Download recent developments in the design and manufacture of plastic pipes

RECENT DEVELOPMENTS IN THE
DESIGN AND MANUFACTURE OF
PLASTIC PIPES
JOHN BREWIN – VINIDEX TUBEMAKERS PTY LIMITED
PETER CHAPMAN– VINIDEX TUBEMAKERS PTY LIMITED
SUMMARY
Since their initial manufacture early this century, plastic pipes have become enormously diverse in
application to the point where they are presently found in all industry groups. In Australia the most
commonly used material for plastic pipes is polyvinyl chloride (PVC) and polyethylene (PE).
These two pipe types account for approximately 90% of the total plastic pipe sales. PVC and PE
pipes have become the material of choice for buried sewer systems and reticulated cold water
supply systems (where pressures are appropriate), they are also making significant inroads into
many other applications traditionally held by metalliferous pipes.
The underlying technical reasons for the increased use of plastic pipes has been the constant
research and development of polymers used and the improved understanding of the physical and
mechanical properties of these polymers hence improved design techniques.
To ensure that pipe line costs are minimised whilst life and performance is maximised, it is essential
that designers understand the physical and mechanical properties of plastic pipes and apply the
appropriate design procedures.
INTRODUCTION
Designers are increasingly recognising that there are four major factors influencing the lifetime of
pipe systems; material factors, environmental factors, loading conditions and installation
techniques. It is generally acknowledged that plastic pipe systems will last in excess of fifty years,
however this acceptance somewhat diminishes when considering lifetimes in excess of one hundred
years. Whilst we as manufacturers are confident that the pipes supplied will last in excess of one
hundred years we are dependent on the pipeline designers to ensure this.
Manufacturing and testing of pipes under laboratory conditions in accordance with the relative
Australia Standards (which are becoming increasingly performance based standards) generally
demonstrate that the pipe materials perform in a ductile manner, ie pressure testing or other tensile
testing exhibiting high strain levels before failure. Failures of pipelines in the field however are
generally brittle failures, suggesting that other system factors are affecting pipeline performance.
Speculation as to the cause of the these failures has included surge, fatigue, pipe quality, installation
techniques, commissioning procedures and localised environmental factors, such as ground
chemicals or differential settlement.
To overcome this problem, the plastic pipes industry has adopted a twofold approach. Firstly to
improve our understanding of material proprieties, hence improve design techniques and secondly
to modify the properties of plastic pipe materials to make them as insensitive, hence resistant to all
of the factors contributing to the failure.
This latter approach has favoured materials which maintain their strength and ductile properties
regardless of the operating conditions and age of the pipeline.
These materials include:
 Molecularly Oriented PVC;
 Modified PVC;
 PE 80’s;
 PE100’s;
which are considerably tougher than previous materials and exhibit other complementary properties.
In March 1999 the UK water industry through WRC published an Information and Guidance Note
(IGN) promulgating the new rules which allows “over-rating” of plastic pressure pipes for short
term surge events as well as new rules for fatigue design due to cyclic loading events. This paper
represents an initial point of discussion for suitable design parameters to suite the Australian
industry for surge, fatigue and bending.
BASIS FOR COMPARISON
Stress Versus Time Regression Curve
The most basic tool for characterisation of the strength of plastic pipes is the stress versus time
regression curve. Plastic pipes are made from a visco-elastic material which means the tensile or
hoop stress capacity varies with time.
Figure 1 illustrates the relative regression curves for the pipes under consideration:
Fig 1
Table 1 lists the various hoop strength for plastic pipe and relevant design safety factors.
