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# What is SDR and how does it influence the pressure rating of the pipe?

The SDR is the ‘Standard Dimensional Ratio' and refers to the geometry of the pipe. SDR is defined as the ratio of the nominal outside diameter to the nominal wall thickness.

SDR = dn/en

Where;

• 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

Where;

• 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

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;

• MRS is in MPa
• 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

The following Table shows the range of available SDRs in PE100 pipe.

 SDR Available to Order Readily Commercially Available 7.4 9 11 13.6 17 17.6 21 26 33 41

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Created Wednesday 03 April 2013

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