As per clause 3.1.2.6 & 3.1.2.7 of API 686
3.1.2.6
The pump suction line shall have a minimum straight run of five pipe diameters between between the pump suction flange and the first elbow, tee, valve,reducer,permanent strainer or other obstruction. The straight run shall be of the same line size as the pump suction nozzle.

Note: the straight run length of five pipe diameter is usually sufficient to ensure stable and uniform at the pump impeller for typical centrifugal. This results in fewer pump failures over the life of the pump due to vibration caused by flow induced turbulence. High speed or high energy centrifugal may require greater suction line straight run lengths. The pump manufactures may be consulted for additional requirements for specific types of pumps.


Please provide your valuable opinion for this straight length requirement , Is this API 686 criteria need to be followed strictly for all type of pump suction piping.I saw at many places(especially in CW pumps) that eccentric reducer are installed just before the suction nozzle flange without providing staright piping length(5D).Does it mean that this clause is not necessary to be satisfied.Please tell me the cases where this clause must be satisfied and why??