How UK polyethylene pipe is rated
UK water mains in polyethylene are made to BS EN 12201, which follows ISO 4427. A pipe's pressure class comes from its material and its standard dimension ratio (SDR), the outside diameter divided by the wall thickness.
The nominal pressure is PN = 20 × MRS / (C × (SDR − 1)), in bar. MRS is the minimum required strength of the resin (10 MPa for PE100, 8 MPa for PE80), and C is the design coefficient, 1.25 for water. For PE100 at SDR 11 that gives 16 bar (PN16); at SDR 17 it gives 10 bar (PN10).
The ratings apply at 20 °C. In warmer water the pipe carries less pressure, so this calculator lowers the rating in a straight line from its full value at 20 °C to 74% of it at 40 °C.
Worked example: 110 mm PE100 SDR 11 main
A 110 mm PE100 pipe at SDR 11 has a wall of 110 / 11 = 10 mm and a bore of 90 mm. Its rating is 20 × 10 / (1.25 × 10) = 16 bar at 20 °C.
Carrying 10 l/s, the water moves at about 1.57 m/s through the 90 mm bore, which is inside the usual range for a water main.
UK HDPE pipe inputs
Pipe outside diameter
The nominal outside diameter in millimetres. PE pipe is sold by outside diameter, such as 32, 63, 110 or 180 mm.
SDR
The outside diameter divided by the wall thickness. A lower SDR is a thicker wall and a higher pressure class.
Polyethylene grade
PE100 has a minimum required strength of 10 MPa and PE80 has 8 MPa.
Water temperature
The rating is for 20 °C and falls as the water gets warmer, down to 74% of it at 40 °C.
Design flow rate
The flow the main must carry, in litres per second. It sets the velocity and the head loss.
CALCULATOR QUESTIONS
UK PE100 HDPE pipe pressure & SDR calculator questions
What does PN16 mean on a PE100 pipe?
PN16 means the pipe is rated for a working pressure of 16 bar with water at 20 °C. For PE100 that is SDR 11.
What is the difference between PE100 and PE80?
PE100 resin has a minimum required strength of 10 MPa and PE80 has 8 MPa. At the same SDR, PE100 carries a higher pressure, or it can use a thinner wall for the same pressure.
How is the head loss worked out?
The head loss uses the Hazen-Williams equation with a roughness coefficient of 150, a common value for new polyethylene pipe. It is an estimate for design checks, not a replacement for a full hydraulic model.