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??

Let me elaborate further

This clause has no referrence regarding pumps like say double Suction Pumps or Specific arrangement of Suction Piping layout.That means say a double suction(side suction side discharge) cooling Tower pump taking suction from Cooling Tower basin . Suction Pipe size 42inch or 36inch & pump Nozzle say 30” .

Based on the API 686 guideline around 4mtr of straight 30” Pipe will be required from Pump suction Nozzle & then say around 600/700 length reducer will come & then only the Suction below , Strainer , EV etc before connecting to Pump basin.

To me layout based on this is not a necessity but we will have to do it if we go for verbatim usage of API 686 clause.

This is because for this side suction side discharge (Double suction ) cooling water pump if there was an elbow in the horizontal plane on the suction pie before entering the pump then water on the outer periphery of the elbow will have higher velocity compared the water in the inner side.

Based on the Bernoulli”s equation concept it will create a higher pressure on the inner side compared to the outer side . Because of this differential pressure gradient there will be flow of water in the elbow from high pressure side to low pressure side. If there is not sufficient straight length ,one side impeller gets higher flow qty & another side impeller will relatively starve. This leads to eff drop as also mismatch of thrust bearing loading may be.

But this type of thing does not happen when there is a straight pipe from basin of the cooling Tower with the ecc reducer just before Pump Nozzle. The way the reducer creates eddies does not create imbalance between the two impeller sides.The turbulence is anyway happening when the water is entering thru the eye of the impeller.

Will this API 686 criteria need to be followed strictly every where? Please provide your valuable opinion and explain it.

Thanks & Regards

API RP 686 is an API Recommended Practice, therefore, unless contractual, it is not mandatory. In this case, better contact the vendor.

Regards,