4. System Design and Calculation

Document Sample

```					  4. System Design and Calculation
4.1 General
In Hong Kong, plumbing design shall follow the requirements
enforced by Water Supplies Department (WSD) and can make
reference to the design guide published by institute. Details can
make reference to:
•   “Hong Kong Waterworks Standard Requirements for Plumbing
Installation in Buildings” - WSD
•   Waterworks Ordinance (Cap. 102)
•   Waterworks Regulations (Cap. 102A)
•   “Plumbing Engineering Services Design Guide” – The Institute of
Plumbing (IOP)
•   “CIBSE Guides” – The Chartered Institution of Building Services
Engineers

37
4. System Design and Calculation
4.2 Water Storage Capacity
•   In Hong Kong, the water storage capacity is governed by WSD.
Potable Water
Residential:
First 10 Units,                        EACH 135L
From 11th Unit and onward,             EACH 90L
Commercial:
Per Fitment,                           EACH 45L
Flushing Water
Per Fitment,                           EACH 45L
•   Storage capacity is the sum of sump tank and storage tank and the
volume shall be in the ratio of 1 : 3 respectively.

38
4. System Design and Calculation
4.2 Simultaneous Demand
•   Simultaneous demand is widely adopted to determine the water flow
rate for plumbing system design.
•   There is a few methods of determine the simultaneous demand but
the mathematical model is originated from probability theory.
•   It is not mandatory for designer to adopt a single method for system
design but due to convenience, method using Loading Unit to
estimate the simultaneous demand is widely used in the field.
•   Two methods will be discussed in this section:
- Binominal and Poisson Distribution Method.

39
4. System Design and Calculation
4.2 Simultaneous Demand
Binominal and Poisson Distribution Method
•   The simultaneous demand can be estimated by:
m = np + c [2np(1 – p)]0.5
m = number of fittings subject to simultaneous use.
n = total number of fittings connected
p = t / T = usage ratio
t = average time a demanding a fitting for each period of use.
T = average time between occasions of use.
c = coefficient representing an appropriate level of acceptability.
[Choosing c = 1.8 will give a 98.9% acceptable service.]
•   The above expression for estimating simultaneous demand only
applicable if all the fittings are of the same type.

40
4. System Design and Calculation
4.2 Simultaneous Demand
Binominal and Poisson Distribution Method
•   Example
System configuration:
Appliance         No. of Fitting, n   Usage Ratio, p   Flow Rate, q (L/s)

Wash Basin               30                0.2               0.15
Sink                     20                0.4               0.30
•    Solution
For Wash Basin
m1 = 30  0.2 + 1.8  [2  30  0.2  (1 – 0.2)]0.5
= 18.6 say 19
Simultaneous demand of wash basin, q1 = 18.6  0.15 = 2.79L/s
Equivalent number of sink subject to simultaneous use
41
m’1= 2.79 / 0.3 = 9.3 say 9
4. System Design and Calculation
4.2 Simultaneous Demand
Binominal and Poisson Distribution Method
•   Solution
Equivalent number of sink that gives the same m’1= 9
9 = n’1  0.4 + 1.8  [2  n’1  0.4  (1 – 0.4)]0.5
n’1 = 11.80 (Solve by Iteration) say 12.
Equivalent system configuration
Appliance             No. of Fitting, n   Usage Ratio, p   Flow Rate, q (L/s)

Equivalent Sink for          12                0.4               0.30
Wash Basin
Sink                         20                0.4               0.30

m2 = 32  0.4 + 1.8  [2  32  0.4  (1 – 0.4)]0.5
= 19.9 say 20
42
Simultaneous system demand, Q = 20  0.3 = 6.0L/s
4. System Design and Calculation
4.2 Simultaneous Demand
•   It is the simplest way to determine the simultaneous demand
published in Plumbing Engineering Services Designs Guide - IOP.