1INSTANT-
ANEOUS
STRESS MPa
MRS2
(50 YEARS)
MPa
SF FOR
INSTANTANEOUS
STRESS
4.7
3.8
2.7
3.1
3.5
DESIGN
SF FOR
STRESS MPa LONG TERM
STRESS
UPVC
52
23.6
11.0
2.10
OPVC
90
47.2
23.6
2.00
mPVC
48
24.5
17.5
1.40
PE100
25
10.0
8.0
1.25
PE80
22
8.0
6.3
1.25
Table 1
It can be seen from table 1 that the response and safety factor of plastic pipes for short term loading
is considerably higher hence allowing a different approach to design for short term events.
DESIGNING FOR SURGE AND FATIGUE
Definition of Surge and Fatigue
Surges are single short term pressure change events, and in pipelines are caused by unforseen
events, such as power failure. Surges can be considerably amplified where there is air in the
pipeline causing damage to the pipe as well as other components such as pumps and valves.
Fatigue is the result of a large number of cyclic amplitude changes such as pump start up and stop.
These events should be considered and designed for as part of the system. Figure 2 illustrates the
difference between a surge and single cyclic amplitude change event.
Cyclic Amplitude
Water
Hammer
Surge
Figure 2 Typical Pressure Fluctuation
1
Typical values generated in constant strain rate tests in uniaxial tensile tests with a universal testing machine
Minimum Required Strength for Australian Standards. Values for PVC and OPVC are mean values, whilst MPVC and
PE are based on 97.5% lower confidence limits
2
The approximate relationships for a water hammer resulting from a change in fluid velocity is given
from Joukowsky’s formula. It is the responsibility to the system designed to choose whether to
conduct a formal surge analysis, however this is recommended whenever the slightest doubt about
operating conditions exist.
H=
CV
I
Where:
 H = Surge Pressure
C = Surge Celerity =
1
1
D
w( 
)
k AE
 V = Velocity Change. W = fluid density.
Joukowsky’s law applies when the change in velocity takes place within a critical time of:
2L
t=
C
L = Length of line.
It should be noted that celerity’s for plastic pipelines are approximately 12 to 30% lower than steel
pipelines, hence surge pressures are lower than steel pipelines where equivalent velocity change is
experienced.
Response of Plastic Pipes to Loading Rate
PVC and PE pipe respond to high loading rates by exhibiting greater strength. Since surges
characteristically are very rapid increases in load, plastic pipes are capable of withstanding surge
pressures in excess of working pressures.
Testing of PVC and PE pipe is given in Figure 3 (Ref 6).
Figure 3: Effect of Pressure Rise Rates on Strength of Plastics Pipes.
The data shows that for typical surge events between 0.1 to 10 bars/sec burst pressures are three to
four times the long term design working pressure of the pipe.
Surge Design
It is the responsibility from the industry as a whole to consider the research and recommend
reasonable limits to surge pressures to ensure reliable long term systems. This paper proposes that a
rational choice of allowable surge factors should be based on commissioning pressures as shown
below. These pressures are the highest pressures the pipe will ever be subjected to whilst having
factors of safety well below the instantaneous values.
Pipe
OPVC
MPVC
PE80
PE100
Surge Factor
1.50
1.30
1.25
1.25
Table 2
Maximum surge pressures are calculated by multiplying the surge factor from Table 2 by the
maximum allowable working pressure. Note, whilst temperature rating principles must be applied
in pipe selection for static pressures, no adjustment need be applied for dynamic design (Ref 11).
FATIGUE OF PVC
The fatigue response of thermoplastics pipe materials, particularly PVC, has been extensively
investigated (Ref 1-6). The results of laboratory studies can be used to establish a relationship
between stress cycle amplitude, defined here as the difference between the maximum and minimum
stress (see Fig 4), and the number of cycles to failure. From these relationships it is possible to
derive load factors that can be applied to the operating pressures to enable selection of an
appropriate class of pipe.
