Caroline  the SDR value of ...

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Caroline  the SDR value of the HDPE pipe will give you the nominal pressure rating of the pipe, Further the HDPE pipe as per AWWA C901 the allowable total pressure during recurring surge is 1.5 X times of the Nominal Pressure and occasional  Surge (Water Hammer) is 2 Times of  the Nominal Pressure. SAY SDR 9 Pipe of say nominal pressure rating of 100 psi then recurring surge would be 150 and occasional surge will be 200 psi. You need to check if your system generate more surge pressure as compared to the stated rated rating of Pipe. As Reuben defined the formula's will be used for calculating the surge pressure.