•   Loading Unit (LU) is a dimensionless figure to represent the
associated water demand of each type of fitment, they are:
Units, LU                                Units, LU
Wash basin domestic use          1.5     WC flushing cistern (9L)          2

Wash basic public use             2      Shower                            3

Wash basin concentrated           3      Sink tap nominal size 15mm        3
use
Bath tap nominal size 20mm       10      Sink tap nominal size 20mm       10

Bath tap nominal size 25mm       22      Spray tap                        0.5

43
4. System Design and Calculation
4.2 Simultaneous Demand
•   The simultaneous demand can be obtained from calculating the total
LU from the pipe sizing chart Graph A1 in design guide issued by
IOP.
•   Using the data of Flow Rate and LU from Graph A1, they can be
represented by the following equation:
Q = 0.0623LU0.681

44
4. System Design and Calculation
4.2 Simultaneous Demand
•   Example
System Configuration
Units, LU   Appliance
Wash basin concentrated use                3           30

Sink tap nominal size 20mm                 10          20

Total LU = 3  30 + 10  20 = 290
Simultaneous demand, Q = 0.0623  (290)0.681 = 3.0L/s

45
4. System Design and Calculation
4.2 Simultaneous Demand
•   The simultaneous demand estimated by the same numbers of fitment
of using Binominal and Poisson Distribution Method (6.0L/s) is double
than that of using Loading Unit Method (3.0L/s).
•   It is due to the usage ratio used in Binominal and Poisson Distribution
Method that is different in the assumption used in Loading Unit
Method.
•   Designer shall pay special attention on making adjustment on the
data utilized for determine the simultaneous demand to suit particular
application.
•   In general, Loading Unit method give satisfactory estimation for
residential and commercial buildings.

46
4. System Design and Calculation
4.3 Pipe Sizing
•   The objectives of pipe sizing is determine the size of pipe to convey
certain amount of water such that
i) No excessive pressure drop in pipe frictional loss.
ii) No excessive flow velocity.
Water Velocity
•   Water velocity within pipeline should be limited to avoid great
pressure fluctuation thus causing water hammer effect.
•   The following table showing the recommended maximum velocity in
pipeline:
Pipe Bore Diameter (mm)   Maximum Water Velocity (m/s)
25                         1.0
50                         1.5
100                         2.0
>150                        3.0
47
4. System Design and Calculation
4.3 Pipe Sizing
Frictional Loss
•   Pipe friction should be limited to avoid excessive pressure drop that
cause insufficient water supply pressure at draw-off point.
•   Darcy Formula
4fl V 2
hf       
d 2g
hf is head loss to friction (m)
f is friction coefficient and is function of Reynolds Number s for
Laminar Flow.
l is length of pipe (m)
d is diameter of pipe (m)
It is suitable for straight pipe loss calculation.

48
4. System Design and Calculation
4.3 Pipe Sizing
Frictional Loss
•   Equivalent Length Method:
 Δp 
Pf   l  le  K n 
                  
       n      l 
Pf if total pressure loss in piping system
K is friction factor.
l is straight pipe length.
le is equivalent length.
∆p/l is pressure drop per unit pipe straight length.
∆p/l & le can be obtained from tables in CIBSE Guide C. For copper
pipe and water at 10 C, it can refer to page 4-40 & 4-41 of CIBSE
Guide C.

49
4. System Design and Calculation
4.3 Pipe Sizing
Frictional Loss
•   K-Factors for Common Pipe Fittings:
Reductions                                 Enlargement
Diameter Ratio                  K           Diameter Ratio                K
3/2                       0.3               3/2                     0.4
2/1                       0.4               2/1                     0.7
3/1                       0.4               3/1                     0.9
4/1                       0.5               4/1                     1.0

Copper Pipe Elbow                                 Valves
Pipe Diameter                  K                Type                     K
10 – 25mm                     1.0           Gate Valve                  0.2
32 – 50 mm                    0.8           Angle Valve                 5.0
65 – 90 mm                    0.5         Tap or Stopcock               10

Tee
Type                       K                Type                     K
Straight Through                0.2             Branch                    0.5
50
4. System Design and Calculation
4.3 Pipe Sizing
•   The pipe sizing shall ensure the total pressure loss in water pipe
distribution system that the required minimum pressure at draw-off
point can be maintained.
