f) Brick in rogh condition (g) Masonry in bad condition (a) Smooth, dressed ashlar Stone-work (b) Rbble set in cement (c) Fine, well packed gravel (a) Reglar srface in good condition (b) In ordinary condition Earth (c) With stones and weeds (d) In poor condition (e) Partially obstrcted with debris or weeds (a) Welded Steel (b) Riveted (c) Sightly tberclated (d) With spn cement mortar lining Cast Iron / ctile Iron (a) Unlined (b) With spn cement mortar lining Asbestos cement Plastic (smooth) FRP 0.01 HPE/UPVC 0.01 Manning s Coefficient of Roghness n for ifferent Materials Case 1 Considering a Stoneware pipe of dia 150mm. As per Manning s eqation for slope, with sewer at fll depth of flow 2 V x n S ( ( 4 ) ) 2/3 V Fll m/s n (From Fig 5 for Stoneware pipes) 0.15m Ths, placing the above vales in the eqation we get S 4.95 x 10-3 l in L Since, Q Fll or S Q Fll Q Fll or Q Fll l in L and V Fll x A Fll π x 4 x cm/sec cm/sec cbic feet/sec 8 / Jly 2017 / Indian Plmbing Today

6 Therefore, if we se stoneware pipe to achieve self cleaning velocity i.e. 0.6 m/s and the depth of flow is 0.8 in a pipe of dia 150mm, we need to provide a slope of 1:202 l in L and the discharge carrying capacity at this velocity and 0.8 depth of flow is Q 0.8 Q (Fll ) x x csec Case 2 Considering PVC pipe of dia 150mm. As per Manning s eqation for Slope, with sewer at fll depth of flow 2 V x n S ( ( 4 ) ) 2/3 V Fll m/s n 0.01 (From Fig 5 for PVC pipes) 0.15m Ths, placing the above vales in the eqation we get S 2.2 x 10-3 l in L or S l in L and Since, Q Fll V Fll x A Fll Q Fll x π 4 x cm/sec Q Fll cm/sec or Q Fll cbic feet/sec Therefore, if we se PVC pipe, to achieve the self-cleaning velocity i.e. 0.6 m/s and the depth of flow is 0.8 in a pipe of dia 150mm, we need to provide a slope of 1:454 l in L and the discharge carrying capacity at this velocity and 0.8 depth of flow is Q 0.8 Q Fll x x csec. Ths, it is clear from the above example that if we se PVC pipe we need to provide a slope of 1:454 and in case of stone ware pipe a slope of 1:202 is reqired nder same parameter. It is recommended to se PVC pipes where ever possible as it reqires less slope de to smooth inner srface. It frther redces the excavation cost, the civil cost of manholes and also the PVC pipes are easier to install. To conclde, we can say that there are nmber of factors which depend pon the sewer pipe size calclations and they all need to be handled careflly in order to achieve an optimm and cost effective soltion. ipt References Manal of Sewerage & Sewage Treatment Systems, Part A Engineering 3 rd Edition, Pblished by Central Pblic Health & Environmental Engineering Organization, Nov Er. SAHIL KANSAL is working as a MEP Consltant, by the name of ESS KAY Consltants in Chandigarh. He has been associated with many local and regional projects with expertise in Plmbing, Fire-fighting and electrical systems. He may be contacted at sahil@esskayconsltants.com Indian Plmbing Today / Jly 2017 / 9