The Islamic University of Gaza Faculty of Engineering Civil Engineering Department Hydraulics - ECIV 3322

1
The Islamic University of GazaFaculty of
EngineeringCivil Engineering Department
Hydraulics - ECIV 3322
Chapter 4
Water Distribution Systems
2
Introduction

• To deliver water to individual consumers with
  appropriate
• quality, quantity, and pressure in a community
  setting requires
• an extensive system of
• Pipes.
• Storage reservoirs.
• Pumps.
• Other related accessories.

Distribution system is used to describe
collectively the facilities used to supply water
from its source to the point of usage .
3
Methods of Supplying Water

• Depending on the topography relationship between
  the source of supply and the consumer, water can
  be transported by
• Canals.
• Tunnels.
• Pipelines.
• The most common methods are
• Gravity supply
• Pumped supply
• Combined supply

4
Gravity Supply

• The source of supply is at a sufficient elevation
  above the distribution area (consumers).
• so that the desired pressure can be maintained

HGL or EGL
Source (Reservoir)
(Consumers)
Gravity-Supply System
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Advantages of Gravity supply
HGL or EGL
Source

• No energy costs.
• Simple operation (fewer mechanical parts,
  independence of power supply, .)
• Low maintenance costs.
• No sudden pressure changes

6
Pumped Supply

• ? Used whenever
• The source of water is lower than the area to
  which we need to distribute water to (consumers)
• The source cannot maintain minimum pressure
  required.
• ? pumps are used to develop the necessary head
  (pressure) to distribute water to the consumer
  and storage reservoirs.

HGL or EGL
(Consumers)
Source (River/Reservoir)
Pumped-Supply System
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Disadvantages of pumped supply

• Complicated operation and maintenance.
• Dependent on reliable power supply.
• Precautions have to be taken in order to enable
  permanent supply
• Stock with spare parts
• Alternative source of power supply .

HGL or EGL
(Consumers)
Source (River/Reservoir)
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Combined Supply(pumped-storage supply)

• Both pumps and storage reservoirs are used.
• This system is usually used in the following
  cases
• 1) When two sources of water are used to supply
  water

Pumping
Source (1)
Gravity
HGL
HGL
Pumping station
City
Source (2)
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Combined Supply (Continue)

• 2) In the pumped system sometimes a storage
  (elevated) tank is connected to the system.

• When the water consumption is low, the residual
  water is pumped to the tank.
• When the consumption is high the water flows
  back to the consumer area by gravity.

Low consumption
High consumption
Elevated tank
Pumping station
Pipeline
City
Source
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Combined Supply (Continue)

• 3) When the source is lower than the consumer area

• A tank is constructed above the highest point in
  the area,
• Then the water is pumped from the source to the
  storage tank (reservoir).
• And the hence the water is distributed from the
  reservoir by gravity.

Pumping
HGL
Gravity
HGL
Reservoir
Pumping Station
City
Source
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Distribution Systems (Network Configurations )

• In laying the pipes through the distribution
  area, the following configuration can be
  distinguished
• Branching system (Tree)
• Grid system (Looped)
• Combined system

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Branching System (tree system)
Branching System

• Advantages
• Simple to design and build.
• Less expensive than other systems.

14
Disadvantages

• The large number of dead ends which results in
  sedimentation and bacterial growths.
• When repairs must be made to an individual line,
  service connections beyond the point of repair
  will be without water until the repairs are made.
  
• The pressure at the end of the line may become
  undesirably low as additional extensions are
  made.

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Grid System (Looped system)
Grid System

• Advantages
• The grid system overcomes all of the
  difficulties of the branching system discussed
  before.
• No dead ends. (All of the pipes are
  interconnected).
• Water can reach a given point of withdrawal from
  several directions.

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Disadvantages

• Hydraulically far more complicated than branching
  system (Determination of the pipe sizes is
  somewhat more complicated) .
• Expensive (consists of a large number of loops).

