72. Pressure pipe systems ratings

Thermoplastic pipes based on the nominated standard sizes

are accorded pressure ratings. The basis of this rating is that

the outside diameter of the pipe remains standard but the

wall thickness is specifi ed according to the strength

requirement. For example the outside diameter of a 32 mm

pressure pipe rated at 10 bar will be the same as that rated

at 16 bar. The higher the pressure that the pipe is required to

hold, the greater the strength requirement and thus the

greater the wall thickness necessary to withstand that

pressure.

2.1 Calculation of pipe wall thicknessThe pipe wall thickness can be calculated by the following

vessel formula:-

2.2 International standards of ratings

There are three classifi cations which are accepted as

International Standards for rating of plastic pipes: -

2.2.1 The PN Nominal Pressure classifi cationThe PN classifi cation rates pipes and components for their

maximum permissible pressure at 20C. Thus a component

rated at PN16 can be used to carry liquid or other fl uid

media at 20C up to a maximum pressure of 16 bar.

This system also rules that pipes, valves and fi ttings of the

same standard size and same nominal pressure rating will

have the same joint dimensions.

2.2.2 The ISO 4065 S series classifi cationThe ISO 4065 S classifi es pipes into pressure ratings using a

series number, therefore pipes with the same series number

are rated to the same pressure.

The series number can be arrived at by use of the

following formula: -

Where e Pipe wall thickness (mm) D Outside diameter (mm) P Permissible operating pressure (bar) at 20 C Permissible circumferential stress (MPa),

HPPE=10 MPa.

20 Constant of proportionality

Due to manufacturing necessity pipes of a small diameter will

have a greater wall thickness than may be expected when

using the vessel formula.

Where S Series number, note this is dimensionless Permissible circumferential stress (MPa),

HPPE=10 MPa.

P Permissible operating pressure (bar) at 20 C 10 Proportionality constant

D Outside diameter (mm) e Pipe wall thickness (mm)

2.2.3 The SDR (Standard Dimension Ratio)The SDR rating (an American classifi cation) denotes the ratio of

the pipes outside diameter to its wall thickness and is found

by using: -

Where SDR Standard Dimension Ratio D Outside diameter (mm) e Pipe wall thickness (mm), this may be calculated using the vessel formula

The SDR may also be calculated from the ISO 4065 S Series (S) by use of the following formula: SDR = 2 x S + 1

Alternatively S can be derived with

Table 2.1 Correlation between SDR and PN

PE100 - Safety Factor S = 1.25

SDR PN

33 5

26 6.3

21 8

17 10

11 16

7.4 25

PN is valid for 20C water for service life of 50 years

Pressure Pipe System Ratings

1

8The required duration of operation for a given working

pressure and temperature must be taken into account when

planning a plastic pipe system. Pressures that can be

sustained for a short time at a certain temperature may not

be sustainable at a higher working temperature; or even at

the same pressure and temperature should the working

duration of the system be extended. It is possible to work

out the maximum permitted working pressures at different

temperatures and the associated safety factors with the

use of regression graphs. Safety factors are used to ensure

that plastic pipeline systems can operate under stress for their

given lifetime without damage or failure and is described as

the ratio between the maximum allowable circumferential

stress which a system can absorb and its operating stress.

Where P Permissible operating pressure (bar) C Safety factor (see table 3.1) 20 Proportionality constant Circumferential stress (MPa), taken from

regression chart (see chart 3.1).

e Pipes wall thickness (mm) D Pipes outside diameter (mm)

Note that fi ttings and other components, with the same

pressure rating as the pipe, are normally thicker walled and

therefore the lowest common denominator of wall thickness

(e) should be used.

Table 3.1 Safety factors for thermoplastics (C)

Thermoplastic material

HPPE PVCu metric PVCu imperial ABS metric ABS imperial

Safety factor* 1.25 2.5 2.1 2.1 2.1

3. Pressure and temperature relationship The following formula is used to calculate the permissible working pressure for a pipeline system: -

* Safety factors are based on 50 year expected life at 20C, with water.

