Standard dimension ratio (SDR) is a method of rating a pipe’s durability against pressure. The standard dimension ratio describes the correlation between the pipe dimension and the thickness of the pipe wall.
Common nominations are SDR11, SDR17 and SDR35. Pipes with a lower SDR can withstand higher pressures.
SDR = dn/en
- dn is the nominal outside diameter of the pipe
- en is the nominal (minimum) wall thickness of the pipe
Therefore a higher SDR indicates a thinner-walled pipe at any given diameter.
The relationship between the SDR and the pressure rating is given by Lames formula for the hoop stress in thick wall cylinders:
s = P(dn – en) / 2 en
This can be rearranged as;
s = P (SDR – 1) / 2
- s is the maximum hoop stress.
- P is the internal pipe pressure.
- The hoop stress is the design stress for the material, which is the (MRS) divided by the overall service (design) coefficient C.
MRS/C = P (SDR – 1) / 2
P = 2 MRS / C (SDR – 1)
This pressure ‘P’ is then defined as the ‘Maximum Operating Pressure’ MOP, or the pressure rating of the pipe.
MOP = 2 MRS / C (SDR – 1)
MRS and MOP are in MPa
MOP = 20 MRS / C (SDR – 1)
- MRS is in MPa
- MOP is in bar.
What is the MOP or pressure rating of an SDR11 PE100 water pipe?
For PE100 the MRS = 10 MPa. For water applications the minimum recommended service design coefficient ‘C’ is 1.25.
MOP = 20 * 10 / 1.25 (11 – 1)
MOP = 16 bar