But, it is the most reliable and used system.
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Combined System
Combined System

• It is a combination of both Grid and Branching
  systems
• This type is widely used all over the world.

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Design of Water Distribution Systems
A properly designed water distribution system
should fulfill the following requirements

• Main requirements
• Satisfied quality and quantity standards
• Additional requirements
• To enable reliable operation during irregular
  situations (power failure, fires..)
• To be economically and financially viable,
  ensuring income for operation, maintenance and
  extension.
• To be flexible with respect to the future
  extensions.

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• The design of water distribution systems must
  undergo through different studies and steps
• Design Phases

Preliminary Studies
Network Layout
Hydraulic Analysis
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Preliminary Studies
Must be performed before starting the actual
design

• 4.3.A.1 Topographical Studies

1. Contour lines (or controlling elevations).
2. Digital maps showing present (and future) houses,
   streets, lots, and so on..
3. Location of water sources so to help locating
   distribution reservoirs.

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Water Demand Studies

• Water consumption is ordinarily divided into
  the following categories
• Domestic demand.
• Industrial and Commercial demand.
• Agricultural demand.
• Fire demand.
• Leakage and Losses.

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Domestic demand

• It is the amount of water used for Drinking,
  Cocking, Gardening, Car Washing, Bathing,
  Laundry, Dish Washing, and Toilet Flushing.
• The average water consumption is different from
  one population to another. In Gaza strip the
  average consumption is 70 L/capita/day which is
  very low compared with other countries. For
  example, it is 250 L/c/day in United States, and
  it is 180 L/c/day for population live in Cairo
  (Egypt).
• The average consumption may increase with the
  increase in standard of living.
• The water consumption varies hourly, daily, and
  monthly

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The total amount of water for domestic use is a
function of
Population increase

• How to predict the increase of population?

Geometric-increase model
Use
P0 recent population r rate of population
growth n design period in years P
population at the end of the design period.
The total domestic demand can be estimated using
Qdomestic Qavg P
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Industrial and Commercial demand

• It is the amount of water needed for factories,
  offices, and stores.
• Varies from one city to another and from one
  country to another
• Hence should be studied for each case separately.
  
• However, it is sometimes taken as a percentage of
  the domestic demand.

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Agricultural demand

• It depends on the type of crops, soil, climate

Fire demand

• To resist fire, the network should save a certain
  amount of water.
• Many formulas can be used to estimate the amount
  of water needed for fire.

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Fire demand Formulas
QF fire demand l/s P population in
thousands
QF fire demand l/s P population in
thousands
QF fire demand flow m3/d A areas of all
stories of the building under
consideration (m2 ) C constant depending on
the type of construction
The above formulas can be replaced with local
ones (Amounts of water needed for fire in these
formulas are high).
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Leakage and Losses

• This is unaccounted for water (UFW)
• It is attributable to

Errors in meter readings
Unauthorized connections
Leaks in the distribution system
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Design Criteria

• Are the design limitations required to get
  the most efficient and economical
  water-distribution network

Pressure
Pipe Sizes
Design Period
Velocity
Head Losses
Average Water Consumption
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Velocity

• Not be lower than 0.6 m/s to prevent
  sedimentation
• Not be more than 3 m/s to prevent erosion and
  high head losses.
• Commonly used values are 1 - 1.5 m/sec.