This type of experimental data inevitably has a degree of scatter and it has been Australian practice,
after Joseph (Ref 2), to adopt the lower bound for design purposes. This approach is retained here
because it ensures the design has a positive safety factor and recognises that pipelines may sustain
minor surface damage during installation, which could promote fatigue crack initiation. For this
reason, the re-rating factors recommended in this note are slightly greater than those given in
Reference 10. Note that for fatigue loading situations, the maximum pressure reached in the
repetitive cycle should not exceed the static pressure rating of the pipe.
Table 3 gives the recommended fatigue re-rating factors for thermoplastics pipes. This is also
graphically presented in Figure 4.
FATIGUE LOAD FACTORS
TABLE 3
Total Cycles
26,400
100,000
500,000
1,000,000
5,000,000
10,000,000
50,000,000
100,000,000
Approx. Nº
Cycles /day
for 50y life
2.7
5
27
55
274
548
2738
5476
UPVC
OPVC
MPVC
HDPE
MDPE
1.00*
1.00
0.62
0.50
0.31
0.25
0.15
0.13
1.00*
1.00
0.79
0.71
0.55
0.50
0.39
0.35
1.00
0.67
0.41
0.33
0.21
0.17
0.10
0.08
1.00
0.80
0.61
0.54
0.41
0.37
0.28
0.25
* Maximum allowable factor for fatigue loading pressure cycles
10
Fa t igue Loa d Fa c t or
1
OPVC
PE80, PE100
Tot a l Numbe r of Cyc le s in pipe life t ime
PVC
0.1
1.E+08
1.E+07
1.E+06
1.E+05
1.E+04
MPVC
Figure 4: Fatigue Load Factors for Thermoplastics Pipes
PROCEDURE
To select the appropriate pipe class for fatigue loading, the following procedure should be adopted:
1. Estimate the likely pressure cycle amplitude, P, ie., the maximum pressure minus the
minimum pressure.
2. Estimate the frequency or the number of cycles per day which are expected to occur.
3. Determine the required service life and calculate the total number of cycles which will occur
in the pipe lifetime
4. Using Table 3 or Figure 4, look up the fatigue load factor for the appropriate pipeline material
and number of cycles
5. Divide the pressure amplitude by the fatigue load factor to obtain an equivalent operating
pressure
6. Use the equivalent operating pressure to determine the class of pipe required.
BENDING OF PIPES
For various reasons it is sometimes required to allow for bending in PVC; getting around long
radius bends, mine subsidence, etc. The amount of bending is limited by the axial flexural stress
and strain levels induced in the pipe.
The maximum strain level  induced in the pipe is related to the ratio of radius of cumulative to
D
diameter.  =
.
2R
Depending on the allowable strain nominated, the R/D ratio varies. See Table 4.
AS2032
AS2566 (lateral loading)
Commonly Used
Pipes with Tappings
Table 4
ALLOWABLE STRAIN
0.38%
1.00%
2.50%
0.15%
R/D
130
50
20
300
R FOR 100MM PIPE
13m
5m
2m
30m
The adopted design value must, however, take into account strains due to all factors acting on the
pipe in service; loading, axial bending and pressure.
The combination of bending strain (constant strain) and strain due to internal pressure (constant
stress) is not well understood, but a reasonable approach is to estimate the stress resulting from a
constant bending strain at the relevant time frame(s) using the creep modulus and apply one of the
established failure criteria for multi-axial stress situations.
The analysis is further complicated by anisotropic mechanical properties for extruded PVC
materials. This is usually ignored for uPVC and mPVC, but maybe significant for OPVC.
The combination of pressure and bend does produce triaxial stress, the radial stress however is
relatively minor and can be ignored, hence the Hoffman criterion for biaxial stress loading provides
a suitably predictive model (Ref 9).
Hoffman criterion for rupture:
12
12
H=
1uc 1ut
1uc 1ut
Where:
+
22
2uc 2ut
+
12
12u
+ 1uc-1ut +
1uc 1ut
2uc-2ut
2uc 2ut
=1
1, 2 are biaxial stresses.
1ut, 1uc, 2ut, 2uc are ultimate stress along the same axis in tension and comparison.
