Why some manufacturer asked for 8D stright pipe in upstream of check valve? If I reduce this amount to 1D what will be happend? Is there anybody who knows this issue.

If there are any fittings, control valves, orifices, etc., close to the check valve they will cause turbulence. This may result in "chatter" and premature failure of the check valve. Our standards require straight runs of 5D upstream and 2D downstream of the check valve. Although you have not mentioned the type of check valve being used, 8D sounds very conservative. A 1D distance is too close for any type of check valve in continuous service.

An excellent source of information concerning this issue is ASME publication 82-PET-12, titled, "Behavior and Wear of Check Valves".  This publication addresses pipe diameters and expected wear that occurs as the result of turbulance caused by fittings and other devices.  This publication is probably out of print.

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