The system of supply in which water is transmitted to the consumers 24 hours a day is called continuous system.
•No use of reservoir or other public tanks
•Favorable for firefighting purposes
•Less pipe diameter is used
•Clear and fresh water
•No suspended materials are present
•Less wastage of water Disadvantages:
•Difficult to repair
•Large size of source is necessary
7.1.2 Intermittent System:
In this system, water is supplied only during fixed hours of the day is called intermittent supply.
•Repair and maintenance are easy in this system
•Wastage of water due to leakage is less
•During emergency and system breakdown, the water stored for the domestic purpose can be utilized.
•Storage tank needs to be built
•Not favorable for firefighting purposes
•Pipe diameter needs to be increased
•No of valves increases
•Clear and fresh water is not available
Reservoirs are the tanks which are used to store water for various purposes. Larger the reservoir size more would be the reliability and high would be the cost. On the basis of use (purposes) it is classified as:
•Clean Water Reservoir
7.3 Clean Water Reservoir:
Clean water reservoir are those reservoirs that stores water that has been treated and clear. The clean water should have the capacity of 14 to 16 hours of daily water demand. The larger the size of the clean water reservoir, higher would be the reliability but higher would be the cost.
7.4 Service Reservoir:
Service Reservoir is constructed after clear water reservoir where clear water is not kept. They are used to provide storage to meet fluctuation in demand of water. It has three main functions as:
•Balancing or equalizing flow
•Break down reserve
7.4.1 Types of service reservoir:
7.4.2Construction of Service Reservoir: Surface Reservoir:
They are also called as ground or non-elevated reservoir. They are rectangular or circular in shape. The supply may be by gravity flow or by pumping. Pumping is avoided as far as possible to minimize the cost. They are constructed with two compartments so that one can be used when other is being repaired. The two compartments are connected by control valves. It consists of an inlet pipe. The reservoir contains ventilation for aeration. Overflow pipes are provided at full supply level to maintain a constant level in the reservoir. The deposited sludge can be removed through washout pipes during occasional cleaning. The outlet pipes are placed at the higher level than the washout pipes.
They are rectangular, elliptical or circular in shape. It consists of the inlet for entry, the outlet for the exit, ventilation for aeration and overflow pipe at full supply level to maintain a constant level. It consists of the ladder to reach the top of the reservoir and manhole for regular inspection.
7.5 Water Consumption Pattern:
It refers to the variation in the amount of water consumed with respect to time. The water consumption pattern depends upon the several factors listed as:
•Type of society
•System of supply
•Cost and metering
•Quality of water
•Distance from the source
7.6 Determination of balancing reservoir capacity:
Tank capacity = Maximum cumulative surplus + Maximum cumulative deficit – total inflow + total demand.
Inflow > Outflow, reduce the tank by the difference of surplus or overflow occurs.
Inflow <= outflow, no consideration.
7.7 Layout of the distribution system:
It is a network of pipeline that conveys water to the consumers in the community.
7.7.1 Dead end/ Tree system/ Branched System:
Dead End System
It is a layout which consists of one supply main from which sub mains are taken and branches are taken from the sub main which ends at service consumers.
•It is a simple layout which is easy to be performed.
•Analysis and calculations are easy.
•Construction cost is less.
•Pipe with less diameter can be used.
• A number of valves required are less.
It is easy to operate and maintain.
•Stagnation of water at the dead end.
•Pressure remain unbalanced.
•The problem occurs during firefighting as the discharge is low.
•Suspended particle settles in the pipe.
•The problem arises when repair need to be done as large part (area) gets affected.
7.7.2 Grid Iron System
Grid Iron System
It is also known as reticulation system. This system is an improvement over dead end system. In this type of system main, sub mains and branch are interconnected. Along the main roads, the main supply line is laid. Submains are then taken in both the directions as shown in the figure. Branches are taken from mains and sub-mains and are interconnected.
•No stagnation of water takes place as there is no dead ends present.
•Water is available in adequate amount for firefighting.
•The suspended particle does not settle in the pipe.
•Only a small part (area) gets affected during repair.
•Analysis and calculations are difficult.
•A number of valves required are more.
