FACTORS DETERMINING THE DESIGN OF ESTATE SEWER DRAINAGE SYSTEM ER. SAHIL KANSAL
COVER STORY
Introduction The general understanding while designing estate services i.e. the site services for Sewage Drainage System and Storm Water Drainage System is that higher the pipe size, less slope is needed for the pipe and hence lower will be the invert level. As a result of a lower invert level, the cost of civil work reduces. This kind of practice is not recommended as the design of these systems involves a mathematical expression which is very useful while designing them. Please note that this method is to be used only for systems which are under gravity flow in circular conduits.
The different materials of sewers which may be used are as under: u
Brick
Brickwork is used for construction of sewers particularly in larger diameters. Many old brick sewers are still in use and the failures are mainly due to the disintegration of the bricks or the mortar joints. Because of the comparatively higher cost, larger space requirement, slower progress of work and other factors, brick is now used for sewer construction only in special cases. The advantage of brick sewers is that these could be constructed to any required shape and size.
Selection of Material While designing the sewerage system it is very important to note that there are many factors which influence 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 scouring; durability and cost including cost of handling and installation. No single material will meet all the conditions that may be encountered in sewer design. Selection should be made for the particular application and different materials may be selected for various parts of a single project. The determination of the suitability of the pipes in all respects, for any work, is to be decided by the concerned engineer.
4
/ July 2017 / Indian Plumbing Today
Egg Shaped Brick Sewers u
Precast Concrete
Plain cement concrete pipes are used in sewer systems on only a limited scale and generally, reinforced concrete pipes are used. Non-pressure pipes are used for gravity flow and pressure pipes are used for force mains, submerged outfalls,
inverted siphons and for gravity sewers where absolute water-tight joints are required. Nonpressure pipes used for construction of sewers and culverts shall confirm to IS 458. Certain heavy-duty pipes that are not specified in IS 458 should conform to other approved standards. u
Stoneware or Vitrified Clay
These pipes are normally available in lengths of 90 cm and the joints need caulking with yarn soaked in cement mortar and packing in the spigot and socket joints which requires skilled labour. The resistance of vitrified clay pipes to corrosion from most acids and to erosion due 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 supportive strength.
coal tar - both carried out at the manufacturer’s works and conforming to the relevant BIS standards/codes of practice. u
Pressure sewer mains, under water river crossings, bridge crossings and necessary connections for pumping stations, self-supporting spans, railway crossing and penstocks are some of the situations where steel pipes are preferred. Steel pipes can withstand internal pressure, impact load and vibrations much better than CI pipes. They are more ductile and withstand water hammer better. u
u
Asbestos Cement
They are subject to corrosion by acids, highly septic sewage and by highly acidic or high sulphate soils. Protective measures as outlined in corrosion protection in sewers shall be provided in such cases. While using AC pipes, strict enforcement of approved bedding practices will reduce possibility of flexible failure. u
Ductile Iron Pipes
Ductile iron pipes are normally prepared using the centrifugal cast process. These pipes are usually provided with cement mortar lining at the factory by centrifugal process to ensure a uniform thickness throughout its length. They have excellent properties of machinability, impact resistance, high wear and tear resistance, high tensile strength and ductility and corrosion resistance. DI pipes, having same composition of CI pipe, will have same expected life as that of CI pipes. They are strong, both inner and outer surfaces are smooth, free from lumps, cracks, blisters and scars. One of the disadvantages of ductile iron pipes is that they are not cost effective, neither are they easy to install due to their weight and jointing method. u
Stoneware Pipes
Steel
uPVC Pipe
The chief advantages of uPVC pipe are resistance to corrosion, light weight for transportation, toughness, rigidity, economical installation, easy jointing and maintenance. To prevent buoyancy the pipes can be tied to poles driven into the ground. IS 15328 deals with non-pressure unplasticized polyvinylchloride (uPVC) for use in underground sewerage system. IS 9271 deals with the uPVC single wall corrugated pipes for drainage.
Cast Iron
The advantage of cast iron pipes are long laying lengths with tight joints, ability (when properly designed) to withstand relatively high internal pressure and external loads and corrosion resistance in most natural soils. They are however subject to corrosion by acids or highly septic sewage and acidic soils. Inside coating shall be by cement mortar and outer coating shall be
uPVC Pipes
Indian Plumbing Today / July 2017 /
5
u
High Density Polyethylene (HDPE) Pipes
The advantages of these pipes are smooth interior surfaces offering relatively highest resistance to corrosion and their availability with a solid wall. When laid in straight gradient, without humps or depressions, they can easily offer longer life cycle. Methods of jointing are usually fusion welded or flange joint depending on straight runs or fittings. Standard specifications have been framed by the BIS in IS 14333 for sewerage application. u
Structured Wall Piping
These pipes can be manufactured in PVC-U, PP and PE as per EN 13476-3 / IS 16098. The walls of these pipes are either double walled or ribbed wall. The BIS for pipes and fittings with PVC-U material having smooth external surface Type A is IS 16098 (Part-1) and for pipes and fittings with PE and PP material having non-smooth external surface Type B is IS 16098 (Part-2). Type B pipes are generally known as Double Walled Corrugated (DWC) pipes. In India, DWC pipes are produced in sizes 75 mm ID to 1,000 mm ID with a standard length of 6 m for easy transportation and handling and to reduce the number of joints required.