1, 2 are the slow stresses developed on planes 1 and 2.
The critical controlling case is the combination of tensile hoop stress and compressive axial bending
stress. In which case the Hoffman criteria for rupture can be rewritten as:
12
12
22
=1
+
12u
1u
22u
Where:
1 are principle stress so ij = 0 and 1uc = iut.
Typical values for PVC materials are given in Table 5 at a bending radius to diameter ratio of 130
(AS2032) a safety of factor of 1.5 is considered a prudent minimum for this material.
Table 5
ALL STRESS AND MODULI IN MPa
Ultimate hoop stress in tension Shut
Ultimate Hoop stress in compression Shuc
Ultimate bending stress in tension Subt
Ultimate bending stress in compression Subc
Effective modulus in hoop direction Eh (a)
Effective modulus in axial direction Eb (a)
Operating hoop stress Sh (b)
Load factor (eg short term field test) k
Design hoop stress S1 = ksh
Bending Radius/ Diameter R/D
Axial bending strain eb = D/(2R)
Axial bending stress S2 = eb/Eb
Hoffman criterion H
Factor of safety = H½
PVC
SHORT
LONG
TERM
TERM
42
23.6
42
23.6
42
23.6
42
23.6
2800
1400
2800
1400
12.3
12.3
1.5
1
18.5
12.3
130
130
0.0038
0.0038
-10.8
-5.4
0.3714
0.4426
1.64
1.50
OPVC
SHORT
LONG
TERM
TERM
72
47.2
72
47.2
42
23.6
42
23.6
3500
1800
2800
1400
23.6
23.6
1.5
1
35.4
23.6
130
130
0.0038
0.0038
-10.8
-5.4
0.381
0.4
1.62
1.67
CONCLUSION
Australian pipeline operating conditions require that a prudent approach is taken to ensure the
longevity of pipeline systems. This is because of the range of applications that plastic pipes are
used, of site conditions that installations are performed under and also the range of fittings
associated with the pipeline (both PVC and metalliferous). The industry is offering designers
plastic pipe with remarkably improved mechanical properties, when compared with traditional
UPVC and type 50 polyethylene. The capital cost born by the manufacturers in developing these
products is enormous. We would therefore ask the designers and specifiers to take advantage of
these developments. There are other products presently being developed in Europe that will
advance the plastic pipe industry in Australia even further, provided that manufacturers are
confident in making the investment in bring these products to Australia.
REFERENCES
1.
2.
3.
4.
5.
6.
Kirby P.C.: Plastics Pipes IV, P.R.I., (1979), Brighton.
Joseph S.H.: Plast. & Rubb.: Proc. & Applns., 4, pp. 325-330, (1984).
Constable I., Williams J.G., Burns D.J.: J. Mech. Eng. Sci., 12, pp. 20-29, (1970).
Dukes B.W.: Plastics Pipes VI, P.R.I., (March 1985), York.
Maddox S.J., Manteghi S.: Plast., Rubb. And Com.: Proc. & Applns., 17, pp. 5-18, (1992).
Marshall G.P., Brogden S: Pipeline Dev Ltd, Shepherd M.A.: Thames Water Utility Ltd.
Evaluation of Surge and Fatigue Resistance of PVC & PE Pipelines Material for use in the UK
Water Industry.
7. Standards Australia, AS2023: 1977. “Installation of PVC Pipe Systems”.
8. Standards Australia AS/NZS2566.1:1998. Buried Flexible Pipelines Part 1: Structural Design.
9. Zagainou G.I., Lozino-Lozinski – G.E. (Editors). “Composite Materials in Aero Space Design”,
Soviet Advanced Composites Technology Series, Chapman and Hall.
10. UK Water Industry Information & Guidance Note IGN 4-37-02, March 1999, Issue WRC plc.
11. Vinidex Water Supply Manuals.