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Pressure

• Pressure in municipal distribution systems ranges
  from 150-300 kPa in residential districts with
  structures of four stories or less and 400-500
  kPa in commercial districts.
• Also, for fire hydrants the pressure should not
  be less than 150 kPa (15 m of water).
• In general for any node in the network the
  pressure should not be less than 25 m of water.
• Moreover, the maximum pressure should be limited
  to 70 m of water

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Pipe sizes

• Lines which provide only domestic flow may be as
  small as 100 mm (4 in) but should not exceed 400
  m in length (if dead-ended) or 600 m if connected
  to the system at both ends.
• Lines as small as 50-75 mm (2-3 in) are sometimes
  used in small communities with length not to
  exceed 100 m (if dead-ended) or 200 m if
  connected at both ends.
• The size of the small distribution mains is
  seldom less than 150 mm (6 in) with cross mains
  located at intervals not more than 180 m.
• In high-value districts the minimum size is 200
  mm (8 in) with cross-mains at the same maximum
  spacing. Major streets are provided with lines
  not less than 305 mm (12 in) in diameter.

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Head Losses

• Optimum range is 1-4 m/km.
• Maximum head loss should not exceed 10 m/km.

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Design Period for Water supply Components

• The economic design period of the components of a
  distribution system depends on
• Their life.
• First cost.
• And the ease of expandability.

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Average Water Consumption

• From the water demand (preliminary) studies,
  estimate the average and peak water consumption
  for the area.

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Network Layout

• Next step is to estimate pipe sizes on the basis
  of water demand and local code requirements.
• The pipes are then drawn on a digital map (using
  AutoCAD, for example) starting from the water
  source.
• All the components (pipes, valves, fire hydrants)
  of the water network should be shown on the
  lines.

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Pipe Networks

• A hydraulic model is useful for examining the
  impact of design and operation decisions.
• Simple systems, such as those discussed in last
  chapters can be solved using a hand calculator.
• However, more complex systems require more effort
  even for steady state conditions, but, as in
  simple systems, the flow and pressure-head
  distribution through a water distribution system
  must satisfy the laws of conservation of mass and
  energy.

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Pipe Networks

• The equations to solve Pipe network must satisfy
  the following condition
• The net flow into any junction must be zero
• The net head loss a round any closed loop must be
  zero. The HGL at each junction must have one and
  only one elevation
• All head losses must satisfy the Moody and
  minor-loss friction correlation

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Node, Loop, and Pipes
Pipe
Node
Loop
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Hydraulic Analysis

• After completing all preliminary studies and
  layout drawing of the network, one of the methods
  of hydraulic analysis is used to
• Size the pipes and
• Assign the pressures and velocities required.

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Hydraulic Analysis of Water Networks

• The solution to the problem is based on the same
  basic hydraulic principles that govern simple and
  compound pipes that were discussed previously.
• The following are the most common methods used to
  analyze the Grid-system networks
• Hardy Cross method.
• Sections method.
• Circle method.
• Computer programs (Epanet,Loop, Alied...)

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Hardy Cross Method

• This method is applicable to closed-loop pipe
  networks (a complex set of pipes in parallel).

• It depends on the idea of head balance method
• Was originally devised by professor Hardy Cross.

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Assumptions / Steps of this method

1. Assume that the water is withdrawn from nodes
   only not directly from pipes.
2. The discharge, Q , entering the system will have
   () value, and the discharge, Q , leaving the
   system will have (-) value.
3. Usually neglect minor losses since these will be
   small with respect to those in long pipes, i.e.
   Or could be included as equivalent lengths in
   each pipe.
4. Assume flows for each individual pipe in the
   network.
5. At any junction (node), as done for pipes in
   parallel,

or
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• Around any loop in the grid, the sum of head
  losses must equal to zero
• Conventionally, clockwise flows in a loop are
  considered () and produce positive head losses
  counterclockwise flows are then (-) and produce
  negative head losses.
• This fact is called the head balance of each
  loop, and this can be valid only if the assumed Q
  for each pipe, within the loop, is correct.
• The probability of initially guessing all flow
  rates correctly is virtually null.
• Therefore, to balance the head around each loop,
  a flow rate correction ( ) for each loop in
  the network should be computed, and hence some
  iteration scheme is needed.