DWC HDPE Pipes Apart from the above Glass Fibre Reinforced Plastic Pipes (GRP), Fibre Glass Reinforced Plastic Pipes (FRP), Pitch Fibre Pipes are also available. Minimum Size of Circular Sewers
In the case of hilly locations, the minimum diameter of 150 mm shall be adopted. The house sewer connection pipe to public sewer shall be (a) minimum 100 mm or higher based on the number of houses/flats connected and (b) subject to the receiving public sewer being of higher diameter. Flow in Circular Sewers If the velocity and depth of flow is the same for the length of a conduit, it is termed steady flow otherwise it is non-steady flow. The hydraulic analysis of sewers is simplified by assuming steady flow conditions though the actual flow conditions are different during morning peak flows and varying flows during other periods of a 24 hour cycle. In the design of sanitary sewers, an attempt shall be made to obtain adequate scouring velocities at the average or at least at the maximum flow at the beginning of the design period. The flow velocity in the sewers shall be such that the suspended materials in sewage do not silt up; i.e., the velocity shall be such as to cause automatic self-cleansing effect. The generation of such a minimum selfcleansing velocity in the sewer, at least once a day is important, because if deposition takes place and is not removed, it will obstruct free flow, causing further deposition and finally leading to complete blocking of the sewer. The smooth interior surface of a sewer pipe gets scoured due to continuous abrasion caused by the suspended solids present in sewage. It is, therefore, necessary to limit the maximum velocity in the sewer pipe. This limiting or nonscouring velocity will mainly depend upon the material of the sewer. Thus the sewers are designed on the assumption that although silting might occur at minimum flow, it would be flushed out during peak flows. Erosion of sewers is caused by sand and other gritty material in the sewer and by excessive velocity.
As per the Central Public Health & Environmental Engineering Organization’s (CPHEEO) ‘Manual Minimum Velocity for Preventing on Sewerage & Sewage Treatment Systems Part Sedimentation A Engineering’ (MSSTS) published in November 2013, the minimum diameter may be adopted As per the CPHEEO’s MSSTS, the minimum design as 200mm for cities having present/base year velocities which needs to be ensured in gravity population of over 1 lakh. However, depending sewers to prevent sedimentation are as under on growth potential in certain areas even 150 mm diameter can also be considered. No Criteria Value However, in towns having present/ 1 Minimum velocity at initial peak flow 0.6 m/s base year population of less than 1 lakh, the minimum diameter of 150 2 Minimum velocity at ultimate peak flow 0.8 m/s mm shall be adopted. 3 Maximum velocity 3 m/s
6
/ July 2017 / Indian Plumbing Today
Manning’s Formula for Sewer under Gravity Flow In case of SI Units 1 x R2/3 x S1/2 V = n Where,
Design Depth of Flow
V = Velocity in metre per second n = Manning’s Coefficient of roughness R = Wetted perimeter in metre =
Area Circumference
(π / 4 x D2) , where D = Diameter of πxD pipe in metre D = 4 S = Slope of Hydraulic Gradient =
Note: In case you need to calculate the Velocity in Imperial Units a factor of K = 1.486 needs to be multiplied to the result Hence, in case of imperial units 1 x R2/3 x S1/2 V = K x n Where, V = Velocity in metre per second n = Manning’s Coefficient of roughness R = Wetted perimeter in metre Area = Circumference
pipe and less slope will be required. Thus, pipes having smooth inner surface need less slope for the same discharge.
(π / 4 x D2) , where D = Diameter of πxD pipe in metre
D 4 S = Slope of Hydraulic Gradient
=
K = 1.486 (A constant multiplied to convert V from SI Units to Imperial Units) Also, the discharge in the pipe can be expressed as, Q=VxA
The sewers shall not run full as otherwise the pressure will rise above or fall below the atmospheric pressure and condition of open channel flow will cease to exist. Moreover, from consideration of ventilation, sewers should not be designed to run full. In case of circular sewers, the Manning’s formula reveals that: • The velocity at 0.8 depth of flow is 1.14 times the velocity at full depth of flow. • The discharge at 0.8 depth of flow is 0.98 times the discharge at full depth of flow. Accordingly, the maximum depth of flow in design shall be limited to 0.8 of the diameter at ultimate 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 full depth of flow, whereas the velocity is 1.14 times at full depth of flow. Comparison in Stoneware and uPVC 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 used to result in higher pipe sizes and the depth at the last reach used to increase. It is always recommended to design the sewer at 0.8 depth of flow until 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. Assuming the sewer is running at full depth of flow,
Q = Discharge in the pipe
Actual Velocity / Desired Velocity or V actual = 0.6 m/s
V = Velocity of the sewer
As we know that for 0.8 depth of flow the
A = Cross sectional area of the pipe
V 0.8
0.6 m/s = 0.526 m/s V Full = 1.14
Where,
Thus, 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 value of ‘n’ more will be the discharge carrying capacity of
= VFull x 1.14
Therefore, if we will design the sewer with
V Full = 0.526 m/s we will get V 0.8 = 0.6 m/s which is desired.
