Pressure - A

First they need to decide what the pipe is to be designed for;

• Internal pressure capability e.g. water or gas distribution main
• 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 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 pipe geometry 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 value of C is 1.25)
• MRS is in MPa; (PE100 = 10 MPa, PE80 = 8 MPa)
• MOP is in bar

To calculate the SDR or MOP for a given standard pipe, use the 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 used by our customers in hydraulic calculations are:

• Colebrook-White equation: k = 0.007 mm
• Hazen-Williams equation: c = 150 (dimensionless coefficient)