•The cost is more for laying pipes than in dead end layout.
Ring and Radial System
7.7.3 Ring System/Circular System:
An area to be served is fixed and then the main is laid around that area. The submains are taken from the mains and are run on inside the area to be served. This system is used for cities with planned roads. It has similar advantages and disadvantages as grid iron system.
7.7.4 Radial System:
In this system the area to be served is divide into smaller zones and each zone is supplied with the distribution reservoir and the water is supplied radially towards the distribution zone. It supplies water with high pressure and low head loss. It is just the reverse of the ring or circular system. During actual practice, only a single way of the layout is not possible so a combination of layouts is used.
7.8 Design of distribution system:
For laying pipes and constructing a distribution system, knowledge on the size of pipes to convey water under a known pressure difference between inlet and outlet section need to be known. It includes pipe hydraulics, design criteria, and design process.
7.8.1 Pipe Hydraulics:
Size of pipes can be determined using:
Q= A * V = ((Pi)*d2)/4 Where,
Q = discharge through the pipe A = Area of cross-section d
= diameter of the pipe V = velocity of flow
The equation indicates that if the velocity is high, the pipe required is of smaller diameter and vice versa.
Head at inlet = Head at outlet + Head loss
Head losses are further classified as major head loss and minor head loss.
Major Head loss is due to the frictional loss. For longer pipes the frictional head loss is more than any other type ho head losses.
Determination of loss of head due to friction:
Where, Hf = head loss in m
L = length of pipe in m
D = diameter of pipe in m
V = mean velocity of flow through pipe in m/s
Q= discharge through the pipe in m3/s g = acceleration due to gravity
f = friction factor which is dimensionless
Equation by Colebrook and White is used to determine the friction factor.
K = roughness of pipe material
Re = Reynold’s number
K = k0 + αt
Where k0 = roughness of the new pipe material
K = roughness at any time t
Α = rate of increase in roughness in time
V = mean velocity of flow in m/s
R = hydraulic mean depth in m
S = slope of energy grade line of head lost per unit length of pipe N = Manning’s roughness coefficient For
Hazen Williams Formula:
V = 0.849CR0.63S0.54
Where, V = mean velocity of flow in m/s
R = hydraulic mean depth of pipe in m
S= slope of energy grade line or head lost per unit length of pipe C = roughness coefficient
Minor Losses are categorized as Entry Loss, Exit Loss, Bend Loss, Expansion Loss, and Contraction Losses.
Entry Loss: Loss of energy at the entrance is called entry loss and is given as: hL= (0.5V2)/2g
Exit Loss: Loss of energy at the exit is called exit loss and is given as: hL=V2/2g
Bend Loss: Loss of energy in the bend provide in the pipe is called bend loss and is given as hL = kV2/2g.
Where, V is the mean velocity of flow in the bend
K is the value which depends upon the total angle of the bend and the relative radius of curvature (R/d). R is the radius of curvature of the pipe axis and d is the diameter of the pipe.
Expansion Loss: Loss of energy due to sudden enlargement in pipe is expansion loss and is given as:hL= (V1– V2)2/ 2g
Where, V1 and V2 are the mean velocities of flow in smaller and larger section of pipe respectively.
Contraction Loss: Loss of energy due to sudden contraction in pipe is contraction loss and is given as:hL= (0.5V2)/2g
Where V is the mean velocity in the smaller section of the pipe.
Loss of energy due to gradual contraction or enlargement in the pipe is given as: hL= k(V1– V2)2/ 2g.
Where, V1 and V2 are the mean velocities of flow in smaller and larger section of pipe respectively.
K is the value which depends upon the angle of convergence or divergence and on the ration of smaller and larger cross-sectional areas of the pipe.
Loss of energy in valves and fittings is given as: kV2/2g.
Where, V is the mean velocity of flow in the bend and k is the value which depends on the type of pipe fittings.
7.8.2 Design Criteria:
Those criteria or conditions that need to be fulfilled during design so that the system will work in a proper manner is said to be design criteria.
The velocity of flow in a pipe need not be too low or too high. Low velocity leads to the settlement of suspended particles in the pipe and high velocity leads to the erosion of pipes. So a suitable velocity termed as recommended velocity is selected.