Indian Plumbing Today / July 2017 /
7
Type of Material
Condition
Salt glazed stone ware Cement concrete pipes (with collar joints)
(a) Good
0.012
(b) fair
0.015
(a) Good
0.013
(b) fair
0.015
Spun concrete pipes (RCC & PSC), with S/S Joints (Design Value)
Masonry
Stone-work
Earth
Steel
Cast Iron / Ductile Iron
Manning's n
0.011
(a) Neat cement plaster
0.018
(b) Sand and cement plaster
0.015
(c) Concrete. steel troweled
0.014
(d) Concrete. wood troweled
0.015
(e) Brick in good condition
0.015
(f) Brick in rough condition
0.017
(g) Masonry in bad condition
0.020
(a) Smooth, dressed ashlar
0.015
(b) Rubble set in cement
0.017
(c) Fine, well packed gravel
0.020
(a) Regular surface in good condition
0.020
(b) In ordinary condition
0.025
(c) With stones and weeds
0.030
(d) In poor condition
0.035
(e) Partially obstructed with debris or weeds
0.050
(a) Welded
0.013
(b) Riveted
0.017
(c) Sightly tuberculated
0.020
(d) With spun cement mortar lining
0.011
(a) Unlined
0.013
(b) With spun cement mortar lining
0.011
Asbestos cement
0.011
Plastic (smooth)
0.011
FRP
0.01
HDPE/UPVC
0.01 Manning’s Coefficient of Roughness n for Different Materials
Case 1 Considering a Stoneware pipe of dia 150mm. As per Manning’s equation for slope, with sewer at full depth of flow Vxn 2 2/3 S = D 4
(( ) )
V Full = 0.526 m/s
n = 0.015 (From Fig 5 for Stoneware pipes)
8
D
= 0.15m
/ July 2017 / Indian Plumbing Today
Thus, placing the above values in the equation we get
S
= 4.95 x 10-3 l in L
or S
= 201.63 l in L and
Since, QFull
= VFull x AFull
QFull
= 0.526 x
QFull
π x 0.152 cum/sec 4 = 0.009 cum/sec
or QFull = 0.318 cubic feet/sec
Therefore, if we use 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 Q0.8= Q(Full ) x 0.98 = 0.318 x 0.98 = 0.31 cusec Case 2 Considering uPVC pipe of dia 150mm. As per Manning’s equation for Slope, with sewer at full depth of flow Vxn 2 2/3 S = D 4 V Full = 0.526 m/s
(( ) )
n
= 0.01 (From Fig 5 for uPVC pipes)
D
= 0.15m
Thus, placing the above values in the equation we get
S
= 2.2 x 10-3 l in L
or S
= 453.66 l in L
provide a slope of 1:454 l in L and the discharge carrying capacity at this velocity and 0.8 depth of flow is Q0.8= QFull x 0.98 = 0.318 x 0.98 = 0.31 cusec. Thus, it is clear from the above example that if we use uPVC pipe we need to provide a slope of 1:454 and in case of stone ware pipe a slope of 1:202 is required under same parameter. It is recommended to use uPVC pipes where ever possible as it requires less slope due to smooth inner surface. It further reduces the excavation cost, the civil cost of manholes and also the uPVC pipes are easier to install. To conclude, we can say that there are number of factors which depend upon the sewer pipe size calculations and they all need to be handled carefully in order to achieve an optimum and cost ipt effective solution. References Manual of Sewerage & Sewage Treatment Systems, Part A Engineering 3rd Edition, Published by Central Public Health & Environmental Engineering Organization, Nov 2013.
and Since,
QFull = VFull x AFull
QFull
QFull
π x 0.152 cum/sec 4 = 0.009 cum/sec
= 0.526 x
or QFull = 0.318 cubic feet/sec
Therefore, if we use uPVC 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
Er. SAHIL KANSAL is working as a MEP Consultant, by the name of ESS KAY Consultants in Chandigarh. He has been associated with many local and regional projects with expertise in Plumbing, Fire-fighting and electrical systems. He may be contacted at
[email protected]
Indian Plumbing Today / July 2017 /
9
