How is PE pipe designed for given operating conditions?

First  it is necessary to decide what the pipe is to be designed for;

• Internal pressure capability e.g. water or gas distribution main or pressure sewer
• External pressure capability e.g. submerged pipeline
• Soil loading e.g. buried sewer pipe
• Dynamic loads, surge and fatigue e.g. sewer rising main

The starting point for the design of a PE pipe is the MRS (Minimum Required Strength) of the grade of PE to be used. Note that the MRS quoted takes into account the creep properties of PE 100 & PE100-RC and applies to operating temperatures up to 20°C. For operating temperatures in excess of 20°C, reference should be made to ISO 13761. The MOP (Maximum Operating Pressure) is related to the MRS of the material used; the pipe geometry (SDR; standard dimension ratio) and operating conditions by the following formulae:

When the SDR is known;

MOP = (20 x MRS) / (C x (SDR-1))

Or when the operating conditions are known;

SDR = 1 + ((20 x MRS) / (C x MOP))

Where:

• C is the ‘overall service (design) coefficient, or Safety Factor . (For PE the minimum values of C are: for gas 2.0; for water & presure sewerage 1.25)
• MRS is in MPa; (PE100/PE100-RC = 10 MPa, PE80 = 8 MPa)
• MOP is in bar

To calculate the SDR or MOP for a given standard pipe, use the SDR-MOP calculator.

The pipe size required is determined by the flow capacity needs. Hydraulic capacity is influenced by the frictional head loss in a pipe, which in turn is influenced by its surface roughness. PE has excellent surface characteristics and as a result the frictional losses are lower than with most other pipe materials, leading to a lower energy requirement to pump water or gas through the pipe.

The roughness coefficients for PE  in hydraulic calculations are:

• Colebrook-White equation: k =  0.00152 mm
• Hazen-Williams equation: c = 150 (dimensionless coefficient)

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