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

Where;

1. dn is the nominal outside diameter of the pipe
2. 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

Where;

1. s is the maximum hoop stress.
2. P is the internal pipe pressure.
3. 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

or rearranging

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)

Where

MRS and MOP are in MPa
Or;

MOP = 20 MRS / C (SDR – 1)

Where

1. MRS is in MPa
2. MOP is in bar.

Example

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.

Hence : 

MOP = 20 * 10 / 1.25 (11 – 1)

MOP = 16 bar