Minimum velocity = 0.3m/s
Maximum velocity = 3 m/s
Minimum velocity for untreated water = 0.6m/s
Same as velocity, the pressure in the pipe also should not be too low or too high. Low velocity leads to the low flow to the required height and high pressure leads to the increment in chances of bursting of pipes. Recommended pressure:
For Private connection:
Maximum Pressure = 15m
Minimum Pressure = 5m For
Public stand post:
Desirable Pressure = 15m
Minimum Pressure = 5m Maximum
Pressure = 55m
c) Pipe Size:
The size of the pipe that is calculated theoretically may not be available commercially. So the pipe size just above the size that has been calculated must be recommended. Commercially available pipe sizes in mm are : 15, 20, 25, 32, 40, 50, 65, 80, 100, 125, 150, 200, 250, 300, 350, 400, 450, 500, 600, 700, 800, 900, 1000, 1200, 1400, 1600, 1800, 2000, 2200, 2400, 2600, 2800 and 3000.
7.8.3 Design Steps:
a) Maps and Surveys:
The topographical map of the area in which the distribution is to be done is studied. The location of the components are marked and the possible ways/routes are determined. The way of serving water i.e. either through pumping or gravity flow is made. The technical survey includes the survey by the use of instruments as Abney level, theodolite. It also includes the study of the contour maps. The cross section of streets, roads. Lanes are prepared and the soil type, hard rock areas, and landslide prone areas are also studied.
The social survey includes the data collection regarding the total water demand, the population growth. A number of commercial, institutional, industrial establishments are also recorded. The dispute that may arise during the planning and implementation should be solved. b) Tentative
It includes the adjustments of mains, sub mains, and the branches. It too includes the position of the proposed water supply components and the route that may be followed.
c) Discharge in Pipelines:
On the basis of data collected (number of households, water demand for domestic and livestock purposes, the number of industrial, commercial and institutional establishments) the discharge desired is calculated. The transmission line is designed for maximum daily demand and the distribution system is designed for maximum hourly demand with the variation in peak factor from 2 to 4.
d) Calculation of Pipe Diameters:
For known discharge through the pipes, the diameter of the pipes can be computed by using DarcyWeisbach or Hazen-Williams formula. The commercially available pipe diameter equal to or greater than the computed pipe size should be used for the distribution system.
e) Computation of residual pressure and velocity:
The residual pressure in the distribution system is computed using
•The pressure available in the upstream
•Head loss in the pipe
When the diameter of the pipe is known, the velocity for the design discharge is computed. The residual pressure and velocity need to conform the design criteria. If they don’t conform the criteria new diameter of the pipe is selected.
7.9Design of Pipe Networks: a) Branched System:
Steps to design the branched system are listed as follows:
1.The population to be served by each section is determined.
2.The discharge to be carried by each section is determined.
3.The allowable head loss in the pipe which depends upon ground levels, residual head pressure available in the upstream and minimum head pressure to be maintained in the pipe is to be determined.
4.Using Darcy-Weisbach or Hazen-Williams equation, the diameter of the pipe in each section is determined.
5.Head loss in each pipe section is determined.
6.Velocity, as well as residual heads, are checked. If they are within the limit, the calculation is completed else change the pipe diameter so as to make velocity and residual heads within the limit.
Hardy Cross method is one of the widely used methods to analyze and design the looped system.
1.ΣQ = 0, at any joint.
2.ΣHf = 0, at any loop.
3.For any closed circuit pipe section Hf α Q.Hf = k1Q1n = k2Q2n, n = 1.852 in the case of Hazen-Williams Coefficient.
Flow correction formula
1.Assume the discharge and its direction.
2.Calculation of head loss of each branch.
3.Correction to flow is computed until reduced to an acceptable limit using
QA = Assumed Flow
Q = Actual flow
= Q – QA
Add it to the flow which is in the clockwise direction and subtract it to the flow in the anticlockwise direction.
If is negative.
Add it to the flow in the anticlockwise direction and subtract it to the flow in the clockwise direction.
For common pipes or networks: correction should be applied in both loops.