The higher the working temperature of a plastic pipe system, the lower will be the working pressure that can be sustained

within the system, please refer to table 3.2.

(PN10) for 3.8 mm wall thickness

and

(PN10) for 3.8 mm wall thickness

and

(PN16) for 5.8 mm wall thickness (PN16) for 5.8 mm wall thickness

Worked example 3.1Calculate the maximum operating pressure for a pipe system

with the following specifi cation:-

Material type: HPPE

Intended operating life: 5 years

Maximum operating temperature: 20 C

Pipe dimensions: 63 x 3.8 mm and 63 x 5.8 mm

SolutionFactor of safety C = 1.25 (from table 3.1).With life span 5 years and temperature t = 20 C fi nd = 10.4

(from chart 3.1).

The formula for determining the operating pressure is used:-

Worked example 3.2Calculate the maximum operating pressure for a pipe system

with the following specifi cation:-

Material type: HPPE

Intended operating life: 25 years

Maximum operating temperature: 30 C

Pipe dimensions: 63 x 3.8 mm and 63 x 5.8 mm

SolutionFactor of safety C = 1.25 (from table 3.1).With life span 25 years and temperature t = 30 C fi nd = 8.5

(from chart 3.1).

The formula for determining the operating pressure is used: -

20 10.4 3.81.25 (63 3.8)

10.7 bar3.8

20 8.5 3.81.25 (63 3.8)

8.7 bar3.8

20 8.5 5.81.25 (63 5.8)

13.8 bar5.8

20 10.4 5.81.25 (63 5.8)

16.9 bar5.8

Pressure andTemperature Relationship

2

9Tab

le 3

.2 P

ress

ure

/tem

per

atu

re r

elat

ion

sip

Ter

rain

HPP

E p

ress

ure

(sa

fety

fac

tor

1.25

)

Temperature C

Operating years

S cl

asifi

cat

ion

2520

1612

.510

.510

8.3

86.

35

43.

22.

52

SDR

cla

ssifi

cat

ion

5141

3326

2221

17.6

1713

.611

97.

46

5

PN r

atin

g

105

4.0

5.0

6.3

7.9

9.4

10.1

12.1

12.6

15.7

20.2

25.2

31.5

40.4

50.5

103.

94.

96.

27.

89.

39.

911

.912

.415

.519

.824

.831

.039

.749

.6

253.

84.

86.

07.

69.

09.

611

.612

.115

.119

.324

.230

.238

.748

.4

503.

84.

75.

97.

58.

99.

511

.411

.914

.819

.023

.829

.738

.047

.6

100

3.7

4.6

5.8

7.3

8.7

9.3

11.2

11.6

14.6

18.7

23.3

29.2

37.4

46.7

205

3.3

4.2

5.3

6.6

7.9

8.4

10.2

10.6

13.2

16.9

21.2

26.5

33.9

42.4

103.

34.

15.

26.

57.

88.

310

.010

.413

.016

.620

.826

.033

.341

.6

253.

24.

05.

06.

47.

68.

19.

810

.112

.716

.220

.325

.432

.540

.7

503.

24.

05.

06.

37.

58.

09.

610

.012

.516

.020

.025

.032

.040

.0

100

3.1

3.9

4.9

6.1

7.3

7.8

9.4

9.8

12.2

15.7

19.6

24.5

31.4

39.2

305

2.8

3.6

4.5

5.6

6.7

7.2

8.6

9.0

11.2

14.4

18.0

22.5

28.8

36.0

102.

83.

54.

45.

56.

67.

08.

58.

811

.014

.117

.722

.128

.335

.4

252.

73.

44.

35.

46.

46.

98.

38.

610

.813

.817

.221

.627

.634

.5

502.

73.

34.

25.

36.

36.

