Calculation Methods 1. CALCULATING WALL THICKNESS Pipe calculation formula in ISO 161-1 is used in order to calculate wall thickness.
The long forms of the symbols in this Formula are as follows: PN Normal (Bar) Pressure s Wall thickness (mm) S Pipe serial (-) σs Hoop stress (N/mm2) SDR Standard Size Rate da External Pipe (mm) diameter The minimum wall thickness according to that:
Hoop stress calculation depends on safety factor.
MRS: Minimum Required Strength
2. HOOP STRESS
PE CLASS
MRS (N/mm2)
STRESS σ (N/mm2)
SAFETY FACTOR (C)
PE 63
6.3
5.0
1.25
WALL WEIGHT THICKNESS (kg/m) s (mm) 10.0 3.14
PE 80
8.0
6.3
1.25
8.1
2.62
PE 100
10.0
8.0
1.25
6.6
2.17
STABILITY PRESSURE
Pipes that are laid under the ground are exposed to loads other than earth load. Those are, as it is the case in sea discharge when pipes are laid straight into the sea, extra loads such as loads that ground water creates, although the pipes are laid under the ground. Apart from these, stability calculation must be done in projects that have overstress such as sleeve concrete made for filling the gap in pipes that engage by sleeve method or additional loads in vacuum pipes that have absorption function.
Stability Pressure Calculation for PE Pipes
Pk Critical Collapse Pressure Ec Elasticity Module Number of Transverse Thermoplasts s Wall Thickness rm Average Pipe Radius
(bar) (N/mm2) (-) (mm) (mm)
3. Acceptable Stability Pressure Calculation for PE Pipes
Pk,zul Acceptable Critical Collapse Pressure f r Reduction Factor s Safety Factor
(bar) (-) (-)
Stability Pressure Calculation for PE Pipes
σk Pk rm s
Stability Pressure Critical Collapse Pressure Average radius Wall thickness
(N/mm2) (bar) (mm) (mm)
4.CALCULATING PIPE DIAMETER The determination of the pipe section; continuity balance is established if passage flow rate at the water passageway is constant.
Q=0.0036 . A .v Q
: Amount of Load
A
: Pipe Section
V
: Flow Rate
If the passage flow rate at the gas and vapour passageway is constant, continuity balance is established. And the formula below is used :
m=0.0036 . A . v . ρ m ρ
: Passage flow rate : Density of the material carried
5. PRESSURE LOSSES
Calculating Separate Pressure Losses In order to calculate the high energy loss or pressure loss due to passage amount, flow rate and pressure decrease in HDPE pipes, the formulas below are used: a)Darc-Weisbach Formula di
: Internal Diameter of the Pipe (mm)
I
: Length of the Pipe Line (mm)
V
: Average Fluid Flow Rate (m/s)
p
: Fluid Density (kg/m3)
I
: Coefficient of Friction (-)
High energy loss stands for the elevation differences intended for the desired flow rate in the line. b)Colebrook-White Formula
Re
: Reynold’s Number
v
: Kinematical Fluidity of Water
k
: Hydraulic Smoothness Value of Internal Pipe Surface
When the previous formula is transformed;
: Flow Rate
V
g
: Gravitational Acceleration
: Energy Line Centering Trend
Je
: Kinematical
Hardness Kb
: Working Smoothness
d
: Pipe Diameter
There are two smoothness values: · Plate smoothness is ‘k’ · Working smoothness is ‘kb’ 6.PRESSURE IMPACT There may be a pressure impact while the valve or the pump is switched on and off. Thus,
a
Ps value:
: Spreading Speed of Press Wave (m/s)
vo
: Flow
p
: Fluid Density (kg/m 3)
Rate
of
the
Fluid
(m/s)
Ps value can be positive or negative in practice. Positive value is observed while the armatures are switched off and the pump is switched on. Ps negative value is observed while the pump is switched off or hydraulic property suddenly changes. Spreading speed of press wave is calculated by the formula below:
Em : Elasticity Module of the Fluid (Esu) p
: Fluid Density (kg/m3)
Er :Elasticity Module of the Pipe (N/m 2) Dm: Pipe Mid-Diameter(m) e
: Pipe Wall Thickness (m)
Short-term changes in pressure and the effect of pressure impact do not cause harm in HDPE pipes. 7.DILATATION Elongation ratio dependent on heat variability must be taken into consideration while the HDPE pipes are laid. At high temperature, there will be an increase, and at low temperature there will be a decrease in the length of the pipe. There will be 0,18 mm increase or decrease in length of 1m. part of the PE pipe, for every ‘K’ amount of temperature change.
MATERIAL
FACTOR mm/m.K
HDPE
0.18
PP
0.15
PVDF
0.14
PB
0.12
PVC
0.07
GFK
0.02
Coefficient of elongation for various plastic materials table For instance, in a line made of PE pipes, when there is an increase or decrease in pipe length depending on temperature, there will be a dislocation in the pipe line, not at the fixed point but at the turning point of the pipe. Let us assume that, for a 12m. pipe, normal working temperature Tv=20 0C, maximum working temperature T1=65 0C and minimum working
temperature
T2=10 0C.
Thus,
length
changes
depending
on
temperature are calculated as follows: Increase
in
length
depending
on
temperature
rise
: Decrease
in
length
depending
Ls
: Fixing Range (mm)
d
: Pipe Outer Diameter(mm)
on
temperature
drop
:
k
: Factor (26 for HDPE, 30 for PP, 33.5 for PVC)
8. FLEXIBILITY
Maximum bending radius for PE pipes:
: Bending Radius(mm)
R
E
: Pipe Elasticity Module
(N/mm2) Dm
: Stress (N/mm 2)
: Average Radius(mm)
HDPE CLASS PE 63 PE 80 PE 100
HOOP STRESS N/mm2 5 6.3 8
The possibility of breaking is the critical point while calculating the bending diameter for thin-walled pipes. For thick-walled pipes, while calculating the bending diameter, stretching-shrinking limit is the critical point. The formula below is used while calculating the acceptable bending radius for thin-walled pipes:
rm
: Average pipe radius (mm)
s
: Wall thickness(mm)
The formula below is used while calculating the acceptable bending radius for thickwalled pipes:
ra (%)
: Pipe external radius (mm)
E
: Stretching-shrinking rate