•   Ps = Pf + Pd + Pz
Ps is water pressure at source
Pd is minimum discharge pressure at draw-off point
Pz is the static head gain or loss. = gH
•   Remark:
Pipe friction dominants in sizing small diameter pipeline whereas
water velocity dominants in sizing large diameter one.

51
4. System Design and Calculation
4.3 Pipe Sizing
•   Example
Water pressure at                         Sink       Wash    Bath
Basin
upstream of main gate
valve is 1.5 bar            Gate
Valve
The draw-off point is 1m
above floor level.
Tap is installed at sink,           25m          8m           6m

wash basin and bath.                 A           B            C

Water flow:
Sink, 0.3L/s
Wash Basin,. 0.15L/s
Bath, 0.3L/s
Minimum pressure at
draw-off point is 1bar
52
4. System Design and Calculation
4.3 Pipe Sizing
•   Solution
Ps = 1.5bar, Pd = 1.0bar, Pz = 0.0981bar
Allowable pressure drop in the piping system,
Pf = Ps – Pd – Pz = 1.5 – 1.0 – 0.0981 = 0.4019 bar = 40,190Pa

53
4. System Design and Calculation
4.3 Pipe Sizing
•   Solution
Section      Mass     Pipe   Pressure   le     K-Factor            Pipe        l + leK     Pf
Flow     Size   Drop                                  Length, l               (Pa)
(kg/s)   (mm)   Gradient                              (m)
(Pa/m)

A         0.75     28       900      1.1   Gate Valve = 3,        25        28.52     25,668
Tee = 0.2
B         0.45     28       375      1.0   Tee = 0.2               8         8.20     3,075
C         0.30     22       650      0.7   Elbow = 1.0             6         15.8     10,270
Gate Valve = 3.0,
Tap = 10
Total   39,013

•         Total pressure drop of piping system = 39,013Pa < 40,190 Pa, the
pipe size is OK.
54
4. System Design and Calculation
4.4 Design Consideration
•   Minimum pressure at draw-off point should be maintained at 10m
head such that most water using equipment can operate properly
such as domestic type instantaneous water heater.
•   Pneumatic booster pump system should be adopted for water supply
in topmost floors if water tank close to the topmost floor that
minimum 10m head cannot be achieved at draw-off point.
•   For flushing water supply, it may not necessary to use pneumatic
booster pump system for the top most floors because even 5m head
could refill the cistern as well. Alternatively using separate down
pipe for the topmost floors can minimize the pressure fluctuation by
minimizing the simultaneous demand of the whole down pipe.
•   Space and elevation of water tank could reduce unnecessary upfeed
and/or booster system.

55
4. System Design and Calculation
4.4 Design Consideration                                                            Elevated
Storage
Pressure Zoning in High Rise Building                                           Tank

•   Maximum pressure at draw-off point
should limit to 60m head to avoid                                Intermediate
Storage/
excessive pressure.                                              Sump Tank
•   Building height in the range of 150 to        Intermediate                            Draw-off
Upfeed Pump                             Point
200m can consider using cascade
upfeed system thus minimize energy
wastage on pressure reducing as well
as lower pressure requirement on
piping accessories.                            Lot Boundary

•   If break tank is practically infeasible to
Water
be provided, pressure reducing valve                  Meter          Upfeed
Pump
is required to regulate the system                    m
pressure zoning.                                          Sump
Tank
Supply                                  56
Main

```
DOCUMENT INFO
Shared By:
Categories:
Stats:
 views: 1742 posted: 9/6/2010 language: English pages: 20