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• After finding the discharge correction, (one
  for each loop) , the assumed discharges Q0 are
  adjusted and another iteration is carried out
  until all corrections (values of ) become
  zero or negligible. At this point the condition
  of
• is satisfied.
• Notes
• The flows in pipes common to two loops are
  positive in one loop and negative in the other.
• When calculated corrections are applied, with
  careful attention to sign, pipes common to two
  loops receive both corrections.

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How to find the correction value ( )
Neglect terms contains
For each loop
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• Note that if Hazen Williams (which is generally
  used in this method) is used to find the head
  losses, then

(n 1.85) , then

• If Darcy-Wiesbach is used to find the head
  losses, then

(n 2) , then
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Example
Solve the following pipe network using Hazen
William Method CHW 100
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12.6
11.4
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D L pipe
150mm 305m 1
150mm 305m 2
200mm 610m 3
150mm 457m 4
200mm 153m 5
25.2
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Example
Solve the following pipe network using Hazen
William Method CHW 120
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Iteration 1
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Iteration 2
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Iteration 3
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Example

• The figure below represents a simplified pipe
  network.
• Flows for the area have been disaggregated to the
  nodes, and a major fire flow has been added at
  node G.
• The water enters the system at node A.
• Pipe diameters and lengths are shown on the
  figure.
• Find the flow rate of water in each pipe using
  the Hazen-Williams equation with CHW 100.
• Carry out calculations until the corrections are
  less then 0.2 m3/min.

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  ?????

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General Notes

• Occasionally the assumed direction of flow will
  be incorrect. In such cases the method will
  produce corrections larger than the original flow
  and in subsequent calculations the direction will
  be reversed.
• Even when the initial flow assumptions are poor,
  the convergence will usually be rapid. Only in
  unusual cases will more than three iterations be
  necessary.
• The method is applicable to the design of new
  system or to evaluation of proposed changes in an
  existing system.
• The pressure calculation in the above example
  assumes points are at equal elevations. If they
  are not, the elevation difference must be
  includes in the calculation.
• The balanced network must then be reviewed to
  assure that the velocity and pressure criteria
  are satisfied. If some lines do not meet the
  suggested criteria, it would be necessary to
  increase the diameters of these pipes and repeat
  the calculations.

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Summary

• Assigning clockwise flows and their associated
  head losses are positive, the procedure is as
  follows
• Assume values of Q to satisfy ?Q 0.
• Calculate HL from Q using hf K1Q2 .
• If ?hf 0, then the solution is correct.
• If ?hf ? 0, then apply a correction factor, ?Q,
  to all Q and repeat from step (2).
• For practical purposes, the calculation is
  usually terminated when ?hf lt 0.01 m or ?Q lt 1
  L/s.
• A reasonably efficient value of ?Q for rapid
  convergence is given by

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Example

• The following example contains nodes with
  different elevations and pressure heads.
• Neglecting minor loses in the pipes, determine
• The flows in the pipes.
• The pressure heads at the nodes.

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Assume T 150C
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Assume flows magnitude and direction
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First Iteration

• Loop (1)

Pipe L (m) D (m) Q (m3/s) f hf (m) hf/Q (m/m3/s)
AB 600 0.25 0.12 0.0157 11.48 95.64
BE 200 0.10 0.01 0.0205 3.38 338.06
EF 600 0.15 -0.06 0.0171 -40.25 670.77
FA 200 0.20 -0.10 0.0162 -8.34 83.42
S -33.73 1187.89
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First Iteration

• Loop (2)

Pipe L (m) D (m) Q (m3/s) f hf (m) hf/Q (m/m3/s)
BC 600 0.15 0.05 0.0173 28.29 565.81
CD 200 0.10 0.01 0.0205 3.38 338.05
DE 600 0.15 -0.02 0.0189 -4.94 246.78
EB 200 0.10 -0.01 0.0205 -3.38 338.05
S 23.35 1488.7
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Second Iteration
14.20
14.20
7.84
14.20