78.

18.

410

.613

.516

.921

.227

.133

.9

405

2.4

3.0

3.8

4.8

5.8

6.1

7.4

7.7

9.6

12.3

15.4

19.3

24.7

30.9

102.

43.

03.

84.

75.

76.

07.

37.

69.

512

.115

.219

.024

.330

.4

252.

32.

93.

74.

65.

55.

97.

17.

49.

211

.814

.818

.523

.729

.7

502.

32.

93.

64.

55.

45.

87.

07.

29.

111

.614

.518

.223

.329

.1

505

2.1

2.6

3.3

4.2

5.0

5.3

6.4

6.7

8.3

10.7

13.4

16.7

21.4

26.8

102.

02.

63.

24.

04.

85.

26.

26.

58.

110

.413

.016

.220

.326

.0

151.

92.

32.

93.

74.

44.

75.

75.

97.

49.

511

.814

.819

.023

.7

605

1.5

1.9

2.4

3.0

3.6

3.8

4.6

4.8

6.0

7.7

9.7

12.1

15.5

19.4

702

1.2

1.5

1.9

2.4

2.9

3.1

3.7

3.9

4.9

6.2

7.8

9.8

12.5

15.7

Op

erat

ing

Ye

ars

Tem

pera

ture

C

3

10

25

20

1

10

1.5

0.5

1

2

2.5

3

3.5

4

5

6

7

8

9

10

100 1000 10000 100000 100000 10000000Time (h)

Years

Circ

umfe

rent

ial S

tres

s (M

Pa)

15

1005025101 5

20C

30C

40C

50C

60C

10C

Chart 3.1 Life Regression for HPPE

Pressure andTemperature Relationship

4

11

It is recommended that if the allowable negative pressure

(Pe) is less than 1 bar then the pipeline system will not sustain vacuum. (1 bar = 0.98 Atmospheres.) Different thermoplastics

have different operating temperatures under a vacuum and

maximum installation temperatures must be observed, as

shown in table 3.3: -

Worked example 3.3A HPPE pipe operates under the following condition: -

Pipe outside diameter: 110mm

Intended service life: 10 years

Safety factor: 2

Modulus of Elasticity 900 MPa

Poissons Ratio 0.4

Determine the collapsing pressure and determine whether the

vacuum pressure can or can not be sustained,

for two cases: -

Pipe wall thickness: 6.6mm

Pipe wall thickness: 3.4mm

SolutionThe collapsing pressure is given by: -

(a) For e = 6.6mm, the collapsing and the vacuum pressure are calculated: -

Therefore the pipe will sustain this condition,

as Pe is greater than 1.

(b) For e = 3.4mm, the collapsing and the vacuum pressure are calculated: -

This pressure is lower than 1 bar; hence the pipe system can

not support this condition.

3.1 Maximum working conditions for

negative pressure

The design safety factor for negative

pressure is 2.

Pipeline systems operating below atmospheric pressure

(1 bar) are subjected to vacuum or negative pressure

and will tend to collapse radially inwards due to the

greater outside pressure.

The collapsing pressure can be shown by the following

formula: -

Where Pc Collapsing pressure (bar) 20 Proportionality constant

E Modulus of elasticity (MPa) (See table 1.1) Poissons ratio (0.4 for plastic) e Pipe wall thickness (mm) D Pipe outside diameter (mm) C Safety factor = 2 (Design safety factor for negative pressure)

The maximum allowable negative pressure (Pe) is obtained from the collapsing pressure (Pc) and safetyfactor (C) with: -

Table 3.3 Maximum installation temperatures for vacuum conditions

Thermoplastic material Maximum temperature under vacuum (C)

PN10 PN16

PE 60 60

PVCu 40 60

ABS 60 60

PP 80 80

20 900 (6.6/110)3 4.63 bar

2.31 bar4.63

20 900 (3.4/110)3 0.63 bar

0.63 0.32

5