- Chemical Resistance
- Safety Data Sheets (SDS)
- Material Properties
- PRO Systems
- PE Pressure Pipe
- PE Pipe Selection
- MAOP for PE Pipes
- Temperature Influences
- E-Modulus
- Selection of Wall Thickness for Special Applications
- Hydraulic Design for PE Pipes
- Surge and Fatigue
- Celerity
- Slurry Flow
- Pneumatic Flow
- Expansion and Contraction
- External Pressure Resistance
- Allowable Bending Radius
- Thrust Block Support
- Conductivity, Vibration and Heat Sources
- Polyethylene Jointing
- Handling and Storage
- Trench Preparation for Buried Pipes
- Relining and Sliplining
- Pipeline Detection
- Above Ground Installation
- Accommodation of Thermal Movement by Deflection Legs
- Service Connections for PE Pipes
- Concrete Encasement
- Fire Rating
- Testing and Commissioning

- PVC Pressure Pipe
- PVC Pressure Pipe Standards
- Pressure Considerations
- PVC Temperature Considerations
- Mine Subsidence
- Buckling
- Water Hammer
- Thrust Support
- Air and Scour Valves
- Soil and Traffic Loads
- Bending Loads
- PVC Pipe Jointing
- Jointing Components with Ductile Iron Flanged Joints
- Service Connections for PVC Pipe
- PVC Pipe Handling and Storage
- Below Ground Installation
- Above Ground Installation for PVC Pipe
- Testing and Commissioning for PVC Pressure Pipe
- Detecting Buried Pipes

- FLUFF – Friction Loss in Uniform Fluid Flow
- Technical Notes

# External Pressure Resistance

The possibility of external pressure (buckling) being the controlling design condition must be evaluated in the design of PE pipelines.

All flexible pipe materials can be subject to buckling due to external pressure or internal vacuum and PE pipes behave in a similar fashion to PVC and steel pipes.

## Unsupported collapse pressure

For pipe of uniform cross-section, the critical buckling pressure (Pc) can be calculated as follows:

where

Pc = critical buckling pressure, kPa

E = modulus, MPa from E Modulus

SDR = pipe SDR from Table 4.1

As the modulus is temperature and time dependent, the advice of Vinidex engineers should be sought for appropriate values.

## Effect of Ovality

For an oval pipe with,

where:

δD = the difference between the maximum outside diameter and the mean outside diameter; and D = the mean outside diameter

Values for C_{1} are summarised in the following table:

Diametral Deflection % | 0 | 1 | 2 | 5 | 10 |

Reduction factor, C_{1}, on Pc |
1 | 0.91 | 0.84 | 0.64 | 0.41 |

Note that these reductions apply to inherent initial ovality as distinct from induced ovality, ie., where ovality has been induced by some external (constant strain) source, as in deflection induced by soil loadings, the pipe is already in a state of elastic strain, and the resistance to buckling is degraded far less. In this case, the correction factor C_{2} is used as given in the following table:

Diametral Deflection % | 0 | 1 | 2 | 5 | 10 |

Reduction factor, C_{2}, on Pc |
1 | 0.99 | 0.97 | 0.93 | 0.86 |

## Supported collapse pressure

Support against buckling is provided by end constraints, fittings or special purpose stiffening rings at intervals around the pipe. Effective support reduces as distance from a stiffened section increases and should be considered zero at a distance of seven diameters.

A buried pipe derives support against buckling from stable soil surround. The effective buckling pressure may be computed from:

Where:

S_{c} is the ring bending stiffness calculated from

Note: S is the stiffness in N/m/m eg. S_{DL} in AS/NZS 2566.1

E’ is the soil modulus in MPa. For values of E’, refer to AS/NZS 2566.1.

For shallow burial, full support is not developed since buckling can occur by vertical lifting of the soil cover. AS/NZS 2566.1 specifies that for cover heights less than 0.5m, P_{c} be used to evaluate the potential for buckling. Where cover heights are greater than or equal to 0.5m, the greater of P_{c} and P_{b} should be used. In both cases, an appropriate factor of safety needs to be incorporated.

## Factors of Safety

The values of Pc and Pb calculated require a factor of safety to be applied. AS/NZS 2566.1 specifies a factor of safety of 2.5 unless an alternative is specified by the designer. A lower factor of safety should only be specified where conditions and performance can be predicted with confidence. For example, in a calculation of unsupported buckling of a pipe under vacuum, a factor of 1.5 may be appropriate. In general terms, PN10 PE pipe should be used as a minimum for pump suction line installations.

## Construction techniques

Where installation conditions potentially lead to negative pressures, consideration may need to be given to modification of construction technique. For example, ducting pipes may need to be sealed and filled with water during concrete encasement. In operation, fluid may be removed from the pipeline faster than it is supplied from the source. This can arise from valve operation, draining of the line or rupture of the line in service. Air valves must be provided at high points in the line and downstream from control valves to allow the entry of air into the line and prevent the creation of vacuum conditions. On long rising grades or flat runs where there are no significant high points or grade changes, air valves should be placed at least every 500-1000 metres at the engineer’s discretion.