• Loop (1)

14.20
Pipe L (m) D (m) Q (m3/s) f hf (m) hf/Q (m/m3/s)
AB 600 0.25 0.1342 0.0156 14.27 106.08
BE 200 0.10 0.03204 0.0186 31.48 982.60
EF 600 0.15 -0.0458 0.0174 -23.89 521.61
FA 200 0.20 -0.0858 0.0163 -6.21 72.33
S 15.65 1682.62
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Second Iteration
7.84
14.20
7.84
7.84

• Loop (2)

7.84
Pipe L (m) D (m) Q (m3/s) f hf (m) hf/Q (m/m3/s)
BC 600 0.15 0.04216 0.0176 20.37 483.24
CD 200 0.10 0.00216 0.0261 0.20 93.23
DE 600 0.15 -0.02784 0.0182 -9.22 331.23
EB 200 0.10 -0.03204 0.0186 -31.48 982.60
S -20.13 1890.60
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Third Iteration

• Loop (1)

Pipe L (m) D (m) Q (m3/s) f hf (m) hf/Q (m/m3/s)
AB 600 0.25 0.1296 0.0156 13.30 102.67
BE 200 0.10 0.02207 0.0190 15.30 693.08
EF 600 0.15 -0.05045 0.0173 -28.78 570.54
FA 200 0.20 -0.09045 0.0163 -6.87 75.97
S -7.05 1442.26
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Third Iteration

• Loop (2)

Pipe L (m) D (m) Q (m3/s) f hf (m) hf/Q (m/m3/s)
BC 600 0.15 0.04748 0.0174 25.61 539.30
CD 200 0.10 0.00748 0.0212 1.96 262.11
DE 600 0.15 -0.02252 0.0186 -6.17 274.07
EB 200 0.10 -0.02207 0.0190 -15.30 693.08
S 6.1 1768.56
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After applying Third correction
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Velocity and Pressure Heads
pipe Q (l/s) V (m/s) hf (m)
AB 131.99 2.689 13.79
BE 26.23 3.340 21.35
FE 48.01 2.717 26.16
AF 88.01 2.801 6.52
BC 45.76 2.589 23.85
CD 5.76 0.733 1.21
ED 24.24 1.372 7.09
13.79
23.85
1.21
21.35
6.52
26.16
7.09
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Velocity and Pressure Heads
Node p/gZ (m) Z (m) P/g (m)
A 70 30 40
B 56.21 25 31.21
C 32.36 20 12.36
D 31.15 20 11.15
E 37.32 22 15.32
F 63.48 25 38.48
13.79
23.85
21.35
1.21
6.52
7.09
26.16
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• Example
• For the square loop shown, find the discharge in
  all the pipes.
• All pipes are 1 km long and 300 mm in diameter,
  with a friction
• factor of 0.0163. Assume that minor losses can
  be neglected.

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• Solution
• Assume values of Q to satisfy continuity
  equations all at nodes.
• The head loss is calculated using HL K1Q2
• HL hf hLm
• But minor losses can be neglected ? hLm 0
• Thus HL hf
• Head loss can be calculated using the
  Darcy-Weisbach equation

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First trial Since ?HL gt 0.01 m,
then correction has to be applied.
Pipe Q (L/s) HL (m) HL/Q
AB 60 2.0 0.033
BC 40 0.886 0.0222
CD 0 0 0
AD -40 -0.886 0.0222
? 2.00 0.0774
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Second trial Since ?HL 0.01 m, then it is
OK. Thus, the discharge in each pipe is as
follows (to the nearest integer).
Pipe Q (L/s) HL (m) HL/Q
AB 47.08 1.23 0.0261
BC 27.08 0.407 0.015
CD -12.92 -0.092 0.007
AD -52.92 -1.555 0.0294
? -0.0107 0.07775
Pipe Discharge (L/s)
AB 47
BC 27
CD -13
AD -53